14.3 Brainstem Functions The major ascending nd descending pathways project through the brainstem. In addition, the brainstem contains the nuclei of the most cranial nerves and the nuclei of the reticular formation. : The nuclei of cranial nerves III - X and XII are in the brainstem o Only collateral branches of cranial nerve Il project to brainstem o Cranial nerve I and XI do not have projections to or nuclei in the brainstem
Sensory input projecting through the brainstem The brainstem receives sensory input from collateral branches of ascending spinal cord pathways and from the axons of most cranial nerves. Sensory input from Cranial nerve II (vision) V (tactile sensation from the face, nasal cavity, and oral cavity) VII (Taste) VIII (hearing and balance) IX (taste and tactile sensation in the throat) X (taste, tactile sensation in the larynx and visceral sensation in the thorax and abdomen) Among these all but lII have sensory nuclei in the brainstem. Fibers of the spinothalamic tracts passing through the brainstem are joined by fibers of the trigeminothalamic tract. This tract carries tactile sensations such as pain and temperature, two point discrimination, and light tough from the face, nasal cavity and oral cavity (including teeth)
Ras Functions of the brainstem The reticular formation is a group of nuclei scattered throughout the brainstem that is involved in regulating cyclical motor functions. . An important part is the RAS- reticular activating system. o This regulates the sleep-wake cycle O Collateral branches of the trigeminothalamic tract neurons project to the RAS, where they stimulate wakefulness and consciousness. Also projecting to the RAS are collateral branches of cranial nerves Il, V, and VIII Also ascending tactile sensory pathways and descending neurons from the cerebrum. o Visual and acoustic stimuli, as well as mental activities, stimulate the RAS to help maintain alertness and attention. Conversely, removal of auditory, visual, and other stimuli may lead to drowsiness or sleep. Damage to these cells can result in a lack of consciousness or coma.
Motor Output and Reflexes Projecting Through the Brainstem.
Both direct and indirect motor pathways pass through the brainstem. : Corticospinal tracts - direct descending pathways originating in the cerebral cortex and passing directly through the brainstem Corticobulbar tract - (from the cerebral cortex) synapse in cranial nerve motor nuclei to initiate movements within the head, such as eye movements. The indirect pathways synapse with brainstem nuclei, which in turn send descending fibers into the spinal cord. Descending fibers from the reticular formation constitute one of the body's most importrnat motor pathways. o Such as respiratory and cardiac rhythms. Brainstem contains motor nuclei of cranial nerves III- VII, IX, X, XII Motor output projecting through the brainstem is classified into two categories: somatic and parasympathetic.
Somatic motor output and Reflexes Craqnial nerves III, IV, and VI are involved in voluntary and reflex eye movements. These nerves are controlled by the superior colliculus, which are midbrain structures that coordinate eye movements. o Voluntary visual tracking of moving obje