MCB 32 Introductory Human Physiology



MCB 32

Notes for Thursday, September 14

Chapter 4, pps.75-76 and pps.78-84

OUTLINE FOR LECTURE

l. Human Brain Anatomy (reorganized the last 2 1/2 pages of lecture 5

here)

ll. Organization of the Brain CNS

lll. Organization of Brain PNS

lV. Cortical Function

l. Human Brain Anatomy

cerebral hemispheres

corpus callosum - large tract of nerve fibers connecting left and right hemispheres

cerbral cortex or gray matter - outer portion of brain

gyri - elevated portions of brain

sulci - depressions between the gyri

longitudinal fissure - runs down middle of brain separating the two hemispheres

central sulcus - separates the frontal and parietal lobes of the brain

lateral sulcus - separates the temporal and parietal lobes

white matter - lies below the cortex having a whiter cast; composed of myelinated

fibers

meninges - four tough membranes which contain the jelly-like substance of the brain

The meniges along with the bony skull help to protect the brain from injury.

Lobes

four of them

frontal, parietal, temporal and occipital

each lobe has a unique function

As stated earlier, although we have the ability to distinquish anatomical divisions of the brain, they work together to allow smooth integration of function.

ll. Organization of the Brain CNS

Telecephalon (Cerebrum)

cerebral hemispheres

thought, intelligence, ability to plan and solve problems

Diencephalon

thalamus and hypothalamus

relay station conducting information from sense organs to other parts of brain

Mesencephalon (midbrain)

relay station for visual and auditory input to the cerebrum

aids in integration of motor output

Metencephalon

cerebellum - coordinates conscious and unconscious movement

pons - aids in connecting the two hemispheres of the cerebellum

- links cerebellum to nuclei in medulla

Myelencephalon

contains many of the structures for the primary function of physiological systems

including the cardiovasculary and respiratory control centers

Ventricular System

series of hollow, liquid-filled chambers at the center of the brain

filled with cerebral spinal fluid (CSF)

CSF

shock absorber; supports the brain within the skull as the brain floats on the surrounding CSF, reducing its effective weight

secreted along parts of the nervous system by a highly vascularized, specialized tissue called the choroid plexus

circulates through the entire central nervous system and then absorbed back in blood via the arachnoid villi

lll Organization of the Brain PNS

The cranial nerves and the spinal nerves make up the PNS.

Spinal cord

contains the spinal nerves which exit the cord between vertebrae

contains both white and gray matter and a central hollow canal containing CSF

Spinal nerves

each nerve is actually composed of two nerves the ventral root and dorsal root, that exit the spinal cord and join a short distance from the cord

dorsal refers to the nerve’s position toward the back and ventral refers to the front

the two parts of the spinal nerve carry information in different directions

a sensory neuron (carries information from the periphery to the brain) may synapse directly with a motor neuron or do so via an interneuron.

lV. Cortical Function

Two experimental methods using anesthetized animals have allowed researchers to construct functional maps of the brain.

In the first method, they exposed a portion of the cortex and apply a very weak electrical current. Any motor activity is noted.

By carefully noting which muscles contract in response to well-localized stimuli, it is possible to construct a detailed map of the areas of the cortex that control specific motor activities.

In a second method, they placed recording electrodes on the brain surface and stimulate some distant part of the body either mechanically or with a weak electrical current. Any evoked electrical activity on the brain surface is recorded.

In this way, it is possible to map the sensory input to the cortex from all parts of the body.

Maps of the cortex have also been constructed from conscious patients undergoing brain surgery.

The cortical map obtained from these experiments showed that specific functions of the brain reside in specific brain areas rather than being intermingled, although elaborate connections exist between essentially all areas of the brain.

Frontal lobes

The area of the frontal cortex that controls muscle movement is called the primary motor cortex.

Immediately in front of the primary motor area is the premotor cortex which is also concerned with voluntary motor activity.

Coordination between the primary motor cortex and the prefrontal cortex is necessary for fine complex control we have over our muscles.

