Chapter 14 The Brain and Cranial Nerves. The Brain The Brain is part of Central Nervous System (CNS)...
-
date post
20-Dec-2015 -
Category
Documents
-
view
217 -
download
1
Transcript of Chapter 14 The Brain and Cranial Nerves. The Brain The Brain is part of Central Nervous System (CNS)...
The Brain• The Brain is part of Central Nervous
System (CNS)• It is divided into 6 major parts:
– The cerebrum– The diencephalon– The mesencephalon– The cerebellum– The pons– The medulla oblongata
The Human Brain
• Ranges from 750 cc to 2100 cc• Contains almost 98% of the body’s
neural tissue• Average weight about 1.4 kg (3 lb)
Protection of the Brain
• The brain is protected by bone, meninges, and cerebrospinal fluid
• Harmful substances are shielded from the brain by the blood-brain barrier
Cerebrospinal fluid
• Protects by absorbing shock (it “floats” the brain.
• Maintains the electrochemical environment (opitmal pH & osmolality).
• Circulates nutrients and wastes.
Meninges
• Three connective tissue membranes lie external to the CNS – dura mater, arachnoid mater, and pia mater
• Functions of the meninges– Cover and protect the CNS– Protect blood vessels and enclose venous
sinuses– Contain cerebrospinal fluid (CSF)– Form partitions within the skull
Dura Mater
• Three dural septa extend inward and limit excessive movement of the brain– Falx cerebri – fold that dips into the
longitudinal fissure– Falx cerebelli – runs along the vermis
of the cerebellum– Tentorium cerebelli – horizontal dural
fold extends into the transverse fissure
Arachnoid Mater• The middle meninx, which forms a loose
brain covering• It is separated from the dura mater by
the subdural space• Beneath the arachnoid is a wide
subarachnoid space filled with CSF and large blood vessels
• Arachnoid villi protrude superiorly and permit CSF to be absorbed into venous blood
Blood flow, O2 and the BBB• Though only 2% of body mass the brain
consumes 20% of the oxygen and glucoseof the resting body.
• No O2 for a minute = dizziness– 4 minutes = brain damage– 6 minutes = death
• The brain is protected by the blood-brain barrier.
Blood-Brain Barrier: Functions
• Selective barrier that allows nutrients to pass freely
• Is ineffective against substances that can diffuse through plasma membranes
• Absent in some areas (vomiting center and the hypothalamus), allowing these areas to monitor the chemical composition of the blood
• Stress increases the ability of chemicals to pass through the blood-brain barrier
The Brain Stem
• The brain stem is comprised of 3 main subdivisions:– The Medulla Oblongata– The Pons– The Mesencephalon
Medulla Oblongata: 3 Groups of Nuclei
1. Autonomic nuclei:– control visceral activities
2. Sensory and motor nuclei:– of cranial nerves
3. Relay stations:– along sensory and motor pathways
Figure 14–6c
The Pons
• Links cerebellum with mesencephalon, diencephalon, cerebrum, and spinal cord
The Pons: 4 Groups
1. Sensory and motor nuclei of cranial nerves V, VI, VII, VIII
2. Nuclei involved with respiration:– apneustic center and pneumotaxic
center – modify respiratory rhythmicity
center activity
The Pons: 4 Groups
3. Nuclei and tracts:– process and relay information to and
from cerebellum
4. Ascending, descending, and transverse tracts:
– transverse fibers (axons) • link nuclei of pons with opposite cerebellar
hemisphere
Functions of the Cerebellum
1. Adjusts postural muscles 2. Fine-tunes conscious and
subconscious movements
Structures of the Cerebellum (1 of 2)
• Folia: – surface of cerebellum– highly folded neural cortex
• Anterior and posterior lobes:– separated by primary fissure
Structures of the Cerebellum (2 of 2)
• Cerebellar hemispheres:– separated at midline by vermis
• Vermis:– narrow band of cortex
• Flocculonodular lobe:– below fourth ventricle
Midbrain nuclei
• Substantia nigra: dopamine release, control of subconscious muscle movents.
• Red nuclei: synapses between neurons of cerebellum and cerebrum. Also origin of oculomotor nerve (CN III) and trochlear nerve (CN IV).
Some major functions mesencephalon
• Cerebral peduncles: contain motor axons connecting cerebrum to brain stem and sensory axons that connect medulla to thalamus
• Tectum:– Superior colliculi: visual reflexes, including
pupillary reflexes.– Inferior colliculi: auditory path from ear to
thalamus. – Startle reflex
The Diencephalon• The Thalamus - switch board for the
cerebrum. Includes the:– Geniculate nuclei (part of the ventral group):
• Medial processes auditory information.• Lateral processes visual input.
