The below mentioned article provides a study note on the Neurons as Structural and Functional Unit of Neural System.
Neurons (= Nerve Cells):
A neuron is a structural and functional unit of the neural tissue and hence the neural system. Certain neurons may almost equal the length of body itself. Thus neurons with longer processes (projections) are the longest cells in the body. Human neural system has about 100 billion neurons.
Majority of the neurons occur in the brain. Fully formed neurons never divide and remain in interphase throughout life. Shortly after birth, new neurons do not develop. Certain neurons have flask shaped cytons and are called Purkinje cells, which occur in the cerebellum of the brain. A neuron consists of main cell body and cytoplasmic processes arising from it.
(i) Cell body (= Cyton or Soma):
It varies in size and form. It may be up to 13.5 µm in diameter and may be irregular, spherical, oval, rounded, star-shaped or pyramidal. Like a typical cell it consists of cytoplasm, nucleus and cell membrane.
It has abundant cytoplasm, called neuroplasm and a relatively large spherical central nucleus with a distinct nucleolus. The cytoplasm has mitochondria, Golgi apparatus, rough endoplasmic reticulum, ribosomes, lysosomes, fat globules, pigment granules, neurofibrils, neurotubules and Nissl’s granules.
Presence of neurofibrils and Nissl’s granules is characteristic to all neurons. Neurofibrils play a role in the transmission of impulses. Neurotubules are in fact, microtubules which maintain the shape of the neuron.
The Nissl’s granules (also called Nissl’s bodies) are irregular masses of rough endoplasmic reticulum with numerous attached and free ribosomes and polysomes. The Nissl’s granules probably synthesize proteins for the cell.
Centrioles, formerly believed to be absent in mature neurons, have been described in neurons and may be associated with the production and maintenance of microtubules. The cytoplasm immediately surrounding the nucleus is loaded with protein-synthetic machinery, and is called perikaryon.
Previously the name perikaryon was given to the cyton (cell body or soma). Ageing neurons contain a pigment lipofuscin (made up of residual bodies derived from lysosomes). Cyton is concerned with metabolic maintenance and growth.
(ii) Neurites:
The processes of neurons are called neurites. These are of two types: dendrites or dendrons and an axon or axis cylinder or neuraxon.
(a) Dendrites (Dendrons):
These are usually shorter, tapering and much branched processes. They may be one to several. The dendrites contain neurofibrils, neurotubules and Nissl’s granules. They conduct nerve impulse towards the cell body and are called afferent processes (= receiving processes).
(b) Axon:
Axon is a single, usually very long process of uniform thickness. The part of cyton from where the axon arises is called axon hillock. Most sensitive part of neuron is axon hillock. The axon contains neurofibrils and neurotubules but does not have Nissl’s granules, Golgi complex, ribosomes, pigment granules, fat globules, etc.
In the absence of Nissl’s granules, the axon depends on the cell body for the supply of proteins. The cell membrane of the axon is called axolemma and its cytoplasm is known as axoplasm. The axon ends in a group of branches, the terminal arborizations (= axon terminals or telodendria).
When terminal arborizations of the axon meet the dendrites of another neuron to form a synapse they form synaptic knobs (= end plates). The synaptic knobs contain mitochondria and secretory vesicles. The part of the sarcolemma (muscle plasma membrane) that lies beneath the axon terminals/nerve endings, is called motor end plate.
Each axon may also possess side branches called collateral fibres which are usually much finer than the main axonal process. The axon conducts nerve impulses away from the cell body, therefore, called an efferent process.
There are two types of axon namely myelinated and non-myelinated. In myelinated nerve fibres Schwann cells form myelin sheath around the axon. The gaps between two adjacent myelin sheaths are called nodes of Ranvier.
Myelinated nerve fibres are found in cranial and spinal nerves. In non-myelinated nerve fibres Schwann cells do not form myelin sheath and are without nodes of Ranvier. They are commonly found in autonomous and somatic neural systems.
Types of Neurons on the Basis of Structure:
1. Non-Polar Neurons:
Each neuron has several branched processes (projections). These neurons are rare in vertebrates but occur in cnidarians (coelenterates) e.g., Hydra.
