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The Central Nervous System Outline of class lecture for Physiology After studying the endocrine system you should be able to:

1. Describe in general, the four major components of the brain. 2. Describe the functions of the components of the brain stem. 3. Explain the functions of the components of the diencephalon. 4. Describe the functions of the components of the cerebrum. 5. Explain the concept of cerebral lateralization. 6. Describe an EEG and the various wave patterns 7. Explain the functions of the limbic system. 8. Explain the functions of the reticular activating system. 9. Explain the protective coverings of the brain. 10. Describe the components and functions of the blood-brain barrier. 11. Describe the functions of cerebrospinal fluid. 12. Discuss the Clinical Applications from the study guide and be able to describe the disorders

from the Applications to Health located at the end of this chapter. General Information

• Adult brain consists of ~100 billion neurons and 1 trillion neuroglia cells. • • Receives ~20% of the total blood flow to the body per minute.

– High rate of blood flow/oxygen content is due to the high metabolic requirements. Brain: Divided into 4 Major Regions

• Brain stem, which consists of: – – –

• Diencephalon, which consists of: – Thalamus – Hypothalamus – Epithalamus

• Cerebellum • Cerebrum, which consists of:

– Right and left cerebral hemispheres

Brain Stem • Brain stem consists of: Medulla

oblongata, pons, and midbrain. • Medulla oblongata (medulla)

– Contains all ascending and descending fiber tracts that – Most tracts cross over from one side of the spinal cord to the other side at an elevated

structure in the medulla called the pyramids. • Results in motor fibers originating in the left cerebral cortex activating muscle on

the right side of the body and vice versa • Results in the left side of the brain receiving sensory information from the

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– Contains centers that regulate autonomic functions such as: • • • • Centers that coordinate swallowing, vomiting, coughing, sneezing, and

hiccupping. – Contains the origins (nuclei) of cranial nerves VIII, IX, X (Vagus), XI, and XII.

• Pons – Bridges the spinal cord to the brain and parts of the brain with each other. – Together with the medulla oblongata help control breathing (respiratory rhythm). – Contains the origins (cell bodies) of cranial nerves V, VI, VII, and VIII.

• Midbrain (Mesencephalon) – Contains the cerebral peduncle and

tectum. – Origin of cranial nerves III and IV. – Cerebral peduncle contains:

• Substantia nigra: Darkly pigmented nuclei whose neurons help coordinate skeletal muscle activity.

• Neurons release dopamine at the basal nuclei

• Loss of these neurons is associated with

– Tectum (corpora quadragemina): Dorsal portion that contains 4 rounded elevations called the superior and inferior colliculi. • Two Superior colliculi: Reflex

centers of movement of the eyes, head and neck in response to visual stimuli

• Two Inferior colliculi: Reflex centers for movements of the

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Cerebellum • Coordinates skeletal muscle contractions required for skilled movements and balance • Refines learned movement patterns controlled at the

– Compares motor commands with sensory info from muscles and joints and performs any adjustments to make the movement smooth.

• White fiber tracts form the arbor vitae or “tree of life” • The cerebellum can be permanently damaged by trauma or stroke or temporarily affected by

drugs such as alcohol. – Damage produces ataxia – lack of

Diencephalon

• Diencephalon consists of the thalamus, hypothalamus, and epithalamus.

• Thalamus: – Relay center through which all sensory information

(except smell) passes on the way to the cerebrum for interpretation.

• Acts like a switchboard that receives sensory signals and sends them to the correct associated cortical areas.

• Acts as a filter along with the reticular activating system, passing on only a small portion of arriving sensory information.

• Hypothalamus: – Control of ANS:

• Initiates the physical responses to emotions. When you think about a dangerous or stressful situation, your heart rate and respiratory rate go up and your body prepares for an emergency. These autonomic adjustments are made by the hypothalamus.

– Regulation of emotional and behavior patterns. • Together with the limbic system participates in expressions of rage, aggression,

loathing, fear, pleasure, and behavior pattern related to sexual arousal. – Regulates

• Hypothalamic neurons monitor blood temperature and receive input from other thermoreceptors in the brain and body periphery. Will initiate cooling (sweating) or heat-generating actions (shivering) to maintain constant body temperature.

– Regulates • Monitors changing blood levels of certain nutrients (glucose and amino acids)

and regulates feelings of hunger and satiety). • When body fluids become too concentrated, hypothalamic neurons called

osmoreceptors become activated and trigger the release of antidiuretic

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hormone (ADH) from the posterior pituitary that causes the kidneys to retain water. Also causes person to feel thirsty and drink more fluids.

