In this article we will discuss about the Nerve Tissue:- 1. Structure of a Typical Nerve 2. Chemical Composition of the Nerve Tissue 3. Metabolism 4. Nerve Impulse 5. Transmission of Nerve Impulse from One Neurone of Other 6. Nature of the Chemical Transmitters 7. Cholinergic and Adrenergic 8. Transmission of an Impulse from the Nerve to a Skeletal Muscle.
- Structure of a Typical Nerve
- Chemical Composition of the Nerve Tissue
- Metabolism of the Nerve Tissue
- Nerve Impulse
- Transmission of Nerve Impulse from One Neurone of Other
- Nature of the Chemical Transmitters Released by Nerve Tissue
- Cholinergic and Adrenergic Nerve Fibres
- Transmission of an Impulse from the Nerve to a Skeletal Muscle
1. Structure of a Typical Nerve:
A nerve consists of groups of nerve fibres. Bundles of nerve fibres are enclosed in a connective tissue sheath known as perineurium. A number of such bundles are bounded together by another sheath of connective tissue known as the epineurium to form a nerve.
2. Chemical Composition of the Nerve Tissue:
Nerve tissue is composed of:
Water……………………………… 80 percent.
Solids……………………………… 20 per cent.
The solids are mainly composed of proteins, lipids, small amounts of organic extracts and inorganic salts. Proteins are about 38 to 40 per cent of the total solids. They include different globulins, nucleoproteins, and a characteristic albuminoid called neurokeratin. The lipid contents are 50 to 54 per cent of the total solids.
The important lipids are phospholipids, cholesterol, cerebrosides, amino-lipids, and sulphur containing lipids. The principal inorganic salts are potassium phosphate and chloride, with smaller amounts of sodium and other alkaline elements. Potassium is highly significant in the nerve impulse.
The water content of brain is little more than that of spinal cord. In brain too the grey matter which represents a concentration of nerve cell bodies contains more water than the whole matter where the nerve fibres are mainly found. A fraction of the brain proteins remains combined with copper forming ceruloplasmin.
An increased deposition of copper is found in the brain tissue in Wilson’s disease.. White matter and the peripheral nerves contain a little more cerebrosides, free cholesterol, and sphingomyelin than the brain grey matter. The considerable increase in the concentration of sphingomyelin is found in Niemann-Pick disease.
3. Metabolism of the Nerve Tissue:
Since the respiratory quotient (R.Q.) of the metabolic nerve is near about 1 this indicates that the nerve tissues utilize carbohydrates almost exclusively for the energy purposes.
Lactic acid and pyruvic acid are formed under anaerobic conditions in the nerve tissue as a result of carbohydrate metabolism and these acids disappear very slowly in the presence of oxygen. Hence, the metabolism of carbohydrates in the nerve tissue appears to be similar to that of muscles.
A minute supply of blood glucose is specially important to the nervous system because of the less glycogen storage in the nerve tissue. This may be the vital cause for the prominence of nervous symptoms such as mental confusion, dizziness, delirium, etc. in hypoglycemia.
As regards to protein metabolism, glutamic acid is the only amino acid metabolized by brain tissue. This acid serves as a precursor of γ-aminobutyric acid (GABA), one of the chemical transmitters, and as a major acceptor of ammonia produced either by brain itself or delivered to brain when the arterial blood ammonia is increased and thus protects the brain tissue against its toxic effects.
Different workers have also demonstrated the synthesis and exchange of lipids in nerve tissue.
4. Nerve Impulse:
Nerve impulse may be defined as an electro-chemical change which is transmitted by nerve fibres. It must not be confused with the stimulus which is the external force (e.g., chemical, physical, biological) which sets up the impulse.
The chemical changes in the nerve fibres are concerned with the recovery processes which follow the activity. The electrical change in the most certain indicator of the development and propagation of the nerve impulse and represents the essential process involved in the transmission of the impulse along the nerve fibre.
5. Transmission of Nerve Impulse from One Neurone of Other:
The successive neurones are separated from one another by a narrow slit called as synaptic cleft; while the junction between one neurone and the next is termed as a synapse. Synapse consists of a presynaptic terminal and a part of perikaryon which is separated by a synaptic cleft having an average width of about 200Å.
The presynaptic terminal contains mitochondria and synaptic vesicles.
The synaptic vesicles contain chemical transmitter which may be excitatory and then the presynaptic terminal is termed as excitatory presynaptic terminal and the parent neurone as excitatory neurone or inhibitory to the next neurone and then the presynaptic terminal is called as inhibitory presynaptic terminal and the parent neurone as inhibitory neurone.
The action potential reaching the presynaptic terminal causes these vesicles to release transmitter substance in the synaptic cleft. The transmitter immediately increases the permeability of sub-synaptic somal membrane either to the Na+ when it is excitatory in nature or to K+ when it is inhibitory in nature.
When the permeability of Na+ increases, Na+ enters the interior of the perikaryon of the next neurone. Thus, it brings about a sequence of changes resulting in the action potential called as excitatory post synaptic potential (EPSP) and thus this neurone gets excited.
But if the permeability increases to K+ due to release of inhibitory transmitter, K moves out rapidly through the membrane because of excess of K+ inside the cell leaving a greater degree of negativity inside the neurone.
This state is known as hyper-polarisation and the potential so developed as Inhibitory Postsynaptic Potential (IPSP) because this results in the inhibition of the next neurone or the effector cell. It is also called as the chemical transmission of nerve impulses due to the involvement of chemical transmitters in the above process.
