MOTOR ORGANISATION IN THE SPINAL CORD

Muscles are directly innervated by motoneurones located in the ventral column of spinal grey matter, or in brainstem motor nuclei (lower motoneurones). Each motoneurone innervates a group of muscle fibres (a motor unit = motoneurone + innervated fibres), which all have the same biochemical characteristics: SO (slow oxidative), FOG (fast oxidative/glycolytic) or FG (fast glycolytic), and which are interspersed among the fibres of other motor units.

They excite muscle fibres by releasing acetylcholine and the modulatory neuropeptide CGRP (calcitonin gene-related peptide) which activates adenylate cyclase to potentiate contractile force.

The motoneurones for a given muscle are distributed in spindle-shaped columns, running 2-4 segments in the cord, which may overlap with the distribution of the motoneurones for agonist muscles. The motoneurones which innervate a muscle are termed the motoneurone pool for that muscle.

In the figure to the left, motor neurones to the soleus (SOL) and medial gastrocnemius (MG) muscles have been stained by injecting the left MG and the right SOL muscle with the protein horseradish peroxidase. The protein is taken up by the nerve endings of the motor neurones and is distributed through the neuron. The neurons can then be visualised by using specific stains. Note the overlapping, spindle-shaped domains, extending over a few segments, occupied by the motor neurones of these agonists. The horizontal, dashed lines (A-E) indicate the levels of the dorsal roots of successive segments, and the levels at which the cross sectional reconstructions were made.

Inhibitory and excitatory influences converge onto the motor neurone pool, and depending upon the net level of excitation will recruit activity in variable numbers of motor units. With low-level excitation of a motor pool, the smallest motor neurones, associated with the smaller, SO type motor units, are first recruited. At higher levels, the intermediate sized neurones, associated with the FOG units, reach threshold. Finally, at the strongest levels of excitation, when the greatest forces are required (vs finely graded effort), the largest, FG units are excited. This is referred to as Henneman's Size Principle, and is probably related to a higher density of synaptic terminals on smaller motor neurones. Motor neurone pools for flexor muscles tend to be found more dorsally in the grey matter of the ventral horn; those for extensor muscles, more ventrally.

Motor pools for proximal limb and axial muscles tend to be located more medially; those for distal muscles, more laterally in the ventral horn. These groups are strategically placed to be influenced respectively, by the descending fibres of the lateral system (lateral cortico- and rubro-spinal pathways) and the medial system (reticulo-, vestibulo-, tecto- and medial cortico-spinal pathways). The lateral group of fibres is responsible for controlling fine fractionated movements of the distal muscles (fingers & toes); the medial for postural control. In the accompanying Diagram, which is designed in part as a mnemonic, extensor motor neurones are indicated by rectangles, flexors by triangles. More proximal motor neurones have larger motor units (larger symbols) than more distal ones.

Proximal and axial motor neurone pools tend to work together in widely separated groups to maintain balance and posture. Accordingly, long propriospinal interneurones, with axons running in the medial column of the white matter, maintain interconnectivity between widely separated levels of the cord. Distal motor neurones tend to work in a more isolated fashion to produce finely graded, fractionated movements. These motor pools are interconnected by short propriospinal interneurones, running in the lateral columns of the white matter. This allows a greater degree of intersegmental autonomy.

Descending pathways terminate in the dorsal horns (sensory modulatory fibres), on interneurones in the intermediate grey matter or on motor neurones (motor fibres). Major afferent influences come from the cutaneous receptors and proprioceptors (muscle spindle and Golgi tendon organs). Descending motor influences come from the medial and lateral descending systems.

Simple stereotyped responses to simple stimuli are termed reflexes. The pattern of interneuronal connections, and their interactions with afferent and descending inputs, determines the character of reflexes. Even the most simple reflexes can alter in strength or character depending upon the modulatory influences of ongoing activity, or through learning. An important role of descending activity is to modulate the reflex pathways in the cord, so that inappropriate reflexes are suppressed and useful ones facilitated.