A conspicuous feature of the cortical map is that the respective brain areas responsible for different parts of the body do not correspond in size to those body parts.

For example, the size of the cortical area controlling hand movement is proportionally much greater than the cortical area controlling arm movement.

This makes sense because the range and fine motor control of hand movements, particularly of the thumb, are far greater than those of the arm.

Another function assigned to the frontal lobes is the control of certain thought and behavorial traits.

For some time it was believed that separating the most anterior portion of the frontal lobes, the prefrontal cortex, from the rest of the brain was a proper cure for many behavioral abnormalities.

Although we know that the the frontal lobes play a crucial role in coordinating and planning complex movement and speech, it is not entirely clear what the relation of the frontal lobes is to personality and behavior.

Parietal lobes

Receive sensory input.

If the area just behind the central sulcus of the parietal lobe is stimulated in an awake patient, the patient will tell the surgeon that they feel something in some localized area of the body.

Similar data can be obtained from mammals, mainly cats. If electrodes are placed on the brain of an anesthetized cat and part of the body is stimulated mechanically, an electrical signal will be recorded from the corresponding area of the cortex.

Both the nonhuman and human cortex are anatomically similar.

Again, the relative size of the brain does not correspond to the size of the body part from which they receive input.

The hand, lips, and tongue occupy far greater cortical areas than their individual sizes represent in proportion to the body’s overall surface area.

The tongue requires many sensory nerves and a correspondingly large cortical projection area.

Your arm, in contrast , is far less sensitive to touch, and has fewer sensory nerves, requiring a relatively small cortical area to which impulses are projected.

Cerebral Lateralization

The two hemispheres control both the motor and sensory reception of the opposite, or contralateral, side of the body.

If the motor cortex of the left side of the brain corresponding to the arm is stimulated, the right arm will move.

Similarly, if recording electrodes are placed on the sensory cortex, evoked potentials are recorded on the opposite side of the brain from which the stimulus was applied.

A cortical evoked potential is one that is produced by a sensory nerve activating a cortical neuron.

Thus, the right hemisphere controls movement of the left side of the body and receives sensory input from the left side.

The two hemispheres are connected by the large bundle of fibers running in the corpus collosum, whose nerves allow the two hemispheres to share information and coordinate integrated function.

Our understanding of the corpus collusum and hemispheric lateralization was enhanced by patients who had their corpus collusom removed.

These “split-brain” patients are able to perform most tasks with no noticeable defects.

However, if an object is placed so that its visual perjection is only to the right side of the brain, the person will see it perfectly well, but may not be able to name it, even though it is a common object.

This demonstrates that the two hemispheres are functional different, each having some strengths and weaknesses not shared by the other hemisphere.

It also demonstrates that information flow between the two hemispheres is necessary for the full range of human responses.

For most people the left hemisphere is largely associated with verbal and written language, as well as analytical abilities.

The right hemisphere, in contrast, processes information having to do with perception of wholes such as face recognition, and emotional behavior.

The latest research into the functional aspects of the two hemispheres indicates that each half of the brain is a functional complement of the other half.

Rather than one hemisphere being a dominant one, they act in concert, sharing information to allow well-coordinated movements and thoughts.

This is not to say that specific brain areas never control specific functions. For most people, the left hemisphere controls speech.

In the mid-nineteenth century, Paul Broca noticed that some people lost the ability to talk intelligibly despite the fact that they had no impairment of motor control of their tongue and vocal cords.

However, they could read and write normally and understand all that was said to them.

He pointed out that they had lesions in an area of the frontal lobe of the brain, now called Broca’s area.

We now know that Broca’s area is a primary speech area, and lesions in this part of the brain interfere with speech.

Aphasia

People with deficits in their ability to speak are said to be aphasic.

Aphasia is a disturbance of language that may take many forms, depending on the exact location of the lesion.

Some patients may be unable to recall specific words or names of objects; others may speak in meaningless phrases and even be unable to repeat short phrases.

The exact deficits experienced depend on the extent of the damage to the frontal lobe.