2. Unipolar Neurons:
The body has only one axon. It is found usually in the embryonic stage.
3. Pseudo unipolar Neurons:
A single process arises from the cyton and then divides into axon and dendrite. They are found in dorsal root ganglia of spinal nerves.
4. Bipolar Neurons:
Each bipolar neuron has one axon and one dendrite. They are present in the retina of eye.
5. Multipolar Neurons:
These neurons have several dendrites and an axon. They are found in cerebral cortex.
Peripheral Neural System:
The nerves which originate from the central nervous system and connect either receptor or effector organs, form peripheral neural system. Those nerves which arise from brain are called cranial nerves while the nerves originating from the spinal cord are termed as spinal nerves.
Cranial Nerves:
These nerves are so named because they pass through various foramina (openings) in the cranial bones (bones of the brain box). There are present 12 pairs of cranial nerves in man.
I. Olfactory Nerve:
Its nerve fibres arise in the olfactory epithelium of the nasal chamber. It enters the olfactory bulb of olfactory lobes of the brain. From the olfactory bulb, the nerve fibres run through the olfactory tract and ultimately reach the temporal lobe of the cerebrum. It is sensory nerve which carries impulses of smell from olfactory epithelium to the brain.
II. Optic Nerve:
The optic nerve fibres originate in the retina of the eye and combine to form the optic nerve. Two optic nerves meet at the floor of the diencephalon where they appear to cross to opposite side and X-shaped structure thus formed is called the optic chiasma.
Only a relatively small portion of the fibres of the optic nerves actually cross at the chiasma, many of them simply bending and remaining on the same side of the brain. Optic nerve fibres lead to the occipital lobe of the brain. It is sensory nerve and carries impulses of sight from the retina to the brain.
III. Oculomotor Nerve:
This nerve has a name meaning “eye mover” because it supplies four of the six extrinsic eye muscles that move the eye ball in the orbit. It arises from the floor of the midbrain. It innervates four eye muscles, viz., inferior oblique, superior rectus, inferior rectus and medial rectus (Fig. 21.13). It is a motor nerve and carries impulses from the brain to these muscles for controlling the movements of eyeball.
IV. Trochlear Nerve:
This nerve’s name means “pulley” because it innervates an extrinsic eye muscle that loops a pulley-shaped ligament in the orbit. The trochlear nerve is the thinnest and smallest cranial nerve. It originates from the floor of the midbrain. It supplies nerve fibres to the superior oblique eye muscle. It is a motor nerve and helps in controlling the movement of the eye-ball.
V. Trigeminal Nerve:
It is the largest cranial nerve. It arises from the ventral surface of the pons varolii. It bears a trigeminal (Gasserian) ganglion at its origin and divides into three branches.
(a) Ophthalmic Nerve:
It is the smallest branch of trigeminal nerve. It runs forward through the eye orbit and innervates the lacrimal glands, the conjunctiva of the eye, the forehead, the eyelids, etc. It is sensory nerve and carries impulses of touch from the above mentioned areas to the brain.
(b) Maxillary Nerve:
It innervates the cheeks, the upper gums, the upper teeth and lower eyelids. It is a sensory branch and carries stimuli from these areas.
(c) Mandibular Nerve:
It is the largest branch of trigeminal nerve. It distributes nerve fibres to the teeth and the gums of the lower jaw, the pinna of the ear, lower lip and the tongue. It is a mixed branch having both sensory and motor fibres that help in controlling these particular organs.
VI. Abducens Nerve:
This nerve is named because it controls the extrinsic eye muscle that abducts the eye ball (turns it laterally). It originates from the pons varolii. It innervates the lateral rectus muscle of the eye ball. It is a motor nerve and controls the movements of the eye-ball.
VII. Facial Nerve:
It arises from the lower part of the pons varolii. It bears geniculate ganglion. It innervates the taste buds of the tongue and muscles of the face. It also innervates the salivary glands. It is mixed nerve. It conveys impulses from the taste buds. It also controls the facial expression.
VIII. Vestibulocochlear Nerve (Auditory Nerve):
It comes from the internal ear (membranous labyrinth) and joins the lateral side of the pons varolii. It is formed by two branches.