– Other ventral nuclei connect motor & sensory areas of cortex to cerebellum & spinal cord
– Anterior & medial nuclei• Anterior connects to limbic system &
hypothalamus• Medial connects with cortex & limbic system
The Thalamus:Filters ascending sensory information for primary sensory cortex.Relays information between basal nuclei and cerebral cortex
Figure 14–9
The HypothalamusFunctions of the hypothalamus• ANS control
– , heart rate, respiration, sweating, digestion, urination, stress response
• Hormones - ADH, oxytocin, GnRH et al.• Basic emotions - rage, sex drive, fear,
pain pleasure.• Thirst & appetite• Temperature & sleep/wake (circadian)
cycles
The Limbic System
• Is a functional grouping that:– establishes emotional states– links conscious functions of cerebral
cortex with autonomic functions of brain stem
– facilitates memory storage and retrieval
Limbic System• Structures located on the medial aspects of
cerebral hemispheres and diencephalon
• Includes the rhinencephalon, amygdala, hypothalamus, and anterior nucleus of the thalamus
• Parts especially important in emotions:
– Amygdala – deals with anger, danger, and fear responses
– Cingulate gyrus – plays a role in expressing emotions via gestures, and resolves mental conflict
• Puts emotional responses to odors – e.g., skunks smell bad
Limbic System: Emotion and Cognition
• The limbic system interacts with the prefrontal lobes, therefore:– One can react emotionally to
conscious understandings– One is consciously aware of emotion
in one’s life• Hippocampal structures – convert
new information into long-term memories
The CerebrumMajor divisions of the Cerebrum• Hemispheres - left and right split down
the middle by the longitudinal fissure.• Lobes:
– Frontal, parietal, occipital, temporal and insula
– Delineated by the sulci (singular sulcus):• Central• Parieto-occipital• Lateral
Figure 14–13
White Matter of the
Cerebrum
Myelinated fibers (axons)1. Association fibers2. Commissural fibers3. Projection fibers
Functions of Basal Nuclei
• Are involved with:– the subconscious control of skeletal
muscle tone– the coordination of learned
movement patterns (walking, lifting)
The Left Hemisphere
• In most people, left brain (dominant hemisphere) controls:– reading, writing, and math– decision-making– speech and language
The Right Hemisphere
• Right cerebral hemisphere relates to:– senses (touch, smell, sight, taste,
feel)– recognition (faces, voice inflections)
Types of Brain Waves• Alpha waves – 8 - 13 Hz, regular and rhythmic,
low-amplitude, slow, synchronous waves indicating an “idling” brain
• Beta waves – 14 - 30 Hz, rhythmic, more irregular waves occurring during the awake and mentally alert state
• Theta waves – 4 - 7 Hz, more irregular than alpha waves; common in children but abnormal in adults
• Delta waves – 1 - 5 Hz, high-amplitude waves seen in deep sleep and when reticular activating system is damped
Brain Waves: State of the Brain
• Brain waves change with age, sensory stimuli, brain disease, and the chemical state of the body
• EEGs can be used to diagnose and localize brain lesions, tumors, infarcts, infections, abscesses, and epileptic lesions
• A flat EEG (no electrical activity) is clinical evidence of death
The 12 cranial nerves
1. On2. Old3. Olympus’4. Towering5. Top6. A7. Frenchman 8. Viewed 9. Greek10.Villagers11.At12.Home
Clinical Stuff
• Epilepsy• CVA & TIA• Degenerative Diseases
– Alzheimers– Huntington’s– Parkinbson’s
Epilepsy
• A victim of epilepsy may lose consciousness, fall stiffly, and have uncontrollable jerking, characteristic of epileptic seizure
• Epilepsy is not associated with, nor does it cause, intellectual impairments
• Epilepsy occurs in 1% of the population
Epileptic Seizures
• Absence seizures, or petit mal – mild seizures seen in young children where the expression goes blank
• Grand mal seizures – victim loses consciousness, bones are often broken due to intense convulsions, loss of bowel and bladder control, and severe biting of the tongue
Control of Epilepsy
• Epilepsy can usually be controlled with anticonvulsive drugs
• Valproic acid, a nonsedating drug, enhances GABA and is a drug of choice
• Vagus nerve stimulators can be implanted under the skin of the chest and can keep electrical activity of the brain from becoming chaotic
Cerebrovascular Accidents (Strokes)
• Caused when blood circulation to the brain is blocked and brain tissue dies
• Most commonly caused by blockage of a cerebral artery
• Other causes include compression of the brain by hemorrhage or edema, and atherosclerosis
• Transient ischemic attacks (TIAs) – temporary episodes of reversible cerebral ischemia
• Tissue plasminogen activator (TPA) is the only approved treatment for stroke
Degenerative Brain Disorders
• Alzheimer’s disease – a progressive degenerative disease of the brain that results in dementia
• Parkinson’s disease – degeneration of the dopamine-releasing neurons of the substantia nigra
• Huntington’s disease – a fatal hereditary disorder caused by accumulation of the protein huntingtin that leads to degeneration of the basal nuclei