Peripheral and Autonomic Nervous System
The peripheral nervous system consists of nerves arising from the brain (cranial nerves) and the nerves arising from the spinal cord (spinal nerves).
- Cranial Nerves
There are twelve pairs of cranial nerves in mammals. The names, nature and distribution of these nerves are given in Table. Sensory nerve fibres conduct nerve impulses from the peripheral tissues or organs such as skin, eye, ear, tongue and nose, to the central nervous system for interpretation or sensation. Motor nerve fibres carry nerve impulses from the central nervous system to the muscles and glands for causing their contraction and secretion, respectively.
- Spinal Nerves
The spinal nerves are the paired segmental nerves. These arise from the spinal cord. Each spinal nerve originates from the spinal cord by two roots, a dorsal root and a ventral root. The dorsal root is in continuation with the dorsal horn of its side and the ventral root is in continuation with the ventral horn of its side. The dorsal root contains sensory fibres, and the ventral root contains motor fibres. The dorsal root bears a ganglion. Both the roots unite to form the spinal nerve. It comes out of the central canal (neural canal) through its intervertebral foramen. Immediately outside the vertebral column, it divides into two branches: (a) Dorsal branch or ramus dorsalis and (b) Ventral branch or ramus ventralis. A fine branch, ramus communicans, arises from the ventral branch of most of the spinal nerves. It joins a ganglion of the sympathetic or autonomic nervous system. The spinal nerves innervate the skin and muscles of the body and contain, therefore, both sensory and motor fibres (mixed nerves). There are 31 pairs of spinal nerves.
The autonomic nervous system is so called because it is partly independent and not under voluntary control. The autonomic system consists of a special set of peripheral nerves that supply nerves to organs such as heart, lungs, digestive tract and other internal organs. It performs a variety of functions which are not under the control of the will. The movements of the stomach and intestine, the rate of heart beat, the secretions of the sweat glands and other glands are all some of the functions controlled by the autonomic nervous system.
The autonomic nervous system consists of two parts: (a) the sympathetic system and (b) the parasympathetic system. The sympathetic system consists of a double chain of ganglia (collection of nerve cells), one on each side of the spinal cord. The sympathetic ganglia are connected to the central nervous system and visceral organs by nerve fibres. The ganglia of the parasympathetic system are also paired but these occur near or within the visceral organs. The nerve fibres of the parasympathetic system originate in the brain and terminate in the posterior part of the spinal cord. The sympathetic and parasympathetic systems regulate the functioning of all the internal organs of the body which are innervated by sympathetic and parasympathetic nerve fibres. The sympathetic system usually speeds up or stimulates the action of a particular organ whereas the parasympathetic system exerts an inhibitory effect on the same organ. Thus the two components of the autonomic nervous system exert opposite effects on the organs. For example, in the working of the heart, the vagus nerve (parasympathetic) inhibits the heart by releasing acetylcholine while the accelerator nerve (sympathetic) releases noradrenalin (norepinephrine) and increases or accelerates the heart beat. In general, all parasympathetic nerves release acetylcholine and most sympathetic nerves release noradrenalin. The substance such as acetylcholine and noradrenalin are called neurotransmitters because they aid in transmission of nerve impulses. These substances are released at the synapses and help in transmitting the impulse from one neuron to the other at the synapses. The autonomic system in relation to the central nervous system and the various organs is shown in the figure below.
The Sympathetic (Broken Lines) and Parasympathetic (Solid Lines) of the Autonomous Nervous System in Relation to the Central Nervous SystemTransmission of Nerve Impulse
The two outstanding properties of the nerve fibre are excitability and conductivity. Excitation arises at the receptors on account of various stimuli such as light, temperature, gravity, pressure, chemical or electrical stimuli which constantly act on the organism. Animals respond to various stimuli by movements, either towards or away from the stimulus. The organ that receives the stimulus is the receptor. The organ effecting the stimulus is called the effector. The excitation generated at the receptor by a stimulus is transmitted along nerve fibres to the effector via the central nervous system. The transmission of excitation is called conductivity.
