the nervous system i: the spinal cord and spinal nerves anatomy & physiology i chapter 9

The Nervous System The Nervous System I:I:

The Spinal Cord and Spinal The Spinal Cord and Spinal NervesNerves

Anatomy & Physiology IChapter 9

Coordination and Control Coordination and Control endocrine and nervous system

maintain internal communication and coordination

◦endocrine system - communicates by means of chemical messengers (hormones) secreted into to the blood

◦nervous system - employs electrical and chemical means to send messages from cell to cell

Functions of the Nervous Functions of the Nervous SystemSystemThe nervous system carries out its task

in three basic steps:

1. Sensory input◦ Information gathered by sensory

receptors about internal and external changes

◦

2. Integration◦ Interpretation of sensory input by the

CNS

3. Motor output◦ Activation of effector organs (muscles

and glands) produces a response

Sensory input

Motor output

Integration

Two Major Divisions of Two Major Divisions of Nervous SystemNervous Systemcentral nervous system (CNS)

◦brain and spinal cord◦Integration and command center

peripheral nervous system (PNS)◦all the nervous system except the

brain and spinal cord◦nerves and ganglia◦nerves carry messages to and from

the CNS

PNS: Nerves and GangliaPNS: Nerves and Ganglianerve – a cable-like bundle of

axons wrapped in connective tissue◦found only in the PNS

ganglion – a knot-like bundle of neuron cell bodies wrapped in connective tissue

Dendrites(receptive regions)

Cell body (Soma)

Axon(impulse generating

and conducting region)

Impulsedirection

Typical Typical NeuronNeuron

(nerve cell)(nerve cell)

Subdivisions of Nervous Subdivisions of Nervous SystemSystem

Brain

Nerves

Ganglia

Peripheral nervoussystem (PNS)

Central nervoussystem (CNS)

Spinal cord

Peripheral Nervous System Peripheral Nervous System (PNS)(PNS) Two functional divisions:

1. Sensory (afferent) division Transmits signal from receptors to the

CNS Somatic sensory fibers—convey

impulses from skin, skeletal muscles, bones, and joints

Visceral Sensory fibers—convey impulses from visceral organs

2. Motor (efferent) division Transmits impulses from the CNS to

effectors Effectors are muscles and glands

Motor Division of PNSMotor Division of PNS

1. Somatic motor division (voluntary) ◦ Conscious control of skeletal muscles

2. Visceral motor division (involuntary) ◦ Also called the autonomic nervous

system (ANS)◦ Regulates smooth muscle, cardiac

muscle, and glands◦ Two functional subdivisions

Sympathetic Parasympathetic

Central nervous system (CNS)

Brain and spinal cordIntegrative and control centers

Peripheral nervous system (PNS)

Cranial nerves and spinal nervesCommunication lines between the

CNS and the rest of the body

Parasympatheticdivision

Conserves energyPromotes house-keeping functions

during rest

Motor (efferent) division

Motor nerve fibersConducts impulses from the CNSto effectors (muscles and glands)

Sensory (afferent) divisionSomatic and visceral sensory

nerve fibersConducts impulses from

receptors to the CNS

Somatic nervoussystem

Somatic motor(voluntary)

Conducts impulsesfrom the CNS toskeletal muscles

Sympathetic divisionMobilizes body

systems during activity

Autonomic nervoussystem (ANS)Visceral motor(involuntary)

Conducts impulsesfrom the CNS tocardiac muscles,smooth muscles,

and glands

StructureFunctionSensory (afferent)division of PNS

Motor (efferent) division of PNS

Somatic sensoryfiber

Visceral sensory fiber

Motor fiber of somatic nervous system

Skin

StomachSkeletalmuscle

Heart

BladderParasympathetic motor fiber of ANS

Sympathetic motor fiber of ANS

Histology of Nervous Histology of Nervous TissueTissue Two principal cell types

1. Neurons—excitable cells that transmit electrical signals

Histology of Nervous Histology of Nervous TissueTissue2. Neuroglia (glial cells)—supporting

cells: Astrocytes (CNS) Microglia (CNS) Ependymal cells (CNS) Oligodendrocytes (CNS) Satellite cells (PNS) Schwann cells (PNS)

NeurogliaNeuroglia

Neuroglia (glial cells)

Protect and nourish nervous tissue

Support nervous tissue

Aid in cell repair

Remove pathogens and impurities

Regulation composition of fluids around and between cells

Neurons (Nerve Cells)Neurons (Nerve Cells)Special characteristics:

◦Long-lived ( 100 years or more)

