Brain and Cranial Nerves

Transcription

1 Brain and Cranial Nerves Brain Consists of the brain stem, diencephalon, cerebrum and cerebellum. Brain Stem Located just superior to the spinal cord, at the foramen magnum. Consists of Medulla oblongata Pons Midbrain

2 Brain Diencephalon Sup and lat to the brain stem. Consists of Thalamus Most sup and lateral to the midline Hypothalamus Inf and medial to the thalamus Cerebrum The largest part of the brain; cognitive functions and memory reside here. Cerebellum Post and inf to the cerebrum; fine motor control and coordination Q30 Coverings Cranial meninges Similar to the spinal meningies Dura mater The cranial version has two layers Outer- periosteal layer adheres to the cranium Inner- meningeal layer sits next to the subdural space and the arachnoid. Between the two layers is a dural sinus which contains venous blood.

3 Coverings Arachnoid Pia mater These layers are similar to their spinal version Cerebrospinal fluid CSF; fills spaces in the brain called ventricles. Makes about 500 ml/day. ~150 ml in ventricles and subarachnoid Ventricles Lateral (2) are located within each cerebral hemisphere Third Med and inf to the lat ventricles Fourth Between the brain stem and cerebellum. Interventricular foramen An opening which connects the lateral ventricles to the third ventricle.

4 Cerebrospinal fluid Cerebral aqueduct Connects the third with the fourth Spinal central canal Passes down the middle of the cord Subarachnoid This spaces surrounds the cord and the brain Choroid plexuses Found within the ventricles, these tufts of capillaries produce CSF, which is very similar to blood plasma without proteins. Cerebrospinal fluid Arachnoid villi Are extensions of the subarachnoid space which project into the Superior saggital sinus This space is found between the two layers of the dura. It is filled with venous blood. CSF passes through the arachnoid villi and it mixes with the venous blood of sinus and is returned to circulation. The sinus eventually drains into jugular veins and then to the heart.

5 Cerebrospinal fluid Function Shock absorber The CSF surrounds the brain and the cord; it creates a watery cushion to absorb the normal shocks of everyday life. Nutrients delivery Glucose, oxygen, some amino acids are carried in CSF and delivered to neural tissues. Removal of wastes Carbon dioxide and waste products of normal metabolism and cell breakdown are removed by CSF.

6 Hydrocephalus Or water on the brain If there is an over production of CSF or if the CSF is not drained properly, then fluid pressure within the ventricles of the brain can increase substantially. The increased pressure will compress neural tissue and this will lead to brain damage and, in some cases, death. Causes of hydrocephalus may include blockage of the the cerebral aqueduct, in which case the CSF accumulates in the lateral and fourth ventricles. A hydrocephalic shunt may have to be implanted to relieve the pressure. Blood supply of the brain The brain requires a constant and steady supply of blood (with oxygen and glucose.) It receives about 15% of the total cardiac output. Interruption Even a short interruption can cause a loss of brain function; a drop in BP can lead to dizziness and blackouts. Unconsciousness If the supply is blocked for longer than 5-10 seconds, unconsciousness will follow Permanent injury Can occur if there is a loss of blood for longer than about 5 mins. This may result in brain injury and perhaps death.

7 Blood supply of the brain Blood brain barrier The BBB is a selective barrier which prevents certain materials from passing into the sensitive cerebral tissues. Selective barrier Slightly permeable to Na +, K +, Cl -. Very permeable to oxygen, carbon dioxide, water, alcohol, and most anesthetics. Not permeable to blood proteins and non-lipid organic molecules Brain stem Medulla oblongata ( or medulla) The most inferior part of the the brain stem; just sup to the cord. Pyramids Descending through the medulla are the pyramids, motor tracts from the cortex of the cerebrum. Decussation As the tracts of both sides pass the junction between the medulla and the cord, they cross over (the decussation) the midline and form the Lateral corticospinal tract These tracts continue down the cord to pass motor signals to skeletal muscles in the periphery.

