Chapter 11

I. SPINAL CORD ANATOMY
        A. Protection and Coverings
            
The CNS is protected by:
              
1. The brain is protected by the cranium, and the spinal cord is protected by the vertebral column- spinal cord is located in the vertebral canal of the spinal column, which is formed by the vertebral foramina of all the vertebrae. 

        2. the brain and spinal cord are covered by three connective tissue layers called the meninges:
                a. Outer most is the dura mater (tough mother) - dense, irregular connective tissue with many blood vessels and nerves. It is attached to the inside of the cranium and forms the periosteum inside the cranial cavity.
                b. The dura mater extends inward between the lobes of the brain, and forms supportive and protective partitions.                         

                        1. an extension of the dura mater called the falx cerebri (CER-e-bree)separates the two halves of the cerebrum.

                        2. a similar dural extension separates the halves of the cerebellum -falx cerebelli. (cer-e-BEL-eye)

                        3. another dural extension separates the cerebellum from the cerebrum and is called the tentorium cerebelli.
               
c. In other areas, the dura mater splits into two layers, forming channels called dural sinuses . Venous blood flows through these channels as it returns from the brain to the heart.
                d. The dura continues down around the spinal cord from foramen magnum, where it is continuous with the dura mater of the brain, down to level of the second sacral vertebra where it is closed ended, forming a sac.
                e. The dura mater around the spinal cord is not attached to the vertebrae, but is separated from it by the epidural space. There is also a cushion of fat and connective tissue in the epidural space, as well as blood vessels.
        
        3. The middle meninx (MEE-ninks) is the arachnoid (mater) - named because of the delicate spider web-like collagen and elastic fibers found in this layer.
                a. between the dura mater and the arachnoid is the subdural space - filled with interstitial fluid. (Can also become filled with blood in cases of head trauma - compressing soft tissue underneath.)
               b. The arachnoid is a thin membrane, which lacks blood vessels. It spreads over the brain, but does not go into the grooves and depressions of the brain and spinal cord.
               c. Many thin strands extend from the underside of the arachnoid and attach to the underlying pia mater.
                d. between arachnoid and pia mater is the subarachnoid space - filled with CSF.

    4. The innermost is the pia mater (pious mother - meant tender mother) thin, nearly transparent layer, made of interlacing bundles of collagen fibers and some fine elastic fibers. Also has many nerves and blood vessels that supply the brain and spinal cord.
            a. The pia mater is attached to the brain and spinal cord, and follows their contours.
            b. Triangular thickenings of the pia mater, called denticulate ligaments, help to suspend the spinal cord within the dura mater. These ligaments are found between the ventral and dorsal roots of the spinal nerves on both sides of the spinal cord. They protect against shock and sudden displacement.

    5. Inflammation of the meninges is called meningitis.

Cerebrospinal Fluid

The brain and spinal cord are protected and nourished by the cerebrospinal fluid (CSF).

1. The CSF contributes to homeostasis in three main ways:
        a. mechanical protection - it "floats" the brain in the cranial cavity, and acts as a shock absorber against bumps and blows.

        b. chemical protection - the CSF provides an optimal chemical environment for the transmission of impulses in the brain and spinal cord. It contains glucose (for energy), proteins, lactic acid, urea, cations (Na+, K+, Ca++, Mg++) and anions (Cl- and HCO3-) and some white blood cells. (Basically a filtrate of blood.)

        c. circulation - it brings nutrients from the blood and removes waste products from the nervous system, which it returns to the blood.
  
2. It is a circulating fluid- flows between the arachnoid and pia mater in the subarachnoid space. Cerebrospinal fluid is formed in the choroid plexuses of the brain. The choroid plexuses are networks of capillaries in the walls of the ventricles in the brain The capillaries of the choroid plexuses are covered by ependymal cells that form the CSF by filtration and secretion. ( facilitated diffusion and active transport) Because the ependymal cells are joined by tight junctions, the only way for substances to reach the CSF from the blood is through the ependymal cells. Thus they form a blood-cerebrospinal fluid barrier that allows some substances through, but not others. This protects the brain and spinal cord from harmful substances in the blood, but also makes it more difficult to get therapeutic drugs into the CSF, say to treat for meningitis.

There are four ventricles: two laterals ventricles located in the center of the hemispheres of the cerebrum, the third ventricle is located above the hypothalamus and between the halves of the thalamus, the fourth ventricle lies between the brain stem and the cerebellum.
    CSF formed in the lateral ventricles flows into the third ventricle through the interventricular foramina . More CSF is added by the choroid plexus in the roof of the third ventricle, and the leaves through the cerebral aqueduct which passes through the midbrain. The fourth ventricle adds more CSF, and then it enters the subarachnoid space by three openings:

 

            a. a median aperture

            b. two lateral apertures

It then circulates through central canal of the spinal cord, and in the subarachnoid space around the brain and spinal cord.