(a) Vestibular Nerve:
It arises from the vestibular part of the membranous labyrinth, (utricle, saccule and semicircular canals). It is a sensory branch which is concerned with equilibrium of the body.
(b) Cochlear Nerve:
It originates in the cochlear part of the membranous labyrinth. It is also a sensory branch and is concerned with hearing.
IX. Glosso-pharyngeal Nerve:
It originates from the side of medulla oblongata. It innervates the taste buds, soft palate, pharynx, tongue and muscles of the pharynx. It is a mixed nerve. It controls secretion of saliva, sense of taste and movement of the pharynx.
X. Vagus Nerve:
This nerve’s name means “wanderer” (as in vagabond) and it is the only cranial nerve to extend beyond the head and neck into the thorax and abdomen. It is the longest cranial nerve. It originates from the side of the medulla oblongata. It bears vagus ganglion.
It innervates the pharynx, larynx, oesophagus, stomach, lungs, heart and intestines. It is a mixed nerve. It controls the visceral sensations, and visceral movements (peristalsis, sound production, respiratory movements, and heart beat).
XI. Accessory Nerve:
Formerly it was called the spinal accessory. It differs from all other cranial nerves because it originates from both the brain (medulla oblongata) and the spinal cord.
It is formed by union of its cranial and spinal roots but these are associated for a short distance only. It arises from the side of the medulla oblongata. It is a motor nerve which innervates the muscles of the pharynx, larynx, neck and shoulder and controls the movements of these organs.
XII. Hypoglossal Nerve:
It originates from the ventral side of the medulla oblongata. It innervates the muscles of the tongue and hyoid apparatus. It is a motor nerve which controls the movements of the tongue.
Spinal Nerves:
Number:
Thirty one pairs of spinal nerves are named and numbered according to the vertebrae with which they are associated. They include eight pairs of cervical nerves, twelve pairs of thoracic nerves, five pairs of lumbar nerves, five pairs of sacral nerves, and one pair of coccygeal nerves.
Structure:
The spinal nerves are formed by the union of the dorsal and ventral roots shortly after they leave the spinal cord. Each spinal nerve has afferent (sensory) and efferent (motor) fibres; in general, efferent come from the ventral root, and afferents go into the dorsal root. Thus, all spinal nerves are mixed nerves because they carry both sensory and motor impulses.
Distribution:
After passing through the intervertebral foramen, each spinal nerve separates into posterior and anterior branches. The posterior branch innervates the muscles and skin of the posterior portion of the body. The anterior branch innervates the limbs and the lateral and anterior portions of the body.
The lumbar, sacral and coccygeal nerves leave the spinal cord before its termination at the level of the first lumbar vertebra and extend downwards inside the vertebral canal in the subarachnoid space below this level. In this way they form a sheaf of nerves which resembles a horse’s tail, called the cauda equina. These nerves leave the vertebral canal at the appropriate lumbar, sacral or coccygeal level.
Certain spinal nerves join to form the following plexuses:
(i) Cervical Plexus:
It innervates the neck and diaphragm.
(ii) Brachial Plexus:
It connects the chest and arm.
(iii) Lumbar Plexus:
It innervates the legs.
(iv) Sacral Plexus:
It connects the pelvic region.
(v) Coccygeal Plexus:
It also innervates the pelvic region.
Autonomic Neural System:
This system controls and coordinates the involuntary activities of various organs. This system is autonomous in the sense that it regulates such activities of the body in which the will power of the animal is not involved, e.g., the secretion of the digestive fluid is always under the control of autonomic nervous system but the animal is not aware of it.
The autonomic neural system is divisible into two parts: sympathetic neural system and parasympathetic neural system.
Sympathetic Neural System:
Sympathetic neural system (Fig. 21.15) consists of the following parts:
(i) Sympathetic chains:
They are paired chains of 21 ganglia which extend from the upper cervical level to the sacrum. Each sympathetic chain possesses 3 cervical, 12 thoracic, 5 lumbar and 1 sacral ganglia.
(ii) Preganglionic fibres:
These are the axons of the neurons present in the spinal cord. They may synapse directly with the postganglionic neuron in the chain ganglia. They may extend through the chain ganglia to collateral ganglia.
