Organization
Of The Nervous System
The nervous system can be divided into three major parts: the autonomic
(ANS), peripheral (PNS) and central (CNS) nervous systems.
The PNS is defined as those nerves that lie outside the brain, brainstem or
spinal cord, while the CNS embraces those cells that lie within these
structures.
Autonomic nervous system
•
The ANS has both a central and peripheral
component and is involved with the innervation of internal and glandular organs
(see Chapter 3): it has an important role in the control of the endocrine and
homoeostatic systems of the body (see Chapter 3, 11). The peripheral component
of the ANS is defined in terms of the enteric (see Chapter 4), sympathetic
and parasympathetic systems (see Chapter 3).
•
The efferent fibres of the ANS originate either
from the intermediate zone (or lateral column) of the spinal
cord or specific cranial nerve and sacral nuclei, and synapse in a ganglion,
the site of which is different for the sympathetic and parasympathetic systems.
The afferent fibres from the organs innervated by the ANS pass via the dorsal
root to the spinal cord.
Peripheral nervous system
•
The PNS consists of nerve trunks made up of both
afferent fibres or axons conducting sensory information to the spinal cord and
brainstem, and efferent fibres transmitting impulses primarily to the muscles.
•
Damage to an individual nerve leads to weakness
of the muscles it innervates and sensory loss in the area from which it conveys
sensory information.
•
The peripheral nerves occasionally form a dense
network or plexus adjacent to the spinal cord (e.g. brachial plexus in the
upper limb).
•
The peripheral nerves connect with the spinal
cord through foramina between the bones (or vertebrae) of the spine (or verte-
bral column), or with the brain through foramina in the skull.
Spinal cord
•
The spinal cord begins at the foramen
magnum, which is the site at the base of the skull where the lower part of
the brainstem (medulla) ends. The spinal cord terminates in the adult at the
first lumbar vertebra, and gives rise to 30 pairs (or 31 if the coccygeal
nerves are included) of spinal nerves, which exit the spinal cord between the
vertebral bones of the spine.
•
The first eight spinal nerves originate from the
cervical spinal cord with the first pair exiting above the first
cervical vertebra and the next 12 spinal nerves originate from the thoracic
or dorsal spinal cord. The remaining 10 pairs of spinal nerves originate
from the lower cord, five from the lumbar and five from the sacral regions.
•
The spinal nerves consist of an anterior or
ventral root that innervates the skeletal muscles, while the posterior
or dorsal root carries sensation to the spinal cord from the skin that
shared a common embryological origin with that part of the spinal cord (see
Chapter 1). The dorsal root fibres have their cell bodies in the dorsal root
ganglia which lie just outside the spinal canal.
•
The spinal cord itself consists of white
matter, which contains the nerve fibres that form the ascending and
descending pathways of the spinal cord, while the grey matter is
located in the centre of the spinal cord and contains the cell bodies of the
neurones (see Chapter 9).
Brainstem,
cranial nerves and cerebellum
•
The spinal cord gives way to the brainstem,
which lies at the base of the brain and is composed of the medulla, pons
and midbrain (or mesencephalon as it is sometimes called, although
this is strictly a term that should be reserved for this region of the brain in
embryonic development) and contains discrete collections of neu- rones or nuclei
for 10 of the 12 cranial nerves, the exceptions being the first (olfactory) and
second (optic) nerves (see Chapter 7).
•
The brainstem and the cerebellum constitute
the structures of the posterior fossa.
•
The cerebellum is connected to the brainstem via
three pairs of cerebellar peduncles, and is involved in the coordination of
move- ment (see Chapter 40).
Cerebral
hemispheres
•
The cerebral hemispheres are composed of four
major lobes: occipital, parietal, temporal and frontal.
On the medial part of the temporal lobe are a series of structures that form
part of the limbic system (see Chapter 45).
•
The outer layer of the cerebral hemisphere is
termed the cerebral cortex, and contains neurones that are organized in
both horizon- tal layers and vertical columns (see Chapter 10).
•
The cerebral cortex is interconnected over long
distances via pathways that run subcortically. These pathways, together with
those that connect the cerebral cortex to the spinal cord, brainstem and nuclei
deep within the cerebral hemisphere, constitute the white matter of the
cerebral hemisphere. These deep nuclei include structures such as basal
ganglia (see Chapters 41 and 42) and thalamus (Chapter 10).
Meninges
•
The CNS is enclosed within the skull and
vertebral column Separating these structures are a series of membranes referred
to as the meninges.
•
The pia mater is separated from the
delicate arachnoid membrane by the subarachnoid space (containing the
cerebrospinal fluid), which in turn is separated from the dura mater by
the subdural space (see Chapter 5).
