Nervous system of man

The nervous system

The body of an animal is frequently exposed to a variety of stimuli in its daily life. To respond to a particular stimulus, usually more than one body part is involved, and their activities are coordinated either by the nervous system or the endocrine system or both.

Nervous system

The system of the body that provides coordination through electric signals among different body parts for the response to a particular stimulus.

Neurology

The study of the structure and functions of the nervous system. The term comes from "neuron" (nerve) and "logia" (the study of).

The human nervous system is the most evolved among all the animals. Nervous coordination in higher animals consists of three basic steps.

1. Reception of the stimulus by receptors.
2. Processing or analysis of the information by the nervous system.
3. Response to the stimulus by an effector.

5.1Receptors are transducersOften examined

A transducer is an electronic device that converts energy from one form to another. A cell that responds to a stimulus is called a receptor cell. Receptor cells are transducers: they convert energy from one form such as light, heat or sound into energy in an electrical impulse (electrochemical energy) within a sensory neuron. So receptors are transducers sensitive to various stimuli.

Transducer

A device or cell that converts energy from one form to another.

Receptor cell

A cell that responds to a stimulus.

Receptors are biological transducers that convert energy from both the external and internal environments into electrical impulses. They may be massed together to form a sense organ, such as the eye or ear, or they may be scattered, as are those of the skin and viscera.

5.2Path of a message transmitted to the central nervous system

Sensory information travels from the body to the spinal cord before reaching the brain. The path is a relay of three neurons, named by their order along the pathway.

In the pain pathway, the second-order neuron may be located in the dorsal horn of the spinal cord, where it receives signals from the first-order neurons and then projects to the thalamus. Somatosensory sensations are pressure, pain, temperature and the body's senses.

5.3NeuronOften examined

Nervous coordination mainly comprises highly specialized cells called neurons (new-ronz). Neurons are the basic structural and functional unit of the nervous system.

Structure of neuron (5.3.1)

Although neurons vary considerably in size and shape, they all have three basic components: a cell body, dendrites and an axon.

Cell body

The cell body is the main part of a neuron. It contains a mass of granular cytoplasm and a cell membrane. The single large nucleus is centrally placed with a prominent nucleolus. Golgi apparatus, mitochondria and other organelles are present. The cytoplasm is characterised by the presence of Nissl's granules, which are groups of ribosomes and rough ER associated with protein synthesis.

Dendrites

Dendrites are short and thin, often highly branched cytoplasmic extensions that gradually taper from their bases to their tips. Axons of other neurons form synapses with the dendrites. The function of the dendrite is to receive stimuli and conduct impulses to the cell body.

Axon

An axon is comparatively a long and thick nerve fibre which has a constant diameter and can vary in size from a few mm to more than a metre in length. It may be branched or un-branched. Axons terminate by branching to form small extensions with enlarged ends called presynaptic terminals. Functionally, axons conduct action potentials from the neuron cell body to the presynaptic terminals, that is, they conduct the signal (information) away from the cell body.

Neuron

A highly specialised cell that is the basic structural and functional unit of the nervous system.

Nissl's granules

Groups of ribosomes and rough ER in the cytoplasm of the cell body, associated with protein synthesis.

Presynaptic terminals

The small extensions with enlarged ends at which an axon terminates.

5.3.2Types of neuronOften examined

All neurons vary somewhat in size, shape and characteristics depending on the function and role of the neuron. Based upon the function there are three types of neurons: sensory neurons, motor neurons and interneurons.

Types of neuron based on structure

Neurons can also be classified by the number of processes that leave the cell body. Fig. 5.4 shows four kinds.

5.3.3Effectors

The two examples of effectors are muscles and glands. The nervous system responds by sending signals to muscles and glands. The signals cause muscles to contract. The signals cause glands to produce secretions. Muscles and glands are called effectors because they cause an effect in response to directions from the nervous system.

Effector

A muscle or gland that responds to the signals of the nervous system (muscles contract, glands secrete).

5.3.4Detailed structure of a sensory neuron, associative and motor neuronOften examined

a. Sensory neurons (afferent neurons)

The cell body or soma is located in the dorsal root ganglion outside the CNS. Sensory neurons often have their cell bodies close to the spinal cord. Dendrites are specialized extensions that receive signals from sensory receptors. They are often long in peripheral sensory neurons. The axon extends from the sensory receptors to the CNS. In some cases they may be myelinated to speed up the signal transmission.

Most sensory neurons are unipolar, having a single elevated process that branches into two. One branch connects to the sensory receptors and the other branch connects to the CNS. The signal pathway starts from the sensory receptor, moves along the axon and reaches the spinal cord or brain where further processing occurs.

b. Motor neurons (efferent neurons)

The cell body is located in the spinal cord (for lower motor neurons) or in the brain's cortex (for upper motor neurons). Motor neurons have many dendrites that receive inputs from interneurons and other neurons. The axon is long and often myelinated to allow rapid signal transmission from the CNS to the muscles or glands. Most motor neurons are multipolar. Motor neurons transmit signals from the CNS to muscles and glands, allowing for movement and glandular response.

c. Interneurons

The cell body is located entirely within the brain and spinal cord. Dendrites are highly branched to receive information from multiple neurons, making them important for processing complex information. Their axons are generally short and unmyelinated, as they primarily transmit signals locally within the CNS. An interneuron is often multipolar, which allows it to communicate with many neurons simultaneously. Interneurons serve as connectors between sensory and motor neurons. They are critical in reflexes, neural processing and communication within the CNS.

5.3.5Relate the structure of neurons with functionsOften examined

The structure of each type of neuron is intricately designed to support its specific function within the nervous system, ensuring efficient communication and response mechanisms. Fig. 5.8 relates each part of a neuron to its function.

