Brainstem

• Cerebrum and cerebral cortex

Table 6.1 Major Levels of CNS Function

Spinal cord

Processes reflexes

Transmits nerve impulses to and from brain

Receives sensory input and initiates motor output

Controls life-sustaining processes (e.g., respiration, circulation, digestion)

Processes, integrates, and analyzes information

Involved with highest levels of cognition, voluntary initiation of movement, sensory perception, and language

Brainstem

Cerebrum and cerebral cortex

The spinal cord is the most anatomically inferior portion of the CNS and its functions are at the lowest level of sophistication (see Table 6.1). As mentioned earlier, the spinal cord receives sensory input from the periphery of the body and contains the cell bodies of motor neurons responsible for voluntary and involuntary movements. Once again, the involuntary and neurologically simple reflexes are processed entirely at the level of the spinal cord. Voluntary, deliberate movements are initiated and controlled by thought processes in the cerebrum. The second important function of the spinal cord is to transmit nerve impulses to and from the brain. Ascending pathways carry sensory input to higher levels of the CNS and descending pathways carry impulses from the brain to motor neurons in the spinal cord.

The brainstem, which consists of the medulla, pons, and midbrain, in evolutionary terms is the oldest and smallest region of the brain. Continuous with the spinal cord, the brainstem receives sensory input and initiates motor output by way of cranial nerves III through XII, which are functionally analogous to the 31 pairs of spinal nerves. Whereas the spinal cord processes sensory and motor activities in the trunk of the body and the limbs, the brainstem processes these activities primarily in the head, neck, and face. The brainstem also controls many basic life-sustaining processes, including respiration, circulation, and digestion. Even with loss of higher cognitive function, this lower level of the brain can sustain these bodily functions essential for survival.

The cerebrum and cerebral cortex, which account for 80% of the total brain weight in humans, constitute the highest functional level of the CNS. The more cognitively sophisticated the specie is, the larger and more highly folded the cerebral cortex is. These convolutions or folds serve to increase the surface area of the cerebral cortex, thus allowing for a greater number of neurons. Therefore, it is not unexpected that the cerebrum is most highly developed in the human. Responsible for the highest levels of processing, integration, and analysis of information, the cerebral cortex plays an important role in the most elaborate neurological functions including intellect; thought; personality; voluntary initiation of movement; final sensory perception; and language.

Essentials of Human Physiology

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