They may synapse directly or extend to the collateral ganglia. The preganglionic fibres of the sympathetic nervous system occur only in the thoracic and lumbar regions. Thus sympathetic nerves arise from the thoracolumbar region of the neural system.
(iii) Collateral ganglia:
There are three collateral or pre-vertebral ganglia situated in the abdominal cavity close to the origins of arteries of the same names. They are the coeliac ganglion, the superior mesenteric ganglion and the inferior mesenteric ganglion.
(iv) Postganglionic fibres:
These are the axons of neurons which may have their cell bodies, either in one of the chain ganglia or in one of the collateral ganglia. Developmentally, the adrenal medullae and sympathetic ganglia are derived from the same tissue, the neural crest.
The adrenal medullae are modified sympathetic ganglia and their cells are similar to the sympathetic post ganglionic neurons. Upon stimulation by the sympathetic preganglionic neurons, the adrenal medullae release about 80% adrenaline (= epinephrine), 20% noradrenaline (= norepinephrine) and a trace amount of dopamine.
Since sympathetic preganglionic neurons release adrenaline, noradrenaline and dopamine, they are called adrenergic. The sympathetic nerves stimulate the adrenal glands to secrete their hormones. Thus sympathetic nerves function with the adrenal glands as a well-integrated sympatheticoadrenal system having wide spread effects.
Parasympathetic Neural System:
Parasympathetic nerural system (Fig. 21.16) consists of preganglionic fibres, parasympathetic ganglia and post-ganglionic fibres.
(i) Preganglionic fibres:
These are the axons of the neurons situated in the midbrain, brainstem and sacral region of the spinal cord. The preganglionic fibres are found accompanying the cranial nerves and spinal nerves.
Those preganglionic fibres which come from brain run along with the III, VII, IX and X cranial nerves while those preganglionic fibres coming from the spinal cord pass through 2, 3 and 4 sacral spinal nerves. Thus parasympathetic nerves arise from the craniosacral region of the neural system. The preganglionic fibres end in the parasympathetic ganglia.
(ii) Parasympathetic ganglia:
These ganglia are located close to or within the viscera. They do not form a chain. The preganglionic fibres synapse with the neurons in the parasympathetic ganglia.
(iii) Postganglionic fibres:
These are the axons of the neurons present in the parasympathetic ganglia. The postganglionic fibres innervate the smooth muscles and glands of the viscera. Cholinergic neurons release the neurotransmitter acetylcholine (Ach).
In the ANS, the cholinergic neurons include:
(a) All sympathetic and parasympathetic preganglionic neurons,
(b) Sympathetic postganglionic neurons that innervate most sweat glands and
(c) All parasympathetic postganglionic neurons.
It is important to note that the preganglionic fibres of both sympathetic and parasympathetic neural system are cholinergic.
Because the autonomic neural system innervates the viscera, it is also called the visceral neural system.
Functions of Autonomic Neural System:
The functions of sympathetic and parasympathetic neural systems are antagonistic to each other. Functions of both the systems are summarised in the following table.
Functions of autonomic neural system
Some structures are supplied by sympathetic neural system alone, e.g., adrenal medulla, most arterioles, ureters, uterus, Fallopian tubes, seminal vesicles, etc. while there are certain other structures having parasympathetic supply alone, e.g., oesophagus, gastric glands, pancreas including the islets of Langerhans, lacrimal glands, etc.
Reflex Action and Reflex Arc:
Definition:
It is a form of animal behaviour in which the stimulation of a sensory organ (receptor) results in the activity of some organ without the intervention of will. Actually it is a spontaneous automatic mechanical response to a stimulus without the will of the animal.
Mechanism of Reflex Action:
If the reflex action is controlled by the spinal cord it is called spinal reflex action and if it is controlled by the brain it is known as cerebral reflex action.
For a reflex action five things are normally essential:
(i) Receptor,
(ii) Sensory nerve fibres,
(iii) A part of the central nervous system,
(iv) Motor nerve fibres and
(v) Effector organ such as muscles and glands.
