Central Nervous System Central nervous system (CNS) brain and spinal cord enclosed in bony coverings...

Central Nervous System

• Central nervous system (CNS)• brain and spinal cord enclosed in bony coverings

• Functions of the spinal cord• spinal cord reflexes• integration (summation of inhibitory and excitatory) nerve

impulses• highway for upward and downward travel of sensory and

motor information• Brain functions

• sensations, memory, emotions, decision making, behavior

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Overview of Spinal Cord

• Spinal cord - Information highway between brain and body

• Each pair of spinal nerves receives sensory information and issues motor signals to muscles and glands

• Spinal cord is a component of the CNS while the spinal nerves are part of the PNS

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Functions of the Spinal Cord• Conduction

• bundles of fibers passing information up and down spinal cord• connecting different levels of the trunk with each other and the brain

• Neural integration - input from multiple sources, integrated, and executed output

• Locomotion• repetitive, coordinated actions of several muscle groups• central pattern generators are pools of neurons providing control

of flexors and extensors (walking)

• Reflexes• involuntary, stereotyped responses to stimuli (remove hand from

hot stove)• involves brain, spinal cord and peripheral nerves

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Anatomy of the Spinal Cord

• Spinal cord - cylinder of nervous tissue that arises from the brainstem at the foramen magnum of the skull

• Passes through the vertebral canal (thick as a finger)• vertebral column grows faster so in an adult the spinal cord only extends to L1

• 31 pairs of spinal nerves arise from cervical, thoracic, lumbar and sacral regions of the cord

• First pair passes between the skull and C1• Rest pass through intervertebral foramina

• Cord segment - part of the spinal cord supplied by each pair of spinal nerves

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Surface Anatomy

Figure 13.1

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Spinal cord

Spinal nerve rootlets

Spinal nerve

Subarachnoid space

Posterior median sulcus

Posterior root ganglion

Rib

Epidural space

C1

C7

S5

Col

T12

L5

Cervicalenlargement

Duralsheath

Subarachnoidspace

Cervicalspinalnerves

Thoracicspinalnerves

Lumbarenlargement

Medullarycone

Cauda equina

Lumbarspinalnerves

Terminalfilum

Sacralspinalnerves

(b)

(a)

Dura mater

Arachnoid mater

Vertebra (cut)

Meninges of the Spinal Cord

• 3 Fibrous layers enclosing spinal cord• Dura mater

• tough collagenous membrane surrounded by epidural space filled with fat and blood vessels

• epidural anesthesia utilized during childbirth

• Arachnoid mater• layer of simple squamous epithelium lining dura mater and

loose mesh of fibers filled with CSF(creates subarachnoid space)

• Pia mater• delicate membrane adherent to spinal cord

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Meninges of Vertebra and Spinal Cord

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Cross-Sectional Anatomy of the Spinal Cord

• Central area of gray matter shaped like a butterfly and surrounded by white matter in 3 columns

• Gray matter = neuron cell bodies with little myelin• White matter = myelinated axons

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Gray Matter in the Spinal Cord

• Pair of dorsal or posterior horns• dorsal root of spinal nerve is totally sensory fibers

• Pair of ventral or anterior horns• ventral root of spinal nerve is totally motor fibers

• Connected by gray commissure punctured by a central canal continuous above with 4th ventricle

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White Matter in the Spinal Cord• White matter of the spinal cord surrounds the gray matter

• White column = bundles of myelinated axons that carry signals up and down to and from brainstem

• 3 pairs of columns or funiculi• Dorsal, lateral, and anterior columns• Tracts or fasciculi - subdivisions of each column• Ascending and descending tract head up or down • Contralateral means origin and destination are on opposite sides while ipsilateral

means on same side

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Ascending Tracts

Figure 13.5a


Gracile fasciculusCuneate fasciculus

Midbrain

Medulla

Spinal cord

Thalamus

Cerebrum

Receptors for body movement, limb positions,fine touch discrimination, and pressure

First-orderneuron

Gracilenucleus

Mediallemniscus

Second-orderneuron

Mediallemniscus

Cuneatenucleus

Third-orderneuron

Somesthetic cortex(postcentral gyrus)

(a)

• Ascending tracts carry sensory signals up the spinal cord

• Sensory signals travel across three neurons from origin in receptors to the destination in the sensory areas of the brain

• First-order neurons: detect stimulus and transmit signal to spinal cord or brainstem

