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The Central Nervous System The Story of Phineas Gage On 13th. September 1848, an accidental explosion of a charge he had set blew his tamping iron through his head. The tamping iron was 3 feet 7 inches long and weighed 13 1/2 pounds. It was 1 1/4 inches in diameter at one end (not circumference as in the newspaper report) and tapered over a distance of about 1-foot to a diameter of 1/4 inch at the other. The tamping iron went in point first under his left cheek bone and completely out through the top of his head, landing about 25 to 30 yards behind him. Phineas was knocked over but may not have lost consciousness even though most of the front part of the left side of his brain was destroyed. Dr. John Martyn Harlow, the young physician of Cavendish, treated him with such success that he returned home to Lebanon, New Hampshire 10 weeks later. I. Formation of the Central Nervous System(CNS) A. The CNS develops from the neural tube (extending down the dorsal median plane of an embryo). 1. The neural tube eventually becomes the brain and spinal cord. B. By the fourth week the tube begins to expand and form the brain. Embryonic development C. The opening of the neural tube becomes the ventricles 1. There are four ventricles which act as chambers in the brain. (2 lateral ventricles, third, and fourth) 2. The ventricles are filled with cerebrospinal Protection: the CSF protects the brain from damage by "buffering" the brain. into fluid. other words, the CSF acts to cushion a blow to the head and lessen the impact. Buoyancy: because the brain is immersed in fluid, the net weight of the brain is reduced from about 1,400 gm to about 50 gm. Therefore, pressure at the base of the brain is reduced. Excretion of waste products: the one-way flow from the CSF to the blood takes potentially harmful metabolites, drugs and other substances away from the brain. Endocrine medium for the brain: the CSF serves to transport hormones to other areas of the brain. Hormones released into the CSF can be carried to remote sites of the brain where they may act. II. Functional Anatomy of the Brain A. There are four main regions of the brain which include: Cerebral hemispheres (Cerebrum), diencephalon, brain stem and the cerebellum B. The Cerebral Hemispheres: The paired left and right superior parts of the brain and make up more than half the mass of the entire brain! This part of the brain involves conscious behaviors including; speech, memory, emotional responses, and voluntary movement, personality. 1. The surface is made of ridges (gyri) and grooves (sulci) 2. Fissures (deep grooves) divide the cerebrum into lobes 3. The outermost area of the cerebrum is made of gray matter (covering an underlying layer of white matter), and called the cerebral cortex. The remaining cerebral hemisphere tissue- the deeper white matter- is composed of bundles of nerve fibers carrying the impulses to or from the cortex. a. One large fiber tract (bundle of nerves) called the corpus callosum, connects the right and left cerebral hemispheres (anterior commisure is the bundle of nerve fibers connecting the two hemispheres) b. The basal nuclei (ganglia) is an internal island of gray matter deep within the cerebral cortex. It helps regulate the voluntary motor activities, learning, and action selection (which behavior to execute at a given time). 4. Surface lobes of the cerebrum include: a. Frontal lobe 1) Primary motor area- allows us to move our skeletal muscles under conscious control 2) Broca’s area- motor speech area that directs muscles involved in speech, usually only present in left hemisphere. 3)High level intellectual reasoning are also believed to be in the anterior portion of the frontal lobe b. Parietal lobe 1. Somatic sensory areas- allows you to recognize pain, coldness and touch - The left side of the sensory cortex interprets impulses from the right side of the body and vice versa. c. Occipital lobe 1)Visual interpretation d. Temporal lobe1) Auditory interpretation, speech, memory Specialized areas of the Cerebrum: Temporal lobe C. The Diencephalon or Interbrain 1. Sits on top of the brain stem and is enclosed by the cerebral hemispheres. 2. Composed of three main parts: Thalamus, epithalamus, and hypothalamus a. Thalamus- A relay station for sensory impulses, allows us to recognize a sensation as pleasant or unpleasant, the impulse is eventually sent to the sensory cortex for localization and interpretation of the sensation b. Hypothalamus- An important part of the autonomic system. Its job is regulating body temperature, water balance, appetite, and metabolism. Also includes the Limbic system- our emotional brain. The Limbic system is involved in our appetite, thirst, pain, sex, and pleasure centers. The pituitary gland is attached to the hypothalamus. c. Epithalamus- Forms the roof of the third ventricle. Important parts include the pineal body(produces melatoninto regulate sleep/wake cycles) and choroids plexus (forms cerebrospinal fluid). D. The Brain Stem 1. Attaches to the spinal cord. 2. There are three parts of the brain stem: midbrain, pons and medulla oblongata. a. Midbrain- is