The sensory nerve fibres bring sensory impulses from the receptor organ to the central nervous system. The motor nerve fibres relay the motor impulses from the central nervous system to the effector organs. Thus an impulse travels a path during reflex action which is called reflex arc.
Examples of Reflex Action:
1. Closing of eyes when strong light is flashed across them.
2. Withdrawal of limbs when they are touched by hot things.
3. Watering of mouth on seeing favourite food.
4. Opening of mouth on hearing loud sound.
5. Withdrawal of limbs in a decapitated frog (whose head has been cut) when dipped in warm water or touched with an acid.
6. Typing, riding a bicycle, knitting etc.
Although animal cannot have any choice in the above mentioned examples but in some of these reactions, the animal may have some knowledge. But there are many examples of reflex actions in which the animal does not have any knowledge of the reflex action.
Examples are given below:
1. Discharge of bile from the gall bladder while the food passes from the opening of the bile duct.
2. Peristalsis of the alimentary canal.
3. Beating of heart.
4. Secretions of glands.
Importance of Reflex Action:
(i) It is an important activity for the survival of the animal.
(ii) It relieves the brain from too much strain.
(iii) The responses of reflex action immediately protect the animal from harmful situations.
Reflexes:
All the reflex activities of an organism can be broadly divided into:
(i) Unconditioned reflexes and
(ii) Conditioned reflexes.
Unconditioned Reflexes are inborn reflexes and are transmitted through heredity. They are also called inborn or inherited reflexes, e.g., Breast feeding and swallowing in newly born babies and blinking of eyes are the examples of unconditioned reflexes.
Conditioned Reflexes (CR):
Conditioned reflexes are acquired reflexes during the life time of an individual. They are absolutely an individual entity and are, therefore, not constant, viz., they may disappear and reappear again. Ivan Pavlov, a Russian physiologist discovered for the first time the existence of conditioned reflexes and, therefore, he is called the father of conditioned reflexes.
Characteristics of conditioned reflexes:
(i) They are acquired in life.
(ii) They depend on previous experience.
(iii) They can be established or abolished.
(iv) They are not transmitted by heredity.
(v) Cortical and subcortical centres ore responsible for them.
Experiments conducted by Pavlov:
Pavlov (1906) had carried experiments with a dog. He rung a bell every time just before the food was placed in the dog’s mouth. Gradually the dog learnt to associate the bell with food.
The dog would salivate when the bell was rung even though no food was placed in its mouth. Pavlov called food, unconditioned stimulus, salivation in response to food an unconditioned response, sound of the bell the conditioned stimulus and salivation in response to bell the conditioned response.
Mechanism of conditioned reflexes:
The reflex arc of a conditioned reflex includes the following parts:
(i) Receptors which perceive the conditioned stimulus,
(ii) A sensory nerve,
(iii) An area in the cerebral cortex which perceives the conditioned stimulus,
(iv) Another area in the cortex which is connected with the centre of the unconditioned reflex,
(v) The motor nerve and
(vi) The effector organ which responds accordingly.
Functions of conditioned reflexes:
(i) Most of our habits are conditioned reflexes. Hence, it is of immense personal and social importance,
(ii) They play an important part in physiology of learning,
(iii) They also help animals to eliminate harmful influences, etc.
(iv) They ensure adaptations of the organisms to the external environment in the course of its life experience and are essential for its better orientation in changing conditions.
i. Nerve Termination:
Nerve fibres terminate in three ways:
(i) The axon of one neuron ends on the dendrites of the next neuron. Such a junction is called synapse.
(ii) The axon of a neuron terminates on either a muscle fibre or a gland cell. When it ends on a muscle fibre, it forms the neuromotor junction. The contact between a neuron and glandular cells is called neuroglandular junction.
(iii) Many nerve fibres, for example in the skin, divide into fine branches known as the sensory nerve endings.
ii. Main Properties of Neural Tissue:
The neural tissue has two outstanding properties: excitability and conductivity.
1. Excitability:
It is the ability of the nerve cells and fibres to enter into an active state called the state of excitation in response to a stimulus. Excitation arises at the receptors on account of various stimuli such as light, temperature, chemical, electrical or pressures which constantly act on the organisms.
