Reflection 59: Areas of the Brain

February 4, 2009

(Copyright © 2009)

 

In the movie Donovan’s Brain, poor Donovan is reduced to a brain hooked up to wires in a jar, sharing his thoughts through a loudspeaker in the lab. We often entertain the careless thought that, like Donovan, the brain represents the consciousness of a whole person. But the brain is only part of a body which keeps it active and alive. A brain abstracted from its body is a dead brain. In discussing activities in different parts of the brain, it is essential to remember that no mental activity would be happening apart from the context the body provides. The brain is not the motor under the hood (“bonnet” in England) that makes us work. Heart, liver, kidneys, fingers, toes, and all the other parts of a functioning organism are implicit in pictures of the brain by itself.

 

Having said that, right now I want to consider the human brain as a complex organ which itself is composed of parts. My focus is on parts of the brain which may contribute to consciousness in particular. If you read books or articles about the brain, you very quickly encounter terms used by neuroscientists, terms which have limited currency outside the lab. Since I myself have uses such terms in earlier posts (amygdala, hippocampus, cerebral cortex, cerebellum), I want to lay the groundwork for future posts so that such terms will be helpful in developing further insights into consciousness. To do that, I will refer to two illustrations. 

 

 

illustration-1-72

Illustration 1. Here, the left side of the cerebral hemisphere is divided into four lobes, each represented by a different color. The frontal lobe is shown in blue, parietal lobe in yellow, temporal lobe in green, and occipital lobe in pink. The cerebellum (not part of the cerebral cortex) is also shown, looking like a ball of yarn in black in white tucked under the temporal lobe.

 

cut-away-view-72

Illustration 2. Here outer layers of the left cerebral hemisphere are cut away to suggest the locations of several parts which lie deeper in the brain near the midline between the two hemispheres. (Clockwise from the top) The corpus callosum consists of nerve fibers connecting the two hemispheres. The cerebral cortex is made up of six layers of nerve cells and the fibers connecting them one to another. The thalamus is a complex relay station between the cerebral cortex and other parts of the central nervous system. The cerebellum contains over half the cells in the brain, and is largely devoted to eye and muscle coordination. The brain stem is the seat of cognitive and emotional arousal systems. The hippocampus facilitates long-term memory storage and retrieval. Not shown, but near the hippocampus is the amygdala which plays an essential role in emotional responses including fear and anxiety. The hypothalamus regulates the autonomic nervous system which maintains life-sustaining processes at appropriate levels; it is a key link between the body’s nervous and hormonal systems.

 

The human brain contains an estimated 100 billion nerve cells, each of which may form 1-10 thousand connections with other cells, producing well over 100 trillion opportunities for activation and feedback. When a given neuron (brain cell) fires, it receives feedback from other cells as a kind of consensus concerning whether it should continue to fire or not. Feedback shapes every activity in the brain and, ultimately, the conscious and unconscious processes governing bodily functions and behavior.

 

 

BBC Brain Map

 

For this post I will give a brief run-through of the functions of the various areas of the brain I have listed above, more or less according to the order in which they have been introduced.

 

Cerebellum. A fixture of the vertebrate brain, the cerebellum is particularly developed in primates and humans. It controls balance, spatial orientation, eye movements, muscle movements (as in reaching and locomotion), and planning of such movements. When there is a discrepancy between movements as planned and executed, the cerebellum facilitates correction.

 

Cerebral Cortex. Cell bodies in the cerebral cortex are arranged in thin layers near the outer surface of the cerebral hemispheres. This arrangement facilitates orderly routing of inputs and outputs between related areas of the brain. The surface of the cortex is larger than that of the inside of the skull because of its folding into valleys (sulci) and hills (gyri), which greatly expands the number of neurons that can fit into a small space. The bulk of the cortex consists of interconnecting fibers. Among vertebrates, humans have the most finely elaborated cortex, allowing detailed planning and execution of a wide diversity of behaviors. The cortex is divided into lobes (named after neighboring bones of the skull) based on prominent anatomical features. These designations are somewhat arbitrary, but the cellular architecture exhibited in different areas lends support to the integrity of different functions within separate lobes (see separate listings below).

 

Corpus Callosum. Nerve fibers connecting corresponding areas of the left and right cerebral hemispheres cross the midline of the brain in a large bundle known as the corpus callosum. These fibers appear white because of the myelin sheaths wrapping individual fibers, greatly increasing the speed of transmission. Each fiber is the axon connecting the body of a particular cell on one side of the cortex to related cells on the opposite side. In general, nerve cells (neurons) on one side of the brain control functions performed on the other.

