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

THALAMUS

• It is a sensory relay station and is important in motor control and play a major role in control of cerebral excitability.
• Located deep within the brain near the basal nuclei forms the part of diencephalon.
• Represented by large grey mass related medially to the third ventricle and at the base of lateral ventricle.
• It mediates connections between cerebral cortex, basal ganglia, hypothalamus, cerebellum, brainstem, reticular formation, and spinal cord. Physiologically thalamus can be considered to have three systems. 
• Specific projection systems-transmits information to and from cortex.
• Nonspecific or diffused projection system-has widespread action on cortex.
• Acts as pacemaker that regulates the cortical activity.
• Nuclei with subcortical connections.

• Specific projection system:  Receives specific afferents from the periphery, which are processed in the thalamus and relayed to specific areas of the cortex for conscious perception.
• Lateral geniculate body:  Receives the visual signals through optic tract and projects the outputs to the visual or the striated area of the cortex.
• Medial geniculate body:  Receives the auditory signals through the cochlear nerve and projects the efferents to the temporal lobe of the cortex.
• Ventrobasal complex:  Receives sensory impulses through medial lemniscus, spinothalamic and trigemina0l nerve and projects its output signals to the somesthetic and gustatory areas of the cortex.
• Posterior nuclear group:  Receives the pain sensations through medial lemniscus and spinothalamic tract and projects them to the cortex.
• Ventrolateral nuclei:  Acts as a motor relay station that receives the input signals from the basal ganglia and cerebellum and projects the efferents to the motor cortex.
• Anterior nuclei:  Receives input signals from the mammillary body of the hypothalamus and projects the efferents to limbic cortex and cingulate gyrus of the cortex.
• Non - specific projection system:  It forms a widespread action on the cortex and thereby regulates cortical activity and consciousness.

Functions

• It acts as a synaptic integrating centre for preprocessing of all sensory information to cortex except olfaction.
• It screens the sensory impulses to appropriate areas of the somatosensory cortex.
• It is capable of recognizing crude awareness of various sensations, but unable to identify and distinguish strength, location, and intensity.
• Somesthetic sensory impulses reach the thalamus through medial, spinal lemniscus to posteroventral nucleus of the thalamus.
• Taste fibers are relayed in arcuate nucleus.
• Arousal, alert, or wakefulness is maintained by part of the thalamus due to the fibers from forebrain and midbrain reticular formation and hypothalamus.
• Act as higher centre for crude sensation and of pain and temperature.