3. The CSF is reabsorbed into the blood through the arachnoid villi,(granulations) which are fingerlike projections from the subarachnoid space into the dural venous sinuses, especially the superior sagittal sinus.

4. The CSF is usually absorbed at the same rate it is produced, about 20 ml/ hour, so the pressure of the CSF remains constant.

5. What would happen if the CSF was produced more rapidly, or didn’t drain?

What would a blockage do? Hydrocephalus or "water on the brain" can be caused developmental abnormalities, infection, injury or brain tumors. in infant with fontanels, the head bulges.
    In adults, whose fontanels are closed, it causes vomiting, headache, optic atrophy, and mental disturbances. Pseudotumor cerebri, called benign intercranial hypertension, produces all the signs of a brain tumor, even though no tumor is present. It can be relieved by removing some of the CSF by spinal tap, or lumbar puncture.

B. External Anatomy of the Spinal Cord

   
The spinal cord begins as a continuation of the medulla oblongata and ends at about the second lumbar vertebra in an adult. In an infant, it extends to the third or fourth lumbar vertebra. (both grow, then spinal cord stops, and vertebral column continues to lengthen).
     Although the spinal cord is not segmented, it appears so due to the 31 pairs of spinal nerves that arise from it. These nerves branch to various areas of the body, and connect the body parts with the CNS.
            The spinal cord contains two enlargements:
                    a. cervical enlargement C4 to T1 - nerves to and from the upper limbs are found in this region.
                b lumbar enlargement - T9-T12 - nerves to and from the lower limbs are found in this region.
                c. Inferior to the lumbar enlargement, the spinal cord tapers to a conical portion called the conus medullaris which ends at the intervertebral disc between L1 and L2.
                d. Coming off of the conus medullaris is the filum terminale (terminal filament), an extension of the pia mater that extends to anchor the spinal cord to the coccyx.
                e. After the conus medullaris the spinal cord divides into the nerves that will leave the vertebral column at lower levels. Its appearance gives it the name cauda equina or horse’s tail. Spinal taps are normally performed between L3 and L4 or between L4 and L5 (to withdrawn CSF, administer anesthetics, antibiotics, chemotherapy, contrast media for myelography.) Why?
        f. Two grooves divide the spinal cord into right and left sides:
                  1. the anterior median fissure - deep, wide groove on front
                  2. posterior median sulcus - shallow, narrow groove on back
        g. Spinal nerves allow the brain and the body to communicate. The areas where the spinal nerves attach to the spinal cord are called roots.
                1. Posterior or dorsal (sensory) roots contain sensory fibers from the periphery of the body. Each root has a swelling called a posterior or dorsal (sensory) root ganglion which contains the cell bodies of these sensory neurons .
                2. Anterior or ventral (motor) roots contain motor neuron axons and conduct impulses from the brain and spinal cord to the periphery.

C. Internal Anatomy of the Spinal Cord
       
The gray matter in the spinal cord forms the letter H in the center of the cord. (Cell bodies, neuroglia, unmyelinated axons and dendrites of internerurons and motor neurons.)
            a. The crossbar of the H is called the gray commissure (2. a connecting bundle of nerve fibers, esp. one joining the right and left sides of the brain.)

            b. In the center of the commissure is a hole called the central canal - continuous with the fourth ventricle of the brain, and running the entire length of the spinal cord.

                c. Anterior to the gray commissure is the white commissure which connects the white matter on the right and left sides of the spinal cord.

                d. Gray matter in the spinal cord is divided into horns:

                        1. anterior (ventral) gray horns - cell bodies of motor neurons to skeletal muscle.

                        2. posterior (dorsal) gray horns

                        3. lateral gray horns - present only in the thoracic, upper lumbar, and sacral segments of the cord. Cell bodies of motor neurons to cardiac, smooth muscle, or glands. (part of autnomic nervous system.)

                e. The white matter (myelinated nerve fibers) is divided into columns (funiculi)

                                1. anterior (ventral) white columns

                                2. posterior (dorsal) white columns

                                3. lateral white columns

        f. Each column is divided into bundles called nerve tracts or fasciculi.  Each tract has a common origin or destination, and carry similar information.
        Ascending (sensory) tracts - impulses toward brain
   
     Descending (motor) tracts - impulses from the brain

II. SPINAL CORD PHYSIOLOGY 

Spinal cord has two functions:
        The white matter tracts serve as information highways to and from the brain.
        The grey matter receives and integrates incoming and outgoing information, especially for spinal reflexes.