 

Frontal Lobe. The frontal lobe of cerebral cortex extends forward of the central sulcus at the top of the brain which separates it from the parietal lobe to the rear. Cells in the primary motor area are arranged along the central sulcus as a topographical map of the muscles in areas of the body which they control, extending from feet represented in the fissure between hemispheres, through legs, trunk, arms, hands, fingers, head, face, and mouth arrayed across the surface of the primary motor area of the cortex. Forward of that is the premotor area where planning of motor behaviors takes place. Speech muscles in tongue and lips are controlled in Wernicke’s area within the frontal lobe. Working memory (or focused attention) integrate three different areas—lateral, orbital, and cingulate—within prefrontal cortex to provide what some researchers believe to be the neurological basis of consciousness.

 

Parietal Lobe. Similar to the motor map in the frontal lobe, a sensory map of the body spreads across neurons at the leading edge of the parietal lobe. Touch, pain, and temperature signals are processed in the parietal lobe, which also processes the interplay between visual and bodily sensations.

 

Occipital Lobe. At the back of the brain, the occipital lobe is devoted entirely to primary and subsequent visual processing. There are some 40 visual processing areas in the brain, but they all depend on signals received from the occipital lobe.

 

Temporal Lobe. Primary auditory processing takes place in the temporal lobe, as well as later stages of visual processing, including image recognition. The amygdala and hippocampus exist in pairs (one on each side of the brain) and are located deep within the temporal lobes near the midline between the two hemispheres.

 

Thalamus. As a relay station, the thalamus receives signals from sensory receptors and sends output to cortical sensory processing areas. Thalamic activity coordinates electrical activity in cortical neurons, giving rise to synchronized waves (basis of the electroencephalogram—the EEG). Consciousness, it is thought, represents synchronized activity throughout the cerebral cortex. Ascending pathways from the brain stem and hypothalamus pass through the thalamus en route to the cerebral cortices, conveying vital signals that maintain arousal, vigilance, and responsiveness to sensory stimuli. Injury to the thalamus can bring about loss of consciousness.

 

Brain Stem. The difference between sleep and wakefulness is largely told by neurons in the brain stem where both autonomic and emotional stimuli come into play with profound implications for arousal, attention, and consciousness. Though this region of the brain is primitive in some senses, its involvement in such basic processes suggests that consciousness is not necessarily a latecomer on the evolutionary totem pole.

 

Hippocampus. A specialized region of cortical cells near the midline of the brain, the hippocampus is essential to laying down and retrieving long-term memories. It is involved in learning and establishing spatial relations. Alzheimer’s disease involves failure of hippocampal function.

 

Amygdala. The amygdala comes into play in threatening situations. Sensory input reaches the amygdala from the thalamus by two different routes, one slow but made clear by the cerebral cortex, the other fast but relatively unprocessed and therefore crude. In emergencies, the fast route may spur immediate, life-saving action. The amygdala generates outputs affecting blood pressure, stress hormones, and both startle and immobilizing reflexes. It also kicks into the hippocampus, enabling long-term memory of emotional situations. Feelings of fear, anxiety, anger, and loathing are conscious signs of activity in the amygdala, location of a sophisticated early warning network.

 

Hypothalamus. The hypothalamus maintains homeostasis across a wide range of circumstances by providing inputs to the autonomic nervous system (which silently regulates bodily functions, including sexual arousal), the hormonal system, and the motivational system. It maintains a feedback loop by which neuronal activity can stimulate hormone production, and hormones in turn can affect brain activation. By such means, the hypothalamus integrates autonomic and hormonal functions with behavior.

 

Anterior Cingulate Cortex. On the medial surface of the cerebral hemispheres, the cingulate cortex arcs above the fibers of the corpus callosum crossing beneath it. Much like the brain stem, anterior cingulate cortex participates in homeostasis, emotions, attention, sleep and wakefulness, learning, and consciousness itself. Cingulate cortex also receives musculoskeletal inputs, and sends a large variety of output signals to motor areas related to speech, movement, and other bodily responses. Lesion studies suggest that patients with damage to this area are deprived of an active inner (conscious) life.

 

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2 Responses to “Reflection 59: Areas of the Brain”

  1. richard said

    “A brain abstracted from its body is a dead brain.”

    While from your perspective this might be true but what about from the brains perspective…the isolated brain could have phantom activity and perceptions much like an amputee still feels the limb.
    Just a random thought on a cold evening in Maine.

  2. Oliver said

    Great concise overview. Thanks for posting!

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