– Regulates circadian rhythm (sleep/wake cycle) and states of consciousness. • Works in conjunction with the

– Production of hormones (ADH and oxytocin) and regulates the anterior pituitary gland via the release of regulatory factors.

• Epithalamus consists of: – Pineal gland: Secretes melatonin

• Function: Promotes sleepiness and helps regulate circadian rhythm.

Cerebrum

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Cerebrum • The cerebrum forms the largest portion of the brain (80% of its mass) and responsible for

higher mental functions. –

• Has an outer cortex of gray matter surrounding an interior that is mostly white matter, except for a few small portions.

– Cortex contains folds called gyri that function to increase the surface area of the brain. • Deep sulci divide each hemisphere into 5 lobes:

– Frontal, Parietal, Temporal, Occipital, and Insula – Why/How are the 1st 4 named? – What does “insular” mean?

Lobes of Cerebrum Frontal Lobe

• Frontal lobe functions: – Reasoning; higher intellectual processes (decision making, planning), – Verbal communication.

• Broca’s Area: Directs muscles of tongue, lips, and throat that are used in speech production and Involved in planning speech production.

• Located in the • Broca’s aphasias: Speech disorder caused by damage to area due to trauma or

stroke. • Person’s speech is slow and poorly articulated (non-fluent) but they can

understand speech. • Can understand a sentence but have

difficulty repeating it. – Voluntary control of skeletal muscles and

• The entire body is represented spatially in the primary motor cortex, i.e., in one region we have neurons controlling hand movements and in another region leg movements, etc.

• Determined by the number of effectors in an area

Frontal Lobotomy • Frontal Lobotomy: Surgical procedure severing the

connection between the • Reasoning for procedure was that this area was somehow

malformed or damaged, and if the nerves were severed they might regenerate into new, healthy connections.

– The main indications for a frontal lobotomy included severe chronic anxiety, depression with risk of suicide, incapacitating obsessive-compulsive disorder and high levels of aggression.

• Thousands were performed between 1930-1950. By the 1960s, the number of operations was in decline as the procedure became much less fashionable.

• Transorbital lobotomy (icepick lobotomy) • Improvements in psychopharmacology and behavior therapy gave the opportunity for more

effective and less-invasive treatment.

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Parietal Lobe • Parietal lobe functions:

– Somatosensory interpretation (cutaneous and muscular sensations)

– Interpretation of Temporal Lobe

• Temporal Lobe functions: – Interpretation of auditory sensations – Memory storage of auditory and visual experiences

• Associations are made between – Understanding speech.

• Wernicke’s area: In both the left temporal and parietal lobes is involved in understanding written and oral words.

• Wernicke’s aphasia: Difficult form person to understand either spoken or written language. Speech is rapid and fluid but without meaning.

• Form real words but they are Occipital Lobe

• Occipital Lobe functions: – Vision – Integration of movements for – Interpretation of visual images and correlating visual

images with previous visual experiences and other sensory stimuli.

Insula • Insula functions:

– Involved with

• Include upset stomach, full bladder, and the feeling that your lungs will burst when you hold your breath too long

• Role in pain experience and the experience of a number of basic emotions, including anger, fear, disgust, happiness and sadness.

Cerebral Lateralization

• Specialization of function in one hemisphere or the other.

– The two hemispheres communicate with each other via the corpus callosum.

• Left hemisphere: specialized for language, analytic

ability and logic. • Right hemisphere: Geared towards musical, artistic

and other creative endeavors. • Most individuals with

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Basal Nuclei • Set of nuclei deep within the white

matter. • Provides subconscious control of

skeletal muscle tone and coordinates learned movement patterns and other somatic motor activities.

– Allows intended movements to occur while inhibiting unintended movements.

• Doesn’t initiate movements but once voluntary movement is underway, they assist in the pattern and rhythm (especially for trunk and proximal limb muscles

Parkinson ’s disease • Each side of the midbrain contains a nucleus called the substantia nigra (SN). • Neurons in the substantia nigra inhibit the activity of • Degeneration of dopaminergic neurons produces the symptoms of Parkinson’s.

Electroencephalogram

• Electroencephalogram (EEG): A record to the electrical currents that occur among the neurons of the cerebral cortex.

• Four type so EEG patterns: – Alpha waves (10-12 cycles/second): Produced when person is awake and relaxed

with the eyes are closed. – Beta waves (13-25 cycles/second): – Theta waves (5-8 cycles/second): Produced in newborn infant

• In adults may indicate severe emotional stress and possible nervous breakdown. – Delta waves (1-5 cycles/second): Common during sleep and in awake infant.