8 Brain stem Medulla oblongata ( or medulla) Nucleus gracilis and cuneatus A nucleus is a collection of somas within a nerve tract within the CNS. These nuclei relay body sensory information to the thalamus. Reticular formation and reticular activating system (RAS) This group of nuclei extend into the pons. They send signals in to the cerebrum to maintain a state of consciousness. If signals are interrupted for any reason, such as a blow to the chin to direct trauma to the medulla, the person will fall into unconsciousness or a coma. Brain stem Medulla oblongata ( or medulla) Cardiac center Regulates the activity of the heart; such as the heart rate and contraction strength. Medullary rhythmicity center Regulates the rate of respiration Vasomotor center Regulates peripheral vasoconstriction and blood pressure. Later. Origins of cranial nerves VIII-XII Vestibular complex Receives information from the vestibule of the inner ear, which helps maintain balance.

9 Brain stem Pons Later. Origins of cranial nerves V-VIII part of VIII Reticular formation Continues from the medulla. Pneumotaxis and apneustic areas Involved in regulating respiratory activities. The former detects the amount of stretch in the lungs and regulates how much the lungs inflate.

10 Brain stem Midbrain (mesencephalon) The most superior part of the brain stem. Corpora quadrigemina On the posterior side of the midbrain; four bodies Superior colliculi These bodies regulate visual reflexes, such as pupil size and focusing. Inferior colliculi Regulate auditory reflexes. If you are exposed to loud sounds, the inferior colliculi sends signals to the middle ear to reduce the level of sound energy transmitted into the inner ear. Brain stem Midbrain (mesencephalon) Substantia nigra Carries signals from the motor cortex which are used to coordinate muscle activity (along with the cerebellum.) Damage to the substantia can lead to involuntary movement and rigidity. Q32

11 Diencephalon Thalamus The lobes of the thalamus are located sup and lat to the brain stem, deep within each hemisphere. Intermediate relay center It receives all of the sensory fibers (except smell) and sends signals to the sensory part of the cerebral cortex. It may be that the thalamus is the crucial structure for the perception of some sensations, rather than the cortex, which may give fine detail to sensations. Origins of voluntary motor pathways Diencephalon Thalamus Brain (cortical cycles) May have their origins here. Emotional expressions Considered the seat of emotions. Receives sensory input and passes in on to the cerebral cortex. Integrates sensations and emotions. Pineal gland/body Located on the posterior surface of the mid-body of the thalamus. Melatonin A hormone produced by the pineal gland that appears to regulate the circadian rhythm; wake-sleep cycles of the brain. Exposure to bright light (such as sunlight at dawn) suppresses the release of melatonin and resets the wake cycle. Melatonin at night may stimulate sleep cycles.

12 Diencephalon Hypothalamus Hormone production Makes two hormones (antidiuretic hormone and oxytocin) which are transported to and released by the pituitary gland. Controls hormones Makes control factors that regulate the release of hormones from the pituitary. Functions Information processing about external/internal environment Next page. Hypothalamus Control of ANS Controls both divisions of the ANS; sympathetic and parasympathetic. Process visceral sensory information Part of the visceral reflex arc. Processes visceral sensory signals and, in turn, activate motor neurons which control visceral effectors. Interface between the nervous and endocrine This is the place where the two control systems overlap and exert mutual control Mind over body The NS can control the release of hormones to control the body. Through various techniques, such as meditation and bio-feed back, it is possible to think your heart slower, for example. Rage/aggression Is closely linked to both the thalamus (associated with emotions) and the hypothalamus (release of hormones, such as adrenaline.)

13 Hypothalamus Body temp Hypothalamus monitors blood as it passes the preoptic area. If varies from normal range (37 C); negative feedback to bring the temp back in range. Food intake regulation Feeding behavior is regulated. Associated with emotions and memory. Appestat Detects blood glucose levels: if glucose drops, feeding behaviors are turned on. Satiety center Eaten enough? Increases in blood glucose and stomach stretch Turns off feeding Hypothalamus Thirst center Water content of blood is monitored Adjusts drinking behaviors Stimulates the release of ADH- anti-diuretic hormone Reduces urine output Helps retain water in the body