mostly composed of nerve fiber tracts. It also contains the cerebral aqueduct which is a tiny canal that connects the third and fourth ventricle. The corpora quadrigemina is dorsally located with rounded protrusions that are reflex centers for vision and hearing. b. Pons- The bulging part of the brain stem that is composed mostly of nerve fiber tracts. It houses important nuclei that control our breathing. ***also important because it contains nerve fibers connecting the cerebellum to the rest of the brain c. Medulla Oblongata- The lowest part of the brain stem that merges into the spinal cord. It controls important centers that help control blood pressure, heart rate, breathing, swallowing, and vomiting. E. Reticular Formation 1. Gray matter that runs along the brain stem. 2. Involved in motor control of visceral organs. It also contains a special group of neurons called the reticular activating system that plays a role in consciousness and awake/sleep cycles. (RAS alerts a person when a friend speaks and enables that person to ignore other sounds and focus on the one sound. Helps prevent sensory overload!) F. The cerebellum 1. Projects dorsally from under the occipital lobe of the cerebrum. It also contains two hemispheres with convoluted surfaces. 2. Its job is providing involuntary control of body movements (balance), and posture. II. Protection of the Central Nervous System A. The scalp and skin provide the first layer of protections. B. The skull and vertebral column provide the second layer. C. A layer of connective tissue called the meninges provides the final protection for the CNS. There are three meningeal layers. 1. Dura mater- the outermost layer that is tough and hard. 2. Arachnoid mater- the middle web-like layer 3. Pia Mater- internal layer that clings to the surface of the brain D. Cerebrospinal fluid- the fluid that forms a watery cushion to protect the brain. 1. Its composition is similar to blood plasma 2. Formed by the choroids plexus 3. Circulated in arachnoid space, ventricles, and central canal of the spinal cord E. Blood Brain Barrier (The BBB)- The barrier that keeps neurons separated from blood borne substances. The brain is dependent upon the constant internal environment more than any other organ in your body. 1. The barrier is created by the least permeable capillaries in the body. 2. The astrocytes contribute to creating this barrier. 3. Exclusion of many substances helps maintain the delicate balance required by the brain. However, the barrier fails to prevent the following substances from entering: Fats and fat soluble molecules, Respiratory gases, Alcohol, Nicotine, Anesthesia III. Traumatic Brain Injuries A. Concussion- a slight brain injury, victim may be dizzy, or lose consciousness briefly but there is no permanent brain damage. B. Contusion- nervous tissue destruction occurs and the tissue does not regenerate. A severe brain stem contusion would cause someone to go into a coma. C. Cerebral Edema (swelling) or hemorrhage (bleeding) - Swelling or bleeding of the brain due to an inflammatory response. This places pressure on brain tissue- this is a serious situation. D. Cerebrovascular Accident (CVA) - commonly called a stroke. It is the result of a ruptured blood vessel supplying a region of the brain. The brain tissue that is supplied with oxygen from the vessel will die. The loss of function depends on the area of the brain affected and the severity of the stroke. E. Alzheimer’s disease- A progressive degenerative disease affecting the brain. Victims experience memory loss, and confusion. (caused by plaques –protein deposits- in between the neurons) F. Parkinson’s disease- A basal nuclei problem resulting from the degeneration of dopamine releasing neurons. Patients experience tremors, a shuffling gait, trouble getting their muscle going, and head nodding. IV. Spinal Nerves A. There is a pair of spinal nerves at the level of each vertebra for a total of 31 pairs B. Spinal nerves are formed by the combination of the ventral and dorsal roots of the spinal cord C. Spinal nerves are named for the region from which they arise D. Complex networks of nerves called plexuses serve the motor and sensory needs of the limbs. 1. Cervical plexus- origin of nerves- C1- C5, control mainly the muscles of the shoulder and neck. 2. Brachial plexus- C5 – C8 and T1, control mainly the arm 3. Lumbar plexus- L1 – L4, controls the lower abdomen, buttocks, leg and thigh 4. Sacral plexus- L4 –L5 and S1 – S4, posterior surface of thigh, and leg, lower leg, foot, and gluteus medius. This plexus includes the sciatic nerve- largest in the body. V. Cranial Nerves A. Twelve pairs of cranial nerves extend from the brain to serve the head and neck region. The exception is the vagus nerve which extends into the thorax and abdomen. B. Most cranial nerves are mixed nerves; however three pairs are purely sensory. The optic (vision), olfactory (smell), and vestibulocochlear (balance and hearing). There is a chart on pg. 231- 232 that identify each cranial nerve by number, origin, function and how to test the nerve. Sheep brain dissection website • http://bpweb.baypath.edu/biology/sheep%20brain/ brain-sheep.html