If a motor nerve is stimulated, a muscle contracts. It is clear that something has passed from the point of stimulation to the other end. This 'something' is called the nerve impulse. In physiological terms a nerve impulse is the overall physiological change (physical, chemical and electrical changes) that occurs in the nerve fibre when it is stimulated. It has been found that this is primarily an electrical phenomenon. The conduction of excitation along the nerve fibre involves the bioelectric phenomenon called the action potential in the nerve tissue. At the synapses the transmission involves certain metabolic events.
Studies of cell membranes, especially in nerve and muscle cells, indicate that when a cell is at rest there is considerable difference between the ion concentration outside and inside the plasma membrane. In a resting neuron (one that is not conducting an impulse), there is a difference in electrical charges on either side of the membrane. This difference, called the potential difference, is partly the result of an unequal distribution of potassium (K+) ions and sodium (Na+) ions on either side of the membrane. In resting neurons, K+ ion concentration inside the cell is about 28to 30 times greater than it is outside. The Na+ ion concentration is about 14 times greater outside than inside. Another significant factor is the presence of large, non-diffusible negatively charged ions trapped in the cell. Most of them are proteins.
Even when a nerve cell is not conducting an impulse, it is actively transporting ions across the membrane. Na+ ions are actively transported out and K+ ions are actively transported in. The cellular system by which Na+ and K+ ions are actively transported simultaneously is called sodium-potassium pump. The operation of the pump requires the expenditure of ATP. Neurons also contain a large number of negative ions, mostly protein anions, on the inside that cannot diffuse outside or diffuse poorly. Since Na+ ions are positive and are actively transported outside the cell by the sodium potassium pump, a positive charge develops outside the membrane. Even though K+ ions are positive and are actively transported to the inside of the cell by the sodium-potassium pump, there is insufficient concentration of K+ ions to equalize the even larger number of non-diffusible negative ions trapped in the cell. The membrane permeability to K+ ions is 100 times that leave the cell, thus, 100 times greater than the number of Na+ ions that enter. The sodium-potassium pump, apart from transporting the Na+ and K+ ions across the membrane, establishes concentration gradients for the ions. The overall result of all these, is that there is a difference in charge on either side of the membrane-the outside is positive and the inside is negative. This difference in charge on either side of the membrane of a resting neuron is the membrane resting potential. Such a membrane is said to be polarised.
Electrical measurements of a polarized membrane indicate a voltage of about 70 millivolts (mv). This means that the inside of the membrane is 70 mv less than the outside, that is, the membrane potential is -70 mv.
Depolarisation and Action Potential
If a stimulus of adequate strength is applied to a polarized membrane, the membranes permeability to Na+ ions is greatly increased at the point of stimulation. Nerve fibres can be stimulated electrically, mechanically (by pinching) and thermally (as by the application of a heated glass rod) or chemically. At this time there are more Na+ ions entering the cell than leaving. This results in a change in the electrical potential. At first the potential inside the membrane changes from -70 mv to 0 mv. At 0 mv the membrane is said to be depolarised. Throughout depolarisation, the Na+ ions continue to come inside until the membrane potential is reversed resulting in the inside of the membrane becoming positive and the outside negative. Electrical measurements, at this stage, indicate that the inside of the membrane is now +30 mv with respect to the outside. Thus the potential inside the membrane changes from -70 mv to +30 mv.
Incitation and conduction of nerve impulseOnce the events of depolarisation have occurred, it can be said that an action potential (nerve impulse) is initiated. It lasts about 1 m sec. The stimulated, negatively charged point on the outside of the membrane sends out an electrical current to the positive point (still polarized) adjacent to it. This local current causes the adjacent inner part of the membrane to reverse its potential from -70 mv to +30 mv. The reversal repeats over and over until the nerve impulse is conducted through the length of the neuron. The nerve impulse is essentially a wave of negativity that self propagates along the outside of a neuron cell membrane. Depolarisation and reversal of potential require only about 0.5m sec. Of all the cells in the body, only muscle and nerve cells produce action potentials. Their ability to do this is called excitability.
