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