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416 P R I N C I P L E S A N D P R A C T I C E O F C R I T I C A L C A R E

Dendrite

Nucleolus

Axon
hillock Nucleus

Initial segment Nissl bodies
Free nerve
endings in skin Oligodendroglial
Schwann cell cell myelin Axon
myelin CNS Myelin
sheath
Node of Ranvier PNS

Afferent cell body Nucleus of
in dorsal root ganglion Schwann
cell
Nucleolus
Nucleus Schwann cell myelin

Nissl
bodies PNS
CNS
Oligodendroglial
cell myelin

Synaptic Neuromuscular
A terminals B Muscle fibre junction
FIGURE 16.2 (A) Afferent and (B) efferent neurons, showing the soma or cell body, dendrites and axon. Arrows indicate the direction for conduction of
action potentials. 3

brain, they can be divided into two general classes: elec- Neurotransmitters
trical synapses and chemical synapses. Electrical synapses A neurotransmitter is a chemical messenger used by
permit direct, passive flow of electrical current from one neurons to communicate in one direction with other
neuron to another in the form of an action potential; they neurons. Unidirectional transmission is required for mul-
are described in Table 16.1. The current flows through gap tineuronal pathways, for example to and from the brain.
junctions, which are specialised membrane channels that Neurons communicate with each other by recognising
connect the two cells. Chemical synapses, in contrast, specific neuroreceptors.
enable cell-to-cell communication via the secretion of
neurotransmitters; the chemical agents released by the Chemically, there are four classes of neurotransmitters:
presynaptic neurons produce secondary current flow in 1. acetylcholine (ACh): the dominant neurotransmit-
postsynaptic neurons by activating specific receptor ter in the peripheral nervous system, released at
molecules (see Figure 16.3). neuromuscular junctions and synapses of the para-
5
Myelin increases conduction velocity. Demyelination of sympathetic division
peripheral nerves, as occurs in the Guillain–Barré syn- 2. biogenic amines: serotonin, histamine, and the
drome, slows conduction and may result in conduction catecholamines dopamine and noradrenaline
block, which manifests clinically as weakness. Con- 3. excitatory amino acids: glutamate and aspartate,
sequently, chronically demyelinated axons become and the inhibitory amino acids gamma-
vulnerable, with axon loss being a major cause of dis- aminobutyric acid (GABA), glycine and taurine
ability. In time, remyelination may occur, requiring the 4. neuropeptides: over 50 of which are known, amino
generation of myelin-competent oligodendrocytes but acid neurotransmitters being the most numerous.
most often does not fully recapitulate developmental In 2009, it was discovered that there is also more than
myelination. one neurotransmitter per synapse; these are called