By the time the impulse has travelled from one point on the membrane to the next, the previous point becomes repolarised, that is, its resting potential is restored. Repolarisation results from changes in the membrane permeability. The membrane now becomes more permeable to K+ ions than it was at its resting potential levels and it is relatively less permeable to Na+ ions. As the K+ ions move, the outer surface of the membrane becomes electrically positive. The heavy loss of positive ions leaves the inner surface of the membrane negative once again. During repolarisation the cell returns to its resting potential, from +30 mv to -70 mv. The neuron is ready to receive another stimulus. The period of time during which the membrane recover is called refractory period.
All or None Principle
Any stimulus strong enough enough to initiate an impulse is referred to as a threshold or liminal stimulus. When a stimulus is of threshold strength the neuron is said to be at its threshold of stimulation. If the stimulus is below the threshold level in strength the nerve cell is not excited and a stimulus beyond the threshold strength does not have any effect on the nerve conduction. That is, the conduction is independent of any further increase in the intensity of stimulus beyond its threshold level. This phenomenon is called all or none principle.
Record of potential changes in a nerve impulseSaltatory Conduction
The step-by-step depolarization in a non-myelinated nerve fibre, like the one just now described, is called continuous conduction. In myelinated fibres, conduction is somewhat different. The myelin sheath surrounding a fibre contains a lipoprotein substance that does not conduct an electric current. However, the myelin sheath is interrupted at various intervals called nodes of Ranvier. At these nodes, membrane depolarisation can occur. But beneath the myelin sheath depolorisation is impossible. When an impulse is transmitted along a myelinated fibre; it moves from one node to another through the surrounding extracellular fluids and through the axoplasm.
Thus the impulse jumps from node to node. This type of impulse conduction, characteristic of myelinated nerve fibres, is called saltatory conduction (saltary = leaping). Since the impulse jumps long intervals as it moves from one node to the next, the speed of conduction is greatly increased. The impulse travels much faster than in the step-by-step depolarization process involved in non-myelinated nerve fibre.
Synapses are interneuronal junctions where the axon of one nerve cell comes very near to the dendrites of another neuron. When the wave of depolarization reaches the axon terminals a neurotransmitter substance (acetyl choline or norepinephrine) is secreted which causes the depolarization and generation of action potential in the second neuron. At synapses the neurons are not in physical contact; there is a gap known as synaptic gap between them. The neurotransmitter bridges the gap between the neurons so that the impulse is transmitted from one neuron to another. The neurotransmitter is quickly degraded enzymically, so that the synapses again can transmit further impulses.
Details of Synaptic Transmission: Synaptic Vesicles fuse with the Presynaptic Membrane and Discharge Transmitter Substances into the Synaptic CleftImpulses are conducted form a neuron to a muscle cell across an area of contact called neuromuscular junction. The area of contact between a neuron and glandular cells is known as neuroglandular junction. In both these types of junctions there is no contact between the neuron and the effector. There is a gap, as in the case of synapses, which is bridged at the time of the transmission of the impulse by a neuro transmitter.
The best studied neurotransmitter substance is acetylcholine which is released by many neurons outside the brain and spinal cord and by some neurons inside the brain and cord. At neuromuscular junctions, acetylcholine binds to receptor sites on the muscle fibre membrane and it increases its permeability to Na+ and K+ ions. This depolarises the membrane and a nerve impulse is generated, causing the muscle fibre to contract. Acetyl choline is quickly inactivated by the enzyme cholinesterase, so that transmission of continuous succession of impulses is prevented. Norepinephrine is another neurotransmitter released from sympathetic nerve endings. A substance called gamma-amino butyric acid (GABA) is believed to be the transmitter substance in the brain.