• Second-order neurons: continues to the thalamus at the upper end of the brainstem

• Third-order neurons: carries the signal the rest of the way to the sensory region of the cerebral cortex

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• Descending tracts - carry motor signals down the brainstem and spinal cord

• Involve two neurons• Upper motor neuron originates in

cerebral cortex or brainstem and terminates on a lower motor neuron

• Lower motor neuron in brainstem or spinal cord

• Axon of lower motor neuron leads the rest of the way to the muscle or other target organ

Descending Tracts

Figure 13.6


Midbrain

Cerebrum

Medulla

Spinal cord

Spinal cord

To skeletal muscles

Motor cortex(precentral gyrus)

Internalcapsule

CerebralpeduncleUpper motorneurons

MedullarypyramidDecussationin medullaLateralcorticospinaltractAnteriorcorticospinaltractDecussation inspinal cordLower motorneurons

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General Anatomy of Nerves and Ganglia

Figure 13.8a


Rootlets

Posterior root

EpineuriumFascicle

Myelin

Perineurium

Endoneurium

Unmyelinated nerve fibers

Myelinated nerve fibers(a)

Anteriorroot

Posterior rootganglion

Spinalnerve

Bloodvessels

• Nerve - a cordlike organ composed of numerous nerve fibers (axons) bound together by connective tissue• Mixed nerves contain both afferent

(sensory) and efferent (motor) fibers

• Blood vessels penetrate connective tissue coverings• Nerves have high metabolic rate

and need plentiful blood supply

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General Anatomy of Nerves and Ganglia

• Ganglion—cluster of neurosomas outside the CNS• Enveloped in an endoneurium continuous with that of the nerve

• Among neurosomas are bundles of nerve fibers leading into and out of the ganglion

• Posterior root ganglion associated with spinal nerves

Figure 13.9


Motor nerve fibers

Posterior root

Anterior root

Spinal cord

Anterior root

Spinal nerve

Posterior root ganglion

Epineurium

Blood vessels

Sensory nerve fibers

Direction ofsignal transmission

Posterior rootganglion

Motorpathway

Sensorypathway

Somatosensoryneurons

To spinal cord To peripheralreceptors and effectors

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Spinal Nerves

• 31 pairs of spinal nerves (mixed nerves)• 8 cervical (C1–C8);

• C1 between skull and atlas • Others exiting at intervertebral foramen

• 12 thoracic (T1–T12)• 5 lumbar (L1–L5)• 5 sacral (S1–S5)• 1 coccygeal (Co)

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Spinal Nerves

• Proximal branches forming spinal nerves• Each spinal nerve has two points of attachment to the spinal cord

• Posterior (dorsal) root is sensory input to spinal cord• Posterior (dorsal) root ganglion—contains the somas of sensory neurons carrying signals

to the spinal cord• Six to eight rootlets that enter posterior horn of cord

• Anterior (ventral) root is motor output out of spinal cord• Six to eight rootlets that leave spinal cord and converge to form anterior root

• These merge to form spinal nerve proper that enters intervertebral foramen• Cauda equina: formed from roots arising from L2 to Co

• Distal branches of spinal nerves• Distal to vertebral foramen, the nerve divides into:

• Anterior ramus: innervates the anterior and lateral skin and muscles of the trunk• Gives rise to nerves of the limbs

• Posterior ramus: innervates the muscles and joints in that region of the spine and the skin of the back

• Meningeal branch: reenters the vertebral canal and innervates the meninges, vertebrae, and spinal ligaments

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Spinal Nerves

Figure 13.11


Anterior root

Posterior root

Spinal nervePosterior root ganglion

Sympathetic ganglion

Communicating rami

Posterior ramus

Anterior ramus Meningeal branch

Anterior

Deep muscles of back

Spinal cord

Posterior

Vertebral body

Transverse processof vertebra

Spinous processof vertebra

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Nerve Plexuses

• Anterior rami branch and anastomose repeatedly to form five nerve plexuses

• Cervical plexus in the neck, C1 to C5 • Supplies neck and phrenic nerve to the diaphragm

• Brachial plexus near the shoulder, C5 to T1• Supplies upper limb and some of shoulder and neck• Median nerve—carpal tunnel syndrome

• Lumbar plexus in the lower back, L1 to L4• Supplies abdominal wall, anterior thigh, and genitalia

• Sacral plexus in the pelvis, L4, L5, and S1 to S4• Supplies remainder of lower trunk and lower limb

