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Spinal cord is situated in the vertebral canal. It extends from the first cervical vertebra to the upper border of second lumbar vertebra.

Rostrally, it is continuous with the medulla oblon-gata and caudally, it ends in filum terminale, which is attached to the coccyx. There are 31 segments in the spinal cord, which are distributed as, cervical 8, thoracic 12, lumbar 5, sacral 5 and coccyx 1. Each segment gives off a pair of spinal nerve (Fig. 4.38).

Each segment shows certain common featu-res, which can be observed in the cross section of the spinal cord. The cross section of the spinal cord shows an outer white matter and an inner grey matter. The white matter contains nerve tracts, in which the myelinated fibers are predominant.

The central grey matter contains nerve cells and glia. The center of the grey matter shows a canal, which is lined by ependymal cells and contains cerebrospinal fluid. The central canal

Fig. 4.38: Formation of spinal nerve

divides the transverse commissure into anterior and posterior grey commissures. The lateral aspect of the grey matter extends into dorsal horn and ventral horn. In the thoracolumbar segments, there is also presence of intermedio lateral cell column which gives rise to preganglionic sympathetic fibers.

Ventral horn of spinal cord

Ventral horn contains motor neurons (Fig. 4.39).

These nerve cells give rise to ventral nerve, which forms the final common pathway.

There are two types of motor neurons in the ventral horn. They are ααααα and γγγγγ motor neurons.

The alpha motor neurons are larger in size, with the diameter showing up to 12-20 μm. The corticospinal tracts end on the alpha motor neurons, either directly or through the inter-neurons. The axons of the alpha motor neurons supply the skeletal muscle and produce volun-tary movement. The gamma motor neurons are smaller in size and give axons to the muscle spindles. This innervation is useful in main-taining the muscle tone. The motor neurons in the ventral horn which executes the motor command of the descending motor tract forms the final common pathway. The ventral horn of the spinal cord shows recruitment of motor units depending on the motor tasks to be performed.

This aspect of deciding which motor units to be activated resides in the spinal cord itself, so that

the motor cortex can be left free for motor planning and execution.

Situated medially in the ventral horn, is another motor neuron which is actually an interneuron, called Renshaw cell. This is an inhibitory interneuron, which receives collateral from the ventral nerve. The stimulation of Renshaw cell inhibits the alpha motor neuron, forming the feedback control. On the basal part of ventral horn is the Clarke’s column which is also a group of nerve cells, sending projections to the cerebellum. The intermediolateral cell column in the thoracolumbar segments, gives rise to preganglionic sympathetic fibers.

The dorsal horn of the grey matter has seven laminae. The laminae II and III form the sub-stantia gelatinosa of Rolando. The nociceptive afferents coming from the periphery synapse here. The dorsal horn acts as a gate for the sensory afferents in regulating the sensory input reaching the nervous system. The descending fibers which inhibit pain afferents, end in the dorsal horn through an interneuron. In the dorsal horn, the larger size afferent fibers carrying touch and pressure inhibit smaller fibers which carry pain.

The white matter of the spinal cord invests the grey matter and contains dorsal, lateral and anterior (ventral) funiculi. The presence of more number of myelinated fibers gives white appearance. These tracts include both ascending and descending fibers. The ascending fibers, not only connect the different segments of the spinal cord, but also the various regions of the brain.

The major ascending tracts of spinal cord are (Fig. 4.40).

Fig. 4.39: Motor neurons of spinal cord

The larger sized ones are α motor neurons and the smaller ones are γ motor neurons. Alpha motor neurons that are situated medially supply proximal musculature, while the laterally situated neurons supply distal musculature

Fig. 4.40: Transverse section of spinal cord showing ascending tracts

Fig. 4.43: Lateral descending pathways in the spinal cord

Dorsal column (fasciculus gracilis and fasciculus cuneatus). These uncrossed fibers carry touch, two point discrimination, vibration, kinesthetic, pressure and stereognosis.

Anterolateral system comprises lateral spino-thalamic and ventral spinospino-thalamic tracts. These crossed fibers carry pain, temperature and crude touch.

Spinocerebellar afferents consist of dorsal and ventral spino cerebellar tracts. The dorsal tract goes ipsilaterally to the cerebellum, while the ventral tract goes to the ipsilateral and contralateral cerebellum. They carry proprio-ceptive impulses (unconscious) and help to coordinate voluntary movements.

Spinotectal tract carries fibers to superior colliculus and help to coordinate visuospinal reflexes.

The principal descending tracts in the spinal cord belong to two divisions, consisting of tracts descending in the lateral and ventral funiculi (Fig. 4.41).

Tracts in the lateral funiculus

Lateral corticospinal tract rubrospinal tract Lateral corticospinal tract is recent in the evolutionary development and is concerned with the execution of fine, skilled voluntary movements from the distal musculature, espe-cially with the digits.

The rubrospinal tract, although occupies the lateral funiculus, is concerned with the voluntary movements of proximal and distal muscles.

The ventral funiculus carries the following tracts:

• Ventral corticospinal

• Tectospinal

• Vestibulospinal

• Reticulospinal

• Olivospinal.

The ventral corticospinal tract is part of the pyramidal tract. It causes contraction of proximal group of muscles. The remaining tracts in the ventral funiculus are all extrapyramidal fibers, concerned with the regulation of muscle tone, posture, and equilibrium and control the voluntary movements.

Interneurons of spinal cord

There are two groups of interneurons present namely, lateral and medial (Fig. 4.42). The descending motor tracts, usually end on the α and γ motor neurons, through these interneurons. The lateral group receives fibers from lateral corticospinal tract (Fig. 4.43), while

Fig. 4.41: Descending pathways in the spinal cord

Fig. 4.42: Interneurons in the white matter of spinal cord

the medial group interneurons receive the descending extrapyramidal fibers and ventral corticospinal tract. The interneurons projecting the lateral and ventral corticospinal tracts end on the α motor neurons, while the extrapyramidal tracts end on the γ motor neurons through the medial group interneurons (Fig. 4.44).

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