A. Sensory and Motor Tracts
   
Names of tracts indicate:
            position in the cord
            where it begins and ends
            and the direction of impulses

(anterior spinothalamic tract - in anterior column, begins in the spinal cord, ends in the thalamus - it is acending, and therefore sensory.)


  REFLEXES
            The second major function of the spinal cord is to serve as an integrating center for spinal   reflexes.
          Reflexes are fast, predictable,  automatic, subconscious responses to changes inside or outside the body.  Somatic reflexes involve contraction of skeletal muscle.  We also have autonomic reflexes which involve responses of smooth muscle, cardiac muscle,  and glands. (heart rate, respiration, digestion, urination)
   Reflex Arc and Homeostasis
     A pathway is the route followed by a series of nerve impulses from their origin  in the dendrites  or cell body of a neuron in one part of the body to destination in another.  A reflex arc is the simplest type of pathway.
  Its functional components are:
      1.  receptor - this is the distal end of a sensory neuron or an associated sensory structure
          which serves as a receptor. It responds to a specific stimulus by producing  a graded
         potential called a generator ( or receptor) potential.  If this generator potential reaches
          the threshold of the neuron, it will trigger one or more impulses.
      2. sensory neuron - impulses travel from the receptor to the axon terminals of the
          sensory neuron, which are located in the grey matter of the spinal cord or brain stem.
      3.  integrating center - This may be as simple as the synapse of the sensory nerve with
            a motor nerve in the gray matter of the spinal cord. This is called a monosynaptic reflex
            arc.  Usually, the integrating center consists of one or more association neurons, which
            may relay the impulse to other association neurons as well as to the motor neuron. A
            polysnaptic reflex arc involves more than two types of neurons and more than one
            CNS synapse.
      4. motor neuron - The integrating center sends impulses out to effector organs through
               the motor neuron.
      5. effector -this is the part of the body which responds to the motor nerve, either muscle
         or gland. The action of the effector is called the reflex.  It is a somatic reflex if skeletal
         muscle is innervated, and is an autonomic reflex if smooth, cardiac muscle or a gland is
           innervated.

    We can use reflexes to test certain nerve pathways for damage. Tapping patellar ligament activates stretch receptors in the quadriceps femoris which synapse in the lumbar spinal cord with motor neurons which cause contraction of the quadriceps and extention of the leg at the knee.  Absence of this reflex indicates damage to the sensory or motor neurons in the lumbar region of the spinal cord.
     Somatic reflexes can be fairly easily tested by tapping or stroking the body surface, but most autonomic reflexes are difficult to examine. The exception is the pupillary light reflex. Synapses in the reflex pathway include synapses in the midbrain and brainstem, and absence of the light reflex can indicate damage to these areas.

Physiology of the Knee-jerk ( patellar tendon or Stretch reflex):
     Stretch reflex is a monosynaptic reflex. One sensory neuron, one motor neuron and one CNS synapse. Can be tested at the elbow, wrist, knee and ankle joints.
     1. Stretch of tendon and muscle  stimulate receptors called muscle spindles which
        monitor changes in the length of the muscle.
     2. The stretched muscle spindle generates one or more impulses along a somatic sensory
         nerve which enters the spinal cord through  the posterior root of the spinal nerve.
      3. In the spinal cord, the sensory neuron synapses with and activates a motor neuron in
          the anterior grey horn.
     4. If the excitation is great enough, the motor neuron fires one or more times, and these
        impulses travel along the axon, out through the anterior root of the spinal nerve. This motor
        nerve then synapses with muscle fibers of the same muscle that has the activated muscle
        spindle.
      5.  ACh is release at NMJ, and causes one or more action potentials in the muscle, and the
        muscle contracts. The contraction of the muscle relieves the stretching of that muscle.
      6. In this reflex arc, the sensory nerves enter the spinal cord on the same side that the
        motor neurons leave it. This is called an ipsilateral reflex arc. All monosynaptic reflexes
        are ipsilateral.
      7. Even though this reflex is monosynaptic,  a polysnaptic reflex arc to the antagonistic
        muscle occurs at the same time. An axon collateral from the muscle spindle sensory
        neuron  also synapses with an inhibitory association neuron in the integrating center.
        The association neuron synapses with and inhibits  a motor neuron tha normally excites
        the  antagonistic muscles.
     This type of circuit which simutaneously causes contraction of one muscle and inhibition of the   antagonistic muscles is called  reciprocal innervation. This coordinates body movements and   prevents conflicts.
      8. Axon collaterals from the muscle spindle sensory neuron also synapse with other cells
        which relay impulses to the brain, so the brain can use the information about the state
        of this muscle to coordinate other muscle movements and posture. The nerve impulses
        to the brain also go to centers which allow you to perceive that the reflex has occurred.
 