◦Amitotic—with few exceptions

◦High metabolic rate—depends on continuous supply of oxygen and glucose

◦Plasma membrane functions in: Electrical signaling

Cell-to-cell interactions during development

Cell Body (Soma)Cell Body (Soma)

Biosynthetic center of a neuron

Spherical nucleus with nucleolus

Well-developed Golgi apparatus

Rough ER called Nissl bodies

Axon hillock—cone-shaped area from which axon arises

Clusters of cell bodies are called nuclei in the CNS, ganglia in the PNS

Dendrites(receptive regions)

Cell body(biosynthetic center

and receptive region)

Nucleolus

Nucleus

Nissl bodies

Axon(impulse generating

and conducting region)

Axon hillock

NeurilemmaTerminal

branches

Node of Ranvier

Impulsedirection

Schwann cell(one inter-

node)

Axonterminals(secretory

region)

Typical Typical NeuronNeuron

ProcessesProcesses

Dendrites and axonsBundles of processes are called

◦Tracts in the CNS◦Nerves in the PNS

DendritesDendritesShort, tapering, and diffusely

branched Receptive (input) region of a

neuronConvey electrical signals toward

the cell body

The Axon (nerve fiber)The Axon (nerve fiber)One long axon per cell

Occasional branches (axon collaterals)

Numerous terminal branches

Knoblike axon terminals (synaptic knobs)

◦Release neurotransmitters to excite or inhibit other cells

Axons: FunctionAxons: Function

Conducting region of a neuron

Generates and transmits nerve impulses (action potentials) away from the cell body

MyelinMyelinmyelin sheath – an insulating

layer around a nerve fiber

◦formed by oligodendrocytes in CNS and Schwann cells in PNS

◦consists of the plasma membrane of glial cells

20% protein and 80 % lipid

MyelinationMyelination

production of the myelin sheath

◦begins the 14th week of fetal development

◦proceeds rapidly during infancy

◦completed in late adolescence

◦dietary fat is important to nervous system development

(A)Schwann cells wrap around the axon, creating a myelin coating.

(B) The outermost layer of the Schwann cell forms the neurilemma.

Space between each myelin sheath is the nodes (of Ranvier).

Formation of a Myelin Formation of a Myelin SheathSheath

Conduction Speed of Nerve Conduction Speed of Nerve FibersFibersspeed at which a nerve signal travels

along a nerve fiber depends on two factors

◦diameter of fiber

◦presence or absence of myelin

signal conduction occurs along the surface of a fiber

◦larger fibers have more surface area and conduct signals more rapidly

◦myelin further speeds signal conduction

Conduction Speed of Nerve Conduction Speed of Nerve FibersFibersconduction speed

◦small, unmyelinated fibers - 0.5 - 2.0 m/sec

◦small, myelinated fibers - 3 - 15.0 m/sec

◦large, myelinated fibers - up to 120 m/sec

slow signals supply the stomach and dilate pupil where speed is less of an issue

fast signals supply skeletal muscles and transport sensory signals for vision and balance

Diseases of Myelin SheathDiseases of Myelin Sheathdegenerative disorders of the myelin

sheath

◦multiple sclerosis oligodendrocytes and myelin sheaths in

the CNS deteriorate

myelin replaced by hardened scar tissue

nerve conduction disrupted (double vision, tremors, numbness, speech defects)

onset between 20 and 40 and fatal from 25 to 30 years after diagnosis

cause may be autoimmune triggered by virus

Diseases of Myelin SheathDiseases of Myelin Sheathdegenerative disorders of the myelin

sheath◦Tay-Sachs disease

a hereditary disorder of infants of Eastern European Jewish ancestry

abnormal accumulation of glycolipid in the myelin sheath; disrupts conduction of nerve signals

normally decomposed by lysosomal enzyme

enzyme missing in individuals homozygous for Tay-Sachs allele

blindness, loss of coordination, and dementia

fatal before age 4

The arrows show the direction of the nerve impulse.

ZOOMING IN • How can you tell

this is motor neuron and not a sensory neuron?

Diagram of a Diagram of a motor neuron motor neuron

Types of NeuronsTypes of Neurons

Sensory neurons (afferent neurons)

◦Conduct impulses to spinal cord, brain

Motor neurons (efferent neurons)

◦Conduct impulses to muscles, glands

Interneurons (association neurons)

◦Conduct information within CNS

Functional Classes of Functional Classes of NeuronsNeurons

1

2

3

Peripheral nervous system Central nervous system

Sensory (afferent)neurons conduct

signals fromreceptors to the CNS.

