Biology Reference
In-Depth Information
2. STRUCTURE AND FUNCTION OF THE PERIPHERAL
NERVES
The neurons of the PNS are responsible for receiving and emitting
impulses to connect through nerves of the CNS with the outside world
(somatic PNS) or the own body (autonomic PNS). Peripheral nerves are
formed by bundles of fascicles grouped by the epineurium, a layer of con-
nective tissue that also contains blood vessels. Nerve fascicles are surrounded
by another layer of connective tissue, the perineurium, and contain groups
of axons that are embedded within a matrix of endoneurium. The fascicles
are functionally and somatotopically organized to provide muscles, sensory
organs, or viscera with motor information and to record sensory stimuli.
Peripheral axons can be unmyelinated or myelinated, and the latter sub-
divided into different classes according to their diameter. The velocity of
impulse conduction and the excitability for electrical stimulation are directly
related to the size of the axons.
Somatic motor neurons have their soma located in the ventral horn of the
spinal cord and are responsible for controlling the skeletal muscles of the
body. Individual motor axons exit the spinal cord to innervate several muscle
fibers constituting a motor unit. An action potential in a motoneuron travels
through the axon jumping fromnode to node up to the axon terminals where,
through the neuromuscular junction, it stimulates the muscle fibers of the
motor unit producing their contraction. Thus, to increase muscle strength,
several motor units need to be recruited, being the slow fatigue-resistant
motor units, the first ones to be activated in a muscle contraction, whereas
large fast-fatigue motor units are activated only when high levels of tension
are needed. On the other hand, sensory neurons, whose soma are in the spinal
ganglia, carry stimuli from different organs to the CNS. Changes in the envi-
ronment of the skin, muscles, and joints that are detected by specific mem-
brane receptors produce action potentials that travel along sensory
myelinatedor unmyelinated axons tobringmechanical, proprioceptive, ther-
mal, or pain information to the CNS. Intensity of stimulation is coded by the
frequency of action potentials and the number of sensory afferents recruited.
After peripheral nerve injury, the control of motor, sensory, and auto-
nomic functions of the denervated areas can be lost. The regeneration of
a severed nerve requires a complex and coordinated process in which