Physiology of the Flexor (Withdrawal) Reflex and Crossed Extensor Reflex:
         The flexor or withdrawal reflex is a polysynaptic reflex.
     1. Stepping on a tack stimulate the dendrites (free nerve endings) of a pain sensing neuron.
     2. The sensory neuron fires, and impulses enter the spinal cord.
     3. In the spinal cord the sensory neuron activates association neurons that extend to
        several spinal cord segments.
     4. The association neurons activate motor neurons in several spinal cord segments, which
        in turn generate impulses which leave the spinal cord through the  anterior roots and go
        to the muscles.
     5. ACh released at NMJ's causes the flexor muscles  in the thigh to contract, withdrawing
        the leg  from the source of the pain.
     6. This reflex is also ipsilateral. In this reflex about 6 different muscles are involved, and
        several  motor neurons must simulateously convey impulses to several muscles. The
        nerve impulses from one sensory neuron accend and descend in the spinal cord and
        activate association neurons in different segments of the spinal cord,  this type of reflex
        is called an intersegmental reflex arc.(also exhibits reciprocal innervation)

When you step on the tack and withdraw your foot, you may also start to lose your balance. The pain impulses from the tack also initiate a balance-maintaining crossed extensor reflex.
     1. Stimulation of pain sensitive neuron in right foot.
     2. Sensory neuron sends impulses into the spinal cord (dorsal root)
     3. In the spinal cord, the sensory neuron activates several association neurons that
        synapse with motor neurons  on the left side of the spinal cord in several spinal cord segments.
        (Signals cross   to the opposite side thru assoc. neurons )
     4. The association neurons excite the motor neurons in the several segments, and the motor
        neurons send impulses to their axon terminals.
     5.  ACh release causes  the extensor muscles in the thigh of the left leg to contract, extending
        the left leg. Now the left leg is ready to support the weight of the body as you lift your
        right foot. A   similar reflex occurs in either upper limb.
     6.  (also exhibits reciprocal innervation)
     7. The crossed-extensor reflex involves a contralateral reflex arc.

            Among clinically important somatic reflexes are the patellar reflex, the Achilles reflex, the Babinski sign,  and the abdominal reflex. Read over, but don't memorize.
 

Brain Development:
 The development of the nervous system begins in the third week with a thickening of the ectoderm, called the neural plate. - forms a groove with raised edges which grow and meet to form a neural tube.
 The brain begins to form in the embryo as the neural tube, which becomes the central nervous system.

Two neural tube defects are associated with low levels of a B vitamin called folic acid.  These are - spina bifida ( failure of  laminae of spine to unite) and anencephaly (absence of the skull and cerebral hemispheres) The incidence of both disorders is greatly decreased when women who may become pregnant take folic acid supplements.

 BRAIN
         A.  Principal Parts
            The four principal parts of the brain are:
                 1. The brain stem is continuous with the spinal cord, and consists of:
                          a. the medulla oblongata
                          b.  pons
                          c. midbrain
                 2. The cerebellum - "little brain"
                 3. The diencephalon - superior to the brain stem
                          a. thalamus
                          b. hypothalamus
                          (c. Pineal gland)
                 4. Cerebrum- largest section
                          a. right and left halves - cerebral hemispheres

      Cerebrum  The cerebrum is the largest part of the brain.  Its cortex, or outer layer, is gray and contains billions and billions of neurons.It contains the cell bodies of about 75% of all the neurons in the nervous system.  During embryonic development, the cortex develops faster than the white matter tracts, and folds in upon itself, forming gyri (convolutions), sulci, and fissures. The cerebrum is made of two large portions called the cerebral hemispheres which are mirror images of each other,  structurally and functionally.
    Between the two hemispheres is the longitudinal fissure In the longitudinal fissure is the falx cerebri,  an extension of the dura mater that encloses the superior and inferior sagittal sinuses.
      The two hemispheres are connected by a thick band of white matter called the corpus callosum.
            The cerebral lobes are named after the bones that cover them :the frontal, parietal, temporal, and occipital lobes.   Two major sulci or fissures divide the lobes: the lateral fissure  separates the frontal and parietals lobe from the temporal lobe. Studies of Einstein's brain show that his inferior  parietal lobe was 15% larger than normal, and the lateral fissure was largely absent.  This area processes mathematical thought, three dimensional visualization, spatial relationships and other mental processes. They feel that this was a anomaly that was present from birth, and may have allowed more neurons to establish more connections and work together more easily.
     The central sulcus  separates the frontal and parietal lobes.  The gyri on either side of the central sulcus are important. The precentral gyrus contains the primary motor area of the cerebral cortex. The postcentral gyrus contains the somatosensory area.(more later)
     There is no distinct boundary between the occipital and parietal lobes.
    The transverse fissure separates the cerebrum and the cerebellum. Extension of dura materis found here called the tentorium cerebelli.
     The insula lies deep to the parietal, frontal and temporal lobes., and is separated from them by the circular sulcus.