Motor (efferent)neurons conduct

signals from the CNSto effectors such asmuscles and glands.

Interneurons(associationneurons) areconfined to

the CNS.

Nerves and TractsNerves and TractsNerve: fiber bundle within PNS

Tract: fiber bundle within CNS

Organized into fascicles

Connective tissue layers

◦Endoneurium

◦Perineurium

◦Epineurium

Anatomy of a NerveAnatomy of a Nerve

RootletsPosterior root

EpineuriumFascicle

Myelin

Perineurium

Endoneurium

Unmyelinated nerve fibers

Myelinated nerve fibers

Posterior rootganglion

Anteriorroot

Spinalnerve

Bloodvessels

Fascicle

EpineuriumPerineurium

Endoneurium

Nervefiber

Bloodvessels

The Nervous System at The Nervous System at WorkWork

Electrical impulses sent along neuron fibers and transmitted between cells at junctions

The Nerve ImpulseThe Nerve ImpulsePlasma membrane carries electrical

charge (potential)Plasma membrane is polarized (negative

charge)Membrane potential reverses, generates

electrical charge (action potential)

◦Resting state

◦Depolarization

◦Repolarization Sodium/potassium (Na+/K+) pump

Myelin sheath speeds conduction

The SynapseThe SynapseJunction point for transmitting nerve

impulseAxon (presynaptic cell) Dendrite (postsynaptic cell)Synaptic cleftNeurotransmitters

◦Epinephrine (adrenaline)◦Norepinephrine (noradrenaline)◦Acetylcholine

Receptors

Neurotransmitters and Neurotransmitters and Psychoactive DrugsPsychoactive Drugs

Psychoactive drugs affect neurotransmitter activity in the brain

Used to treat depression, anxiety, obsessive-compulsive disorder (OCD)

Selective serotonin reuptake inhibitors (Example: Prozac)◦Block serotonin uptake

Others block norepinephrine, dopamine.

(A)The end-bulb of the presynaptic axon has vesicles containing neurotransmitter, which is released into the synaptic cleft to the membrane of the postsynaptic (receiving) cell.

(B) Close-up of a synapse showing receptors for neurotransmitter in the postsynaptic cell membrane.

A SynapseA Synapse

The Spinal CordThe Spinal Cord

Links PNS and brainHelps coordinate impulses within

CNSContained in and protected by

vertebrae

Nerve plexuses (networks) are shown.

(A) Lateral view. (B) Posterior view. 

ZOOMING IN • Is the spinal cord

the same length as the spinal column? How does the number of cervical vertebrae compare with the number of cervical spinal nerves?

Spinal Cord and Spinal Spinal Cord and Spinal Nerves Nerves

Structure of the Spinal Structure of the Spinal CordCordUnmyelinated tissue (gray matter)

◦Dorsal horn◦Ventral horn◦Gray commissure◦Central canal

Myelinated axons (white matter)◦Posterior median sulcus◦Anterior median fissure◦Ascending and descending tracts

(A) Cross-section of the spinal cord showing the organization of the gray and white matter. The roots of the spinal nerves are also shown.

(B) Microscopic view of the spinal cord in cross-section (x5).

The Spinal CordThe Spinal Cord

Meninges of Vertebra and Meninges of Vertebra and Spinal CordSpinal Cord

Fat in epidural space

Spinous process of vertebra

Spinal nerve

Posterior root ganglion

Spinal cord

Denticulate ligament

Subarachnoid space

(a) Spinal cord and vertebra (cervical)

Posterior

Anterior

Meninges:Dura mater (dural sheath)Arachnoid materPia mater

Vertebral body

Reflex ArcReflex Arc Components of a reflex arc (neural path)

1. Receptor—site of stimulus action2. Sensory neuron—transmits afferent

impulses to the CNS3. Interneuron – (Integration center) within

the CNS4. Motor neuron—conducts efferent impulses

from the integration center to an effector organ

5. Effector—muscle fiber or gland cell that responds to the efferent impulses by contracting or secreting

Receptor

Sensory neuron

Integration center

Motor neuron

Effector

Spinal cord(in cross section)

Interneuron

Stimulus

Skin

1

2

3

4

5

Reflex ArcReflex Arc

Numbers show the sequence of impulses through the spinal cord (solid arrows). Contraction of the biceps brachii results in flexion of the arm at the elbow.

  ZOOMING IN • Is this a somatic or an

autonomic reflex arc? What type of neuron is located between the sensory and motor neuron in the CNS?

