Biomedical Engineering Reference
In-Depth Information
axons) that is 2-4 mm thick and contains over 50 billion neurons and 250 billion glial cells
called neuroglia. The thicker inner layer is the white matter that consists of interconnecting
groups of myelinated axons that project from the cortex to other cortical areas or from the
thalamus (part of the diencephalon) to the cortex. The connection between the two cerebral
hemispheres takes place via the corpus callosum (Figure 3.30b). The left side of the cortex
controls motor and sensory functions from the right side of the body, whereas the right side
controls the left side of the body. Association areas that interpret incoming data or coordi-
nate a motor response are connected to the sensory and motor regions of the cortex.
Fissures divide each cerebral hemisphere into a series of lobes that include the frontal lobe,
the parietal lobe, the temporal lobe, and the occipital lobe. Each of these lobes has different
functions. The functions of the frontal lobe include initiating voluntary movement of the skel-
etal muscles, analyzing sensory experiences, providing responses relating to personality, and
mediating responses related to memory, emotions, reasoning, judgment, planning, and
speaking. The parietal lobe responds to stimuli from cutaneous (skin) and muscle receptors
throughout the body. The temporal lobes interpret some sensory experiences, store mem-
ories of auditory and visual experiences, and contain auditory centers that receive sensory
neurons from the cochlea of the ear. The occipital lobes integrate eye movements by directing
and focusing the eye and are responsible for correlating visual images with previous visual
experiences and other sensory stimuli. The insula is a deep portion of the cerebrum that lies
under the parietal, frontal, and temporal lobes. Little is known about its function, but it seems
to be associated with gastrointestinal and other visceral activities.
The diencephalon is the deep part of the brain that connects the midbrain of the brain
stem with the cerebral hemispheres. Its main parts include the thalamus, hypothalamus,
and epithalamus (Figure 3.30b). The thalamus, the major switchboard of the brain, is
involved with sensory and motor systems, general neural background activity, and the
expression of emotion and uniquely human behaviors. Due to its two-way communication
with areas of the cortex, it is linked with thought, creativity, interpretation and understand-
ing of spoken and written words, and identification of objects sensed by touch. The hypo-
thalamus is involved with integration within the autonomic nervous system, temperature
regulation, water and electrolyte balance, sleep-wake patterns, food intake, behavioral
responses associated with emotion, endocrine control, and sexual responses. The epithala-
mus contains the pineal body that is thought to have a neuroendocrine function.
The brain stem connects the brain with the spinal cord and automatically controls vital
functions such as breathing. Its principal regions include the midbrain, pons, and medulla
oblongota (Figure 3.30b). The midbrain connects the pons and cerebellum with the cerebrum
and is located at the upper end of the brain stem. It is involved with visual reflexes, the move-
ment of eyes, focusing of the lenses, and the dilation of the pupils. The pons is a rounded
bulge between the midbrain and medulla oblongata that functions with the medulla oblon-
gata to control respiratory functions, acts as a relay station from the medulla oblongata to
higher structures in the brain, and is the site of emergence of cranial nerve V. The medulla
oblongata is the lowermost portion of the brain stem and connects the pons to the spinal cord.
It contains vital centers that regulate heart rate, respiratory rate, constriction and dilation of
blood vessels, blood pressure, swallowing, vomiting, sneezing, and coughing.
The cerebellum is located behind the pons and is the second largest part of the brain.
It processes sensory information that is used by the motor systems and is involved with
