Ch 5 Nervous System

Introduction 1. the nervous system and the endocrine system are 2 control systems that help to regulate

activities in the body 2. nervous system coordinates rapid responses by way of nerve impulses 3. the endocrine system coordinates slower responses by secreting hormones 4. The Nervous System is highly complex. It is the seat of our conscious experience,

personality, and behavior. Since the brain is the center of consciousness it gives us our sense of self.

5. brain controls skeletal muscle contractions, emotions, the ability to form images, etc. 6. the brain is capable of storing memories and of complex thought to include problem

solving and creative thinking

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Organization of the Nervous System 1. Only one NS that is divided into logical parts with the major divisions: CNS and PNS 2. Central Nervous System (CNS) a. brain (100 billion neurons) and spinal cord (100 million neurons) b. command center of the NS 3. Peripheral Nervous System (PNS) a. found outside of the CNS b. consists of cranial and spinal nerves, and all of their branches, ganglia, motor end

plates, and sensory receptors c. 2 Divisions 1. sensory (afferent) division of the PNS - neurons that carry impulses from sensory

receptors in the periphery to the CNS (e.g., pain, touch, smell, taste, pressure, touch) a. somatic sensory – impulses from receptors in the skin, skeletal muscles, bones,

and joints b. visceral sensory – impulses from internal viscera mainly in the thoracic and

abdominal cavities (e.g., heart, lungs, kidneys, intestine) 2. motor (efferent) division of PNS a. somatic motor (voluntary) 1. somatic motor neurons that conduct impulses from CNS to effector cells:

skeletal muscles 2. allow conscious control of skeletal muscle b. autonomic (involuntary or visceral) motor 1. autonomic motor neurons that conduct impulses from CNS to effector cells:

smooth muscle, cardiac muscle, and glands (e.g., sweat glands, pancreatic acinar cells, gastric glands)

c. 2 ANS divisions: parasympathetic and sympathetic

Structural Classification of Neurons (3 types)

Multipolar, Bipolar, and Unipolar

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3 Functional Classes of Neurons 1. Sensory (Afferent) in the PNS a. conduct impulses from sensory receptors (e.g., pain, touch, temperature, pH, O2, CO2)

into the brain and spinal cord b. 2 types of sensory neurons based on structure: 1. unipolar sensory neurons 2. bipolar sensory neurons – found in only 3 locations: retina - vision, olfactory mucosa

- smell, Organ of Corti in cochlea - hearing c. afferent neurons are found primarily in the PNS; only the latter part of their axons and

their axonal endings are found in the CNS 2. interneurons a. found exclusively in the CNS (brain and spinal cord) b. multipolar in structure c. about 99% of mammalian neurons are interneurons d. the human brain and spinal cord is estimated to contain 100 billion interneurons e. involved with planning, memory, intellect, personality, processing sensory

information, controlling skeletal muscle contractions, creativity, problem-solving 3. Motor (efferent) neurons a. send impulses from the CNS where their cell bodies are found out their axons into the

PNS where they effect effector cells b. multipolar in structue c. 2 types of motor neurons and their effector cells 1. somatic motor neurons a. effector cells: skeletal muscle cells 2. autonomic motor neurons a. effector cells 1. smooth muscle 2. cardiac muscle 3. glandular epithelial cells (e.g., pancreas, liver, intestinal lining, sweat and oil

glands)

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Spinal Cord 1. extension of brain 2. starts at the brainstem, goes through the foramen magnum and down the vertebral canal 3. spinal cord ends at L2

Structure of Spinal Cord (thick as a finger) 1. 31 pairs of spinal nerves: cervical (C1-C8), thoracic (T1-T12), lumbar (L1-L5), sacral (S1-S5),

and coccygeal (Co) 2. spinal nerves are only about 1-2 cm in length (less than an inch); they branch into named

nerves and plexuses soon after leaving the spinal cord 3. cauda equina a. collection of spinal nerves that come off the conus medullaris at the base of the spinal

cord b. resemble a horses’ tail, hence the name

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Cross-Section of Spinal Cord - consists of a gray H area made of gray matter surrounded by white matter

Gray matter 1. area within the spinal cord that resembles an H or a butterfly 2. mostly the cell bodies and dendrites of interneurons and motor neurons; also the axons of

unipolar sensory neurons

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3. presence of cell bodies creates a gray color 4. horns of the gray matter (also called columns when viewed longitudinally) a. dorsal horn (posterior) 1. cell bodies of interneurons that carry sensory info to brain 2. axon endings of unipolar sensory neurons 3. axons of the sensory neurons enter the spinal cord by way of the dorsal root 4. cell bodies of unipolar sensory neurons in dorsal root ganglion b. ventral horn (anterior) 1. cell bodies of somatic motor neurons that innervate skeletal muscle 2. axons of motor neurons leave the spinal cord by way of the ventral root c. lateral horn 1. only in thoracolumbar and sacral regions of spinal cord gray matter 2. lateral horns contain the cell bodies of autonomic motor neurons (sym and

parasym) that innervate visceral effector cells (cardiac and smooth muscle, glands) 5. gray commissure a. commissure means to unite b. thin strip of gray matter that connects the right and left sides of the gray matter 6. central canal a. CSF-filled canal that runs down the center of the spinal cord b. receives CSF from the IV ventricle of the brain c. terminates at the end of the spinal cord (blind-ended) White Matter 1. surrounds the gray matter 2. contains mostly myelinated axons that give it a white color 3. does not contain any cell bodies of any neurons 4. Nerve Tracts run within the white matter a. ascending tracts carry sensory info up the spinal cord to the brain (e.g., spinothalamic) b. descending tracts carry motor info down the spinal cord to somatic and autonomic

motor neurons (e.g., corticospinal)

