VPB 121: Veterinary Physiology - II (2+1)

MIDBRAIN

• It is a relay centre. Extends from pons to thalamus concerned mainly with auditory and visual relay system.  It has dorsal portion namely the corpora quadrigemina which consists of two pairs of colliculi and ventral portion namely the cerebral peduncle. Roof of the midbrain is known as Tectum which has pair of brain centres vis namely optic lobes in non mammalian species. It has a pair of inferior colliculi.

• Corpora quadrigemina:Represented as paired colliculi, anterior pair concerned with visual relay is known as Superior colliculi and posterior pair concerned with auditory relay is known as Inferior colliculi.
• Superior colliculi: It is similar structure as optic lobes in mammals functions as reflex optical response (orientation towards visual stimuli, focusing etc.) Visual reflex centres houses many nuclei.  Optic tract via lateral geniculate body of thalamus leaves collaterals to the suprachiasmatic nucleus of the hypothalamus, pretectal nucleus and superior colliculus.  The pretectal region immediately anterior to superior colliculus receives inputs from retina, superior colliculus, lateral geniculate body and frontal eye field.  Through these they modify pupillary light reflexes, accommodation, and ocular fixation reflexes.  Through hypothalamic connections they modify the activities of limbic system and visual control.  Through the connections with hypothalamus they also influence light-related endocrine secretions, circadian rhythm, sleeping cycle, and eating and drinking.  Through connections with accessory optic system with flocculonodular lobe and inferior olivary nucleus, eyeball movements in relation to position and movements of head in space is integrated.Superior colliculi do receive collaterals through optic tract which connects visual and non-visual information via circuit.  These neural circuits integrate visual information with non-visual information for coordinating head, eye, and body movements (postural reflexes).
• Inferior colliculi: It is concerned with hearing. It is an important relay station for the integration of ascending and descending auditory signals.  These provide functional link between brainstem and telencephalic organization of auditory information.  It receives the afferents from superior olivary nucleus which forms the major ascending auditory relay from cochlear nuclei.  It involves lateral lemniscus to exhibit reflex vocalization and correlation of information of equilibrium.
• Cerebral peduncle:  consists of tegmentum, substantia nigra, and basis peduncle.
• Tegmentum: It is a posterior part of midbrain that posses fine control of muscles. Act as a relay in integrating functions of sensory and motor functions.  It forms the ascending reticular formation of the pons.  It contains key structure of somatomotor neuron RED NUCLEUS along with the nuclei of trochlear and oculomotor nerves.
• Red nucleus: Located within the rostral portion of the midbrain and extend up to the posterior part of thalamus and hypothalamus.  It is well connected with other structures of CNS via corticorubral (from cortex), pallidorubral (basal ganglia), and cerebellorubral (cerebellum-dendate nucleus) through the respective tracts and through axons to subthalamus.  Motor fibers from red nucleus are projected to spinal cord via rubro-spinal tract, to reticular formation via rubro-thalamic tract, to olivary nucleus through rubro-olivary tract.  They do send efferents via axons to III, IV, and VI cranial nerves.Red nucleus maintains muscle tone and equilibrium.  It act as a centre for all righting reflexes except visual righting reflex.  It integrates various impulses and transmits them to spinal cord to control somatic activity.
• Substantia nigra: Functions as integration centre for skilled muscular activity by receiving sensory information from the body surface, ear, eye, and nose.  It inhibits extra-pyramidal motor movements.

• Basis peduncle: Forms the passage for corticospinal and corticopontile tracts.