Careful analysis of aphasics has shown that control of language resides in the left hemisphere in over 95% of all people. About 70% of left handed people have left hemispheric control of speech.

Another type of aphasia indicates that the left frontal lobe is intimately concerned with comprehension of language.

Temporal Lobes

The temporal lobe is also involved in speech comprehension.

Patients with damage to Wernicke’s area, in the posterior part of the temporal lobe, have the ability to speak well-formed words and syllables but their speech in not intelligible.

Words are used in an almost random manner conveying no meaning. These people also have difficulty understanding both written and verbal language.

Also located in the temporal lobe is the primary center for hearing.

Lesions to the posterior part of the temporal lobe may cause word deafness. The person may be able to read and write with little or no abnormalities. However, spoken language may be severely disturbed.

The temporal lobe is closely associated with language, both spoken and written, yet a deficit in one may occur without a deficit in the other.

In some manner, not understood, the temporal lobes integrate written and oral symbols, using different neural circuits, so as to make those symbols meaningful.

Occipital lobes

Lying in the rearmost portion of the cortex is the occipital lobe. It is here that the primary visual area resides.

If this area is stimulated in an awake patient, the response is a visual one.The person will report seeing some type of light, but not necessarily an object or scene.

Hindbrain

The hindbrain, also called the brainstem, is located just above the spinal cord.

It comprises three major divisions: the medulla, the pons and the cerebellum.

Structures in this part of the brain play an important role in emotional responses, sleep patterns, and some pleasureable sensations.

Medulla

The medulla contains groups of neurons, or nuclei, that control cardiovascular and respiratory function.

Information from the lungs, heart, and vascular receptors is relayed to these centers, where they are integrated with other neural and hormonal stimuli.

The result is to keep blood pressure at the proper level and respiration at the appropriate rate and depth; all done at the subconscious level.

Pons

The pons is located immediately above the medulla and contains nerve tracts connecting the spinal cord with other areas of the brain.

Within the pons are the origins of the V, Vl and Vll spinal nerves, as well as nuclei that control, in part, the rhythmicity of respiration.

Cerebellum

The cerebellum lies just below and behind the occipital lobe and frontal lobes of the cerebrum.

The cerebellum receives information from the spinal cord relating to the position of muscle groups in many parts of the body.

This suggests the cerebellum is an important controller of muscle tone, particularly of the muscles responsible for maintaining posture.

In addition, fine control over repetitive movements seems to rely on a well-functioning cerebellum. The cerebellum also appears necessary for the rapid, coordinated movements of a piano player or typist.

Subcortical Function

Beneath the cortex of the brain are many structures that receive and send information to other parts of the brain.

Electrophysiological experiments have allowed researchers to map the connections and determine some of the functions of those structures.

Thalamus

A large oval mass lying alongside the third ventricle is the thalamus.

The largest portion of the thalamus is composed of cell bodies that receive input from sensory fibers. For this reason it is often called the relay station of the brain.

It integrates neural activity from both the cerebrum and the periphery, relaying that information to the cerebellum, the cortex, and other brain areas.

It also connects with the cerebral cortex and the hypothalamus and is involved with motor functioning and emotional processing.

Hypothalamus

Lying just below and forming part of the floor and sides of the third ventricle is the hypothalamus.

It is area of densely packed cell bodies that regulate many of the homeostatic mechanisms of the body.

It helps regulate biological rhythmns such as sleeping and waking, body temperature, blood pressure, salt and water balance, heart rate and behavioral drives such as thirst, hunger, and sex.

Limbic System

The limbic system is not a separate, well defined structure, but a ring of structures that includes part of the thalamus and hypothalamus.

This complex set of interconnecting pathways plays ani important role in emotion, behavior and motivation.

Stimulating special parts of the limbic system with electrodes implanted in the brain of conscious animals elicits emotional changes. A quiet, passive animal may exhibit rage and aggressive behavior when stimulated in a particular area of the limbic structure.

Conscious humans may report feelings of great pleasure if the electrode is placed appropriately.

