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The Central Nervous System

Nervous System:
The Central Nervous System
The Brain
Essential Information and Problems


Student Performance Objectives - for the lecture
1. Name the 3 initial swellings that compose the embryonic brain and the 5 final swellings that form the mature brain.
2. Explain the value of convolution seen in the cerebral cortex.
3. Explain the difference between commissural, association and projection tracts.
4. Explain what the term "limbic system" means.
5. List the 4 major activities of the cerebrum.
6. Explain the difference between translation, interpretation and integration occurring in the cerebral cortex.
7. Explain the significance of the angular gyrus, Wernicke's area and Broca's area.
8. Describe the significance of the precentral and postcentral gyri of the cerebral cortex.
9. Explain why the sensory and motor homunucli look out of proportion for human bodies.
10. List and describe 4 functions for the thalamus.
11. List and describe 4 functions for the hypothalamus.
12. List and describe 2 functions for the reticular system of the midbrain.
13. List 3 sources of neural input to the cerebellum and describe the function of the cerebellum.
14. Explain decussation.
15. Describe 4 functions for the brainstem.

Lesson Outline
A. In General: A biological discussion of the human brain could disappoint beginning anatomy and physiology students hoping that a study of the brain will uncover what the mind is and will reveal why they feel, remember and think the things that they do. Even courses in psychology may not provide such answers. Questions about the mind and consciousness with relation to the physical brain are the subject matter of important areas of philosophy and theology and, within these disciplines, there are many points of view. In the anatomy and physiology laboratory, students anticipate their dissection of sheep or human brains expecting some intense revelation, or at least some sparks to illuminate the mind's mysteries. What they get is the odor of formaldehyde and lots of neuroanatomical terms to memorize. However, neuroscientists have learned a great deal about the anatomical organization of the brain and the pattern of its electrochemical circuits. Neuroanatomists have sectioned and stained the brain in great detail, neurophysiologists have imaged living, thinking brains with CT, MRI and PET scans correlating their knowledge with that of neurologists and neurosurgeons, and neuropharmacologists have learned a great deal about our neurotransmitters through the use of behavior and mood altering drugs. And so we can gain some insight into the relationship between the brain and the mind through study of the brain's anatomy and physiology, although we may not provide the ultimate answers.

. In the Beginning
    1. The nervous system's beginning - We have no brain or nervous system in the first two weeks of our gestation. We do develop three primary germ layers early on and the outermost one, ectoderm, begins forming the nervous system during our third week of existence. For your sense of perspective on when this occurs, your mother would be one week late in the menstrual phase of her ovulatory-menstrual cycle and would be pretty sure she is pregnant. The early embryo's beginning nervous system then goes through a series of rapid changes from a neural streak, where the ectoderm changes to become a neuroectoderm, to a thickened neural plate, an infolded neural groove with sides called neural folds, and then a neural tube, which is our primitive spinal cord and brain, with budded off neural crest (which will form various
    2. The brain's beginning
         a. The first three swellings - By the 4th week of gestation the anterior region of the neural tube develops 3 swellings called, generally, the forebrain, midbrain, and hindbrain. [In more technical terminology, the swellings are referred to, respectively, as prosencephalon, mesencephalon, and rhombencephalon].
         b. The final 5 swellings - Within a week, the forebrain divides into two parts- telencephalon and diencephalon, as does the hindbrain - becoming metencephlon and myelencephalon. The midbrain, or mesencephalon, does not subdivide.
    3. The parts of the mature brain - The fate of the 5 embryonic brain swellings
  (1) The telencephalon will form the cerebral hemispheres.
             (2) The diencephalon forms the:
                    (a) thalamus
                    (b) hypothalamus
                    (c) pineal gland.
             (3) The midbrain contains many structures: from the back of the midbrain to the front, some important structures we will discuss are the:
                   (a) Corpora quadrigemina
(b) Cerebral aqueduct
                   (c) Reticular formation
                   (d) Medial lemniscus
                   (e) Red nucleus
                   (f) Substantia nigra
                   (g) Cerebral peduncles
             (4) The metencephalon forms the:
                   (a) pons
                   (b) cerebellum
(5) The myelencephalon forms the medulla oblongata.