Some of the well characterised interneurones include the following:

Renshaw Cells: are excited by recurrent collaterals from the motoneurones themselves, and which inhibit motoneurones in the same, and agonist, motor pools, while inhibiting the inhibitory interneurones in the motor pools of antagonist muscles. The neurotransmitter in these inhibitory interneurones is the amino acid glycine. Their role seems to be to stabilize activity in the motor pool and to reduce the subliminal fringe (by lateral inhibition). The poisonous alkaloid strychnine, blocks transmission at glycinergic synapses, and causes convulsions.

Propriospinal Interneurones: mediate connections between different levels (segments) of the cord. These have short axons in the lateral column (connecting distal muscle groups within a limb) and long axons medially (connecting axial motor pools at different spinal levels throughout the cord). Via these connections, weak stretch reflexes at one level of the cord (e.g. legs), may be strengthened by exerting force in the muscles at another level (e.g. arms). This is known as the Jendrassik manoeuvre, and may operate by facilitation of g -motoneurone activity.

Ia Inhibitory Interneurones: provide a route whereby Ia afferent inputs, from muscle spindle receptors, which excite motor neurones of the homonymous muscle (the same muscle from which the afferents came), can inhibit inter-neurones of the antagonist muscles. Descending fibres which excite motor neurones also excite the Ia inhibitory inter-neurones of the antagonist muscles. This illustrates the general principle of reciprocal innervation. Descending inhibition of these inter-neurones can permit co-contraction of antagonists.

Ib Inhibitory Interneurones: are excited by the afferents of Golgi tendon organs and inhibit motor neurones of the homonymous muscle. The strength of this effect can be modulated by inhibitory and excitatory inputs from descending and cutaneous afferent input.

Stimulation of the Ia afferent stretch receptors causes monosynaptic activation of homonymous motor neurones, and, via polysynaptic pathways, facilitation of agonists, and inhibition of antagonists (above). The sensitivity of this response, the "myotatic reflex", depends on the level of activity in the -motor neurones, which is responsible for the tone of the muscle. This reflex serves to stabilise postural muscles, acting as a length servomechanism. Stimulation of the Golgi Tendon Organs serves as a tension feedback control system, and may also help to prevent auto-injury. Together the muscle spindles and Golgi tendon organs regulate muscle stiffness. Reflexes elicited via these deep proprioceptors are termed deep reflexes.

Stimulation of nociceptive endings can produce a clearly protective withdrawal reflex via excitatory inter-neurones, in which flexor motor neurones are excited and extensors inhibited (reciprocal innervation) over several segments. Medial inter-neurones, synapsing on the other side of the cord, elicit contraction of the extensors, and relaxation of the flexors (crossed extension reflex). This may serve primitively to stabilize posture - offering contralateral support as the ipsilateral limb is withdrawn. Stimulation of itch receptors can induce reflex scratching - which goes on longer than the applied stimulus, due to the existence of reverberating circuits. It also is directed to the point of origin of the stimulus (local sign). These two, along with the superficial abdominal reflex (contraction of the abdominal muscles in response to cutaneous scratch in a quadrant of the abdomen); the Babinski reflex (downward curling of the toes in response to scratch across the sole of the foot); and the cremasteric reflex (contraction of the scrotal dartos muscle in response to stroking the inner thigh) are examples of superficial reflexes.

The last three require an intact corticospinal tract for their normal expression. Determining whether changes in reflex responses result from segmental lesions or disruption of descending pathways, and a what level, are major objectives of clinical evaluation of reflex responses.

Not all spinal activity is reflexive. Locomotor activity for example is controlled by locomotory pattern generators in each limb, which once activated can generate the basic "swing/stance" cycle of walking, independently of sensory feedback. The "pattern generators" in each limb are loosely and reciprocally coupled. Their output is modulated by both sensory feedback, and voluntary control however, and can be turned on and off via a mesencephalic locomotory area.

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