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Roots of the Spinal Cord and Spinal Nerves 1. Dorsal Root (sensory root) – each consists of 6-8 rootlets that converge to form one root a. contain axons of unipolar sensory neurons b. Dorsal root ganglion – swelling on the dorsal root that contains the cell bodies of

unipolar sensory neurons 2. Ventral root (motor root) – each consists of 6-8 rootlets that converge to form one root a. contains the axons of somatic and autonomic motor neurons b. only contains axons 3. both roots come together to form the spinal nerves Nerve Tracts 1. nerve tracts are bundles of axons enclosed within connective tissue that travel within the

CNS 2. Ascending (sensory) Tracts a. consist of axons of mostly interneurons in the spinal cord that conduct sensory impulses

up the cord and into the brain b. decussation (crossing-over) 1. this term describes the fact that sensory impulses from one side of the body cross

over to the other side of the CNS for processing in the brain 2. decussation generally occurs in the spinal cord or in the medulla a. touch decussates in the medulla b. pain and temperature decussate at the level of the spinal cord where the

sensory fibers enter 3. because of decussation, sensations on the left side of the body are interpreted on

the right side of the brain and sensations from the right side go to the left side of the brain

3. Descending (motor) Tracts a. consist of axons of interneurons that start in the brain and conduct motor impulses

down the spinal cord to stimulate somatic and autonomic motor neurons b. Decussation 1. decussation of motor impulses occurs within the brain or spinal cord 2. because of decussation, the left side of the brain controls the muscles on the

opposite side of the body and the right side of the brain controls the muscles on the left.

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Ascending and Descending tracts 1. Corticospinal Tract: Descending Pathway for Skeletal Muscle Contraction (2-Neuron

pathway) a. thought that want to contract specific skeletal muscle (e.g., biceps brachii in anterior

part of arm) b. thought stimulates pyramidal neurons that make up a “map” of body in primary motor

cortex of frontal lobe (precentral gyrus) that corresponds to biceps brachii c. pyramidal neurons in primary motor cortex send impulses down their axons that travel

out in corticospinal tracts d. corticospinal tracts go through the cerebrum then into the brainstem e. corticospinal tracts decussate (cross over) in the medulla oblongata of the brainstem

and then go down the white matter in the outer part of the spinal cord f. axons of pyramid neurons leave corticospinal tract and enter ventral horn of spinal

cord gray matter where they stimulate somatic motor neurons to biceps brachii g. impulses travel out from spinal cord on axons of somatic motor neurons to biceps

brachii (motor unit) h. biceps brachii contracts

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Pain and Temperature Pathways

2. Spinothalamic Tract: Ascending Pathway for Pain (3 neuron pathway) a. strike thumb with a hammer b. tissue damage results in dead and dying cells that release pain-stimulating chemicals

(e.g., bradykinin) that bind to receptors (nociceptors) on the dendritic zone of the unipolar sensory neuron to trigger an impluse

c. dendrite at end of unipolar sensory neuron associated with the modality of pain sends impulses along its axon; this neuron is referred to as a 1st order neuron

d. the axon enters a peripheral nerve e. impulse travels along axon into the spinal cord by way of the dorsal root (cell bodies of

unipolar sensory neurons are in the dorsal root ganglion) f. within the spinal cord the impulse stimulates an interneuron (2nd order neuron) whose

axon decussates or crosses over to the other side of the CNS g. the axon of the interneuron travels up the spinal cord in the spinothalamic tract (two

other tracts carry pain, but this is the most important one) h. the interneuron stimulates another interneuron (3rd order neuron) in the thalamus of

the brain (within the diencephalon)

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i. the 3rd order neuron takes the info to two areas of the brain found in the cerebral cortex

1. pain interpretation center in the somatosensory association cortex a. part of the somatosensory association cortex of the parietal lobe of the

cerebrum b. provides the sensation of pain (“ouch”); one feels pain in the brain, but not the

body part 2. primary somatosensory cortex a. on the post-central gyrus of the parietal lobe b. contains a “map” of the body that allows the cerebrum to “localize” the source

of the pain (neurons that make up this area form a map of the body) Effects of Advil (ibuprofen) and Aspirin (acetylsialicylic acid) 1. tissue injury damages cell membranes. 2. A fatty acid named arachidonic acid escapes from injured cells. It is acted on by an enzyme

called cyclo-oxygenase and converted to prostaglandins which stimulate nociceptors. 3. Ibuprofin (Advil, Motrin) and aspirin decrease the activity of the enzyme, thus decrease the

production of prostaglandins and decrease the pain associated with tissue damage.