C. Systematic Survey of the Human Brain
     1. Cerebrum
a. In general - This is the largest part of the human brain occupying its superior surface. It is divided into hemispheres by the longitudinal fissure.
             (1) Cerebral Cortex
                  (a) Convolutions - The surface of the cerebrum, the cerebral cortex, is about 3 mm thick and is folded into convolutions consisting of gyri (singular = gyrus) and sulci (singular = sulcus) that increase the cortex's functional surface area.
The 15 billion neurons located in the cerebral cortex would require a much larger brain volume if the cortex was smooth. The folding (convolutions) permits a larger surface area to fit into a smaller volume. It has been estimated that one's head would have to be the size of a 5 gallon beer keg to accommodate a smooth cerebral cortex fitting in 15 billion neurons.
                (b) Neuronal types - The cortex contains two major types of neurons: pyramidal cells and granule cells (stellate cells), all of which form approximately 1 trillion synapses forming the many different types of circuits required to create a mind. Unlike the neuromuscular junctions that utilize acetylcholine as their neurotransmitter, the majority of cerebral synapses utilize glutamate (glutamic acid) as their excitatory neurotransmitter.
             (2) Below the cortex
                   (a) Beneath the cerebral cortex is the cerebral white matter which consists of commissural tracts like the corpus callosum and the anterior and posterior commissures, all of which interconnect the two cerebral hemispheres. The cerebral white matter also contains association tracts that interconnect different regions of the same cerebral hemisphere, and projection tracts that link the cerebrum with other brain regions and with the spinal cord.
                   (b) Also beneath the cerebral cortex and embedded within the cerebral white matter are islands of gray matter called basal ganglia (more properly called basal nuclei). The neurons composing these areas form feedback loops with motor areas of the cerebrum and the cerebellum to coordinate muscular movements.
                   (c) An additional cerebral nucleus is the amygdala. It appears to work with other brain regions, most notably the hypothalamus, in the area of human emotion. Also part of the cerebrum is the hippocampus, which is involved in memory. The term "limbic system" is often used to indicate a brain pathway, involving many brain regions, for the initiation, response to and control of human emotions. This system is thought to involve the cerebrocortical region called the cingulate gyrus, and also the amygdala, hypothalamus (with its mammillary body),
hippocampus, and the fornix.
        b. Cerebral activities can be divided into 4 broad areas: the processing of sensory information, the analysis of information, the production of motor responses, and the storage of information as memory.
            (1) Processing of sensory information and information analysis involves a number of steps that we understand in a general way.
                 (a) The first step in sensory information processing is translation and it occurs in the primary sensory areas of the cerebral cortex. Bioelectrical signals become sensations. E.g., nerve impulses reaching the primary auditory area become sounds; those reaching the primary visual area become sights.
                 (b) The second step in sensory information processing is interpretation and it occurs in the sensory association areas of the cerebral cortex. Bioelectrical signals that have already been translated into specific sensations move into the adjacent cortical regions called association areas and take on meaning. E.g., a noise becomes a word with a meaning; a sight becomes a recognizable object or person.
                 (c) The third step in sensory information processing is integration and it occurs in several cerebral cortical regions like the angular gyrus, Wernicke's area and Broca's area. Here the information from various association areas are integrated together. There is an analysis of information to give one a more complex and thorough understanding of the information. E.g., in Wernicke's area, spoken and written sentences take on increased meaning as information from auditory and visual association areas is processed; in the angular gyrus, written symbols (words) we observe are processed so they can be spoken; Broca's area prepares the muscular vocal regions of the respiratory system for the speech we are about to deliver. This example illustrates that understanding, writing and speaking a language is an area of cerebral function requiring much integration of knowledge and much information analysis.
