The spinal cord

The spinal cord is a glistening while tube running from the medulla oblongata of the brain to the level of the sixth or seventh lumbar vertebra, where it breaks up into several terminal spinal nerves forming a structure known as the Ctiiulti rquiιιbgcolor=white>Motor Supplies the extrinsic muscles of the eye, enabling the eye to make minute, delicate movements; accurate movement of the eye is essential for survival IVTrochIear Motor Supplies the extrinsic muscles of the eye V Trigeminal Mixed Sensory fibres are carried from the skin around the eye and the face; motor fibres supplied to the muscles of mastication - mainly the temporal and masseter Vl Abducens Motor Supplies the extrinsic muscles of the eye Vll Facial Motor Supplies the muscles of facial expression including those associated with the movement of the lips, ears and skin around the eyes Vlll Vestibulocochlear (auditory) Sensory Vestibular branch carries the sensation of balance from the semicircular canals of the inner ear; cochlear branch carries the sensation of hearing from the cochlea of the inner ear l? Glossopharyngeal Mixed Carries the sensation of taste from the taste buds on the tongue and pharynx; supplies motor fibres to the muscles of the pharynx X Vagus Mixed Carries sensory fibres from the pharynx and larynx; supplies motor fibres to the muscles of the larynx; parasympathetic visceral motor fibres to the heart and various thoracic and abdominal organs including the gastrointestinal tract down to the descending colon Xl Accessory (spinal accessory) Motor Supplies the muscles of the neck and shoulder Xll Hypoglossal Motor Supplies the muscles of the tongue

Forehmb - supplied by nerves from Ce6 to T2: the brachial plexus

Hindlimb - supplied by nerves from L4 to S2: the sacral plexus

Each spinal nerve consists of two components (Fig.

1.

2. Λ ventral root carries motor fibres away from the spinal cord. There are no ganglia outside the cord - the ganglia of the ventral root are in the grey matter of the cord.

In some nerve pathways, there may be one or more intenalated neurons lying in the grey matter between the sensory and motor neurons. The sensory and motor nerve fibres of each spinal nerve come together in the same myelin sheath and leave the vertebral col- umn via the relevant intervertebral foramen as a mixfdnerve (Fig. 5.9).

Reflex arcs

A rellex arc is a fixed involuntary response to certain stimuli. The response is always the same. It is rapid and automatic and involves only nerve pathways in the spinal cord, using the appropriate spinal nerve. Reflex arcs are a means of protection and produce a prompt response to potent tally damaging phenomena such as heat or sharp objects.

Common reflexes observed in the dog and cat are:

■ Withdrawal or pedal reflex

■ Λnal reflex

Patella reflex

Panniculus rellex

Palpebral reflex.

Using the patella rellex as an example, we can follow the pathway taken by the nerve impulse. The tendon of the quadriceps Ieinoris muscle passes over the patella as the straight patella ligament and inserts on the tibial tuberosity of the proximal tibia. Within the muscle Iibres are stretch receptors known as muscle spindles, whose function is to monitor muscle lone. The nerve pathway is as follows:

1. When the tendon is lightly tapped, the muscle fibres stretch

2. This initiates a response in the muscle spindles and a nerve impulse passes along the sensory nerve to the spinal cord

T Within the grey matter of the spinal cord, the sensory nerve synapses with the motor nerve and an impulse passes along it to the muscle Iibres of the quadriceps femoris, causing contraction

4.

This is an example of a monosynaptic reflex as there is only one set of synapses. A polysynaptic reflex. e.g. the withdrawal rellex. involves one or more intercalated neurons and several synapses within the grey matter.

A rellex is unconscious and will still occur even if the spinal cord has been severed. However, if the web of the dog’s foot is pinched and it withdraws its foot and then yelps or bites you. this indicates that it has felt pain. There is conscious perception. involving the transmission of impulses up the tracts of while matter of the spinal cord to the brain and back to the muscles of the jaw. as well as along the pathway of the rellex arc. A conscious response indicates that the spinal cord is intact.

Rctlcxes may be described as being:

1. Unconditional - these are unconscious and automatic (the type already described). They cannot be overcome

2. Conditional - these can be overcome by conscious thought, e.g. keeping your hand on a hot iron requires a determined effort to prevent your muscles contracting and pulling the hand away.