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Brain - Adult brain contains 85- 100 billion neurons and a trillion neuroglial cells; it weighs about 3 lbs (1300 g). The brain is the center of motor control, sensation, emotion, thought, personality, memory, dreams, and plans for the present and future. 1. Brain receives about 15-20% of cardiac output (amount of blood pumped by the heart each

minute) and brain neurons consume about 20% of the body’s oxygen and glucose to support its high metabolic rate.

a. This volume of cardiac output to the brain does not change even during exercise. In contrast, resting skeletal muscle receives about 20% of the cardiac output at rest. This volume increases several fold during exercise.

b. A 10-second interruption in blood flow to the brain can result in a loss of consciousness. An interruption of 1-2 minutes can impair brain function and 4 minutes can result in irreversible brain damage

2. Because of the high blood flow to the uninsulated brain, it radiates considerable heat during the winter - wear a hat to conserve body heat. In fact, the brain has a high metabolic rate that continually produces a lot of heat that must be shed to prevent brain damage.

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3. 4 main regions of brain a. cerebrum (lateral ventricles, functional areas, corpus callosum, fornix, septum

pellucidum) b. cerebellum c. diencephalon (thalamus, hypothalamus, pituitary gland) d. brainstem (medulla oblongata, pons, midbrain) Cerebrum 1. Largest portion of the brain (over 80% of total mass) and occupies most of the cranial vault

(hollow cavity in the skull). It governs higher mental processes. The longitudinal fissure divides the cerebrum into right and left hemispheres

2. Function a. Motor Areas: In response to thoughts, initiates and controls voluntary skeletal muscle

contractions b. Sensory Areas: Interprets and localizes information from sensory receptors (e.g.,

hearing, vision, touch, taste, pain). Brain is aware of sensations. c. Association Areas: serves as an association center that carries out complex higher brain

functions (1) intelligence, (2) language: the ability to read, write, and speak, (3) consciousness, (4) creativity, (5) memories that allow us to retain past experiences, (6) plan for the future and envision our own death; (7) abstract thought, (8) manipulation of symbols, (9) judgment, (10) emotions and personality, (11) problem-solving

3. Cerebral cortex - the surface of the cerebrum is composed of gray matter 2 to 4 mm (1/8" - ¼”) thick. Despite being thin, the cerebral cortex is highly folded, hence represents a large surface area (it’s surface area is comparable to 4½ 8.5” x 11” pages).

Matter in the Nervous System 1. Gray – contains dendritic zones of neurons (cell bodies have a grayish color) 2. white – contains only axons (myelinated axons are white in color)

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Functional areas of cerebral cortex of human brain Functional areas of the human cerebral cortex (areas specialized for different activities) Sensory, Motor and Associative Areas Sensory Areas and Sensory Association Areas of the Cerebrum (sensory areas scattered throughout the parietal, temporal, and occipital lobes of the cerebrum) 1. Somatosensory Areas

a. Primary Somatosensory Cortex (General Sensory Area) 1. Location: Postcentral gyrus in each parietal lobe (ridge directly behind the central

sulcus) 2. It receives sensory information that we perceive (i.e., are consciously aware of). 3. The entire body is spatially represented on it as if it were a map. 4. Receives nerve impulses from somatic sensory receptors (skin and joints) for touch,

pressure, proprioception (joint and muscle position), pain, and temperature. 5. Major Function: Localize through projection the exact point on the body where

sensations originate (precisely distinguishes the specific area of sensory stimulation). "Sensory stimuli are coming from the tip of my index finger." Localization does not establish the nature of the modality or sensation.

b. Somatosensory Association Cortex (association area) 1. Location: posterior to the primary somatosensory area in parietal lobe of cerebrum 2. Receives input from sensory neurons (e.g., pain, touch, temperature) 3. Function: site in cerebrum where one feels conscious sensations like touch,

pressure, temperature change, pain, hot, cold; Neurons organized by sensations a. Integrate and analyze different somatic sensations. Meaning of incoming stimuli

analyzed and compared against memories

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b. Integrate and interpret sensory stimuli and compare it to sensations associated with previous experiences (memories). "Ouch!" - the index finger is in pain. This area permits one to determine the exact shape and texture of an object without looking at it (e.g. determine an object in pocket by feeling it). Memories about objects are stored in this area.

2. Visual Areas a. Primary Visual Cortex (sensory)

1. Location: medial surface of occipital lobe 2. Sensory input: retina of the eye via the optic nerve (cranial nerve II); axons of retinal

neurons stimulate neurons in the thalamus which then project to the visual areas 3. Function: take in information about color, shape, and movement and form images 4. If damaged, a person would be blind in at least part of their visual field

b. Visual Association Area 1. Location: occipital lobe 2. Input: receives sensory impulses from the primary visual area (and the thalamus) 3. Function: relates present to past visual experiences to determine what is being seen 4. If damaged, a person might see normally, but not recognize the face of a friend. For

example, a person might see letters within a word, but not recognize their meaning 3. Auditory Areas (hearing)

a. Primary Auditory Cortex - Located in superior part of temporal lobe. It interprets the basic characteristics of sound (e.g. pitch and rhythm)

b. Auditory Association Area - located below the primary auditory area. It determines if a sound is speech, music, or some other noise. It also interprets the meaning of speech by translating words into thoughts.