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Damage to SN neurons

Decrease in dopamine secretion

Increased activity of basal

nuclei

Gradual increase in muscle tone

Appearance of symptoms of Parkinson’s disease: tremor,

slow movement, inability to move, rigid gait, reduced facial

expression

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Limbic System • Limbic system: Includes nuclei and tracts along the

border between the cerebrum and the diencephalon. – Prominent structures include the hypothalamus,

amygdala, hippocampus, and fornix. • More of a functional grouping rather than

anatomical structure. – Functions:

• Center for basic emotional drives – called the emotional brain

– Motivation; goal directed behavior;

• Processes memory storage and retrieval – particularly the hippocampus and amygdala

The Reticular Activating System (RAS)

• Functions of RAS: – Maintains cerebral cortical alertness. Sends impulses to

the cerebral cortex keeping the cortex alert. – Filters out repetitive stimuli from sensory inputs such as

pressure from your watch, clothes, etc. Works to help person fall asleep by decreasing sensory stimuli.

• RAS is depressed by

• RAS Extends through the central core medulla oblongata, pons and midbrain with connections to the thalamus to relay sensory information.

Sedatives and the RAS

• A common over-the-counter sedative is a H1 antihistamine (H1-receptor antagonist) and includes diphenhydramine (Benadryl).

• For Sedation: The exact mechanism is unclear. It reduces the arousal component of the RAS. The RAS includes a group of cholinergic neurons (neurons that release acetylcholine) in the brain stem that project to the thalamus; these neurons enhance the transmission of sensory information from the thalamus to the cerebral cortex. Diphenhydramine may block the action of acetylcholine (anticholinergic effect similar to atropine) on muscarinic receptors and causes drowsiness by decreasing the activity of the RAS.

• Benadryl can cross the blood-brain barrier and cause drowsiness. The antihistamines that don’t cause drowsiness, such as Claritin, cannot cross the blood-brain barrier.

• Drowsiness caused by benzodiaszpines (such as Valium), barbiturates, alcohol, and most anesthetic gases is due to the ability of these agents to enhance the activity of GABA receptors. Increased ability of GABA to inhibit the RAS then reduces arousal and promotes sleepiness.

– GABA is the principal inhibitory neurotransmitter in the brain

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Protection and Coverings • The brain is protected by:

– Cranial bones – Cranial meninges

• Dura mater – • Arachnoid Mater –

– Subarachnoid space contain CSF

• Pia mater – – Blood-Brain Barrier – Cerebrospinal fluid

Blood-Brain Barrier • Blood-brain barrier: Regulates

passage of substances from the blood into the brain tissue and formed by:

– Presents difficulties in chemotherapy of brain diseases because certain drugs cannot enter the brain.

• Ex: Parkinson’s disease can be treated with increased levels of dopamine but dopamine cannot pass the BBB. Solution: levodopa (L-dopa) is used which crosses the BBB and is then converted to dopamine.

• Endothelial cells: • Astrocyte processes that press up against capillaries and help maintain the

permeability characteristics. Cerebrospinal Fluid (CSF)

• CSF consists of: Clear fluid with Glucose, proteins, lactic acid, urea, electrolytes, and lymphocytes

• Has three major functions: – Mechanical

protection: Shock-absorber and buoys the brain so that it “floats” in the cranial cavity.

– Chemical protection: Maintains optimal chemical environment of neuronal signaling

–

• CSF Formation takes place at the choroid plexuses located in the ventricular walls.

– CSF is produced at a rate of ~500 ml/day

• Total volume of CSF at

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• CSF circulates from the choroid plexus thru the ventricles and the central canal of the spinal cord.

– CSF is reabsorbed at the sagittal sinus into the venous blood at the same rate it is formed.

• If its drainage is blocked, a condition called hydrocephalus occurs. • Results in increased pressure on the • A shunt can be inserted to drain excess CSF

Spinal Cord and Reflexes • Reflex: Involuntary action that occurs in response to a stimulus. • Reflex arc: Pathway followed by nerve impulses that produce a reflex. • Reflex arc components:

– Sensory receptor: Respond to a stimulus by generating nerve impulses. – Sensory neuron: Carry nerve impulses toward the CNS. – Integrating Center: Determines the response to the stimulus – Motor neuron: Carry nerve impulses away from the CNS to an effector. – Effector: Responds to the motor nerve impulse.

• Somatic reflex involves skeletal muscle • Autonomic (Visceral) reflex involves smooth muscle, cardiac muscle, or a gland