| Peripheral Nervous
System |
The peripheral nervous system (PNS) refers
those nerves (neurons) that are not part of the
brain or spinal cord. Nerves inside the brain
and spinal cord are part of the central
nervous system (CNS). PNS nerves have their cell bodies (ganglia)
inside the CNS but their length outside it. The PNS consists of neurons
with both myelinated and unmyelinated axons.
All the cranial nerves (the 12 pairs of
nerves leaving the base of the braincase) are part of the peripheral nervous
system although the second cranial nerve (the optic nerve) can be considered
as part of the brain.
There are two kinds of nerve in the Peripheral Nervous
System:
-
Motor or efferent nerves.
These are the nerves that connect to muscles in the limbs, torso, face
and internal organs. They carry nerve signals from the CNS and make body
parts move. The cell bodies of the motor nerves connect to CNS in the shaft
of grey matter in the spine called the anterior
horn. They terminate at a neuromuscular junction known as the motor
plate or motor end plate. Neurotransmitters released by the motor nerves
into the motor plate cause the muscle to contract.
-
Sensory or afferent nerves.
These are nerves that connect to sensory organs and sensors in the skin,
muscle and internal organs. They take relay nerve signals to the CNS. They
convey heat, touch, position (proprioceptive), balance, sound and other
information from the limbs and sense and internal organs. Sensory nerves
terminate in the CNS at the root ganglion.
The peripheral nervous system is divided into two parts:
-
The sensory-somatic nervous
system (SNS).
The SNS is performs all our interactions with the physical
world such as control of our limbs and receiving of conscious information
from our senses. It consists of the 12 pairs of cranial nerves and the
31 pairs of spinal nerves.
-
The autonomic nervous
system (ANS).
The ANS consists of both motor and sensory nerves which
run both to and from the central nervous system and fulfills "automatic"
functions that we are seldom aware of such as heart activity and the release
of hormones. The autonomic nervous system is further divided into the sympathetic
nervous system and the parasympathetic
nervous system.
Peripheral Nervous System links:
The
Peripheral Level
The
Nervous System (Peripheral Nervous System)
The
Peripheral Nervous System
The
Motor End Plate
Nervous
System Lecture Notes
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