• Coccygeal plexus, S4, S5, and Co

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The Sacral and Coccygeal Plexuses

Figure 13.18


Superior gluteal nerve

Inferior gluteal nerve

Pudendal nerve

L5

L4

S1

S2

S3

S4

S5

Co1

Posterior divisions

Roots

Anterior divisions

Sciatic nerve: Common fibular nerve

Lumbosacraltrunk

Tibial nerve

Posterior cutaneousnerve

Reflexes

• Reflexes - quick, involuntary, stereotyped reactions of glands or muscle to stimulation

• Automatic response to change in environment

• Integration center for spinal reflexes is gray matter of spinal cord

• Examples• somatic reflexes result in skeletal muscle contraction• autonomic (visceral) reflexes involve smooth & cardiac muscle and

glands.• heart rate, respiration, digestion, urination, etc

• Reflexes can be: • simple

• involve peripheral nerves and the spinal cord• spinal reflexes

• learned (acquired)• involve peripheral nerves and the brain

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Reflex Arc

• Specific nerve impulse pathway -5 components of reflex arc• Somatic Receptor

• Registers stimuli • Sensory neuron - afferent nerve fibers

• Carry information from receptors to posterior horn of spinal cord or the brainstem

• Integrating center• Part of the CNS that processes the information and generates response

• Motor neuron• Transmits the response to the effector

• Effector • Muscle or gland

• 4 important somatic spinal reflexes• Stretch reflex - when a muscle is stretched, it “fights back” and contracts • Tendon reflex - contraction of a muscle when its tendon is tapped

• in response to excessive tension on the tendon

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The Patellar Tendon Reflex Arc

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Central Nervous System-The Brain

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• Evolution of the central nervous system shows spinal cord has changed very little while brain has changed a great deal• Greatest growth in areas of vision, memory, and motor control of the

prehensile hand

The Brain

• Longitudinal fissure - cerebral hemispheres.• gyri = folds; sulci = grooves• cortex = surface layer of gray matter • nuclei = deeper masses of gray matter • tracts = bundles of axons (white matter)

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Median Section of the Brain

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• Three major portions of the brain

• Cerebrum is 83% of brain volume; cerebral hemispheres, gyri and sulci, longitudinal fissure, corpus callosum

• Cerebellum contains 50% of the neurons; second largest brain region, located in posterior cranial fossa

• Brainstem is the portion of the brain that remains if the cerebrum and cerebellum are removed; diencephalon, midbrain, pons, and medulla oblongata

Gray and White Matter

• Gray matter = neuron cell bodies, dendrites, and synapses• forms cortex over cerebrum and cerebellum• forms nuclei deep within brain

• White matter = bundles of axons• forms tracts that connect parts of brain

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Meninges of the Brain

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Brain Ventricles

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• Ventricles - four internal chambers within the brain• Two lateral ventricles: one in each cerebral hemisphere

• Interventricular foramen—a tiny pore that connects to third ventricle• Third ventricle: single narrow medial space beneath corpus callosum

• Cerebral aqueduct runs through midbrain and connects third to fourth ventricle• Fourth ventricle: small triangular chamber between pons and cerebellum

• Connects to central canal, runs down through spinal cord• Ependyma - neuroglia that lines the ventricles and covers choroid plexus

• Produces cerebrospinal fluid

• Choroid plexus - spongy mass of blood capillaries on the floor of each ventricle

Cerebrospinal Fluid

• Fills ventricles and subarachnoid space

• Brain produces and absorbs 500 ml/day• choroid plexus creates by filtration of blood

• Functions• floats brain so it is neutrally buoyant• cushions from hitting inside of skull• chemical stability -- rinses away wastes

• Escapes (4th ventricle) to surround brain

• Absorbed into venous sinus by arachnoid villi

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Ventricles and Cerebrospinal FluidCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Arachnoid villus

Choroid plexus

Third ventricle

Lateral aperture

Fourth ventricle

Median aperture

Dura mater

Arachnoid mater

1

2

3

4

56

7

7

8

1

2

3

4

5

6

7

8

Superiorsagittalsinus

Subarachnoidspace

Cerebralaqueduct

Central canalof spinal cord

Subarachnoidspace ofspinal cord

CSF is secreted bychoroid plexus ineach lateral ventricle.

CSF flows throughinterventricular foraminainto third ventricle.