4. Primary Gustatory Area (taste) - Located in parietal lobe and receives impulses from tongue associated with taste. The gnostic area or common integrative area is the association area that interprets this information.

5. Primary Olfactory Area (smell) - Located in temporal lobe and receives impulses associated with smell. The gnostic area or common integrative area is the association area that interprets this information.

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Motor Areas and Motor Association Areas (all of these motor areas are in the frontal lobe) 1. Primary Motor Cortex (conscious planning of skeletal muscle movements

a. Location: precentral gyrus of the frontal lobe b. Input: thoughts and stimuli from the premotor area c. Spatial representation or “map” of the body on it d. Function 1. pyramidal neurons send nerve impulses to the brainstem and spinal cord to

stimulate somatic motor neurons 2. The somatic motor neurons then control voluntary contractions of specific muscles

or groups of muscles.

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3. Electrical stimulation of any point in the primary motor area results in contraction of specific skeletal muscle fibers on the opposite side of the body; 80-90% of decussation occurs in the medulla, while the rest occurs in the spinal cord

e. Long axons from the pyramidal cells in primary motor area from the corticospinal or pyramidal tracts in spinal cord. Damage to these tracts causes flaccid paralysis. These axons run through and make up most of the cerebral peduncles. Pyramidal cells are those that respond to conscious thought to contract skeletal muscle

2. Premotor cortex (motor association area) a. Location: anterior to primary motor area in frontal lobe b. Function – store instructions that initiates movements that we learn to do (e.g., walk,

run, type, throw a ball) 1. This is where we plan our skeletal muscle movement (motor) behavior. This area of

the brain determines the degree and sequence of skeletal muscle contractions for an action such as walking, dancing, typing, or running.

2. Most of its neurons project into the primary motor area. Some of the neurons of the premotor area (perhaps 15%) directly influence muscle control and axons from this region travel within the pyramidal tracts

3. Controls learned skilled motor activities that involve the sequential contraction of groups of skeletal muscles. It coordinates learned motor activity

4. It serves as a memory bank for these learned movements (walking, writing, typing, piano playing, running...)

c. For a given movement, the instruction sets of the premotor cortex do the following 1. determine which skeletal muscles contract 2. determine the sequence of muscle contractions (which ones contract first, then

second, etc.) 3. Determine the force and direction of contraction

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Associative or Integrative Functions 1. prefrontal cortex of frontal lobe (higher brain functions) a. this is one area of the cerebrum that sets humans apart from other animals b. contains very complex neural circuits associated with intelligence, complex learning,

personality, ability to reason and make judgments (weigh options), ability to plan ahead, “see” the future (envision and prepare for death) and manipulate abstract symbols (letters of the alphabet and numbers). It is also linked with mood and empathy for others. Our brains allow for the development of complex social structures and technology.

2. associative areas within the brain allow for memory, sleep and wakefulness, and emotional responses

3. Memory and learning - no satisfactory explanation for how we learn and remember information

4. Human brain cells know they exist. They have consciousness. Brain cells of one person can “talk” to those of another person

Cerebellum 1. works with motor areas of the cerebrum to coordinate skeletal muscle contractions and

maintain balance; operates unconsciously, thus we have no awareness of what it is doing 2. second largest portion of the brain (about 10% of total mass of brain) 3. Determination of Motor Error (similar to a coach) a. The cerebellum compares what you intend to do with what you are actually doing and

determines Motor Error b. Motor Error is the difference between intended movement and the actual movement

taking place

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c. The correction of the motor error is then sent to the premotor cortex in the cerebrum and can be added to the “motor program” stored there.

4. Input a. motor areas of cerebral cortex so that the cerebellum is aware of what want to do with

our muscles in order to compare it with what is actually occurring (the cerebellum monitors the cerebral motor center’s intentions); motor centers of cerebrum via nerve impulses notify cerebellum of their intent to initiate voluntary muscle contractions

b. proprioceptors in joints and muscle (muscle movement and force of contraction) send impulses to the cerebrum by way of the spinocerebellar tracts so that the cerebellum is aware of actual body movements (e.g., force of contraction and direction of movement)

c. eyes (visual images) and ears (semicircular canals transmit impulses about position of head relative to the body) in order to monitor information about balance

5. Output: primarily to the motor areas of the cerebral cortex to inhibit or activate skeletal muscle; cerebellum allows for precise timing of muscle contractions so that get coordinated movements

6. Function – the cerebellum monitors muscle contractions and the movement of body parts to help a person coordinate their movements. It is also involved in spatial perception.

