The spinal cord and spinal reflexes

Published on 03/03/2015 by admin

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Last modified 03/03/2015

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Chapter 13

The spinal cord and spinal reflexes

Basic anatomy and function of the spinal cord

Gross anatomy

The spinal cord begins superiorly at the foramen magnum where it is continuous with the medulla oblongata (S2.10). It continues inferiorly within the vertebral column protected in the vertebral canal by the meninges and cerebrospinal fluid (S2.8). The lowest part of the spinal cord lies at the first lumbar vertebra (level L_1/2). Along the length of the spinal cord, there are two swollen regions: the cervical and lumbar enlargements, which represent the origins of the brachial and lumbosacral plexus, respectively.

Grey and white matter

In cross-section (Fig. 13.1), the spinal cord can be seen to consist of the inner grey matter (H shaped) and the remaining area termed the white matter. These are so named because of their microscopic appearance and a consequence of the cells located within the area. The white matter consists of myelinated axons and represents the existence of the ascending and descending tracts (S2.14, 15). The grey matter is formed by the cell bodies of the same neurons and includes motor neurons and many interneurons which make connections within segments and between segments of the spinal cord. The grey matter is described in terms of the anterior/ventral horn and posterior/dorsal horn (Fig. 13.1). In the thoracic and upper lumbar region there is also a lateral horn, which represents the sympathetic trunk, part of the autonomic nervous system.

Figure 13.1 A cross-section of the spinal cord.

Central pattern generators (CPG)

A central pattern generator is a network of neurons within the spinal cord which underlie the production of most of our rhythmic motor output, such as walking, chewing/swallowing and breathing. These are by nature stereotyped, repetitive and complex movements but unlike reflex activity, are still under voluntary control. The network of neurons is thought to work in two opposing halves which when activated switch each other on and off to create a repetitive rhythmic action. The stereotyped movement pattern created avoids the need for descending commands from higher centres, however changes in the environment may require the CPG to be modulated and adapted to meet the changing needs. This modulation primarily comes from the higher centres but the CPGs also modulate themselves by responding to sensory feedback from the body. A small change in function may result in a long-term change to the whole CPG network as it adapts to achieve a motor goal. In terms of neurologically impaired patients, this may be positive or negative in relation to function.

Spinal nerves

At each spinal level there are a pair (left and right) of spinal nerves which exit the vertebral canal via the intervertebral foramen. There are 31 pairs of spinal nerves in total (8 cervical, 12 thoracic, 5 lumbar and 5 sacral). The spinal nerve consists of a sensory root and motor root (Fig. 13.1) from the spinal cord. The sensory root transmits information into the spinal cord and its cell body is found in the dorsal root ganglion just adjacent to the spinal cord but within the vertebral canal. The motor root carries information out of the spinal cord and its cell body is located in the anterior/ventral horn of the grey matter of the spinal cord. The sensory and motor roots at one level combine to form a spinal nerve and are part of the peripheral nervous system. Spinal nerves from several levels of the spinal cord will combine to form a peripheral nerve. For example, the median nerve consists of the spinal nerves from levels C_5/6/7/8 and T₁, whereas the musculocutaneous nerve includes spinal nerves from levels C_5/6/7.