Page 127 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Nervous tissue (textus nervosus) 109

Function of the chemical synapse ture of the G protein, resulting in the formation of a second
VetBooks.ir Chemical transmission of neural impulses at the synapse messenger molecule that opens ion channels located
is performed by neurotransmitters. These substances elsewhere in the membrane (e.g. adrenergic receptors).
Inactivation of the neurotransmitter occurs primar-
conduct the nerve impulse to other neurons or to effec-
tor cells. Neurotransmitters of the peripheral nervous ily through reuptake into the presynaptic membrane. This
system include biogenic amines (e.g. noradrenaline) and process is facilitated by high affinity of the transmitter for
carboxylic acid esters (acetylcholine). In the central ner- the membrane proteins. Reacquired neurotransmitters are
vous system, impulses are transmitted by biogenic amines recycled. In other cases, the neurotransmitter is broken
(noradrenaline, dopamine, serotonin and adrenaline), ace- down enzymatically into inactive molecules (e.g. acetyl-
tylcholine and other molecule classes (e.g. neuropeptides choline is degraded by acetylcholinesterase into choline
such as enkephalins, β-endorphins and substance P, amino and acetate).
acids including γ-aminobutyrate [GABA], glycine and
glutamate). Small-molecule neurotransmitters are pre- Neuromuscular synapse (motor end plate)
dominantly synthesised in the cytosol of the end bulbs, Motor end plates are specialised chemical synapses in
while neuropeptides are produced in the perikaryon. which motor nerve fibres transmit neural impulses to
Despite the wide variety of neurotransmitters, impulse skeletal muscle (Figure 5.13). They are also referred to as
transmission appears to proceed according to the same neuromuscular (or myoneural) junctions. Towards the
basic mechanism in all synapses. The sequence of events end of the nerve fibre, the myelin sheath ends, and the
comprises: axon divides into branches that come to lie in concavities
in the muscle cell. Copious synaptic vesicles containing the
· release of the neurotransmitter from the presynap- neurotransmitter acetylcholine are located in the free ends
tic nerve terminal into the synaptic cleft, of the axon. The postsynaptic membrane features multiple
· interaction of the neurotransmitters with specific junctional folds. The synaptic cleft, including the spaces
postsynaptic receptors, between the folds, contains the external lamina of the sur-
· alteration of voltage across the postsynaptic mem- rounding neurolemmocyte (see below). Mitochondria,
brane as a result of altered ion permeability and ribosomes and glycogen granules are present under the
· inactivation of the chemical signal by enzymatic thickened postsynaptic membrane.
cleavage of the neurotransmitter or reuptake of the The release of acetylcholine into the synaptic cleft
neurotransmitter by the nerve terminal. results in depolarisation of the plasmalemma of the mus-
cle cell. The action potential passes to the T system, from
A nerve impulse arriving at a synaptic end bulb (action which it is transmitted to the sarcoplasmic reticulum
potential) leads to transient depolarisation of the (Chapter 4, ‘Muscle tissue’).
presynaptic membrane. This promotes the influx of extra- The transmission of sensory information from skeletal
2+
cellular Ca . Under the influence of the intracellular Ca muscle involves specialised receptors termed neuromus-
2+
ions, synaptic vesicles are transported to the presynaptic cular and neurotendinous spindles. These are described
membrane (possibly with the aid of neurotubules) and further in Chapter 16, ‘Receptors and sense organs’.
neurotransmitters are released into the synaptic cleft.
The liberated neurotransmitter induces a local voltage Nerve fibre (neurofibra)
change in the postsynaptic membrane. The term nerve fibre refers to the axon of a neuron and
The postsynaptic membrane contains specific recep- its outer covering. The latter is formed by glial cells (oli-
tors formed by membrane proteins. Upon binding with a godendrocytes in the central nervous system [forming the
neurotransmitter, the receptors undergo conformational white matter]; Figures 5.11 and 5.12, Schwann cells [neu-
alterations that result in opening of ion channels (e.g. for rolemmocytes] in the peripheral nervous system). As well
+
–
+
+
Na , K and Cl ). An increase in the passage of Na ions as being distinguished by the cells from which their sheaths
into the cell results in depolarisation of the postsynaptic originate, nerve fibres can be classified on the basis of func-
membrane. tion. Based on differentiation of neuronal pathways in the
There are two main types of postsynaptic membrane somatic and autonomic (vegetative) nervous system, nerve
receptors. Ionotropic receptors contain ion channels, such fibres are divided into:
that the conformational change induced by binding of the
neurotransmitter leads directly to opening of the associ- · somatic afferent sensory nerve fibres: convey
ated channel (e.g. nicotinic ACh receptors). impulses from the periphery to the spinal cord or
Metabotropic receptors do not incorporate an ion brain, and from some organs of special sense to the
channel, rather their intracellular domain is linked to a G brain (e.g. the auditory and visual senses),
protein. Binding of the neurotransmitter alters the struc-

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