            (2) The production of motor responses involves the interactions of cerebral motor areas with basal nuclei (located deeper in the cerebrum) and the cerebellum. Feedback loops between these brain regions (reberverating circuits) permit repetitive activities like walking and repeated complex movements like knitting or smoothly swinging a baseball bat. All motor movements involve the actions of several major cell types: upper motor neurons located in the cerebral cortex's prefrontal gyrus, that synapse with lower motor neurons in the brainstem or spinal cord that then relay the message to contract to the skeletal muscles. Monitoring all this cerebral electrical activity are the cerebellum's Purkinje cells that coordinate balance (proprioceptive) feedback from joints, muscles, tendons, and the inner ear, so that cerebrum-directed movements are smooth and appropriate for the intended outcome.
            (3) Memory
   (a) In general, memory is thought to be a path through the cerebral cortex in the form of a pathway of synapses. The pathway may travel through one cerebral lobe utilizing only local association neurons and their axons (short association fibers), or the pathway may travel through several cerebral lobes in one hemisphere utilizing widely separated association neurons and much longer axons (long association fibers).
                  (b) Types of memory - memory is commonly subdivided into short term and long term. Short term memory is thought to involve reberverating neural circuits with possible facilitation of synaptic transmission to permit the memory to last a bit longer than just for the immediate moment. Long term memory is thought to involve actual growth of new dendrites and axons on existing neurons to form new synapses and circuits specific for that particular memory (in addition to the facilitation of synaptic transmission).
        c. Cerebral subdivisions - The cerebrum has 5 lobes, four being obvious from a surface view. The four major cerebral lobes are named for the cranial bones that they underlie. Some cerebral functions appear to be unique to a given cerebral lobe (e.g., the visual sensory area is mostly in the occipital lobe and auditory sensory area is mostly in the temporal lobe), whereas other higher brain functions such as "learning" may occur in several cerebral areas simultaneously. Some key cerebral areas lie at the junction of the various cerebral lobes. For example Wernicke's area and the angular gyrus lie approximately at the intersection of the parietal, temporal and occipital lobes. The following discussion emphasizes cerebral lobe functions that are unique to the lobe in question.
           (1) The Frontal lobes are concerned with many areas of biological intelligence such as our ability to sense time and to plan for the future, aspects of language (Broca's region), memory and personality traits. The frontal lobes also contain the precentral gyri concerned with voluntary movements of skeletal muscles.
There is a representation of the human body on the precentral gyrus called the motor homunculus. This human image is strange looking because some body parts are larger than other parts. The face, lips, tongue and hands, particularly the thumb, are disproportionately larger than other parts of the body because the number of motor neurons innervating and providing fine motor control for these areas is disproportionately larger than other bodily areas. The olfactory bulbs leading to the olfactory tracts (cranial nerve I - olfactory) are located just beneath the frontal lobes.
           (2) The Parietal lobes, containing the postcentral gyri, are concerned with the translation and interpretation of sensory signals (touch, pressure, temperature, pain) from skin and the tongue (sense of taste). Just as the precentral gyrus has a motor homunculus, the postcentral gyrus has a sensory homunculus. It is as strange looking as the sensory homunculus because some body parts are larger than others. The face, lips, tongue and hands, particularly the thumb, are disproportionately larger than the rest of the body because the number of sensory neurons in these areas is disproportionately larger than other bodily areas.
           (3) The Temporal lobes are concerned mainly with the senses of hearing and smell.
           (4) The Occipital lobes are mainly concerned with the sense of sight.
           (5) The Insula is observed in frontal and horizontal sections of the brain, but not from the surface. Its lobes appear to be concerned with the sense of taste and with the processing of visceral sensations.
        d. Differences between the hemispheres: The cerebral hemispheres do not work identically. The left cerebral hemisphere appears to be more concerned with analytical reasoning in which information is broken down and dissected in some logical way. It is also more concerned with written and spoken language. The right cerebral hemisphere, in contrast with the left, appears to be more concerned with the ability to see broad spatial relationships, to grasp things intuitively, and to appreciate and demonstrate artistic talents.
            (1) Handedness - this description of the differing abilities of the cerebral hemispheres is found to hold true for the vast majority of people who are right handed. However, in the majority of left handed people the cerebral roles are reversed. Only in a small percentage of left handed individuals are the cerebral roles are the same as for right handed people.