2. Conductivity:
The excitation does not remain at the site of its origin. It is transmitted along nerve fibres. The transmission of excitation in a particular direction is called conductivity.
iii. Stimulus:
A stimulus is sudden change in the environment (external or internal) which is strong enough to excite the nerve or muscle or organism as a whole. If the stimulus is capable to excite a given tissue, it is called threshold stimulus (adequate stimulus). If the stimulus is not capable to excite any response, it is called sub-threshold stimulus (inadequate stimulus).
iv. Summation:
As stated above, a sub-threshold stimulus is unable to generate a nerve impulse. A series of sub-threshold stimuli applied to a nerve fibre may succeed in initiating an impulse. This additive effect of several sub-threshold stimuli is called summation.
v. All-or-none Principle:
It states that a neuron either conducts or does not conduct an impulse. If it conducts an impulse it is always of maximum size. Therefore, according to the all-or- none principle, a neuron can be thought of as being either “on” or “off”.
vi. Sodium-Potassium Pump:
The process of expelling out sodium ions and drawing in potassium ions against concentration gradient and electrochemical gradient is called Sodium Potassium pump.
Thus it transports sodium from inside the cell to outside and potassium from outside into the cell by active transport in which a considerable amount of energy (ATP) is spent. It operates with the help of Na+, K+ and ATPase enzyme located in the cell membrane. This pump is present in all the cells of the body.
The result of sodium-potassium exchange pump is that there is a difference in charge on either side of the membrane—positive outside and negative inside. This difference in charge on either side of the membrane of a resting neuron is the resting membrane potential and such a membrane is said to be polarized (resting potential).
Nerve Impulse:
A nerve impulse may be defined as wave of depolarization of the membrane of the nerve cell. The nerve impulse travels along a neuron or across a synapse (junction), between one neuron and another, or between a neuron and an effector, such as a muscle or gland.
Transmission of Nerve Impulse:
The nerve cells remain bathed in the extracellular fluid (ECF) or interstitial fluid containing a large amount of sodium chloride and bicarbonates. In addition, it contains nutrients and oxygen for supplying to the cell and carbon dioxide and other metabolic wastes released into it by the body cells.
However, the intracellular fluid (cytoplasm of the neurons) contains a large amount of potassium and magnesium phosphates in addition to complex proteins and other organic molecules. Most of the solutes in extracellular fluid and the cytoplasm of the neuron are electrically charged particles or ions (positively charged cations or negatively charged anions).
Membrane or Ionic Theory of Nerve Impulse:
This theory was proposed by English neurophysiologists Hodgkin and Huxley in the late 1930s. This theory states that electrical events in the nerve fibre are governed by the differential permeability of its membrane to sodium and potassium ions and that these permeability’s are regulated by the electric field across the membrane.
The interaction of differential permeability and electric field makes a critical threshold of charge essential to excite the nerve fibre.
According to this theory, the process of nerve impulse conduction is divisible into two main phases— resting membrane potential of nerve and action membrane potential of nerve. Resting membrane potential has been described above. Action membrane potential is to be explained below under the heading depolarization.
Generation and Conduction of Nerve Impulse (Conduction of nerve impulse along the axon):
Polarisation (= Resting Potential). In a resting nerve fibre (a nerve fibre that is not conducting an impulse), the axoplasm (neuroplasm of axon) inside the axon contains high concentration of K+ and negatively charged proteins and low concentration of Na+.
In contrast, the fluids outside axon contains a low concentration of K+ and a high concentration of Na+ and thus form a concentration gradient. These ionic gradients across the resting membrane are maintained by the active transport of ions by the sodium-potassium pump which transports 3 Na+ outwards and 2 K+ inwards (into the cell).
As a result, the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged, and, therefore, is polarised. The electrical potential difference across the resting plasma membrane is called as the resting potential. The state of the resting membrane is called polarised state.
Thus to maintain resting potential sodium-potassium pump operates. In Na+ — K+ pump of active transport there is efflux of Na+ and influx of K+. It means Na+ is out and K+ is in.
