The spinal cord and spinal reflexes

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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 L1/2). 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.

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 C5/6/7/8 and T1, whereas the musculocutaneous nerve includes spinal nerves from levels C5/6/7.

Lower motor neuron/final common pathway and upper motor neuron

Understanding the anatomy of the spinal cord and its divisions is vital for ensuring your assessment of a neurologically impaired patient is appropriate and accurate. One important division is the differentiation between an upper motor neuron and lower motor neuron pathology and their clinical presentations.

The lower motor neuron/final common pathway

The neurons indicated as nerve roots in Figure 13.1 are the starting point of the lower motor neuron division. Anything more peripheral is included in this division. In effect, damage to these neurons is interpreted as a deficit of the peripheral nervous system, for example, Guillain–Barré syndrome and motor neuron disease. However, different clinical presentations can occur within this division depending upon the location of the lesion. For example, damage to the spinal root or spinal nerve would present very differently to that of a peripheral nerve.

Spinal root or spinal nerve

A single spinal root or nerve could be damaged with a very specific injury such as an intervertebral disc prolapse or in association with other trauma. The injury itself could relate to:

Therefore the clinical presentation resulting from damage to a single spinal root or spinal nerve will be very localized in terms of reduced sensation and/or a weakness in a specific muscle. Total paralysis of a muscle is unlikely as a single spinal root level innervates only a fraction of the muscle. For example, the triceps brachii muscle is supplied by the radial nerve (C5/6/7/8 and T1), so damage to root level C5 would present as mild weakness and not total paralysis because the muscle is still innervated by the remaining undamaged spinal root levels.

Basic anatomy and function of spinal reflexes

A reflex is an involuntary, almost instantaneous movement, in response to a stimulus that does not require the higher centres of the brain. However, information received from both the periphery and higher centres is used to modulate the reflex activity at the level of the spinal cord.

The neural pathway involved in a simple reflex arc includes:

The most important reflex arcs in relation to movement control are:

Stretch reflex

The stretch reflex has both a phasic and a tonic component. The phasic component adapts rapidly to a stimulus and responds to a change in muscle length. The tonic component which is continually active is more responsive to a constant stretch.

Phasic stretch reflex

This component of the stretch reflex is primarily monosynaptic:

Function

The phasic component is evident in the intact nervous system at rest and is the component that is assessed during reflex testing (S3.22). An imposed stretch on the muscle by the clinician gives information on nerve integrity and the reflexic properties of muscle. This technique is referred to as a tendon jerk.

Tonic stretch reflex (TSR)

This component is primarily polysynaptic:

Function

The tonic component is not apparent in an intact nervous system unless the muscle is already contracting. The TSR serves to set resting muscle tone (S3.21), the base level of muscle activity. The reflex arc also responds to changes in the environment adjusting the level of activity as appropriate to functional needs, e.g. lowering muscle activity when we lie down and raising it in preparation to stand up against gravity.

Golgi tendon organ (GTO)

This reflex arc is polysynaptic:

Flexor withdrawal and crossed extensor reflex

This reflex is polysynaptic and initiated from the flexor withdrawal arc:

Additional neuronal activity associated with the reflex arc

For the reflex arcs described above, additional circuitry exists to ensure the efficiency of the response and allow modification as necessary.

First, the afferent neuron within the circuit includes the following branches which synapse upon:

Second, the efferent neuron output from the reflex arc can be modulated by neurons from higher centres. This occurs via the descending tracts (S2.14), which synapse upon the alpha motor neuron directly or indirectly via interneurons. This descending control involves altering the level of output from the AMN to the muscle by enhancing either the inhibitory or excitatory synapses as required.

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