            (2) Gender - women appear to have a better ability than men to have functions from one hemisphere take over if there is loss to the other hemisphere. This is observed with stroke victims in that men are more likely than women to suffer permanent language impairment (a condition known generally as aphasia).
    2. Diencephalon - In general, the diencephalon is located between the cerebral hemispheres and is composed of three major structures: thalamus, hypothalamus and the pineal gland.
        (1) Thalamus -
the thalamic hemispheres make up most (80%) of the mass of the diencephalon. The cerebrum is an outgrowth of the thalamus, embryologically, and there is a radiation of nerve fibers from the thalamus up into the cerebrum. The thalamic hemispheres are connected by the intermediate mass.
(a) Sensory signal relay station - All ascending, sensory signals, except for olfaction, pass through the thalamus before entering the cerebrum. It generally requires 3 neurons for a sensory signal to reach the cerebrum: a first order neuron carries the signal in from the receptor to the spinal cord or brainstem. A second order neuron carries the signal from the spinal cord or brainstem to the thalamus. Finally, a third order neuron carries the sensory signal from the thalamus into the specific primary sensory area of the cerebrum. The brain's "emotional system" (limbic system) also communicates with the cerebrum through impulses passing through the mammillary bodies to the thalamus and then to the cerebrum.
              (b) Rough translation - cutaneous sensation undergo some translation in the thalamus, but without precise localization. Without the operation of the cerebral somatosensory cortex (postcentral gyrus), you would sense "cold" if an ice cube were placed on your neck, but you would not know exactly where the cold sensation was coming from. The cerebrum not only translates nerve impulses into sensations, but also provides localization of where the sensation is coming from. The thalamus does not roughly translate visual or auditory sensory input, only cutaneous.
              (c) Concentration - by acting as a filter for information traveling into the cerebrum, the thalamus aids in our ability to concentrate: some signals reach the thalamus and others are blocked. The thalamus works in this regard with the reticular formation of the brainstem.
              (d) Motor signal circuits - some motor signals pass directly from the cerebrum to the brainstem or spinal cord (the corticospinal tracts, also called pyramidal tracts), bypassing the thalamus. But some descending motor signals pass to the pons and then to the cerebellum for processing. Also, many motor signals pass from the cerebrum to the basal nuclei for processing. All these processed signals, from the cerebellum and the basal nuclei, then pass back up to the cerebrum via the thalamus for further processing before being sent to the brainstem and spinal cord.
        (2) Hypothalamus - this area of the diencephalon is sometimes called "the brain within the brain" because of the number of control centers it contains and the influence it has on other regions of the nervous system, on the endocrine system, and the body as a whole. The optic nerves (cranial nerve II - optic) cross at a point called the optic chiasm that is just below the hypothalamus, although not part of the hypothalamus. The following body functions are regulated through the hypothalamus:
             (a) Thirst and body water regulation, working through the posterior pituitary.
Hunger for food and feelings of satiety .
             (c) Heart rate and blood pressure, working through brainstem nuclei.
             (d) Body temperature regulation which controls vasodilation and vasoconstriction of blood vessels to the skin, sweating, shivering, and control of the metabolic rate.
             (e) Peristalsis and glandular secretion in the stomach and intestines.
             (f)  Pupillary diameter
             (g) Hormone secretion, working through the anterior pituitary.
             (h) Biological rhythms, including sleep and wakefulness.
             (i)  Basic human emotional drives, including sex, anger, joy, fear, and pleasure.
             (j)  Memory, working as a pathway (through the mammillary bodies) from the hippocampus to the thalamus and cerebrum.
        (3) Pineal gland - this gland, once thought to be the location of the human soul, secretes 2 monoamine hormones - serotonin (which you also know as a neurotransmitter whose reuptake is inhibited by mood elevating drugs like Prozac) during the day, and melatonin at night. Melatonin may play a role in the timing of puberty and in human sleep cycles.
    3. The Midbrain is a connecting link between the forebrain (cerebrum and diencephalon), and the hindbrain (brainstem and cerebellum). Its parts include:
         a. Corpora quadrigemina - the two superior colliculi functioning in visual reflexes in which you respond to an object that enters your visual field by turning your eyes and head toward it, and the two inferior colliculi, functioning in auditory reflexes in which you turn your head toward a sound you have suddenly heard.
b. Cerebral aqueduct - this is the passageway for the flow of cerebrospinal fluid from the third ventricle to the fourth ventricle.
         c. Reticular formation - this extensive region of gray matter runs through the midbrain and also the entire brainstem (pons and medulla). It consists of over 100 nuclei that regulate:
             (1)  Sleep and wakefulness through their connections to the thalamus and cerebrum. The reticular activating system learns to rouse the cerebrum from sleep based on external signals - an alarm for instance, while ignoring even louder sounds that might occur earlier.
             (2) Concentration (working with the thalamus) through control of which sensory signals pass through and reach the cerebrum.
             (3) Heart rate and blood pressure control- these nuclei are in the part of the reticular formation that is within the medulla oblongata. They act as reflex centers for blood pressure control and heart rate. They also can be controlled from the hypothalamus.
             (4) Balance and posture maintenance through connections with the motor cortex and by interconnecting signals from peripheral receptors (eyes and balance receptors in the inner ear) with the cerebellum.
         d. Medial lemniscus - consists of a continuation of sensory spinal tract carrying cutaneous and proprioceptive signals to the thalamus.
         e. Red nucleus - a large nucleus concerned with muscle control that communicates with the cerebellum.
         f.  Substantia nigra - this is a motor modulating nucleus that sends inhibitory signals to the the thalamus and basal nuclei helping to control muscle contractions. Disease affecting this nucleus, like Parkinson's disease, results in uncontrolled muscular movements.
         g. Cerebral peduncles
- these descending (motor) white matter tracts (corticospinal tracts) are just passing through the midbrain.
         h. The midbrain also contains the nuclei for cranial nerves III (oculomotor) and IV (trochlea), which control eyeball movements (along with cranial nerve VI).
    4. Metencephalon - consists of the pons and cerebellum.
         a. Pons
  (1) A relay station for ascending and descending signals traveling in white matter tracts. The cerebellum receives most of its input from tracts running through the pons.
            (2) Contains the nuclei for cranial nerves V (trigeminal - motor and sensory to the face), VI (abducens - eyeball movements), VII (facial - motor and sensory to the face), and VIII (auditory - hearing and equilibrium).
            (3) The reticular formation runs through the center of the pons and contains nuclei that regulate aspects of respiration, posture and sleep.
         b. Cerebellum - containing about 100 million neurons (1/2 of all the neurons in the brain), the cerebellum is a major brain center for the integration of signals relating to the smooth, precise, and coordinated movements of skeletal muscles. The cerebellum, divided into two, finely convoluted cerebellar hemispheres, is attached to and communicates with the brainstem and, from there, the rest of the brain, through 3 pairs of white matter tracts. 
             (1) Input to the cerebellum comes from:
                  (a) Cerebral cortex through the pons.
                  (b) Inner ear including signals for balance and hearing.
                  (c) Eyes.
                  (d) Reticular formation through the red nucleus.
                  (e) Muscle spindles and joint receptors (balance or proprioceptors).
            (2) Output from the cerebellum goes to:
                  (a) Cerebral cortex through the thalamus.
                  (b) Postural muscles of the arms and legs through tracts running through the pons, medulla oblongata and spinal cord (reticulospinal tracts).    
    5. Myelencephalon forms only one structure - the medulla oblongata. This brain region is continuous with the spinal cord and begins just at the foramen magnum, ending at the pons. Ascending and descending white matter tracts run through the medulla and it is here that most of them decussate so that sensory messages from the left side of the body are received by the right cerebral cortex and vice versa. Similarly motor responses originating from the right cerebral motor cortex, lead to movements on the left side of the body, and vice versa. Decussation does not apply to sensory and motor signals from the head, only to areas below the head. The reticular formation runs through the medulla and contains nuclei that reflexively regulate the heart rate, blood pressure, respiratory rate and depth and sweating. The medulla contains nuclei for cranial nerves IX (glossopharyngeal), X (vagus, XI (accessory), and XII (hypoglossal). A number of complex reflexes are processed through the medulla utilizing these cranial nerves, including swallowing, coughing, sneezing and vomiting