Typical Reflex Arc Typical Reflex Arc

Reflex ActivitiesReflex Activities

Simple reflex

◦Rapid

◦Uncomplicated

◦Automatic

Spinal reflex

◦Stretch reflex

Medical Procedures Medical Procedures Involving theInvolving theSpinal CordSpinal CordLumbar puncture (spinal tap)

◦Cerebrospinal fluid (CSF) removed for testing

Drug administration

◦Anesthetic (an epidural)

◦Pain medication

Diseases and Other Disorders Diseases and Other Disorders of theof theSpinal CordSpinal Cord

Multiple sclerosis (MS)Amyotrophic lateral sclerosisPoliomyelitisTumors Injuries

◦ Monoplegia◦ Diplegia◦ Paraplegia◦ Hemiplegia◦ Tetraplegia

The Spinal NervesThe Spinal Nerves31 pairsEach nerve attached to spinal

cord by two roots◦Dorsal root

Dorsal root ganglion

◦Ventral root

Nerves near end of cord travel together in the cord until each exits from its respective intervertebral foramen

Mixed nerves

Branches of the Spinal Branches of the Spinal NervesNervesCervical plexus

◦Phrenic nerveBrachial plexus

◦Radial nerveLumbosacral plexus

◦Sciatic nerveDermatomes

A dermatome is a region of the skin supplied by a single spinal nerve.

ZOOMING IN • Which spinal

nerves carry impulses from the skin of the toes? From the anterior hand and fingers?

Dermatomes Dermatomes

Disorders of the Spinal Disorders of the Spinal NervesNerves

Peripheral neuritis

Sciatica

Herpes zoster

Guillain-Barré syndrome

Autonomic Nervous System Autonomic Nervous System (ANS)(ANS)

The ANS consists of motor neurons that:

◦Regulate the action of smooth and cardiac muscle and glands

◦Make adjustments to ensure optimal support for body activities

◦Operate via subconscious control

Central nervous system (CNS) Peripheral nervous system (PNS)

Motor (efferent) divisionSensory (afferent)division

Somatic nervoussystem

Autonomic nervoussystem (ANS)

Sympatheticdivision

Parasympatheticdivision

Motor Divisions: Somatic vs. Motor Divisions: Somatic vs. Visceral (ANS)Visceral (ANS)

Somatic and Autonomic Somatic and Autonomic Nervous Systems Nervous Systems

The two systems differ in

◦Effectors

◦Efferent pathways (and their neurotransmitters)

◦Target organ responses to neurotransmitters

EffectorsEffectorsSomatic nervous system

◦Skeletal muscles

ANS

◦Cardiac muscle

◦Smooth muscle

◦Glands

Efferent PathwaysEfferent PathwaysSomatic nervous system

◦ A, thick, heavily myelinated somatic motor fiber makes up each pathway from the CNS to the muscle

ANS pathway is a two-neuron chain

1.Preganglionic neuron (in CNS) has a thin, lightly myelinated preganglionic axon

2.Ganglionic neuron in autonomic ganglion has an unmyelinated postganglionic axon that extends to the effector organ

Neurotransmitter EffectsNeurotransmitter EffectsSomatic nervous system

◦All somatic motor neurons release acetylcholine (ACh)

◦Effects are always stimulatoryANS

◦Preganglionic fibers release ACh◦Postganglionic fibers release

norepinephrine or ACh at effectors◦Effect is either stimulatory or

inhibitory, depending on type of receptors

Skeletal muscle

Cell bodies in centralnervous system Peripheral nervous system Effect

+

+

Effectororgans

ACh

AChSmooth muscle

(e.g., in gut),glands, cardiac

muscle

Ganglion

Adrenal medulla Blood vessel

ACh

ACh

ACh

NE

Epinephrine andnorepinephrine

Acetylcholine (ACh) Norepinephrine (NE)

Ganglion

Heavily myelinated axon

Lightly myelinatedpreganglionic axon

Lightly myelinatedpreganglionic axons

Neuro-transmitterat effector

Unmyelinatedpostganglionic

axon

Unmyelinatedpostganglionic axon

Stimulatory

Stimulatoryor inhibitory,dependingon neuro-

transmitterand

receptorson effector

organs

Single neuron from CNS to effector organs

Two-neuron chain from CNS to effector organs

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Divisions of the ANSDivisions of the ANS

1.Sympathetic division2.Parasympathetic divisionDual innervation

◦Almost all visceral organs are served by both divisions, but they cause opposite effects