a. one is not consciously aware when the cerebellum is active b. it helps to coordinate skeletal muscle contractions that occur during movements such

as walking, running, throwing, etc. c. Compares the intentions for movement with the actual movement and makes

corrections in the contraction of skeletal muscles participating in the activity via its output to the motor cortex

d. Coordinates skilled skeletal muscle activity and helps to maintain one's balance e. important to many sports - dancing, tennis, gymnastics, diving, kicking soccer ball,

basketball 4. Ataxia (a-without, taxi-movement; without coordinated movements)

a. damage to cerebellum results in ataxia - movements are jerky and uncoordinated - cannot coordinate body movement with a sense of where a body part is

b. blind-folded individuals with ataxia could not touch the tip of their nose with a finger c. ataxic individual may knock over a glass of water while trying to pick it up and they have

difficulty climbing stairs d. ataxic patients have difficulty putting pegs in a peg board

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Diencephalon (III ventricle, thalamus, hypothalamus, pineal gland, pituitary gland, mammillary bodies are part of the hypothalamus). Some consider the diencephalon to be part of the brainstem. 1. Surrounded by the cerebral hemispheres and most of it cannot be observed from the

outside of the brain 2. Thalamus (relay station); makes up nearly 80% of diencephalon a. “gateway” to the cerebral cortex for nearly all sensory input (e.g., taste, hearing, vision,

equilibrium, touch, pain, pressure, heat, cold); nearly all sensory information to cerebrum passes through the thalamus

b. receives sensory information from most peripheral sensory receptors c. routes the information to the sensory areas of the cerebral cortex for localization and

interpretation; synapse of second and third order interneurons 3. Hypothalamus

a. Located below the thalamus. It forms the sides and floor of the third ventricle. Cup-shaped with a hollow interior (III ventricle) filled with CSF

b. Major visceral control center involved with many homeostatic mechanisms. c. Some functions

1. the hypothalamus secretes hormones (often called releasing or inhibitory factors) that control the release of pituitary hormones (GH, FSH, LH, TSH, ACTH...). These hormones control a wide variety of metabolic processes within the body to include growth, basal metabolic rate, reproductive functions, and stress responses.

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2. Thermoregulation – the hypothalamus acts like a “thermostat” - monitors temperature of blood and stimulates processes that regulate body temperature within very narrow limits a. too hot (above 98.6oF or 37oC ) – stimulates vasodilation of blood vessels in the

skin and stimulates sweat glands to make sweat (evaporative cooling); this occurs via sympathetic neurons

b. too cold (less than 98.6oF) - increase muscle tone in skeletal muscle (shiver), increase release of thyroxine to step up metabolic rate; also stimulates arrector pili muscles to contract to give goosebumps (piloerection)

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3. Water Balance - osmoceptors sensitive to hypertonicity of blood a. hypertonic blood plasma perfusing hypothalamus (dehydrated) - increase

release of ADH from pituitary (conserve body water because urinate less) b. hypotonic (overhydrated) - inhibit ADH release (form more urine to expel excess

water) 4. also monitors nutrient levels in the blood to control the release of growth hormone

and ACTH

4. Pituitary Gland (hypophysis) a. anterior pituitary (adenohypophysis) 1. made up of mostly epithelial cells 2. the release of hormones from the anterior pituitary is controlled by the

hypothalamus (FSH, LH, GH, ACTH, TSH, prolactin) b. posterior pituitary (neurohypophysis)– part of the diencephalon since it is an extension

of the hypothalamus and secretes hormones produced in the hypothalamus like ADH and OXT

5. Pineal gland (within the epithalamus of the diencephalon) a. secretes a sleep-inducing hormone called melatonin (amine hormone derived from

serotonin) that helps to regulate sleep-wake cycles (diurnal or circadian rhythm of 16 hours up and 8 hours asleep)

b. the pineal gland receives input from the retina of the eye so that tends to regulate the release of melatonin such that one is active during the day when the sun is up and sleepy at night. Sunlight decreases melatonin release.

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c. more melatonin is released during darkness than during lighted conditions (10x more). Melatonin induces sleep. Overnight its secretory rate decreases. Melatonin levels are rather low during the approx. 16 hours when one is awake.

d. overproduction of melatonin can lead to drowsiness and depression. This can occur during long dark winter months up north and lead to Seasonal Affective Disorder (SAD). Treatment for SAD is 2-4 hours of exposure to full-spectrum bright lights.

Brainstem 1. Components - midbrain (cerebral peduncles, cerebral aqueduct and corpora quadrigemina),

pons, and medulla 2. Medulla Oblongata (medulla) - Found between the pons and the spinal cord. It contains

several important reflex centers that control vital autonomic functions a. Cardiac Center - regulate heart rate and the strength of the heart beat b. Vasomotor Center - regulate blood pressure via sympathetic output to arteriole smooth

muscle (vasoconstriction) and reroutes blood from one part of the body to another c. Respiratory Center - operates in conjunction with a center in the pons to control the

rate and depth of breathing (controls contraction of the diaphragm); sends impulses to somatic motor neurons whose axons travel out to the diaphragm by way of spinal nerves originating from the C2-C4 section of the spinal cord

