Page 55 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

2 Nerve and Muscle, Physical Work

rate of ca. 25 cm/day. Slow axon transport (ca.
Neuron Structure and Function
1 mm/day) plays a role in the regeneration of
An excitable cell reacts to stimuli by altering its severed neurites.
membrane characteristics (! p. 32). There are Along the axon, the plasma membrane of
two types of excitable cells: nerve cells, which the soma continues as the axolemma (! A1,2).
transmit and modify impulses within the The axolemma is surrounded by oligodendro-
nervous system, and muscle cells, which con- cytes (! p. 338) in the central nervous system
tract either in response to nerve stimuli or au- (CNS), and by Schwann cells in the peripheral
tonomously (! p. 59). nervous system (! A1,2). A nerve fiber con-
The human nervous system consists of sists of an axon plus its sheath. In some neu-
10
more than 10 nerve cells or neurons. The neu- rons, Schwann cells form multiple concentric
ron is the structural and functional unit of the double phospholipid layers around an axon,
nervous system. A typical neuron (motor neu- comprising the myelin sheath (! A1,2) that in-
ron, ! A1) consists of the soma or cell body sulates the axon from ion currents. The sheath
and two types of processes: the axon and den- is interrupted every 1.5 mm or so at the nodes
drites. Apart from the usual intracellular or- of Ranvier (! A1). The conduction velocity of
ganelles (! p. 8ff.), such as a nucleus and mi- myelinated nerve fibers is much higher than
tochondria (! A2), the neuron contains neuro- that of unmyelinated nerve fibers and in-
fibrils and neurotubules. The neuron receives creases with the diameter of the nerve fiber
afferent signals (excitatory and inhibitory) (! p. 49 C).
from a few to sometimes several thousands of A synapse (! A3) is the site where the axon
other neurons via its dendrites (usually ar- of a neuron communicates with effectors or
borescent) and sums the signals along the cell other neurons (see also p. 50 ff.). With very few
membrane of the soma (summation). The axon exceptions, synaptic transmissions in mam-
arises from the axon hillock of the soma and is mals are mediated by chemicals, not by electri-
responsible for the transmission of efferent cal signals. In response to an electrical signal in
neural signals to nearby or distant effectors the axon, vesicles (! p. 1.6) on the presynaptic
(muscle and glandular cells) and adjacent neu- membrane release transmitter substances
rons. Axons often have branches (collaterals) (neurotransmitters) by exocytosis (! p. 30).
that further divide and terminate in swellings The transmitter diffuses across the synaptic
called synaptic knobs or terminal buttons. If the cleft (10–40 nm) to the postsynaptic mem-
summed value of potentials at the axon hillock brane, where it binds to receptors effecting
exceeds a certain threshold, an action poten- new electrical changes (! A3). Depending on
tial (! p. 46) is generated and sent down the the type of neurotransmitter and postsynaptic
axon, where it reaches the next synapse via the receptor involved, the transmitter will either
terminal buttons (! A1,3) described below. have an excitatory effect (e.g., acetylcholine in
Vesicles containing materials such as pro- skeletal muscle) or inhibitory effect (e.g., gly-
teins, lipids, sugars, and transmitter sub- cine in the CNS) on the postsynaptic mem-
stances are conveyed from the Golgi complex brane. Since the postsynaptic membrane nor-
of the soma (! p. 13 F) to the terminal buttons mally does not release neurotransmitters
and the tips of the dendrites by rapid axonal (with only few exceptions), nerve impulses
transport (40 cm/day). This type of antero- can pass the synapse in one direction only. The
grade transport along the neurotubules is pro- synapse therefore acts like a valve that ensures
moted by kinesin, a myosin-like protein, and the orderly transmission of signals. Synapses
the energy required for it is supplied by ATP are also the sites at which neuronal signal
(! p. 16). Endogenous and exogenous sub- transmissions can be modified by other (exci-
stances such as nerve growth factor (NGF), tatory or inhibitory) neurons.
herpes virus, poliomyelitis virus, and tetanus
toxin are conveyed by retrograde transport
42
from the peripheral regions to the soma at a
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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