THE BRAIN AND CRANIAL NERVES

Transcription

1 THE BRAIN AND CRANIAL NERVES The Brain - made up of a trillion neurons - weighs about 3 lbs - has four principle parts 1. Brain stem - medulla oblongata, pons, midbrain (mesencephalon) 2. Diencephalon - thalamus and hypothalamus 3. Cerebrum - largest part of the brain 4. Cerebellum Meninges (see the chapter on the Spinal Cord for more information) 1. Dura Mater - consists of two layers - endosteal layer - thicker part - adheres to cranial bones - meningeal layer - thinner part - has a mesothelial layer 2. Arachnoid Mater 3. Pia Mater

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3 Cerebrospinal Fluid (CSF) Production and distribution of CSF (see diagram above) 1. Produced by the choroid plexus of the lateral ventricles (2) 2. Passes through the interventricular foramen (foramen of Monroe) into the third ventricle. 3. The choroid plexus of the third ventricle adds CSF 4. The CSF then passes through the cerebral aqueduct (aqueduct of Sylvius) into the fourth ventricle. 5. The choroid plexus of the fourth ventricle adds CSF. 6. The CSF then passes through the lateral apertures (foramen of Luschka) and medial aperture (foramen of Magendie) into the subarachnoid space around the brain and spinal cord; and into the central canal of the spinal cord. 7. Reabsorbed into the veins through the arachnoid villi. Composition of CSF - about 3 to 5 oz total of CSF - clear colorless liquid - consistency of water - contains protein, glucose, urea, and salts, and lymphocytes - serves as a shock absorber - delivers nutritive substances - removes wastes and toxic substances Blood Supply to the Brain - the brain needs lots of oxygen and nutrients (glucose) - the brain is only 2% of body weight but uses 20% of oxygen - blood flow is effected by the concentrations of carbon dioxide and oxygen in the blood - an increase in CO 2 leads to an increase in H ions 122

4 CO 2 + H 2 O > HCO H + - an increase in H + causes the cerebral arteries to vasodilate and the blood flow rate increases in the brain to ensure that an adequate supply of oxygen reaches the brain. Blood Brain Barrier - controls passage of substances into the brain - glucose, oxygen and carbon dioxide pass readily - creatine, urea, chloride, insulin, and sucrose pass slowly - proteins and antibodies do not pass at all - astrocytes combine with blood capillaries to make up the barrier Brain Stem Medulla Oblongata - derived from myelencephalon - continuation of the upper portion of the spinal cord - forms inferior part of the brain stem - contains all ascending and descending tracts running between brain and spinal cord - the medulla contains 2 vital reflex centers 1. Cardiovascular Center - which regulates heart rate and force of contraction, and the diameter of blood vessels 2. Respiratory Center - regulates the basic rhythm of breathing The Pons 123

5 - means bridge - connects the medulla and spinal cord with the cerebrum - contains middle cerebellar peduncles which connect with the cerebellum via transverse fibers - contains the pneumotaxic area and apneustic area that help regulate respiration Midbrain 124

6 - the cerebral aqueduct runs through the middle - contains the cerebral peduncles - motor and sensory fiber tracts that connect the brain stem to the cerebrum - the tectum or roof of the midbrain contains the Corpora Quadrigemina which in turn is composed of: Superior Colliculi - reflex centers for movements of the eyeballs and head in response to visual and other stimuli Inferior Colliculi - reflex centers for movement of the head and trunk in response to auditory stimuli The Reticular Formation (see figure in reference to pons) 125

7 - runs through medulla, pons and midbrain, spinal cord, and diencephalon - functions include: - aids in regulating muscle tone - tells the cerebral cortex that sensory information is incoming from the spinal cord - reticular activating system maintains consciousness, arousal from sleep The Diencephalon (see figure on the first page) - two principle parts 1. Hypothalamus. 2. Thalamus Thalamus (see figure two pages back) - forms the lateral walls of the third ventricle - the thalamus is the interpretive center for sensory impulses like: - pain. temperature light touch pressure The Hypothalamus - is a collection of nuclei found in four major regions - several nuclei that regulate autonomic activities - The hypothalamus forms the floor and lower walls of the third ventricle - receives information from the external environment via peripheral sense organs conveying information related to: - sound taste smell somatic receptors visceral receptors - monitors - water concentration - hormone concentration - temperature of blood - release of hormones from the pituitary gland - Functions of the hypothalamus 1. Controls and regulates the ANS. 2. Reception and integration of sensory impulses from the viscera. 3. Intermediate between nervous system and endocrine system. 4. Center for mind over body phenomenon. 5. Rage and aggression. 6. Controls normal body temperature. 7. Has hunger and satiety centers. 8. Thirst center. 9. Waking and sleeping states. 10. Controls biological rhythms 126