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Bony Protection of CNS 1. the brain and spinal cord are encased in bone 2. skull surrounds brain 3. spinal cord runs through the vertebral canal formed by the vertebral arches (vertebral

foramina) of adjacent vertebrae Meninges (sing. meninx) 1. CT coverings over the brain and spinal cord 2. consist of 3 layers of CT 3. Dura mater (outer) a. dense fibrous CT (thick as a rubber kitchen glove) a. cranial dura (two-layered) 1. periosteal layer (forms periosteum of overlying bone) 2. meningeal layer (conforms to the surface of the brain and continuous with spinal

dura) b. spinal dura (only consists of one layer) c. folds of the cranial dura 1. falx cerebri – extension of dura into longitudinal fissure that attaches to crista

galli of ethmoid bone 2. falx cerebelli – separates the cerebellar hemispheres 3. tentorium cerebelli – in transverse fissure; separates the cerebellum from the

cerebrum b. dural sinuses 1. splits in the cranial dura mater that serve as veins that drain blood off the brain to

the jugular veins then eventually back to the heart

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2. cerebrospinal fluid drains into the dural sinuses from the subarachnoid space through arachnoid villi

c. dural sinuses carry blood to the jugular veins that carry blood back towards the heart d. epidural space 1. fat-filled space between the spinal dura and the overlying vertebral arches 2. Epidural anesthetic given during labor just prior to birth a. inject a local anesthetic into the epidural space around L3 b. diffuses into the spinal nerves and deadens sensory impulses from nociceptors

(pain) c. reduces most of the pain of childbirth from mid-chest to toes 4. arachnoid mater (middle meninx) a. thin, delicate membrane that adheres to the underside of the dura b. subarachnoid space 1. CSF-filled space beneath the arachnoid 2. held open by CT plates called trabeculae (cobwebby or spidery appearance); thread-

like extensions of arachnoid that connect to the pia mater 3. arachnoid – means spidery (arachnophobia – fear of spiders) c. arachnoid villi (also called granulations) 1. project up into the dural sinuses 2. CSF is secreted through them into the blood plasma within the dural sinuses 5. pia mater (inner meninx) a. thin and translucent (clear) b. adheres to surface of CNS (only meninx that contacts brain) c. conforms to ridges and sulci (follows topography of the brain and spinal cord) 6. Filum terminale a. filament-like extension of pia mater b. it begins at L2 where the spinal cord ends c. it travels through the sacral canal and attaches to the coccyx d. Function: keep the spinal cord taut at all times (no slack in it)

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Dura mater and dural sinuses

Ventricles of the Brain and Cerebrospinal Fluid 1. ventricles consist of 4 interconnected cavities within the brain 2. ventricles are continuous with the central canal of the spinal cord and the subarachnoid

space within the meninges; ventricles are lined with ependymal cells (type of neuroglial cell) 3. 4 ventricles a. lateral ventricles (I and II) 1. one in each cerebral hemisphere 2. separated by a thin partition called the septum pellucidum 3. each is connected to the III ventricle by an interventricular foramen (foramen of

Munro) b. III ventricle 1. small slit-like cavity in the diencephalon 2. the hypothalamus forms the sides and floor of the III ventricle 3. drains into the cerebral aqueduct, which then drains into the IV ventricle c. IV ventricle 1. found between the cerebellum and the medulla 2. the IV ventricle is continuous with the central canal and drains into the

subarachnoid space by way of the median (1) and lateral apertures (2)

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4. Cerebrospinal Fluid (CSF) a. CSF is modified blood plasma (mostly water and dissolved solutes) that is filtered

through ependymal cells and into the ventricles and the subarachnoid space b. choroid plexus 1. capillary beds on the sides of each of the 4 ventricles that continuously secrete CSF

into the ventricle cavities 2. capillaries are covered with ependymal cells joined by tight junctions; the ciliated

ependymal cells help to circulate the CSF 3. the choroid plexuses as well as cells that line the canals and the subarachnoid space

secrete about 150 ml of CSF every 8 hours (about 500 mL per day). The choroids plexuses make about 30% of the CSF

c. CSF circulation 1. choroid plexuses secrete CSF into each ventricle 2. CSF flows from the lateral ventricles into the III ventricle through the interventricular

foramina 3. CSF flows from the III ventricle into the cerebral aqueduct 4. CSF in the cerebral aqueduct flows into the IV ventricle 5. CSF flows out of the IV ventricle and into the central canal of the spinal cord 6. CSF flows out of the IV ventricle through the median and lateral apertures into the

subarchnoid space 7. CSF flows through the arachnoid villi and into the blood plasma within the dural

sinuses 8. the rate of secretion equals the rate of reabsorption, hence the volume of CSF stays

relatively constant over time

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d. Function 1. the CSF acts primarily as a protective “jacket” of water around the brain and spinal

cord that absorbs shocks that would otherwise be transmitted to the neurons of the CNS

2. shock absorption function helps to protect the delicate neurons 3. CSF reduces the weight of the brain by 97% (brain tends to “float” within the CSF)

Hydrocephalus (“water on the brain”) 1. may occur in infants (can occur in adults as well) if structural

problems occur (e.g., tumor, congenital defects) that block drainage of the CSF out of the 4th ventricle and into the subarachnoid space; if this happens, then CSF accumulates in the ventricles

2. hydrocephalus leads to an enlarged head in newborns since the cranial sutures have not united and due to the presence of the fontanels (soft spots)

3. Damage from Hydrocephalus a. over time, the increased fluid pressure within the brain

ventricles can compress neurons and lead to brain damage

b. compress blood supply to the brain and cut off oxygen and nutrients

4. Treatment a. surgically correct the problem b. insert a tube into the IV ventricle that runs under the skin and empties into the

abdominal cavity; this shunts CSF out of the IV ventricle and allows it to simply drip into the abdominal cavity

Cerebrospinal Fluid and Brain Size 1. The brain is suspended in CSF and has neutral buoyancy. A human brain out of the body

weighs about 1500g, but when suspended in CSF its effective weight is about 50g. Consider how easy it is to lift a person in water as compared to land.