Pterygopalatineganglion

EyeLacrimal

gland

Nasalmucosa

Ciliaryganglion

Pterygopalatineganglion

Submandibularganglion Submandibular

and sublingualglands

CN III

CN VIICN IXCN X

Otic ganglion

Parotid gland

Heart

Lung

Liver andgallbladder

Stomach

Pancreas

Urinarybladder

and ureters

Smallintestine

Largeintestine

S2

Pelvicsplanchnic

nerves

Genitalia(penis,

clitoris, and vagina)

Rectum

Celiacplexus

Inferiorhypogastric

plexus

Cardiac andpulmonaryplexuses

S4

Preganglionic

Postganglionic

Cranial nerve

Superiorcervicalganglion

MiddlecervicalganglionInferiorcervicalganglion

Sympathetic trunk(chain) ganglia

Pons

L2

T1

White ramicommunicantes

Liver andgallbladder

Stomach

Spleen

Kidney

Adrenal medulla

Smallintestine

Largeintestine

Genitalia (uterus, vagina, andpenis) and urinary bladder

Celiac ganglion

Inferiormesenteric

ganglion

Lesser splanchnic nerveGreater splanchnic nerve

Superior mesenteric ganglion

Lumbarsplanchnic

nerves

EyeLacrimal gland

Nasal mucosa

Blood vessels;skin (arrector pili

muscles andsweat glands)

Salivary glands

Heart

Lung

Rectum

Cardiac andpulmonaryplexuses

PreganglionicPostganglionic

Sacralsplanchnic

nerves

Sympathetic nervous Sympathetic nervous systemsystemThoracolumbar areaAdrenergic systemActivated in the four E’s:

excitement, emergency, embarassment, exercise

Role of the Sympathetic Role of the Sympathetic DivisionDivisionMobilizes the body during activity;

is the “fight-or-flight” system

Promotes adjustments during exercise, or when threatened

◦Blood flow is shunted to skeletal muscles and heart

◦Bronchioles dilate

◦Liver releases glucose

Parasympathetic nervous Parasympathetic nervous systemsystem

Arise in craniosacral areas

Cholinergic system

Role of the Role of the Parasympathetic DivisionParasympathetic DivisionPromotes maintenance activities

and conserves body energyIts activity is illustrated in a person

who relaxes, reading, after a meal

◦Blood pressure, heart rate, and respiratory rates are low

◦Gastrointestinal tract activity is high

◦Pupils are constricted and lenses are accommodated for close vision

Sympathetic Effects…Sympathetic Effects… On the iris - Pupillary dilation On the sweat glands – secretion On piloerector muscles – hair erection (goose

bumps) On the heart – increased heart rate and force On blood vessels of skeletal muscle – vasodilation On blood vessels of skin – vasoconstriction On the bronchi and bronchioles – bronchodilation On the kidneys – reduced urine output On the GI Tract – decreased motility and secretion On the Liver – glycogen breakdown On the pancreas – decreased insulin secretion;

decreased digestive enzyme secretion On the reproductive system – stimulation of

orgasm and relaxation of the uterus

Parasympathetic Effects...Parasympathetic Effects...On the iris - Pupillary constriction

On the heart – decreased heart rate and force

On blood vessels of skin – vasodilation

On the bronchi and bronchioles – bronchoconstriction

On the bladder wall – contraction

On the GI Tract – increased motility and secretion

On the Liver – glycogen synthesis

On the pancreas – increased digestive enzyme secretion

On the reproductive system – stimulation of penile and clitoral erection

The diagram shows only one side of the body for each division.

ZOOMING IN• Which division

of the autonomic nervous system has ganglia closer to the effector organ?

Autonomic Nervous Autonomic Nervous System System

Cellular ReceptorsCellular Receptors“Docking sites” on postsynaptic cell

membranes

Two types:Cholinergic receptors

◦Nicotinic (bind nicotine) on skeletal muscle cells

◦Muscarinic (bind muscarine, a poison) on effector cells of PNS

Adrenergic receptors

◦Found on receptor cells of sympathetic nervous system

◦Bind norepinephrine, epinephrine

Drugs and the Nervous Drugs and the Nervous SystemSystemsympathomimetics enhance sympathetic activity◦stimulate receptors or increase

norepinephrine release cold medicines that dilate the bronchioles or constrict nasal

blood vessels

sympatholytics suppress sympathetic activity◦block receptors or inhibit norepinephrine

release beta blockers reduce high BP interfering with effects of

epinephrine/norepinephrine on heart and blood vessels

parasympathomimetics enhance activity while parasympatholytics suppress activity

End of PresentationEnd of Presentation