8 The figure below illustrates the structure of the Hypothalamus. Notice that the hypothalamus is not a single structure, but instead, consists of many nuclei in the wall of the third ventricle. The Cerebrum 127

9 - the surface of the cerebrum is composed of gray matter and is known as the cerebral cortex - the center of the cerebrum contains white matter - the cortex is highly folded with: - gyri (gyrus) - the fold (or bumps) - sulci - the shallow grooves between the gyri - deep grooves - Longitudinal fissure - separates cerebral hemispheres - Lateral fissure - separates frontal lobe from temporal lobe - Transverse fissure - separates cerebrum from cerebellum - Central sulcus - separates the frontal lobe from the parietal lobe White Matter of the Brain 128

10 - myelinated axons running in three directions 1. Association fibers - run between gyri of the same hemisphere 2. Commissural fibers - run between gyri of one hemisphere to the same gyri on the opposite hemisphere. There are three important commissural fibers: 3. Projection fibers - run between the cerebrum and the spinal cord (and other parts of the brain) Limbic System - functions: - emotional aspects of behavior related to survival - memory - pleasure and pain - rage Functional Areas of the Cerebral Cortex cortex generally divided into: 129

11 - sensory areas - motor areas - association areas Sensory Areas 1. Primary Somesthetic Area (General Sensory) - receives sensory information from the cutaneous, muscular and visceral receptors from the entire body via the thalamus. 2. Somesthetic Association Area - receives input from the thalamus and primary somesthetic area - integrates and interprets sensations 3. Primary Visual Area - receives input from the retina of the eye - interprets shape, color, and movement 4. Visual Association Area - receives input from the primary visual area - relates past to present visual experiences 5. Primary Auditory Area - receives input from cochlea of ears - interprets basic character of sound 6. Auditory Association Area - receives input from primary auditory area - determines if sound is music, speech, or noise - interprets words into language 7. Primary Gustatory Area - receives input from taste buds - interprets sensations related to taste 8. Primary Olfactory Area - receives input from taste buds - interprets sensations related to smell Motor Areas 1. Primary Motor Area - controls specific muscles or groups of muscles. 2. Frontal Eyefield Area - voluntary scanning movements of the eyes 3. Language area - translate thought into speech - also called Broca's area or motor speech area 130

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14 Electroencephalogram (EEG) - millions of action potentials are occurring in the brain at any moment. The electrical activity they generate produce wave forms on a physiograph that we interpret as brain waves, of which there are four types: 1. Alpha Waves - 8 to 13 cycles per second - present in normal individuals when awake with eyes closed - not present when asleep 2. Beta Waves - 14 to 30 cycles per second - present in individuals experiencing sensory input and an active mind. 3. Theta Waves - 4 to 7 cycles per second - present in children and adults in emotional stress 4. Delta Waves - 1 to 5 cycles per second - present during deep sleep Brain Lateralization (split-brain concept) - cerebral hemispheres not exactly alike structurally or functionally Left Hemisphere - is more important for: - right- hand control - spoken and written language - numerical and scientific skills - ability to use and understand sign language - reasoning 133

15 Right Hemisphere - is more important for - left-hand control - musical and artistic awareness - space and pattern perception - insight and imagination - generating mental images of sight, sound, touch, taste, and smell in order to compare relationships Cerebellum - motor area of the brain - controls coordinating subconscious movements of skeletal muscle - receives input from: - proprioceptors in muscles, tendons and joints - receptors for equilibrium in ear - visual receptors in the eye - interprets info from all sources and produces smooth, coordinated movements - predicts the future position of a body part - plays a role in emotional development Cranial Nerve 134

16 - 12 pairs - originate in the brain and exit through the foramina of the skull - some carry only sensory info - sensory nerves - some carry sensory and motor info - mixes nerves 135