Depolarization:
When a stimulus of adequate strength (threshold stimulus) is applied at a site (Fig. 21.20, e.g., point A) on the polarized membrane, the membrane at the site A becomes freely permeable to Na+. This leads to a rapid influx of Na+ followed by the reversal of the polarity at that site, i.e., the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged.
The polarity of the membrane at the site A is thus reversed and hence de-polarised. The electrical potential difference across the plasma membrane at the site A is called the action potential, which is in fact termed as a nerve impulse. At sites immediately ahead, the axon (e.g., site B) membrane has a positive charge on the outer surface and a negative charge on its inner surface.
As a result, a current flows on the inner surface from site A to site B. On the outer surface current flows from site В to site A (Fig. 21.20) to complete the circuit of current flow.
Hence, the polarity at the site is reversed and an action potential is generated at site B. Thus, the impulse (action potential) generated at site A arrives at site B. The sequence is repeated along the length of the axon and consequently the impulse is conducted.
The rise in the stimulus-induced permeability to Na+ is extremely short lived. It is quickly followed by a rise in permeability to K+. Within a fraction of a second, K+ diffuse outside the membrane and restores the resting potential of the membrane at the site of excitation which is called repolarization and the fibre becomes once more responsive to further stimulation.
Refractory Period:
The repolarization period returns the cell to its resting potential. The neuron is now prepared to receive another stimulus and conduct it in the same manner. In fact until repolarization occurs neuron cannot conduct another impulse.
It now becomes necessary to restore the normal resting membrane potential by expelling Na+ and taking K+ back in (the sodium potassium pump starts working). The time taken for this restoration is called refractory period, because during this the membrane is incapable of regenerating another impulse.
One more benefit of refractory period is that impulses travel in the axon only in forward direction —unidirectional impulse conduction. The refractory period is very short, being only about one millisecond (1/1000 of a second). Thus a nerve fibre can transmit about 1000 impulses per second.
Speed of Nerve impulse:
In man, the nerve fibres can transmit impulses at a maximum speed of about 130 metres per second, whereas in frog its speed is only about 30 meters per second.
Salutatory Conduction of Nerve Impulse:
The properties of impulse conduction described so far apply to un-myelinated neurons. However, the myelin sheath of many axons in the body insulates those axons except at the nodes of Ranvier. When an impulse travels along a myelinated neuron, depolarization occurs only at the nodes. It leaps over the myelin sheath from one node to the next.
This process, the saltatory conduction, gets its name from the root word saltere, which means to leap. Saltatory conduction accounts for the greater speed of an impulse travelling along a myelinated neuron than along a non-myelinated one.
Less energy is required for saltatory conduction than for conduction along a non-myelinated neuron because smaller amounts of ATP are used to operate the sodium pump. It is up to 50 times faster than the non-myelinated nerve fibre.
The Synapse:
The synapse is an area of functional contact between one neuron and another for the purpose of transferring information. Synapses are usually found between the fine terminal branches of the axon of one neuron and the dendrites or cell body of another.
This type of neuron is called axo-dendrite synapse. Sir Charles Sherrington (1861-1954) was the first person who used the term ‘synapse’ to the junctional points between two neurons.
Types of Synapse:
On the basis of transmission of impulses, the synapse is of two types: Electrical synapse and chemical synapse. However, generally the term synapse refers to a chemical synapse.
Structure of Synapse:
A typical (generalized synapse) consists of a bulbous expansion of a nerve terminal called a pre-synaptic knob lying close to the membrane of a dendrite.
The cytoplasm of the synaptic knob contains mitochondria, smooth endoplasmic reticulum, microfilaments and numerous synaptic vesicles. Each vesicle contains neurotransmitter (chemical substance) responsible for the transmission of the nerve impulse across the synapse.
The membrane of the synaptic knob nearest the synapse is thickened and forms the presynaptic membrane. The membrane of the dendrite is also thickened and is called the postsynaptic membrane. These membranes are separated by a gap, the synaptic cleft. The post synaptic membrane contains large protein molecules which act as receptor sites for neurotransmitter and numerous channels and pores.
The two main neurotransmitters in vertebrate nervous system are acetylcholine (ACh) and noradrenaline although other neurotransmitters also exist.
