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Organization Of The Nervous System

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.

Organization Of The Nervous System

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).