Structure and function of each type

• Motor neurons have long axons, which are essential for covering the considerable distances between the central nervous system and the target muscles or organs. The axons are myelinated, which enhances the speed of signal transmission, allowing the rapid communication necessary for quick muscle responses.
• Sensory neurons possess long dendrites and axons, which are vital for receiving signals from sensory receptors and transmitting them over distances to the central nervous system. Sensory neurons are myelinated and often located in ganglia, which are clusters of neuron cell bodies. This arrangement aids in processing sensory information and contributes to reflex actions.
• Interneurons are generally shorter than both motor and sensory neurons, which is suitable for their role in local processing. Interneurons have numerous branches of axons and dendrites, allowing them to form multiple connections with other neurons. Their partial myelination and clustering in large groups help to enhance the speed of signal transmission despite their shorter length.

5.3.6Differences between myelinated and non-myelinated neuronsOften examined

The axons may be categorized into myelinated (with sheath) and non-myelinated (without sheath). The myelinated fibres are present in the white part of the brain and also in the spinal cord, while the non-myelinated nerve fibre is present in the autonomic nervous system. Myelinated axons tend to be larger in diameter than the non-myelinated axons.

5.4Reflex arcOften examined

Reflex action

An immediate, automatic and involuntary response to external and internal environmental changes.

Reflex arc

The path of the nerve impulse during a reflex action.

A typical reflex arc includes five fundamental parts: receptors, sensory neurons, interneuron, motor neuron and effectors.

5.5Nerve impulseOften examined

Nerve impulse

Information or signal about a stimulus that is transmitted from receptors to the CNS and from the CNS to the effectors. In technical terms, a nerve impulse is a wave of electrochemical changes that travel along the length of a neuron from one end to the other.

Generation and transmission of nerve impulse (5.5.1)

The word "electrochemical" refers to the electrical potential (a capacity to do electrical work) that exists on the neuron membrane. In the case of a neuron, the electrical potential is termed the membrane potential, which is exhibited in two different forms: the Resting Membrane Potential (RMP) and the Active Membrane Potential (AMP).

5.5.2Resting membrane potentialOften examined

The resting membrane potential is characterised by a more positive outer surface of the neuron membrane than the inner surface. This state is also referred to as the polarised state and the neuron is supposed to be at rest. This means that there is an unequal distribution of ions on the two sides of the nerve cell membrane. The potential measures about 70 mV, with the inside of the membrane negative with respect to the outside. So the resting membrane potential is expressed as −70 mV, and the minus sign indicates that the inside is negative relative to (compared to) the outside.

It is called a resting membrane potential because it occurs when a membrane is not being stimulated or conducting impulses. The resting membrane potential is established by the following factors.

1. Distribution and active movement of Na⁺ and K⁺ ions

The concentration of potassium (K⁺) is about 30 times greater in the fluid inside the neuron than outside, and the concentration of sodium ions (Na⁺) is nearly 10 times greater in the fluid outside the cell than inside. These ions are continuously moved against their concentration gradient through sodium-potassium pumps by the expenditure of energy. For every two K⁺ that are actively transported inward, three Na⁺ are pumped out. So the inside becomes more negative than the outside of the neuron membrane.

2. Negative organic ions

There are many types of organic compounds in the neuron cytoplasm that also have negative charges. These ions include some amino acids, many proteins and RNA. The presence of these ions in the neuron cytoplasm makes the inside of the neuron more negative than the outside.

3. Leakage of K⁺ ions

The cell membrane of a neuron also has many channel proteins called gates. K⁺ ions leak continuously through the leaky K⁺ gates. This also makes the outside of the neuron more positive than the inside.

Overall there are more positive charges on the outside than on the inside. This is known as the resting membrane potential. It will be maintained until the membrane is disturbed or stimulated by a sufficiently strong stimulus (threshold), when an action potential will be produced.

5.5.3Active membrane potentialOften examined

The active membrane potential (also called action potential) is characterised by a more positive inside of the neuron than outside (depolarised state). This happens when positive charges tend to move inside the neuron on receiving a particular stimulus. This electrochemical change appears on a short region of the neuron for a brief period of time, followed by the recovery of the polarised state. In this way a wave of action potential begins to move towards the other end of the neuron.

Threshold stimulus

Threshold stimulus

A stimulus that is capable of producing an action potential in a neuron.

Sub-threshold stimulus

A stimulus that is not capable of exciting the neuron or of raising any response.

Influx of Na⁺ ions (depolarization)

When a neuron fibre is stimulated by a threshold stimulus, it causes the opening of voltage-regulated Na⁺ gates. As a result the Na⁺ gates permit the influx of Na⁺ ions by diffusion. Since more Na⁺ ions enter than leave, the electrical potential of the membrane changes from −70 mV towards zero and then reaches about +50 mV. This reversal of polarity across the two sides of the membrane is called depolarization. This electropositivity inside and electronegativity outside lasts for about one millisecond, until the Na⁺ gates are closed.

Repolarization of neuron fibre

A fraction of a second after the sodium gates open, depolarization of the axon membrane causes the potassium gates to open. Potassium therefore diffuses out of the cell. Since the potassium is positively charged, this makes the inside of the cell more negative and starts the process of repolarization.

Hyperpolarization

At the peak of the action potential, the sodium gates start to close again. Sodium permeability therefore declines. The sodium-potassium pump continues to work during this time, so it gradually begins to restore the original resting potential. This repolarization is shown by the falling phase of the action potential spike and results in the membrane potential returning to its original level. In fact, there is a slight overshoot into a more negative potential than the original resting potential. This is called hyperpolarization. It is due to the slight delay in closing all the potassium gates compared with the sodium gates.

Refractory period

After an action potential, the nerve fibre undergoes a period of recovery in which it regains its original ionic distribution and polarity and prepares itself for the next stimulation. This period of recovery of the nerve fibre is called the refractory period. Although a repolarised neuron fibre has the same polarity as that of a polarised neuron fibre, it has a different ionic distribution: it has more K⁺ outside and more Na⁺ inside. So the repolarised nerve fibre undergoes a refractory period of a few milliseconds, during which the original ionic distribution is restored by the sodium-potassium pump, which actively transports Na⁺ ions out and K⁺ ions in. This returns the membrane to its resting potential, that is, −70 mV. The refractory period lasts for about 4 milliseconds, so a neuron can conduct 250 impulses per second.