     The white matter is under the cortex and makes up the bulk of the cerebrum. It consists of mostly myelinated axons running in three principal directions:
      1. between gyri in the same hemisphere.
      2. from gyri in one hemisphere to the corresponding gyri in the opposite hemisphere of the
        cerebrum. The primary commissural fibers  are the corpus callosum (Also  the anterior and
        posterior commissures.)
      3. Projection fibers form descending and ascending tracts that transmit impulses from the
        cerebrum and other parts of the brain to the spinal cord or from the spinal cord to the  brain.
Functions of the cerebrum:
    Functional Areas of the Cerebral Cortex
     Specific types of signals are processed in certain regions of the cerebrum. The sensory areas of the cerebral cortex are concerned with the interpretation of sensory impulses.  The motor areas are the regions that govern muscular movement.  The association areas are concerned with more complex integrative functions, such as memory, emotions, reasoning, will, judgement, personality traits, and intelligence.
     The primary motor areas of the cerebral cortex are found in the frontal lobes, just in front of the central  sulcus in the precentral gyrus and anterior wall of the sulcus. Each region controls voluntary contractions of specific muscles or groups of muscles on the opposite side of the body.  Areas are proportional to the number of motor units in a particular muscle of the body.
      In addition to the primary motor areas, certain other regions of the frontal lobe control motor functions. Broca's area (ant. to prim. Motor cortex and sup. to the lat. fissure.) coordinates muscle movements which make speech possible.
     Frontal eye field - controls voluntary movements of the eyes and eyelids
     Other areas - coordinate movements of head that direct the eyes
               Controls hands and fingers and makes writing possible.
    Sensory input flows mainly to the posterior half of the cerebral hemispheres, posterior to the central sulcus. The primary sensory areas  have the most direct connection with the peripheral sensory receptors.  Secondary sensory areas and sensory association areas often are adjacent to the primary areas and receive input from the primary areas and from other regions of the brain.
     Primary somatosensory area - postcentral gyrus. This area receives impulses for touch,  proprioception (where body parts are) pain,  and temperature. Certain spots within this region  receive input from specific parts of the body. (picture) The area given to each part depends not on the size of the body part, but by the number of sensory receptors in that body part. The major function of this area is to localize the points on the body where these sensations originate.
     The posterior parts of the occipital lobe are responsible for vision; the posterior dorsal parts of the temporal lobe contain the centers for hearing. Smell- deep with in the cerebrum (uncus). Taste- near the base of the central sulcus, along the lateral sulcus.
     Most sensory fibers cross over in the brain stem and spinal cord, however sensory areas concerned with sight and hearing receive information from both sides (why is this important?)
     Association Areas- regions of the cerebral cortex that are not primary motor or sensory in function, which interconnect with other areas of the brain. The secondary and association areas interpret the sensory input into meaningful patterns of recognition and awareness. Person with damage to primary visual cortex has a scotoma. Damage to visual association area may see normally, but be unable to interpret or recognize what he sees (a friend's face).
      These areas also help provide memory, reasoning, verbalization, judgement, and emotions.(Don't memorize all of the assoc. areas.)
Hemisphere dominance:
     In over 90 % of the human population, the left hemisphere is dominant for the spoken and written language.(Controls the right side of the body). This side of the brain also controls numerical and scientific skills, and  reasoning . The right side of the brain controls the muscles on the left side of the body, musical and artistic awareness, space and pattern perception, insight, imagination, and generates mental images to compare spatial relationships.
 In left handed people, the right side of the brain would be dominant, and in some individuals they are equally dominant.