6. Nature of the Chemical Transmitters Released by Nerve Tissue:
Loewi in 1921 first discovered the process of chemical transmission. He applied the fluid from a perfused frog heart to a second heart and showed that when the vagus nerve supplying to the first heart was stimulated, the fluid from this heart also caused the typical vagal effects of depression of the second heart.
Similarly, the stimulation of sympathetic nerve supply to the first heart also caused the stimulation of second heart. Loewi called the substance so liberated by vagus stimulation as ‘vagusstoff’ and that liberated by sympathetic stimulation as “acceleranstoff” Later on these substances had been shown to be acetylcholine and noradrenaline respectively.
A substance to act as a transmitter must satisfy the following needs:
a. Nerve stimulation must release the substance (the transmitter) from the nerve terminals.
b. Drugs which antagonise the action of that substance at the post-synaptic membrane should also block the effects of the nerve stimulation transmitted by that very substance.
c. Substances (drugs) which block the synthesis or storage of this substance should block the effects of nerve stimulation transmitted by this substance.
d. There must be mechanism for the destruction or removal of the proposed transmitter, e.g., the enzyme acetyl cholinesterase is needed for the destruction of acetylcholine; so this enzyme must be present in the neurons whose transmitter is acetyl choline.
e. It must be synthesized and stored within the neurones.
f. The action of the substance on the innervated organ if added from outside must be the same as the effects on stimulation of the nerves which release that substance.
g. Drugs which affect the destruction or removal of transmitter must also influence the response to nerve stimulation in an appropriate way.
The transmitters which satisfy the above needs are acetylcholine, epinephrine, norepinephrine. The possible transmitters are γ-aminobutyric acid (GABA), serotonin, glutamic acid, glycine, and dopamine.
7. Cholinergic and Adrenergic Nerve Fibres:
Dale expressed that the nerve fibres which form and release acetylcholine as transmitter are termed as cholinergic fibres; while those which form and release noradrenaline are termed as adrenergic fibres.
Cholinergic nerve fibres are as follows:
a. All the pre-ganglionic fibres of both parasympathetic as well as sympathetic systems.
b. All the post-ganglionic fibres of the parasympathetic systems.
c. Certain post-ganglionic fibres of the sympathetic system, e.g., nerve fibres to the sweat glands.
d. The motor nerve fibres to skeletal muscles.
e. Some of the neurones of the central nervous system.
f. Efferent fibres supplying to adrenal medulla.
The adrenergic fibres are as follows:
a. Most of the post-ganglionic fibres of the sympathetic system.
b. Some of the neurones of the central nervous system.
8. Transmission of an Impulse from the Nerve to a Skeletal Muscle:
Figure 40.11. A. shows a junction between a motor nerve fibre and a skeletal muscle fibre which is said to be neuromuscular junction. A motor neurone together with the group of muscle fibres which it innervates is called a motor unit.
As the myelinated nerve fibre approaches to a skeletal muscle fibre, it loses its medullary sheath and branches at its terminal end into terminal ramifications. At the tips of these terminal ramifications of the nerve fibre are still other structure called as sole feet which make an extensive contact with a specialised part of the muscle fibre membrane to form the actual neuromuscular junction.
The part of muscle fibre membrane lying in contact with a sole foot is called as motor end plate (Fig. 40.11 B). It is thrown into several folds which increase the surface area at which the synaptic transmitter can act.
The large aggregates of the enzyme cholinesterase which is capable of destroying the synaptic transmitter (acetylcholine) exist around the rim of the motor end plate.
The sole foot is separated from the synaptic gutter (motor end plate) by a gap known as synaptic cleft which remains filled with a gelatinous “ground substance” through which the extracellular fluid diffuses. The sole foot contains a number of small vesicles which store the synaptic transmitter synthesized probably by mitochondria which also supply the energy for synthesis.
As a result of the discussion of the structural details of neuromuscular junction, the sequence of events that occur at the neuromuscular junction during the transmission of a nerve impulse from motor nerve fibre to a skeletal muscle fibre can be discussed as follows:
a. Arrival of an impulse at motor nerve terminal.
b. Rupture of synaptic vesicles containing acetylcholine by these calcium ion (Ca++) through the membrane of the sole foot and thus release of acetylcholine into the synaptic cleft and its attachment to the end plate receptor (cholinergic receptors). Acetylcholine release is inhibited by magnesium ion (Mg++).
c. Movements of Ca++ (hypothetical) from the extracellular fluid into membrane of the sole foot.
e. Propagation of the so produced end-plate potential when it reaches a certain critical magnitude (threshold).
e. Increase in permeability of muscle membrane to Na+ allowing rapid influx of Na+ into the muscle fibre and thus production of a potential called the local end-plate potential which is entirely analogous to the excitatory post-synaptic potential.
f. Removal of acetylcholine from its receptor site within about two milliseconds partly by diffusion back into sole foot and partly by destruction by cholinesterase. The sequence of these events is represented in the figure 40.12.
When the nerve fibre is stimulated at a rate greater than 150 times per second continuously for many minutes, the quantity of acetylcholine release with each impulse diminishes; so that impulses even fail to pass into the muscle fibre.
This is referred to as the fatigue of the neuromuscular junction and is analogous to fatigue of a synapse. However, under normal conditions, fatigue of the neuromuscular junction does never occur since it is rate that more than 150 impulses per second reach even the most active neuromuscular junction.