5.5.4Velocities of nerve impulseOften examined

The velocities of nerve impulse in the axon membrane and in the synaptic cleft are variable. In human non-myelinated fibres, nerve impulses travel at 1 to 3 metres per second. Myelinated fibres conduct at speeds of up to 120 metres per second.

What makes a nerve impulse faster

• Saltatory conduction. The rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another, skipping the myelinated regions of membrane. It is up to 50 times faster than conduction through the fastest unmyelinated axons because the impulse does not have to travel through every single space before moving to the next.
• Myelin sheath as an insulator. Myelin acts as an insulating sheath and prevents loss of energy, so myelinated neuron fibres require less energy.
• Diameter of the fibre. Thick neuron fibres conduct impulses faster than thin fibres, because resistance to electrical current flow is inversely proportional to the cross-sectional area of the conductor. With an increase in thickness of the neuron fibre there is a decrease in resistance of the fibre to the nerve impulse.

Saltatory conduction

The rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another, skipping the myelinated regions of membrane.

Synaptic delay

The time of about a millisecond that the short journey across the synapse takes, which is longer than a nerve impulse takes to travel the same distance in an axon.

5.5.5Role of local circuits in saltatory conductionOften examined

Local circuits

Circuits of current that create depolarization in the next section of the axon membrane. They play a key role in saltatory conduction, the process by which nerve impulses jump from node to node in myelinated nerve fibres.

Local circuits, or the diffusion of sodium ions along the axon, trigger the depolarization of the next section of the axon membrane. This process causes the action potential to appear and jump from one node of Ranvier to the next.

5.6SynapseOften examined

Synapse

The junction between the axon terminal of one neuron and the dendrite of another neuron, where information from one neuron is transmitted or relayed (handed over) to another neuron.

Structure of synapse (5.6.1)

The neurons are not in direct contact at a synapse. There is a gap, called the synaptic cleft, between them. A single neuron may form synapses with many incoming fibres of different neurons.

Presynaptic neuron

The neuron that carries an impulse toward a synapse.

Postsynaptic neuron

The neuron that receives the impulse after it crosses the synapse.

Synaptic cleft

The gap between the axon terminal of the presynaptic neuron and the postsynaptic neuron.

5.6.2Mechanism of synaptic transmission of nerve impulseOften examined

The movement of an impulse across the synapse is called synaptic transmission. It takes place by the formation of a message which is transmitted across the synapse in the form of a chemical messenger called a neurotransmitter. The axons usually have several rounded synaptic knobs at their distal ends, which dendrites lack. These knobs contain numerous membranous sacs, called synaptic vesicles, and when a nerve impulse reaches a knob, some of the vesicles respond by releasing a neurotransmitter.

Synaptic transmission

The movement of an impulse across the synapse, by means of a chemical messenger.

Neurotransmitter

A chemical messenger released from the synaptic vesicles that carries the message across the synaptic cleft.

Excitatory postsynaptic potential (EPSP)

The depolarization of the postsynaptic membrane towards threshold level caused by the neurotransmitter, which can generate an action potential.

5.6.3Classifications of neurotransmitters

Neurotransmitters are classified as excitatory and inhibitory.

Excitatory neurotransmitters

Neurotransmitters that cause increased membrane permeability to sodium ions and, thus, trigger nerve impulses are said to be excitatory. Acetylcholine is an excitatory neurotransmitter of the peripheral nervous system, whereas biogenic amines (amino acid derivatives) are important neurotransmitters in the central nervous system. They include epinephrine, norepinephrine, serotonin and dopamine, all of which also function as hormones. Epinephrine and norepinephrine increase the heartbeat rate during stress. Serotonin and dopamine affect sleep, mood, attention and learning.

Inhibitory neurotransmitters

Other neurotransmitters cause decreased membrane permeability to sodium ions, thus causing the threshold of stimulus to be raised. This action is called inhibitory, because it lessens the chance that a nerve impulse will be transferred to an adjoining neuron, for example amino acids gamma-aminobutyric acid (GABA) and glycine. The endorphins are peptides that function as both neurotransmitters and hormones, decreasing our perception of pain.

5.7Main components of nervous systemOften examined

The human nervous system consists of the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is a coordinating centre and it lies in the midline of the body, whereas the PNS transmits information from receptors to the CNS and transmits orders and commands from the CNS to effectors.

5.7.1Architecture of human brain and spinal cord and their functionsOften examined

The central nervous system consists of the brain and spinal cord, and both are hollow. The brain and spinal cord are covered with three protective membranes called meninges (singular: meninx). The brain is enclosed within the cranium, while the spinal cord is enclosed within the vertebral column.

• Dura matter (next to the cranium)
• Arachnoid matter (middle membrane)
• Pia matter (next to the nervous tissue)

Between the arachnoid and pia matter there is a fluid, the cerebrospinal fluid (CSF), which helps to cushion the brain from shock.

Meninges

The three protective membranes covering the brain and spinal cord: dura matter, arachnoid matter and pia matter.

Cerebrospinal fluid (CSF)

The fluid between the arachnoid and pia matter that helps to cushion the brain from shock.

5.7.1The brainOften examined

The brain is divided into three parts: forebrain, midbrain and hindbrain. The forebrain consists of the cerebrum, thalamus and limbic system. The hindbrain consists of the cerebellum, medulla oblongata and pons.

Forebrain

Cerebrum

The cerebrum is the largest part of the human brain. It is divided into two cerebral hemispheres which are interconnected with each other by a band of axons, called the corpus callosum. Each hemisphere contains four surface lobes: frontal, parietal, temporal and occipital lobe.