Learning and memory
     Learning is the ability to acquire new knowledge or skills through instruction or experience.
      Memory is the ability to store and recall thoughts and involves persistent changes in the brain, a capability called plasticity.  This ability to change involves changes in individual neurons, which may make new proteins or form new dendritic connections ( new research indicates that dendrites can and do rapidly form new synapses and break old ones). There may also be changes in the strengths of synaptic connections. Portions of the brain associated with memory are the frontal, temporal, parietal and occipital lobes, the limbic system, and the diencephalon.
     Memory is generally classified into two kinds: short-term and long-term memory.
    Short-term memory is the temporary ability to recall a few pieces of information. If the information has no particular significance, or if it is not repeated and reinforced, it is forgotten. Short-term memory  is related to electrical events, and lasts as long as the stimulation continues. It may be caused by reverberating circuits.
      Long-term memory lasts from days to years. The reinforcement due to the frequent retrieval of a piece of information is called memory consolidation. Long term memory is related to anatomical and biochemical changes at synapses.
     When neurons are kept active they show an increase in the number and size of  synaptic end  bulbs, and an increase in the branching of the dendrites. There may be loss of neurons or their abilities if they go unused.

     The basal ganglia are paired masses of gray matter (nuclei) in the cerebral hemispheres.
     These are the caudate nucleus ,putamen and globus pallidus.
     The basal ganglia are interconnected by many nerve fibers an d receive input from and provide output to the cerebral cortex, thalamus and hypothalamus ( to spinal cord). They produce most of the inhibitory neurotransmitter dopamine. They help to control automatic muscular movements and muscle tone. These degenerate in Parkinson's disease.

     The diencephalon is located between the cerebral hemispheres and above the brain stem. It surrounds the third ventricle, and is mostly gray matter. Here we find the dumb-bell shaped thalamus, and the hypothalamus which is made up of many small nuclei. Here we also find the optic tracts and optic chiasm, the infundibulum, the posterior pituitary, mammillary bodies and the pineal gland. The mamillary bodies serve as relay stations for the sense of smell. Infundibulum attaches pituitary gland and makes regulating hormones for the anterior pituitary.
     The pineal gland, named for its resemblance to a pine cone) lies on the midline in the third ventricle. This structure is usually easy to find on the brain when you are dissecting, just remember it lies dorsally, and don't get it confused with the pituitary gland. This gland is part of the endocrine system, and is sometimes referred to as the "third eye" because it responds to light.  It secretes the hormone melatonin, and more of this hormone is released during darkness. It is thought to promote sleepiness. The pineal gland also seems to contribute to the body‘s biological clock. In some animals it controls seasonal cycles, such as breeding.
     The thalamus makes up about 80 % of the diencephalon. It is the principle relay station for sensory impulses to the cerebral cortex. These sensations include hearing, vision, taste and the somatic sensations of touch, pressure, vibration, heat, cold and pain. It also plays a role in certain emotions and memory. As shown by the case of Karen Ann Quilan, the thalamus plays an important role in awareness, as well as the acquisition of knowledge, or congition.
     The hypothalamus is one of the major regulators of homeostasis. Nerve fibers connect the hypothalamus with the cerebral cortex, thalamus and parts of the brain stem.
Its chief functions are:
     1. Control of the Autonomic nervous system. (Heart rate and arterial blood pressure, and control of movements and glandular secretions of the stomach and intestines) It controls and integrates the activities of the ANS: heart rate, contraction of smooth muscle in the G.I tract and elsewhere, and the secretion of glands.
     2. Control of the pituitary gland. The pituitary gland is often referred to as the"master gland" because it controls so many functions of the body : it regulates hormones which affect the gonads, the basal metabolic rate, the adrenal secretions,  and lactation. But all of the hormones released by the anterior pituitary are regulated by releasing and inhibiting factors produced by the hypothalamus. These factors are carried to the anterior pituitary by their own blood vessel network. The posterior pituitary is actually an extension of the hypothalamus, and the hormones released by the post. pit. are manufactured in the hypothalamus. Oxytocin and antidiuretic hormone ( water and electrolyte balance)
     3. Regulation of eating and drinking. The feeding (hunger) center sends you feelings of hunger. When you have eaten enough, the satiety center sends impulses that inhibit the hunger center.  The thirst center responds to increased osmotic pressure in the blood but producing the sensation of thirst. Increased water  intake reduces the osmolarity of the blood.
     4. Control of body temperature - the hypothalamus measures the temperature of the blood flowing through it, and if it is too high or too low, it instructs the ANS to bring about changes to fix it. (What would these changes be?)
     5. Regulation of diurnal (daily) rhythms and states of consciousness.
 It other words, it establishes patterns of sleep that occur on a daily basis.
     6. Regulation of emotional and behavioral  patterns. Together with the limbic system, the hypothalamus  regulates feelings of rage, aggression, pain, and pleasure, and the behavioral patterns of sexual arousal.