2. If the brain weren’t suspended and it rested on the floor of the cranial vault, then its weight would crush and kill the neurons on the bottom.

3. Suspension in the CSF allows the brain to be much larger than it would be otherwise without damage to the delicate neurons.

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Peripheral Nervous System (PNS) 1. the PNS consists of the cranial and spinal nerves and their branches that connect the CNS

to other body parts 2. Afferent or Sensory Pathways of the PNS: sensory neurons that convey impulses from

receptors into the CNS 3. Efferent or Motor Pathways of the PNS - axons of motor neurons that convey motor

impulses from the CNS to effector cells Efferent (motor) pathways of the PNS 1. Somatic (motor) Nervous System a. made up of somatic motor neurons that fire impulses in response to conscious thoughts

about contracting skeletal muscle b. somatic motor neurons originate in the brainstem (axons go out cranial nerves) and in

the ventral horns of the spinal cord gray matter (axons go out spinal nerves) c. control skeletal muscle contraction d. axons release ACh e. Cholinergic effects of ACh to stimulate skeletal muscle cells to contract 2. Autonomic (motor) NS (parasympathetic and sympathetic divisions) a. made up of autonomic motor neurons that automatically control the activity of visceral

organs (internal organs); they are under unconscious control b. originate in brainstem (axons go out cranial nerves) or in the lateral horns of the spinal

cord gray matter (axons go out spinal nerves) c. ANS effector cells 1. cardiac muscle (beating of the heart) 2. smooth muscle (walls of blood vessels, esophagus, stomach and intestine, urinary

bladder, uterus, etc.) 3. glandular epithelial cells (pancreas, liver, sweat glands)

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ANS has 2 divisions (parasympathetic and sympathetic) 1. parasympathetic division a. parasympathetic neurons originate in the brainstem (axons go out cranial nerves) and

from the lateral horns in the sacral segments (around L2) of the spinal cord gray matter (axons go out sacral spinal nerves)(craniosacral origin)

b. Vagus Nerve (Xth cranial nerve) arises from the medulla of the brainstem and carries about 80% of all the parasympathetic neurons in the body (parasympathetic fibers travel to visceral organs in the thoracic and abdominal cavities. Most parasympathetic neurons arise from the medulla oblongata.

c. parasympathetic neurons release ACh at cholinergic synapses d. ACh may inhibit or stimulate effector cells e. parasympathetic neurons are concerned with resting and digestive activities. 1. the parasympathetic system tends to be active all of the time (tonically active)

although its effects can be overridden by sympathetic discharge 2. help to maintain resting heart rate, resting breathing rate, help to empty bladder of

urine; helps to prevent the heart and lungs from working too hard while at rest 3. helps to digest food during and after a meal (stimulates all digestive activities)

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4. emotions like grief, sorrow and sadness stimulate the parasympathetic system (among other things, this will decrease heart rate and blood pressure and may make a person cry)

5. parasympathetic system is subordinate to the sympathetic system. Sympathetic discharge has stronger effects that overwhelm parasympathetic effects

2. sympathetic division a. sympathetic neurons originate within the lateral horns of the spinal cord in the thoracic

and lumbar regions (thoracolumbar origin) b. postganglionic sympathetic neurons axons release mostly norepinephrine (NEpi or

NorEpi); NorEpi is also called noradrenaline c. adrenergic effects of NEpi may inhibit or stimulate effector cells d. prepares the body for a response to some physiological or psychological emergency

(the sympathetic system is sometimes called the Fight or Flight Response System) e. in general, the sympathetic system is only activated when one experiences a

psychological or physiological stressful emergency; sympathetic effects are stronger than parasympathetic effects and overpower them

1. physiological stress: exercise (most common stressor to body), injury (tissue trauma), low blood sugar levels, hypothermia (extreme cold when core temperature drops below 95F or 35C)), high body temperatures (normal range of BT’s is 97.7-99.5 F or around 98.6F or37C)

2. psychological stress: fear, anxiety (fear of unknown), guilt, relationship problems (divorce, on-the-job tensions)

f. stress turns the sympathetic system on which results in 1. sympathetic neurons secreting NEpi 2. cells of the medulla of the adrenal gland (medulla) releasing Epi into bloodstream g. removal of the stress turns the sympathetic system off h. Sympathetic effects that occur when stressed 1. increase the heart rate and blood pressure during exercise 2. raise blood sugar (glucose) levels if they fall too low between meals 3. increase blood flow to skeletal muscle during exercise 4. stimulate activity of sweat glands if body temperature goes too high as in exercise i. the sympathetic system is either on or off. Stress turns it on and the removal of stress

turns it off

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Epinephrine (Epi) from the Adrenal Glands 1. the adrenal glands are on top of the kidneys 2. A sympathetic response of the body to stress is to send sympathetic impulses to the

adrenal gland and stimulate the adrenal gland to secrete primarily epinephrine (adrenaline) into the blood.

