Physiology of spinal cord

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Passive stretch of the muscle activates the spindle stretch receptors and causes an increased rate of action potentials in the afferent nerve. Contraction of the extrafusal fibers removes tension on the stretch receptors and lowers the rate of action potential firing.

As the ends of the intrafusal fibers contract in response to gamma motor neuron activation, they pull on the center of the fiber and stretch the receptor. The black arrows indicate direction of action-potential propagation.

The Brain Stem The Superior Colliculi receive impulses from the Occipital (Visual) Cortex of the Cerebrum for reflex movements of the Eyes, such as when following objects that are moving across the visual field. The Inferior Colliculi are part of the Auditory Pathway to the Cerebrum. Some fibers pass to the Superior Colliculus, producing Eye movements in response to sound, such as when turning the eyes toward the source of a sound. The Cell Bodies giving rise to the Third (Oculomotor) & Fourth (Trochlear) Cranial nerves lie in the MidBrain. Lesions in the MidBrain typically affect Auditory and Visual reflexes and, if the Peduncles are involved, produce deficits in Voluntary Movement.

The Midbrain (Mesencephalon) This is the most superior part of the brain stem. The corpora quadrigemina, the red nucleus, the substantia nigra, the cerebral peduncles, and the cell bodies of two cranial nerves are located in the midbrain. The corpora quadrigemina consists of the tectum which is the roof of the brain stem, and of four protrusions located on the tectum which are called colliculi. The two superior colliculi are involved in vision. They relay information to the lateral geniculate bodies of the thalamus.

The Midbrain (Mesencephalon) The two inferior colliculi are involved in hearing. They relay information to the medial geniculate bodies of the thalamus. The red nucleus is part of the extrapyramidal tract and connects the cerebellum to the thalamus and spinal cord. The substantia nigra is a group of dark colored cell bodies which produce dopamine. It is also part of the extrapyramidal tract. The cerebral peduncles connect the pons to the cerebrum. The nuclei of cranial nerve III, the oculomotor cranial nerve, and of cranial nerve IV, the trochlear cranial nerve which both provide innervation for eye movement are also located in the midbrain.

Postural changes with stepping. (Left) Normal standing posture Postural changes with stepping. (Left) Normal standing posture. The line of the center of gravity falls directly between the two feet. (Right) As the left foot is raised, the whole body leans to the right so that the center of gravity shifts and is over the right foot.

Diencephalon The diencephalon is the part of the forebrain that contains such important structures as the thalamus, hypothalamus, posterior portion of the pituitary gland, and pineal gland. The hypothalamus performs numerous vital functions, most of which relate directly or indirectly to the regulation of visceral activities by way of other brain regions and the autonomic nervous system.

The medulla, the most caudal segment of the brain stem, appears as a conical expansion of the spinal cord. Both the pons and the medulla are separated from the overlying cerebellum by the fourth ventricle, and cerebrospinal fluid entering the fourth ventricle from the cerebral aqueduct passes into the cisterna magna, a subarachnoid space surrounding the medulla and the cerebellum, via foramina in the lateral recesses and in the midline of the ventricle