            The limbic system is found in the cerebral hemispheres and diencephalon. Some of the visible   parts are the mammary bodies of the diencephalon and the olfactory bulbs.
 The limbic system governs emotional aspects of behavior and also in memory. Because this   system controls emotions and memory, events which produce a strong emotional response are   remembered better that those that do not. (worm in your dinner?)  Damage to the limbic system can result in the loss of short-term memory and the ability to learn.  Stimulation of different parts of the limbic system in animals can produce behaviors associated with rage, pain, pleasure, fear, anger, affection and sexual feelings. The limbic system is sometimes called the "emotional brain."

         Brain Stem
 The brain stem connects the spinal cord to the diencephalon, and is made up by the medulla oblongata, pons, and midbrain.
     1.The midbrain or mesencephalon, runs from the pons to the diencephalon, and contains the  cerebral aqueduct that connects the third and fourth ventricles. Area for passage of fibers between areas of the brain, and for coordination of motor activity.
          a. The cerebral peduncles, located on the anterior surface of the midbrain, contain tracts
            which  carry some of the motor fibers (corticospinal tracts) from the cerebral cortex to
            the pons, medulla, and spinal cord. These tracts also have sensory fibers going from the
            medulla to the thalamus.
              b.  The posterior portion of the midbrain is called the tectum (roof), and has four
              round elevations called the corpora quadrigemina. The superior colliculi are 
            reflex centers  for movements of the eyes, head and neck  in response to visual and
            other stimuli.  The inferior colliculi are reflex centers for movements of the head
             and trunk in response to sounds.
          c. The right and left red nuclei are so named because of their rich blood supply and an
              iron containing pigment in their cell bodies.  Fibers from the cerebrum and the
            cerebellum synapse here, and this area functions to help coordinate muscle movements
            and provides reflexes that maintain posture.

 The pons ("bridge") is superior to the medulla.  It connects the spinal cord with the brain and links parts of the brain with one another by way of tracts.
  There are several nuclei in the pons. Several of these nuclei are the origins of cranial nerves.
  Other nuclei along with the medullary rhythmicity area, control breathing.


The medulla oblongata
     1.  The medulla is continuous with the upper part of the spinal cord. It begins at the foramen  magnum, and extends upward to the pons. In the medulla are all the ascending (sensory) and descending (motor) tracts that connect the brain to the spinal cord.
          a.  Most sensory and motor tracts cross over from one side of the body to the other
            as they pass through the medulla. On the anterior side of the medullar are two
            bulges called  the pyramids. These contain the largest motor tracts which travel from
            the cerebrum to the spinal cord. Just above where the spinal cord joins the medulla, 
            these tracts cross over, right to left and left to right. This is called the decussation of 
            pyramids.
(Which side of the brain controls the left side of the body?)
 It also contains nuclei that regulate some vital body functions:
          The cardiac center regulates the heart rate and force of contraction of the heart.
          The vasomotor center sends impulses to the smooth muscle in the walls of the blood vessels, causing them to constrict and raise blood pressure. A decrease in the activity of these cells allows the smooth muscle to relax and the vessels dilate.
          The respiratory center (The medullary rhythmicity area) which controls the rate, rhythm and depth of breathing.
          Other centers coordinate swallowing, vomiting and  some modified respiratory    movements, such as coughing,  sneezing, vomiting and hiccuping
 
The reticular formation  is a net-like formation of white and gray matter  that extends through a large portion of the brain stem, running from the spinal cord up into the diencephalon. It has both sensory and motor functions. It receives input from higher brain centers that control skeletal muscle, and contributes to regulating muscle tone. It also alerts the cerebral cortex to incoming sensory signals.  This part of the reticular formation is called the reticular activating system (RAS) and is responsible for maintaining consciousness and awakening from sleep. Incoming signals from the ears, eyes and skin stimulate the RAS.( RAS wakes up the cerebral cortex)