Acetylcholine (ACh) was the first neurotransmitter to be isolated and obtained by Otto Loewi in 1920 from the endings of parasympathetic neurons of the vagus nerve in frog heart. Neurons releasing acetylcholine are described as cholinergic neurons and those releasing noradrenaline are described as adrenergic neurons.
Transmission of Impulses:
1. Transmission of Nerve Impulses at an Electrical Synapse:
At electrical synapse there is continuity between the presynaptic and postsynaptic neurons. The continuity is provided by the gap junction between the two neurons. The gap junctions are small protein tubular structures that allow free movement of ions between the two neurons. Because of this, the action potential reaching the presynaptic terminal produces potential change in the post-synaptic neuron.
In electrical synapse there is minimal synaptic delay because of the direct flow of electrical current from one neuron into the other through gap junction. Thus impulse transmission across an electrical synapse is always faster than that across a chemical synapse.
At an electrical synapse, the transfer of an impulse occurs by purely electrical means without involving any chemical (neurotransmitter). However, electrical synapses are relatively rare. It is found in the cardiac muscle fibres, smooth muscle fibres of intestine and the epithelial cells of lens. Most impulse transmission across the synapse between neurons takes place at the chemical synapses.
2. Transmission of Nerve Impulse at a Chemical Synapse:
The process of chemical transmission across synapses was discovered by Henry Dale (1936). The physiological importance of synapse for the transmission of nerve impulses was established by McLennan in 1963.
A brief description of the mechanism of synaptic transmission is given below:
(i) When an impulse arrives at a presynaptic knob, calcium ions from the synaptic cleft enter the cytoplasm of the presynaptic knob.
(ii) The calcium ions cause the movement of the synaptic vesicles to the surface of the knob. The synaptic vesicles are fused with the presynaptic membrane and get ruptured (exocytosis) to discharge their contents (neurotransmitter) into the synaptic cleft.
(iii) The synaptic vesicles then return to the cytoplasm of the synaptic knob where they are refilled with neurotransmitter.
(iv) The neurotransmitter of the synaptic cleft binds with protein receptor molecules on the post synaptic membrane. This binding action changes the membrane potential of the postsynaptic membrane, opening channels in the membrane and allowing sodium ions to enter the cell.
This causes the depolarization and generation of action potential in the postsynaptic membrane. Thus the impulse is transferred to the next neuron.
(v) Having produced a change in the permeability of the postsynaptic membrane the neurotransmitter is immediately lost from the synaptic cleft. In the case of cholinergic synapses, acetylcholine (ACh) is hydrolysed by an enzyme acetyl cholinesterase (AChE) which is present in high concentration at the synapse.
(vi) The products of the hydrolysis are acetic acid and choline which are reabsorbed into the synaptic knob where they are resynthesized into acetylcholine, using energy from ATP.
Neurotransmitters:
Neurotransmitters are chemicals released from a presynaptic neuron that interact with specific receptor sites of a postsynaptic neuron. At least thirty chemicals thought to have the capacity to act as neurotransmitters, have been discovered, most of them in brain tissue, and more are likely to be found. Some of these neurotransmitters are discussed here.
Acetylcholine:
The neurotransmitter acetylcholine is released at all neuromuscular junctions between motor neurons and skeletal muscle cells, at all synapses between preganglionic and postganglionic in the autonomic neural system, and at certain synapses between neurons in the central neural system.
The enzyme acetyl cholinesterase is present on the membrane of the muscle cell or the postsynaptic neuron, where it breaks down acetylcholine into acetate and choline and terminates the action of the transmitter.
Norepinephrine:
Another transmitter, norepinephrine (formerly called noradrenalin) is secreted by some neurons of the sympathetic neural system and also by some neurons of the central neural system. Norepinephrine is usually inactivated by the action of an enzyme monoamine oxidase.
Gamma amino-butyric acid (GABA):
Gamma amino-butyric acid is released by synaptic knobs of the fibres of some intemeurons in central neural system. It inhibits postsynaptic regeneration of action potential, hence it is called inhibitory neurotransmitter. Other neurotransmitters are dopamine (DA), serotonin, glycine, histamine, glutamic acid, aspirate, substrate P and nitric oxide (NO).