The cerebral cortex, a thin layer of gray matter comprising the outer portion of the cerebrum, is the center of the conscious mind. The adult human brain contains almost 98% of all the neuron cell bodies of the nervous system. The cerebral cortex is involved with awareness, communication, sensation, memory, understanding, and the initiation of voluntary movements. Its gray matter contains dendrites, neuron cell bodies, glia, and blood vessels. It lacks fiber tracts but contains six layers in which there are billions of neurons. The cerebral cortex is approximately 2–4 mm thick, yet it makes up approximately 40% of the overall brain mass. Its surface area is nearly tripled by its many convolutions.
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| The lobes of the cerebrum. |
Beneath the cerebral cortex is white matter, comprising most of the cerebrum. It contains myelinated axon bundles, some of which pass from one cerebral hemisphere to the other. Others carry impulses from the cortex to nerve centers of the brain and spinal cord.
The lobes of the cerebral cortex are:
■■ Frontal lobe: Forms the anterior portion of each cerebral hemisphere .
■■ Parietal lobe: Lies posteriorly to the frontal lobe.
■■ Temporal lobe: Lies below the frontal and parietal lobes.
■■ Occipital lobe: Forms the posterior part of each cerebral hemisphere.
■■ Insula: Lies under the frontal, parietal, and temporal lobes.
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| A coronal section through the cerebrum. The area of gray matter is greatly increased by the folding of the surface into gyri, sulci, fissures, and the insula. |
In most people, one side of their cerebrum acts as the dominant hemisphere, controlling the use and understanding of language. The left side of the cerebrum is usually responsible for activities such as speech, writing, reading, and complex intellectual functions. The nondominant hemisphere controls nonverbal functions and intuitive and emotional thoughts. The dominant hemisphere controls the motor cortex of the nondominant hemisphere.
📖 Imaging Anatomy Brain and Spine
Function
Aside from sensory and motor control, memory, and reasoning, the cerebrum also coordinates intelligence and personality. It is the “executive suite” of the body. Functions overlap between regions of the cerebral cortex. The three functional areas of the cerebral cortex are the motor, sensory, and association areas.
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| Functional regions of the cortex. |
All neurons in the cerebral cortex are interneurons. Each cerebral hemisphere controls the motor and sensory functions of the contralateral (opposite) side of the body. The hemispheres are not exactly equal in function, even though their structure is closely matched. Cortical functions are specialized, which exhibits a phenomenon known as lateralization. No functional area acts individually and conscious actions use the entire cortex in varying ways.
Motor Areas
Most of the cerebral cortex motor areas are located in the frontal lobes and are further defined as the primary motor cortex, premotor cortex, Broca’s area, and frontal eye field. Impulses from large pyramidal cells in the motor areas travel through the brain stem into the spinal cord via the corticospinal tracts that form synapses with lower motor neurons. Their axons leave the spinal cord, reaching the skeletal muscle fibers. The primary motor cortex is also known as the somatic motor cortex. It is located in the precentral gyrus of the frontal lobe of both hemispheres. The mapping of the CNS structures of the body is referred to as somatotopy. The premotor cortex lies just anterior to the precentral gyrus in the frontal lobe and helps to plan movements. Broca’s area is found anterior to the inferior region of the premotor area and is more prevalent in the left hemisphere. It has a motor speech area, and also becomes active just before speaking or when planning other voluntary motor activities. The frontal eye field is superior to Broca’s area, located partly in and anterior to the premotor cortex. It controls voluntary eye movements. The central sulcus separates the primary motor areas from the somatosensory areas.
Sensory Areas
Sensory areas of the cerebrum interpret impulses such as skin sensations, which are picked up in the anterior portions of the parietal lobes. The posterior occipital lobes affect vision, whereas the temporal lobes affect hearing. Taste and smell receptors are located deeper within the cerebrum. Sensory fibers also cross similarly to motor fibers. Additional sensory areas include the insular and occipital lobes. The primary somatosensory cortex lies in the postcentral gyrus of the parietal lobe. It is just posterior to the primary motor cortex and its neurons receive input from the somatic sensory receptors of the skin. It also receives input from position sense receptors in the joints, skeletal muscles, and tendons. The somatosensory association cortex is found just posterior to the primary somatosensory cortex, is interconnected, and functions primarily to integrate temperature, pressure, and related information. The primary visual cortex, also called the striate cortex, is not only mostly buried in the calcarine sulcus of the occipital lobe, but also extends to the extreme posterior occipital tip. It is the largest cortical sensory area, receiving visual information from the retinas of the eyes. The visual association area uses visual experiences from the past to interpret color, form, movement, and other visual stimuli. Each primary auditory cortex lies in the superior margin of the temporal lobe and receives impulses from the inner ear, interpreting location, loudness, and pitch. Posteriorly, the auditory association area perceives sound stimuli such as speech, music, and environmental noises. The vestibular (equilibrium) cortex controls balance and is located in the posterior insula and the nearby parietal cortex. The primary (olfactory) smell cortex is present on the medial temporal lobe in the piriform lobe area, which is signified by its uncus, a hook-like structure. The olfactory cortex is part of the rhinencephalon, a primitive structure that includes the orbitofrontal cortex, uncus, and related regions on or inside the medial temporal lobe as well as the olfactory tracts and bulbs extending to the nose. The gustatory (taste) cortex is located in the insula, deep in the temporal lobe. The visceral sensory area controls visceral sensations and lies in the cortex of the insula, just posterior to the gustatory cortex. Its sensations include bladder fullness, stomach upset, and tightness in the lungs (such as from holding your breath).
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