Each lobe further contains different functional areas, for example auditory (hearing) and visual areas. Each functional area consists of three sub-areas: sensory area, association area and motor area. The sensory area receives impulses from different body parts. The association area interprets or analyses the incoming information. The motor area controls the responses of the body. The cerebrum also functions in the analysis and interpretation of memory, reasoning, judgement, thoughts and dreams.

Thalamus and limbic system

The thalamus is below the cerebrum. It receives all sensory impulses (except the sense of smell) and channels them to the limbic system and to the appropriate regions of the cortex for interpretation. The limbic system is a complex set of structures that lies on both sides of the thalamus, just under the cerebrum. It includes the hypothalamus, the amygdala, the hippocampus and several other nearby areas.

• Hypothalamus: on the ventral side of the thalamus. It maintains homeostasis and contains centres for regulating hunger, sleep, thirst, body temperature, water balance and blood pressure, menstrual cycle and sleep-wake cycle. It also controls the pituitary gland and thereby serves as a link between the nervous and endocrine system.
• Amygdalae: two almond-shaped masses of neurons on either side of the thalamus. They control feelings and emotions of love, hate, anger, fear, rage and sexual arousal.
• Hippocampus: consists of two "horns" that curve back from the amygdala. It appears to be very important in converting things that are "in your mind" at the moment (in short-term memory) into things that you will remember for the long run (long-term memory).

Midbrain

The midbrain is reduced in humans. It acts as a relay station for tracts passing between the cerebrum and the spinal cord or cerebellum. The midbrain contains the reticular formation, which is a relay centre connecting the hindbrain with the forebrain.

Hindbrain

The hindbrain consists of the cerebellum, medulla oblongata and pons.

• Cerebellum controls equilibrium, that is, body position, and coordination of the actions of individual muscles to produce complex activities such as walking, running, riding bicycles and doing delicate work with the hand. The cerebellum is also involved in learning and memory storage for behaviour.
• Pons acts as a bridge between the cerebellum, medulla and cerebrum. It also controls the rate and pattern of heartbeat and breathing.
• Medulla controls the automatic functions of the body, such as heartbeat, blood pressure, respiration and swallowing.

The brain is a hollow structure as it has cavities called ventricles. There are four ventricles in the brain.

In a cross-section of the brain the cerebral cortex forms the outer grey matter and the cerebral medulla forms the inner white matter. The arrangement of grey and white matter in the brain is opposite to that of the spinal cord.

5.7.1Spinal cordOften examined

The spinal cord is the most important structure between the body and the brain. It extends from the medulla to the level of the lumbar vertebrae. It is a vital link between the brain and the body.

A transverse section of the adult spinal cord shows white matter in the periphery, grey matter inside and a tiny central canal filled with CSF at its centre. The grey matter is shaped like the letter "H" or a "butterfly".

Several pairs of spinal nerves originate from the ventral and dorsal horn of the grey matter. The dorsal root of the spinal nerves also contains ganglia present just beside the spinal cord.

5.7.2Cranial and spinal nerves in manOften examined

Cranial nerves

The peripheral nervous system consists of the nerves that branch out from the central nervous system and connect it to other body parts. The peripheral nervous system includes cranial nerves, which arise from the brain, and spinal nerves, which arise from the spinal cord.

A number of cranial nerves send electrical signals between the brain and different parts of the neck, head and torso. These signals help us to smell, taste, hear and move our facial muscles. The cranial nerves begin toward the back of the brain. There are twelve cranial nerve pairs. Each nerve pair splits to serve the two sides of the brain and body. For example, we have one pair of olfactory nerves: one olfactory nerve is on the left side of the brain and one is on the right side of the brain. The twelve nerves are numbered from I to XII using Roman numerals according to the order in which they emerge from the front of the brain to the back of the brain. The twelve cranial nerves each have a specific function. The cranial nerves have been categorised based on number and function.

Spinal nerves

A spinal nerve carries motor, sensory and autonomic signals between the spinal cord and the body. Each spinal nerve is a mixed nerve. It is formed from the combination of nerve root fibres from its dorsal and ventral roots.

There are 31 pairs of spinal nerves, one on each side of the vertebral column. These include eight cervical nerve pairs (nerves starting in the neck and running mostly to face and head), twelve thoracic nerve pairs (nerves in the upper body that extend to chest, upper back and abdomen), five lumbar nerve pairs (nerves in the low back that run to legs and feet), five sacral nerve pairs (nerves in the low back extending into the pelvis) and one pair of coccygeal nerves. The spinal nerves are part of the peripheral nervous system.

Outside the vertebral column, the nerve divides into branches. The dorsal ramus contains nerves that serve the posterior portions of the trunk. The ventral ramus contains nerves that serve the remaining anterior parts of the trunk and the upper and lower limbs.

5.7.3Peripheral nervous systemOften examined

The peripheral nervous system can also be subdivided into the somatic and autonomic nervous systems.

Somatic nervous system

Generally, the somatic nervous system consists of the cranial and spinal nerve fibres that connect the CNS to the skin and skeletal muscles; it is involved in conscious activities.

Autonomic nervous system

The autonomic nervous system includes those fibres that connect the CNS to the visceral organs, such as the heart, stomach, intestines and various glands. It is concerned with unconscious activities. The autonomic system is divided into the sympathetic and parasympathetic systems. Both of these systems function automatically and usually subconsciously, in an involuntary manner.

Sympathetic division

The sympathetic division controls various autonomic functions during the state of emergency. It prepares the body for the fight or flight response. It consists of only spinal nerves. These nerves arise from the first thoracic segment to the second lumbar segment of the spinal cord.

Parasympathetic division

A few cranial nerves, including the vagus nerve, together with nerves that arise from the sacral portion of the spinal cord, form the parasympathetic division. It controls various autonomic functions during the state of rest. In short, the parasympathetic system returns the body functions to normal after they have been altered by sympathetic stimulation. In times of danger, the sympathetic system prepares the body for violent activity. The parasympathetic system reverses these changes when the danger is over.