3. Epi is a neurohormone that circulates within the blood and triggers sympathetic effects on target cells throughout the body

4. 50% of all sympathetic effects are due to release of epinephrine into the bloodstream by the adrenal medulla when one is stressed; the remainder is due primarily to norepinephrine from postganglionic sympathetic neurons acting at synapses with effector cells

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Two Characteristics of ANS Dual innervation (brake and gas pedal analogy) and Opposing Functions 1. most visceral organs receive input from parasympathetic and sympathetic neurons 2. Opposing Functions: if one division stimulates, then the other inhibits 3. Example: Effect of ANS on heart rate a. parasym neurons go to heart and release Acetylcholine. ACh inhibits (brake) slows the

heart rate down b. sym neurons go to the heart and release of norepi. NorEpi stimulates (gas pedal)

increases the heart rate. Sym neurons go to the adrenal gland to stimulate the release of Epi which will increase heart rate

4. Example: Effect of ANS on digestive activities a. parasym neuron release of ACh stimulates digestive activities (secretion of enzymes,

secretion of pancreatic juice, peristalsis) b. sym neuron release of NEpi inhibits digestive activities (peristalsis). Sym neurons

stimulate the release of Epi from the adrenal gland which will decrease digestive activities

Two Neuron Outflow 1. both branches of the ANS are characterized by 2 neuron outflow 2. there are two neurons that travel out from the brainstem or spinal cord to effector cells;

they synapse inside a nerve or inside an organ a. preganglionic neuron (dendritic zone in CNS and axon goes into PNS) b. postganglionic neuron (peripheral neuron) 3. the neurotransmitter released from the postganglionic neuron is the one that regulates the

activity of the effector cell 4. Postganglionic neurotransmitters of the ANS a. Parasympathetic postganglionic fibers release acetylcholine from their axonal endings 1. acetylcholine mediates the parasympathetic effect 2. acetylcholine is released from cholinergic synapses b. Sympathetic postganglionic fibers release norepinephrine (noradrenaline) as a

neurotransmitter; Epi is released by special postganglionic cells in the adrenal gland into the blood as a neurohormone

1. NEpi and Epi produce adrenergic effects 2. Norepi and epi mediate the effects of the sympathetic nervous system (there are a

few exceptions to this, but will not be discussed); about 50% of effect are NEpi and 50% from Epi

3. Epinephrine is released from specialized sympathetic neurons in the medulla of the adrenal gland. Another name for Epi is adrenaline

c. Acetylcholine and norepinephrine are the two major neurotransmitters of the ANS; epinephrine acts mostly via the bloodstream as a neurohormone

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Exercise 1. stresses the cardiovascular and skeletomuscular systems of the body 2. exercise is the most common stimulus for the release of Epi and NEpi by the sympathetic NS 3. Adrenergic Effects a. incr HR, incr BP, incr SV, incr CO

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b. increase blood flow to skeletal muscle; decrease blood flow to skin, kidney and digestive organs

c. increase the availability of glucose for ATP synthesis; increase lipolysis to provide FA’s for ATP synthesis

d. bronchiodilation to increase diameter of airways in order to move more air into lungs when breath more heavily during exercise

ANS Receptor Types 1. Cholinergic receptors for ACh a. nicotinic (N) 1. binding of ACh to nicotinic receptors opens nonspecific cation channels that allow

mostly a Na+ influx that depolarizes the cell b. muscarinic (M) 1. binding of ACh to muscarinc receptors activate G proteins that stimulate second

messenger systems (e.g., cAMP) 2. Adrenergic receptors for NEpi and Epi a. 2 Major Classes: Alpha (α) and Beta (β) b. Subclasses 1. Alpha 1 (α1) and Alpha 2 (α2) 2. Beta 1 (β1) and Beta 2 (β2) c. all adrenergic receptors are coupled to G proteins 1. binding of Epi or NEpi to alpha 1 receptors stimulates an excitatory response in

effector cells via G protein induced increases in the IP3/Ca2+ second messenger system (e.g., contract smooth muscle in wall of blood vessel to cause vasoconstriction)

2. binding of Epi or NEpi to alpha 2 receptors blocks the production of cAMP in the target cells to bring about an inhibitory response (e.g., inhibit the contraction of smooth muscle)

3. binding of neurotransmitter to beta receptors (Beta 1 and Beta 2) brings about effects by stimulating cAMP production via G proteins in the target cells

a. Beta 1 receptors are usually associated with excitatory responses (e.g., increase the rate and force of the heart contraction)

b. Beta 2 receptors are usually associated with inhibitory responses (e.g., dilation of bronchioles as smooth muscle relaxes)

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