Experiments, stimulating different regions of the brain, indicate that facial expressions characteristic of emotion are not learned responses but are hard-wired in the limbic system.

At birth, the emotional cues such as happiness, fear and sadness are already present in the deep structures of the brain.

A strong piece of evidence that facial signals are not learned is that people who are blind smile when happy and display other signs of emotion when appropriate, yet have never seen these expressions.

Reticular Formation

A network of fibers running from the spinal cord up through the brain stem is called the reticular formation.

The reticular formation is composed of widely spread neurons that signal the cortex promoting arousal, alertness, and attention.

When researchers selectively destroy some areas of this network, the animal exhibits hyperarousal. Destruction of other areas of the reticular system results in coma.

lll. Brain Neurotransmitters

The major neurotransmitters we talked about earlier, acetycholine, epinephrine, norepinephrine and dopamine are also important for communication between neurons in the brain. However, in addition to these chemicals, others play important roles in transferring information from one neuron to another.

Monoamines

Monoamines are modified amino acids including epinephrine, norepinephrine, serotonin, histamine and dopamine.

Neuropeptides

This class of neurotransmitters is composed of short chains of amino acids.

Some of these neuropeptides, such as arginine, vasopressin and angiotensin ll, are also hormones.

Within this class of neurotransmitters are the enkephlins and endorphins, which are sometimes called opioid neurotransmitters.

They bind the same receptor sites as does morphine.

Endorphins create a general sense of well-being ranging from reduced perception of pain to euphoria.

The well-known runner’s high is believed to result from the release of endogenous opioid neurotransmitters.

In addition to these neuropeptides many others that have been isolated and shown to affect neurotransmission and their number gets larger every year.

The brain makes many polypeptide neurotransmitters that play some still unknown role in neurotransmission and storage of information in the brain.

Nitric Oxide

Nitric Oxide (NO)

Nitric oxide is a new neuroactive compound that has been discovered and seems to have wide biological effects.

NO is a very reactive gas and is a highly toxic one.

NO is one of a class of highly reactive compounds known as free radicals; these chemicals, generated normally, are thought to be damaging to cellular molecules.

Fortunately, each cell possesses other chemicals or enzymes that rapidly destroy or inactivate free radicals.

Many neurons of the CNS, as well as other cell types, contain receptors for NO, and these receptors control many functions.

NO has been shown to play an important role in regulating blood pressure by its action on the cardiovascular center of the brain.

Alzheimer’s Disease is one of the most common degenerative diseases of the brain.

For reasons that are unknown, large areas of the brain in some people atrophy. The weight of the brain decreases as does the number of functioning neurons.

The most consistent finding in these patients is that the activity of the enzyme required for making acetylcholine is severely reduced.

The implication of this finding is that cholinergic neurons, the neurons that use acetylcholine for neural signaling are selectively lost.

The loss appears greatest in the cortex and deeper nuclei in the brain.

People who have this disease exhibit progressive mental deteriorization, memory loss, and disorientation of time and place.

They may completely lose contact with the world about them, but they appear to be able to see, hear, and talk normally.

Reseachers have shown that the loss of cognitive ability is proportional to the loss of the enzyme required for the synthesis of acetylcholine.

Parkinson’s disease

Patients exhibit lesions in an area of the brain called the basal ganglia. The basal ganglia ensures smooth and coordinated movement Cells of the basal ganglia secrete the neurotransmitter dopamine.

The first symptoms of this disease is a reduction in strengh of muscles, over time a rhythmic tremor, and gradual progression to paralysis. There is no loss of mental abilities.

Autopsies determined there were regions of the brain with lower than normal levels of dopamine.

In animal models of this disease researchers demonstrated that supplying dopamine alleviated the symptoms of the disease. These results led to technique that holds promise in humans.

Human tissue obtained from spontaneously aborted fetuses, if surgically implanted

could increase the level of dopamine in the basal ganglia. Fetal tissue has the advantage that it can be transplanted and unlike tissue from the adult will not be rejected.

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