5.7.4Sensory receptors and their workingOften examined

The body must detect what is occurring inside and outside the body, and this is performed by sensory receptors. Here we discuss receptors for smell, taste, touch and pain.

Olfactory receptors

The smell or olfactory receptors are chemoreceptors, stimulated by chemicals dissolved in liquids. The olfactory organs, which contain the olfactory receptors, are present in the upper part of the nasal cavity. The olfactory receptor cells are neurons. These cells are surrounded by columnar epithelial cells having cilia at the distal ends. Chemicals that stimulate the olfactory receptors enter the nasal cavity as gases. They must dissolve at least partially in the watery fluids that surround the cilia before they can be detected.

Taste receptors

Taste buds occur primarily on the surface of the tongue and are associated with tiny elevations called papillae. Each taste bud includes a group of modified epithelial cells, the taste cells, which function as receptors. The taste bud has an opening, the taste pore, on its surface. Tiny projections, called taste hairs, protrude from the outer ends of the taste cells and just protrude through the taste pore. There are four primary taste sensations: sweet, sour, salty and bitter, which are situated at various regions on the tongue. All the four regions overlap at certain places.

Sensory receptors in human skin

The dermis of the skin contains receptors for touch, pressure, temperature and pain. Meissner's corpuscles and Merkel disks are touch receptors. These consist of small, oval masses of flattened connective tissue cells. Two or more sensory nerve fibres branch into each corpuscle. Meissner's corpuscles are especially numerous in the lips, fingertips, palm and soles. Paccinian's corpuscles are also encapsulated nerve endings present in the fatty layer deep into the skin. They are concerned with the sensation of pressure. Receptors for touch and pressure are also called mechanoreceptors. The skin also has cold and heat receptors to detect the temperature variations.

Pain receptors are technically called nociceptors. Pain receptors are located at the top of the skin in the epidermis area to detect pain. These receptors are free nerve endings that respond to chemicals released by damaged tissues or excess stimuli of heat or pressure. These receptors are widely distributed throughout the skin and inter tissues, except in the tissue of the brain.

5.8Effects of drugs on nervous coordinationOften examined

A narcotic is a group of substances which, when administered, diminish the perception of pain. Narcotics bind to certain painkilling sites in the brain. With constant use, they build up in the brain and block the production of endorphins, the brain's natural painkilling chemicals. Their side effects are inhibition of the endocrine and autonomic nervous system and so on. The narcotics are the drugs that act as agents which interact with normal nervous activity.

Narcotic drugs

Drugs that act as agents which interact with normal nervous activity; they diminish the perception of pain by binding to painkilling sites in the brain.

Endorphins

The brain's natural painkilling chemicals, whose production is blocked when narcotics build up with constant use.

5.8.1Narcotic drugsOften examined

Common narcotic drugs are heroin, Cannabis, nicotine and alcohol and so on.

Heroin

Heroin gives a feeling of euphoria along with relief of pain. Side effects can include nausea, vomiting, respiratory and circulatory depression leading to death.

Cannabis

It is the dried flowering tops, leaves and stem of the Indian hemp plant Cannabis sativa. It includes marijuana and hashish. Usually the users report a mild euphoria, along with alterations in vision and judgement. Intoxication is recognised by the presence of hallucinations, anxiety and depression.

Nicotine

It is an alkaloid derived from tobacco. When smoking a cigarette, nicotine is quickly distributed to all body organs. In the peripheral nervous system, nicotine stimulates postsynaptic receptors (like acetylcholine) and leads to increased skeletal muscular activity. It also increases heartbeat rate and blood pressure.

Alcohol

Alcohol acts as a depressant and slows down nervous communication. Its short-term effects are impairment of vision, judgement and alertness. Long-term chronic drinking can damage the nervous system, liver and pancreas.

Withdrawal symptoms of alcohol

Individuals who eliminate addictive substances from their lives often feel withdrawal symptoms. Alcohol withdrawal refers to a group of symptoms that may occur from suddenly stopping the use of alcohol. The symptoms are: feeling of anxiety, irritability, depression, headache and hallucinations.

Inhalants

These are volatile organic chemicals, commonly referred to as "glue sniffing". Inhalant abuse now includes aerosols (for example hair spray) and anaesthetics (for example ether). Inhalants rapidly start euphoria followed by central nervous system depression. Deep breathing of the toxic vapours may result in hallucinations or even death.

5.8.2Drug addiction and drug toleranceOften examined

Drug addiction

A dependence on an illegal drug or a medication. You may want to quit, but most people find they can't do it on their own.

Drug tolerance

A person's diminished response to a drug, which occurs when the drug is used repeatedly and the body adapts to the continued presence of the drug, for example caffeine and nicotine.

a. Caffeine

Caffeine is a drug, and individuals can develop both tolerance and a form of dependence.

• Caffeine tolerance: tolerance to caffeine means that the body becomes accustomed to its effects, requiring higher doses to achieve the same level of alertness or stimulation.
• Caffeine addiction: it does not cause classic addiction the way some drugs do.
• Harmful effects of caffeine: insomnia, rapid heart rate, anxiety, frequent urination, digestive issues, fatigue, high blood pressure and irritability tremors.

b. Nicotine

Nicotine is the chemical in tobacco that makes it hard to quit.

• Nicotine tolerance: it occurs when the body requires increasing amounts of nicotine to achieve the same effects initially felt with lower doses.
• Nicotine addiction: nicotine produces pleasing effects in your brain, but these effects are temporary. So you reach for another cigarette. The more you smoke the more nicotine you need to feel good.
• Harmful effects of nicotine: cancer, nausea, dizziness, emphysema, infertility, insomnia, cardiovascular diseases, headache and mouth ulcer.

