Physiology of spinal cord and brain stem Physiology of spinal cord and brain stem. Physiology of the cerebral cortex and cerebellum

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.

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.

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

Cerebellum Connected to brainstem by cerebellar peduncles White matter (arbor vitae) visible in sagittal section Sits atop the 4th ventricle

FUNCTION OF CEREBELLUM 1. Regulation of posture and equilibrium, and muscle tone 2. Coordination of posture and slow determined movements 3. Coordination of fast determined movements

Cerebellar Inputs and Outputs The 3 deep nuclei are: (1) fastigial - concerned with balance; sends information mainly to the vestibular and reticular nuclei (2) dentate and (3) interposed - both concerned with voluntary movement; send axons mainly to the thalamus and red nucleus All 3 receive inputs from sensory afferent tracts and from the cerebellar cortex.

FUNCTIONAL AREAS OF CORTEX SENSORY AREAS OF THE CORTEX The somatic senses include sensations of touch, pressure, temperature, body position, and similar perceptions that do not require complex sensory organs. The special senses include vision, hearing, and other types of perception that require complex sensory organs. The postcentral gyrus serves as a primary area for the general somatic senses. Under central gyrus presents secondary somatic sensory area. It also has topic structure.

Functional Regions of the Lateral Side of the Left Cerebral Cortex

Demonstration of Cortical Activities During Speech The figures show the pathway for reading and naming something that is seen, such as reading aloud. PET scans show the areas of the brain that are most active during various phases of speech. Red indicates the most active areas; blue indicates the least active areas.

Topography of the Somatic Sensory Cortex Cerebral cortex seen in coronal section on the left side of the brain. The figure of the body (homunculus) depicts the nerve distributions; the size of each body region shown indicates relative innervation. The cortex occurs on both sides of the brain but appears on only one side in this illustration. The inset shows the somatic sensory region of the left hemisphere (green).

Topography of the Primary Motor Cortex Cerebral cortex seen in coronal section on the left side of the brain. The figure of the body (homunculus) depicts the nerve distributions; the size of each body region shown indicates relative innervation.

Cerebral Medullary Tracts

The (a) alpha and (b) beta rhythms of the EEG.

Effects of Aging on the Nervous System As a person ages, there’s a gradual decline in sensory function because the number of sensory neurons declines, the function of remaining neurons decreases, and CNS processing decreases. In the skin, free nerve endings and hair follicle receptors remain largely unchanged with age. Meissner’s corpuscles and pacinian corpuscles, however, decrease in number. The capsules of those that remain become thicker and structurally distorted and, therefore, exhibit reduced function. As a result of these changes in Meissner’s corpuscles and pacinian corpuscles, elderly people are less conscious of something touching or pressing on the skin, have a decreased sense of two-point discrimination, and have a more difficult time identifying objects by touch. These functional changes leave elderly people more prone to skin injuries and with a greater sense of isolation.