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