5.8.3Effects of drug addiction and tolerance on the central nervous system

Drugs are chemicals that interfere with the way neurons normally send, receive and process information. Some drugs can activate neurons because their chemical structure mimics that of a natural neurotransmitter. Drugs interact with the brain and body to alter moods, emotions and behaviours by changing brain chemistry. The regions of the brain affected by drug abuse are the brain stem, limbic system and cerebral cortex.

• Depressants all work by slowing down the functioning of the central nervous system.
• Stimulants can produce a number of effects on the body such as increased heart rate, improved concentration and increased respiratory rate.

5.9Pain medicines reduce or numb pain in human bodyOften examined

Analgesics are also called painkillers. These medicines reduce or numb pain in the body through various mechanisms that affect the nervous system. There are different types of pain medications and each works in a unique way to interfere with pain signals. The main categories of pain medications and how they function are as follows.

• a. Non-steroid anti-inflammatory drugs (NSAIDs): these medicines work by inhibiting the activity of an enzyme called cyclooxygenase (COX), which plays a key role in the production of prostaglandins. Prostaglandins are chemicals released at the site of injury or inflammation that cause pain, swelling and fever. By reducing the production of prostaglandins, NSAIDs help to reduce inflammation and numb pain.
• b. Acetaminophen (paracetamol): it is believed that it works by acting on the brain's pain centre. It likely inhibits COX enzymes in the brain, which reduces the perception of pain without the anti-inflammatory effect.
• c. Opioids: these are also called narcotics. Opioids bind to opioid receptors in the brain, spinal cord and other parts of the nervous system. These receptors are involved in the transmission and perception of pain. When opioids bind to these receptors they block pain signals from being transmitted to the brain and increase the brain's threshold for pain, essentially dulling or eliminating the sensation of pain.
• d. Local anaesthetics: these work by blocking sodium channels in nerve cells. Sodium channels are essential for the transmission of electric signals along nerves. By blocking these channels, local anaesthetics prevent pain signals from travelling from the site of injury to the brain. These are commonly used for procedures where localised numbing is required, such as dental work or minor surgeries.
• e. Antidepressants and anticonvulsants: these medicines are used to treat chronic pain, particularly nerve pain. They work by modulating the neurotransmitters involved in pain perception, such as serotonin and norepinephrine.
• f. Corticosteroids: they reduce swelling and inflammation, which in turn reduces pain. They work by inhibiting the production of inflammatory chemicals and suppressing the immune response.
• g. Other medicines: these numb a localised area. Some are used to relax muscles.

5.9.1Certain pain medications are addictive

Physicians typically prescribe opioids to manage moderate to severe pain. However, opioids can become addictive because they not only dull pain, but also produce a sense of euphoria (intense happiness). This, combined with tolerance build-up (needing to increase doses to produce the same effect), can lead to opioid use disorder.

Opioids also cause neurons that produce dopamine to respond. Dopamine is a neurotransmitter that plays a role in how we feel pleasure, or strong reactions to something. This creates feelings of euphoria. Physicians reduce the length and strength of opioids to try to prevent addiction.

5.10Disorders of the nervous system

The disorders of the nervous system may be classified as vascular, infectious, structural, functional and degenerative. While classifying, the site of involvement is also considered. The book discusses the causes, symptoms and treatment of a few diseases of the major categories.

5.10.1Vascular disorders of the CNS

Any disorder of the nervous system which occurs due to abnormality in blood circulation is called a vascular disorder of the nervous system, for example strokes and brain haemorrhage.

Stroke

It occurs due to rupture of small cerebral arteries.

• Cause: the cause and risk factors for stroke include hypertension, cigarette smoking, diabetes mellitus, high alcohol intake, thrombosis, blood disorders, blood embolism and cocaine abuse.
• Symptoms: sudden loss of function in one region of the brain. Weakness and heaviness occur in the arm, leg or face. Paralysis occurs on the side of the body opposite the cerebral infarction (a portion of the tissue that is dying because the blood supply to it has been cut off). Aphasia (inability to express through words) may be present.
• Treatment: medical treatment is aimed at preventing further attacks and stroke. Anticoagulants and platelet aggregation inhibitors (such as aspirin) are given. Blood pressure management and nursing care are essential.

Cerebral infarction

A portion of the brain tissue that is dying because its blood supply has been cut off.

Aphasia

Inability to express through words.

5.10.2Infectious disorders of the CNS

Infections of the central nervous system can be caused by almost any infectious agent, including viruses, bacteria, fungi, protozoa and Platyhelminthes.

Meningitis

It is an inflammation of the meninges.

• Cause: bacterial or viral infection of the meninges.
• Symptoms: usually include stiffness in the neck, headache and fever. In severe cases, meningitis may also cause paralysis, coma or death.
• Treatment: for viral meningitis there is no specific treatment. Bacterial meningitis is treated with antibiotics and steroids.

5.10.3Structural disorders of the CNS

Several disorders disturb the structure of the brain and are referred to as structural disorders, such as tumours.

Tumour

A tumour (tumor) is an abnormal mass of neuroglial cells produced as a result of uncontrolled cell division.

• Cause: it is caused by mutation, which may occur at any age in the brain and spinal cord.
• Symptoms: these vary widely, depending on the location of the tumour, but may include headaches, severe nerve pain, paralysis, seizures, coma and death.
• Treatment: surgical removal of the tumour.

5.10.4Functional disorders of the CNS

Headache

Headache is the pain anywhere in the region of the head or neck. It can be a symptom of a number of different conditions of the head and neck. The brain tissue itself is not sensitive to pain because it lacks pain receptors. Rather, the pain is caused by disturbance of the pain-sensitive structures around the brain.

Treatment: several analgesic drugs are available for the treatment of any kind of headache.

5.10.5Degenerative disorders of the CNS

Many diseases cause degeneration in different parts of the nervous system without an identifiable external cause. Genetic factors are known to be involved. An example of such diseases is Alzheimer's disease.