Choroid plexus in thirdventricle adds more CSF.

CSF flows down cerebralaqueduct to fourth ventricle.

Choroid plexus in fourthventricle adds more CSF.

CSF fills subarachnoid space andbathes external surfaces of brainand spinal cord.

At arachnoid villi, CSF is reabsorbedinto venous blood of duralvenous sinuses.

CSF flows out two lateral aperturesand one median aperture.

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Blood Supply and the Brain Barrier System

• Brain is only 2% of the adult body weight, and receives 15% of the blood - 750 mL/min

• Blood is also a source of antibodies, macrophages, bacterial toxins, and other harmful agents

• Brain barrier system—strictly regulates what substances can get from the bloodstream into the tissue fluid of the brain

• Two points of entry must be guarded• Blood capillaries throughout the brain tissue• Capillaries of the choroid plexus

Hindbrain - Medulla Oblongata

• Cardiac center • adjusts rate and force of heart

• Vasomotor center • adjusts blood vessel diameter

• Respiratory centers • control rate and depth of breathing

• Reflex centers • for coughing, sneezing, gagging, swallowing,

vomiting, salivation, sweating, movements of tongue and head

• Most of the nerve fibers are crossing over• Left cortex controls right side of body

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Pons

• Bulge in brainstem, superior to medulla• Ascending sensory tracts• Descending motor tracts• Pathways in and out of cerebellum

• Nuclei • concerned with posture, sleep, hearing,

balance, taste, eye movements, facial expression, facial sensation, respiration, swallowing, and bladder control

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Cerebellum

• Two hemispheres connected by vermis

• Cortex = surface folds called folia

• Output comes from deep gray nuclei

• granule and purkinje cells

• White matter (arbor vitae) visible in sagittal section

• Evaluation of sensory input• coordination and locomotor ability• spatial perception

• Timekeeping center• predicting movement of objects

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Midbrain - Cross Section

• Tegmentum • connects to cerebellum and

helps control fine movements through red nucleus

• Substantia nigra • sends inhibitory signals to

basal ganglia and thalamus (degeneration leads to tremors and Parkinson disease)

• Central gray matter = pain awareness

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Reticular Activating System

• Scattered nuclei in medulla, pons & midbrain

• Reticular activating system• alerts cerebral cortex to sensory signals (sound of

alarm, flash light, smoke or intruder) to awaken from sleep

• maintains consciousness & helps keep you awake with stimuli from ears, eyes, skin and muscles

• Motor function is involvement with maintaining muscle tone

• Somatic motor control

• Cardiovascular control

• Pain modulation

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Diencephalon: Thalamus and Hypothalamus

• Thalamus Functions• Relays signals from cerebellum to motor

cortex• Emotional and memory functions

• Hypothalamus Functions• hormone secretion• autonomic NS control• thermoregulation• food and water intake (hunger and satiety)• sleep and circadian rhythms• memory (mammillary bodies)• emotional behavior

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Cerebrum -- Gross Anatomy

• Cerebral cortex - 3mm layer of gray matter• extensive folds increase surface area - divided into lobes

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Functions of Cerebrum - Lobes

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• Frontal lobe• Voluntary motor functions • Motivation, foresight, planning, memory, mood, emotion, social judgment, and

aggression

• Parietal lobe• Receives and integrates general sensory information, taste, and some visual

processing

• Occipital lobe• Primary visual center of brain

• Temporal lobe• Areas for hearing, smell, learning, memory, and some aspects of vision and

emotion

• Insula (hidden by other regions)• Understanding spoken language, taste and sensory information from visceral

receptors

Tracts of Cerebral White Matter

• Most of cerebrum is white matter• Types of tracts

• projection tracts • from brain to spinal cord, forms internal

capsule

• commissural tracts• cross to opposite hemisphere

• corpus callosum• anterior and posterior commissures

• association tracts• connect lobes and gyri within a

hemisphere

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Limbic System

• Loop of cortical structures• amygdala, hippocampus and cingulate gyrus (arches over the top of the corpus callosum

in the frontal and parietal lobes)

• Role in emotion and memory • pleasure and aversion centers

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Memory

• Information management • requires learning, memory and forgetting

• Amnesia• anterograde amnesia - no new memories• retrograde amnesia – can’t remember old ones

• Hippocampus • organizes sensory and cognitive information into a new memory

• Cerebellum • helps learn motor skills

• Amygdala • emotional memory

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General Properties of the Autonomic Nervous System