 Cerebellum
            The cerebellum is the second largest part of the brain, and is located interiorly and posteriorly.
 It is separated from the cerebrum by the transverse fissure and by an extension of the dura mater called the tentorium cerebelli. It consists of two hemispheres and a central, constricted area called the vermis ("worm").  The falx cerebelli separates the two hemispheres, and each hemisphere is divided into lobes by deep fissures.
  a. The cerebellar cortex is made of gray matter in as series of ridges called folia.
  b. The white matter tracts in the cerebellum  are called the arbor vitae ("tree of life")
  c. Cerebellar peduncles connect the cerebellum to other parts of the brain..
  d. The cerebellum compares intended movement programmed by the cerebrum, with    what is actually happening.  It receives input from proprioceptors (senses where body parts are), receptors for equilibrium, and visual receptors through the inferior peduncles. The middle peduncle sends information from the cerebral cortex about the desired position of theses body parts.  The cerebellum then compares the two sources.  If the intent of the cerebrum is not being met, the cerebellum detects the variations, and from  the dentate nucleus and out through the superior peduncles  sends correcting impulses to the midbrain.  The corrections are incorporated into the motor impulses traveling through the brain stem.  This produces smooth, coordinated muscle movements.  The cerebellum also coordinates posture and balance.
    Most of what we know about what the cerebellum does comes from people who have damage to that region. Patients with cerebellar damage walk with the feet wide apart, in part due to loss of equilibrium., they show inaccurate movements of voluntary muscles and "intention tremors."  They also show a loss of muscle tone.

Peripheral nerves:
     Consists of nerves that branch from the central nervous system, and connect the CNS to other parts of the body. The PNS includes the 31 pairs of spinal nerves and the cranial nerves that arise from the brain. We can have nerves that are sensory nerves and  motor nerves, but most are mixed nerves.
    Composition and Coverings
 Each cranial and spinal nerve is surrounded by connective tissue coverings.
     Individual neurons, myelinated or unmyelinated, are wrapped in endoneurium.
     Groups of axons are arranged in bundles called fascicles, and each fascicle is wrapped in   perineurium.
     The fascicle which form the nerve are all wrapped together in epineurium.
     The dura mater of the spinal meniges fuses with the epineurium as the nerve passes through the   intervertebral foramen.
 Note the many blood vessels in the coverings.

 Dermatomes
     Each spinal nerve contains both sensory and motor neurons, and serves a specific, constant area of the body. Most of the skin of the face and scalp is served by cranial nerve V,  the trigeminal nerve.
The area of skin that provides sensory input to one pair of spinal nerves or to cranial nerve V is called a dermatome. (Shingles - herpes zoster - dorsal root ganglion) The nerve supply in neighboring dermatomes over laps somewhat, in some cases considerably. This overlap provides a backup system in case one of the nerves supplying a dermatome is damaged.
     Skeletal muscles receive their innervation from the motor neurons in a single spinal segment called a myotome. Myotomes roughly underlie the corresponding dermatomes.
 Dermatomes and myotomes help physcians locate areas of nerve damage. Because of the overlap, if a doctor wishes to achieve complete anaesthesia of a region, at least three adjacent spinal nerves must be cut or blocked by drugs.

Autonomic Nervous system:
     Is divided into sympathetic and parasympathetic nervous systems.
     All fibers are efferent or motor fibers and consist of two nerves: a preganglionic fiber, whose cell body is in the CNS and a postganglionic fiber, whose cell body is in an autonomic ganglion outside the CNS. In the sympathetic NS the preganglionic fibers arise from the lateral horns. Also associated with the symp. N.S. is the medulla of the adrenal glands, which releases epinephrine and norepinephrine.
 
 


 

The Autonomic vs. The Somatic Nervous System


Somatic Nervous System                                     The Autonomic Nervous System

Both Sensory and motor neurons                                   Motor neurons

Input from the special senses, proprioreceptors,             General visceral sensory neurons
and general somatic receptors

Operates with conscious control                                     Usually without conscious control

Effect of motor neurons is excitation                                 Effects can be excitatory or inhibitory

Single motor neuron                                                         Two motor neurons in series

Releases Acetylcholine at synapses                                     Releases acetylcholine or
                                                                                            norepinephrine at synapses
 
 

Sympathetic vs Parasympathetic nervous systems:

Sympathetic N.S.                                                         Parasympathetic N. S.
 

Originates in the thoracic and lumbar segements                 Originates in the brain stem and sacral
of the spinal cord : Thoracolumbar system                         segments: Craniosacral system

Synapses near the spinal cord                                             Synapses near visceral effector

Preganglionic neuron synapses with neurons to                     Preganglionic neuron synapses with
several organs                                                                     neurons to a single organ

Effect is wide spread                                                             Effect is local

"Fight-or-Flight" - "E's":                                                         "Feed and Breed" - "SLUD"
emergency, excitement, exercise, embarrassment                     salivation, lacrimation, urination,
                                                                                              defecation

Postganglionic fibers release norepinephrine                             Postganglionic fibers release ACh

Receptors are alpha and beta                                                   Receptors are nicotinic and
                                                                                                muscarinic