Alzheimer's disease

Alzheimer's disease is a slowly progressive disease of the brain that is characterised by impairment of memory and eventually by disturbances in reasoning, planning, language and perception. Although the onset of this disease occurs in aged people, it is not particularly associated with ageing.

• Cause: there is genetic predisposition, so it tends to run in families.
• Symptoms: the most prominent symptom is short-term memory loss.
• Treatment: there is no effective treatment for this disease.

5.11Principles of diagnostic tests for nervous disordersOften examined

These days a number of diagnostic tests have been developed for nervous disorders. The principles of EEG, CT scan and MRI are discussed here.

Electroencephalography (EEG)

Neurons within the cerebral cortex continuously generate electrical activity. This activity can be recorded by electrodes attached to precise locations on the scalp, producing an electroencephalogram, and this technique is called electroencephalography (EEG). An EEG pattern is commonly called brain waves.

Computed Tomography scan (CT scan)

Computerised tomography is more commonly known by its abbreviated name, CT scan. It is an X-ray procedure that combines many X-ray images with the aid of a computer to generate cross-sectional views and, if needed, three-dimensional images of the internal organs and structures of the body. A CT scan is used to define normal and abnormal structures in the body and/or assist in procedures by helping to accurately guide the placement of instruments or treatments.

Magnetic Resonance Imaging (MRI)

A Magnetic Resonance Imaging (MRI) scan is a radiology technique that uses magnetism, radio waves and a computer to produce images of body structures. The MRI scanner is a tube surrounded by a giant circular magnet. The patient is placed on a moveable bed that is inserted into the magnet. The patient is exposed to a strong magnetic field and a beam of radio waves. The receiver information is processed by a computer and an image is produced. The image and resolution produced by MRI is quite detailed and can detect tiny changes of structures within the body.

STEAM Activity 5.1

Investigating reflex actions

Researchers can carry out a number of investigations to determine the effect of a specific factor on human reaction times. A suitable investigation could be the effect of caffeine or the amount of background noise in the room. A simple method to measure the effect is to use the ruler drop test.

Important: 1 millisecond (ms) is one thousandth of a second (1/1000 s).

Example results

The mean catch distance is greater with noise (168\div5=33.6 cm) than without noise (92\div5=18.4 cm). A longer catch distance means a slower reaction, so background noise slowed the reaction time. Using the conversion table, 18.4 cm lies between 15 cm (170 ms) and 20 cm (200 ms), so the reaction time without noise is about 190 ms. A catch distance of 33.6 cm is beyond the last value of the table (30 cm, 250 ms), so the reaction time with noise is more than 250 ms.

Key terms

Nervous system

The system of the body that provides coordination through electric signals among different body parts for the response to a stimulus.

Neuron

A highly specialised cell that is the basic structural and functional unit of the nervous system; it has a cell body, dendrites and an axon.

Receptor cell

A cell that responds to a stimulus; it is a transducer that converts the stimulus energy (light, heat, sound) into an electrical impulse in a sensory neuron.

Effector

A muscle or gland that responds to the signals of the nervous system.

Nissl's granules

Groups of ribosomes and rough ER in the cell body of a neuron, associated with protein synthesis.

Dendrites

Short, thin, branched extensions of a neuron that receive stimuli and conduct impulses to the cell body.

Axon

A long, thick nerve fibre of a neuron that conducts action potentials away from the cell body to the presynaptic terminals.

Sensory neuron

A neuron that carries sensory information from receptors to the brain and spinal cord (afferent neuron); its cell body is in the dorsal root ganglion.

Motor neuron

A multipolar neuron that carries information from the CNS to an effector, a muscle or gland (efferent neuron).

Interneuron

A multipolar neuron within the CNS that passes signals between sensory and motor neurons; also called a relay or association neuron.

Reflex action

An immediate, automatic and involuntary response to external and internal environmental changes.

Reflex arc

The path of the nerve impulse during a reflex action: receptor, sensory neuron, interneuron, motor neuron and effector.

Nerve impulse

A wave of electrochemical changes that travels along the length of a neuron from one end to the other.

Resting membrane potential

The potential of a neuron that is not conducting an impulse (polarised state): about −70 mV, the inside negative relative to the outside.

Active membrane potential

The action potential: a brief state in which the inside of a neuron is more positive than the outside (depolarised state).

Depolarization

The reversal of polarity across the membrane (inside becoming positive) caused by the influx of Na⁺ ions through voltage-regulated Na⁺ gates.

Repolarization

The return of the inside of the membrane towards negative, as K⁺ ions diffuse out of the cell.

Hyperpolarization

A slight overshoot to a potential more negative than the resting potential, because the K⁺ gates close slightly later than the Na⁺ gates.

Threshold stimulus

A stimulus that is capable of producing an action potential in a neuron.

Refractory period

The recovery period (about 4 ms) after an action potential in which the Na⁺-K⁺ pump restores the original ionic distribution and −70 mV.

Saltatory conduction

The rapid transmission of a nerve impulse along a myelinated axon as the action potential jumps from one node of Ranvier to the next.

Synaptic delay

The time of about a millisecond that an impulse takes to cross the synapse, longer than it takes to travel the same distance along an axon.

Local circuits

Circuits of current that depolarise the next section of the axon membrane and so carry the impulse between nodes in saltatory conduction.

Synapse

The junction between the axon terminal of one neuron and the dendrite of another, where information is passed on.

Synaptic cleft

The gap between the presynaptic and postsynaptic neurons at a synapse.

Presynaptic neuron

The neuron that carries an impulse toward a synapse.

Postsynaptic neuron

The neuron that receives the impulse after it crosses the synapse.

Neurotransmitter

A chemical messenger released from synaptic vesicles that carries the message across the synaptic cleft.

Excitatory postsynaptic potential (EPSP)

The depolarization of the postsynaptic membrane towards threshold, caused by a neurotransmitter opening Na⁺ channels.

Excitatory neurotransmitter

A neurotransmitter that increases membrane permeability to Na⁺ and triggers nerve impulses, such as acetylcholine, epinephrine, norepinephrine, serotonin and dopamine.