D. Summary of the Cranial Nerves

Cranial Nerve General Function Cranial Exit
I Olfactory Smell Cribriform Plate of the Ethmooid
II Optic Sight Optic Foramen
III Oculomotor Eye Movement Superior Orbital Fissure
IV Trochlear Eye Movement Superior Orbital Fissure
V Trigeminal Face: sensory, motor Superior Orbital Fissure
VI Abducens Eye Movement Superior Orbital Fissure
VII Facial Facial Expressions Stylomastoid Foramen
VIII Vestibulocochlear Hearing and Balance Internal Acoustic Meatus
IX Glossopharyngeal Toung and Throat - motor and sensory Jugular Foramen
X Vagus Parasympathetic Jugular Foramen
XI Accessory Head, neck and shoulder movement and swallowing Jugular Foramen
XII Hypoglossal Speech, chewing and swallowing Hypoglossal Canal


Biomedical Terminology:   Define each term:

angular gyrus
association fibers
Broca's area
cerebral peduncles
commissural fibers
corpora quadrigemina
corpus callosum
granule cells
optic chiasm
projection fibers
Purkinje cells
pyramidal cells
reticular formation
Wernicke's area

Nervous System Problems

 1. Choose one of the problems described below.
 2. Prepare your solution as a word document.
 3. Send it to your professor as an email attachment. You will receive an     email response.

Problem #1: An individual approached by police during a robbery resists arrest and is subdued only by the combined efforts of 3 very strong officers. The officer's impressions, and later chemical analyses, reveal the individual to be heavily under the influence of cocaine. Utilize the Internet to research the effects of cocaine on the human nervous system and other systems of the body. 
     Your report should include
          1. A description of cocaine and a list of other chemicals in its class.          
          2. An explanation of the effects of cocaine on the nervous system.
          3. An explanation of why the individual was difficult to subdue.
          4. The effects of cocaine use, both short and long term, on the organs of the human body.

Problem #2: A recent book: "Women are from Venus, Men are from Mars" has popularized the idea that there are significant differences between the brains of men and women. Utilize the Internet to answer the following questions:
         1. Are there anatomical differences between the brains of men and women? If yes, what are they?
         2. What are some of the differences cited between male and female behavioral patterns? Are any of these differences related to anatomical differences?
         3. Is there evidence that men and women are born "programmed" with different physiological responses to stimuli, or are the different responses of men and women to stimuli based on learning throughout life?

Problem #3: A child having problems concentrating in school is given Prozac by a doctor. A college student suffering from depression is given Prozac to combat the depression. In fact, 30 million Americans have been given Prozac or drugs like it. Utilize the Internet to evaluate the pros and cons of Prozac therapy.
   Your report should include
        1. A description of the postulated basic mechanism of action of Prozac. What is neurogenesis and does Prozac stimulate this process?
        2. Undesirable side of effects of Prozac therapy. Is altered brain microanatomy an indication of brain damage?
        3. Alternatives to Prozac therapy.


Practice Quiz

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