• Autonomic nervous system (ANS)—a motor nervous system that controls glands, cardiac muscle, and smooth muscle

• Also called visceral motor system• Primary organs of the ANS

• Viscera of thoracic and abdominal cavities• Some structures of the body wall

• Cutaneous blood vessels• Sweat glands• Piloerector muscles

• Carries out actions involuntarily: without our conscious intent or awareness

• Visceral effectors do not depend on the ANS to function; only to adjust their activity to the body’s changing needs

• Denervation hypersensitivity - exaggerated response of cardiac and smooth muscle if autonomic nerves are severed

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Visceral Reflexes

• Visceral reflexes - unconscious, automatic, stereotyped responses to stimulation involving visceral receptors and effectors and somewhat slower responses

• Visceral reflex arc• Receptors: nerve endings that detect stretch, tissue damage, blood

chemicals, body temperature, and other internal stimuli• Afferent neurons: leading to the CNS• Interneurons: in the CNS• Efferent neurons: carry motor signals away from the CNS• Effectors: that make adjustments

• ANS modifies effector activity

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Visceral Reflexes

• Example of homeostatic negative feedback loop

• High blood pressure detected by arterial stretch receptors (1), afferent neuron (2) carries signal to CNS, efferent (3) signals travel to the heart, then (4) heart slows reducing blood pressure

Figure 15.1


Vagus nervetransmitsinhibitorysignalsto cardiacpacemaker

3

Glossopharyngealnerve transmits signalsto medulla oblongata

2

Baroreceptorssense increasedblood pressure

1

Common carotidartery

Heart ratedecreases

4

Terminalganglion

Autonomic Nervous System

• Visceral motor neurons control• heart rate• breathing rate• digestion• blood pressure• salivation

• Nerve impulses of these motor neurons start in the CNS (medulla oblongata and pons)

• Pathway through:• Spinal cord• Cranial nerves

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Divisions of the ANS

• Two divisions innervate same target organ• May have cooperative or contrasting effect• Prepares body for physical activity: exercise, trauma, arousal,

competition, anger, or fear• Increases heart rate, BP, airflow, blood glucose levels, etc.• Reduces blood flow to the skin and digestive tract

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Sympathetic Division

• The sympathetic division is called the “fight or flight” system

• when the body needs to generate energy• exercise, excitement, emergency, and embarrassment

• Fight or flight response• increases heart rate, blood pressure, respiration rate, blood flow

to skeletal muscles, glucose metabolism • decreases the activities that are not essential at the moment

(digestive system organs are subdued- decreased blood flow to that system

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Parasympathetic Division

• The parasympathetic division is called the “rest and digest”

• activated when the body needs to conserve energy• digestion, defecation, and diuresis (urination)

• Promotes necessary changes during these activities• decreases heart rate, blood pressure, respiration rate, blood flow

to skeletal muscles, glucose metabolism • increases the activity of and blood flow to the digestive system

organs

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Somatic effectors(skeletal muscles)

Visceral effectors(cardiac muscle,smooth muscle, glands)

Autonomicganglion

Unmyelinatedpostganglionic fiber

ACh or NEACh

Myelinatedpreganglionic fiber

Myelinatedfiber

ACh

Autonomic efferent innervation

Somatic efferent innervation

Autonomic Output Pathways

ANS—two neurons from CNS to effectors• Presynaptic neuron cell body is in CNS

• Postsynaptic neuron cell body is in peripheral ganglion

Figure 15.2


Efferent Sympathetic vs. Parasympathetic

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NE – Norepinephrine – Adrenergic fibers – adrenergic receptors

Ach – Acetylcholine – Cholinergic fibers-cholinergic reseptors

Organization of the Sympathetic and Parasympathetic Division

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Effects of Neurotransmitters of the Autonomic Nervous System

• The cells of each organ controlled by the ANS have both ACh and NE receptors

• organs are dually controlled• The response of the organ is determined by the identity of the

neurotransmitter released• the binding of ACh to its receptor will cause the effector to respond

in one way• the binding of NE to its receptor will cause the effector to respond in

the opposite way• The effect of ACh and NE is effector specific

• NE increases heart rate, ACh decreases heart rate• NE decreases the secretion of saliva, ACh increases the secretion of

saliva

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Central Nervous System Central nervous system (CNS) brain and spinal cord enclosed in bony coverings...

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