Inhibitory neurotransmitter

A neurotransmitter that decreases membrane permeability to Na⁺ and raises the threshold, such as GABA, glycine and the endorphins.

Meninges

The three protective membranes covering the brain and spinal cord: dura matter, arachnoid matter and pia matter.

Cerebrospinal fluid (CSF)

The fluid between the arachnoid and pia matter that cushions the brain from shock.

Cerebrum

The largest part of the human brain, divided into two cerebral hemispheres, each with frontal, parietal, temporal and occipital lobes.

Thalamus

A part of the forebrain that receives all sensory impulses (except smell) and channels them to the limbic system and cortex.

Limbic system

A complex set of structures on both sides of the thalamus, just under the cerebrum, including the hypothalamus, amygdala and hippocampus.

Hypothalamus

The part of the limbic system on the ventral side of the thalamus that maintains homeostasis and controls the pituitary gland.

Amygdalae

Two almond-shaped masses of neurons that control feelings and emotions such as love, hate, anger, fear and rage.

Hippocampus

A structure of the limbic system that converts short-term memory into long-term memory.

Cerebellum

The part of the hindbrain that controls equilibrium and the coordination of muscles.

Pons

A part of the hindbrain that acts as a bridge between the cerebellum, medulla and cerebrum and controls the rate and pattern of heartbeat and breathing.

Medulla

A part of the hindbrain that controls automatic functions such as heartbeat, blood pressure, respiration and swallowing.

Grey matter

Neuron cell bodies and non-myelinated parts of fibres; inside the spinal cord (H-shaped) and the outer cortex of the brain.

White matter

Bundles of myelinated fibres; at the periphery of the spinal cord and inside the brain.

Cranial nerves

The 12 pairs of nerves of the peripheral nervous system that arise from the brain.

Spinal nerves

The 31 pairs of mixed nerves of the peripheral nervous system that arise from the spinal cord.

Somatic nervous system

The cranial and spinal nerve fibres that connect the CNS to the skin and skeletal muscles; involved in conscious activities.

Autonomic nervous system

The fibres that connect the CNS to the visceral organs; concerned with unconscious activities and divided into sympathetic and parasympathetic divisions.

Sympathetic division

The part of the autonomic system, from T1 to L2 of the spinal cord, that prepares the body for "fight or flight" in an emergency.

Parasympathetic division

The part of the autonomic system (a few cranial nerves including the vagus, and sacral nerves) that controls functions at rest and reverses the changes of the sympathetic division.

Nociceptors

Pain receptors: free nerve endings in the epidermis that respond to chemicals from damaged tissue or excess heat or pressure.

Narcotic drugs

Drugs that interact with normal nervous activity and diminish the perception of pain by binding to painkilling sites in the brain.

Drug addiction

A dependence on an illegal drug or a medication that most people cannot quit on their own.

Drug tolerance

A diminished response to a drug that occurs when the drug is used repeatedly and the body adapts to it, for example caffeine and nicotine.

Analgesics

Painkillers: medicines that reduce or numb pain through various mechanisms affecting the nervous system.

Stroke

A vascular disorder caused by rupture of small cerebral arteries, with sudden loss of function in one region of the brain.

Meningitis

Inflammation of the meninges caused by a bacterial or viral infection.

Electroencephalography (EEG)

A technique that records the electrical activity of the cerebral cortex with electrodes on the scalp, giving an electroencephalogram (brain waves).

CT scan

Computed tomography: an X-ray procedure combining many X-ray images with a computer to give cross-sectional and 3-D images of the body.

MRI

Magnetic resonance imaging: a radiology technique using magnetism, radio waves and a computer to produce detailed images of body structures.

FBISE 2026 questions

Questions from the FBISE HSSC-II 2026 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

Exam focus

1. Describe the mechanism of synaptic transmission and draw the labelled synapse (synaptic knob, vesicles, cleft, receptors, postsynaptic membrane). (long)
   Asked in 2026 three times: Set B Q.2 (v) (identify and label the junction), Set B Q.2 (vi) OR (release of neurotransmitter and its effect on the postsynaptic membrane) and Set D Q.2 (vi) (describe a synapse and draw it). Also book long question 4.
   Go to answer
2. Explain how a nerve impulse is generated: resting potential, depolarization, repolarization, hyperpolarization. (long)
   Set D Q.2 (viii) (what is a nerve impulse and how is it generated) and Set B Q.2 (v) OR (identify X, Y, Z on the action potential graph). Book exercise MCQ 2 and 9b also test the ion movements.
   Go to answer
3. Read and label the action potential graph: threshold, resting potential, rising limb, falling limb, undershoot. (short)
   Set B Q.2 (v) OR gave the graph and asked for X, Y and Z with a short description of each.
   Go to answer
4. State what the sodium-potassium pump does (3 Na+ out, 2 K+ in) and how the resting potential of -70 mV is set up. (short)
   Set D MCQ 6 asked the pump ratio directly; the resting potential factors are a standard short question.
   Go to answer
5. Drug addiction and tolerance and their effects on the nervous system (narcotics, endorphins, depressants and stimulants, caffeine, nicotine). (short)
   Set D Q.2 (xiii) OR asked to associate addiction and tolerance with nervous system functioning (1.5 + 1.5 marks). Book long question 9 covers narcotic drugs.
   Go to answer
6. Describe the structure of a neuron and the three types of neuron with their functions. (long)
   Not asked as a paper question in 2026, but it is the base of the unit (book long questions 2 and 3) and the labelled neuron diagram is a likely draw-and-label.
   Go to answer
7. Name the parts of the human brain and the function of each (corpus callosum, cerebellum, thalamus, hypothalamus). (short)
   Set B MCQ 5 (corpus callosum connects the two hemispheres). Book long question 6 and MCQs 11, 12 and 17 ask the same brain-part functions.
   Go to answer