THE NERVOUS SYSTEM & NERVOUS TISSUE Human Anatomy Sonya Schuh-Huerta, Ph.D.

THE NERVOUS SYSTEM & THE NERVOUS SYSTEM & NERVOUS TISSUENERVOUS TISSUE

Human AnatomyHuman Anatomy

Sonya Schuh-Huerta, Ph.D.Sonya Schuh-Huerta, Ph.D.

Leonardo Da Vinci, 1508

Nervous System

• Master control & communication system

• What makes us uniquely “human”Great intelligence

Emotions & empathy

Reasoning & problem solving

Strategizing & predicting

Culture (religion, etc.)

…

Nervous System

• 3 Overlapping Functions:

• Sensory receptors monitor changes inside & outside body

• Change a stimulus• Gathered information sensory input

• Processes & interprets sensory input• Makes decisions integration

• Dictates a response by activating effector organs

• Response motor output

Nervous System

Sensory input

Motor output

Integration

Basic Divisions of Nervous System

• Central nervous system (CNS)– Brain & spinal cord– Integrating & command center– Personality traits, emotions, intelligence, etc.

Basic Divisions of Nervous System

• Peripheral nervous system (PNS)– Outside the CNS– Consists of nerves extending from brain & spinal

cord:• Cranial nerves• Spinal nerves

– Peripheral nerves link all regions of body to CNS– Ganglia are clusters of neuronal cell bodies

Basic Divisions of the Nervous System

Brain

Spinalcord

CNS

Nerves

GangliaPNS

Sensory Input & Motor Output

• Sensory (afferent) signals picked up by sensory receptors– Carried by nerve fibers of PNS to the CNS

• Motor (efferent) signals are carried away from the CNS– From brain/spinal cord to organs (muscles, glands)

Sensory Input & Motor Output

• Divided according to region they serve:– Somatic body region– Visceral body region

• Results in 4 main subdivisions of NS:– Somatic sensory– Visceral sensory– Somatic motor– Visceral motor (=autonomic nervous system)

Types of Sensory & Motor InformationCentral nervous system (CNS) Peripheral nervous system (PNS)

Parasympatheticdivision

Motor (efferent) divisionSensory (afferent) division

Somatic nervoussystem

Sympathetic division

Autonomic nervoussystem (ANS)

Brain and spinal cord

Integrative and control centers

Cranial nerves and spinal nerves

Communication lines between the CNSand the rest of the body

Conserves energy

Promotes house-keeping functionsduring rest

Motor nerve fibers

Conducts impulses from the CNSto effectors (muscles and glands)

Somatic and visceral sensorynerve fibersConducts impulses fromreceptors to the CNS

Somatic motor(voluntary)Conducts impulsesfrom the CNS toskeletal muscles

Mobilizes body systemsduring activity

Visceral motor(involuntary)Conducts impulsesfrom the CNS tocardiac muscles,smooth muscles,and glands

Somatic sensoryfiber

Visceral sensoryfiber

Motor fiber of somatic nervous system

Skin

StomachSkeletalmuscle

Heart

BladderParasympathetic motor fiber of ANS

Sympathetic motor fiberof ANS

Structure

Function

Sensory (afferent)division of PNS

Motor (efferent)division of PNS


• Somatic sensory – General somatic senses receptors are

widely spread • Touch• Pain• Vibration• Pressure• Temperature

(receptors discussed later in Ch 14)


• Somatic sensory (cont.)– Proprioceptive senses detect stretch in

tendons & muscle • Body sense position & movement of body in space

– Special somatic senses (Ch 16)• Hearing• Balance• Vision• Smell & Taste


• Visceral sensory– General visceral senses stretch, pain,

temperature, nausea, & hunger• Widely felt in digestive & urinary tracts, &

reproductive organs

– Special visceral senses• Taste & smell often considered special visceral

senses


• Somatic motor– General somatic motor signals contraction

of skeletal muscles• Under our voluntary control • Often called “voluntary nervous system”


• Visceral motor– Regulates the contraction of smooth & cardiac m.– Makes up autonomic nervous system (ANS)– Controls function of visceral organs– Often called “involuntary nervous system”

• “Fight or Flight”• =Autonomic nervous system (later, Ch 15)

Nervous Tissue

• Cells are densely packed &

intertwined – 2 main cell types:

• Neurons transmit electrical signals• Support cells (neuroglial or glial cells)

– Nonexcitable– Support growth & function

of neurons – Surround & wrap neurons

The Neuron

• The human body contains billions

of neurons!!!– Basic structural unit of the

Nervous System• Specialized cells that conduct electrical

impulses along the plasma membrane – Nerve impulse (= action potential)

The Neuron

• Special characteristics:1.) Excitability conduct electrical impulses

2.) Longevity can live & function for a lifetime!

3.) Do not divide fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception (olfactory neuron regeneration is an example)

4.) High metabolic rate require abundant oxygen & glucose!

• Neurons die after 5 minutes without oxygen!!!

The Cell Body

• Cell body (= soma) – Perikaryon cytoplasm around the nucleus– Size of cell body varies from 5–140 µm – Contains usual organelles plus other structures

• Chromatophilic bodies (Nissl bodies)– Clusters of rough ER &

free ribosomes– Stain darkly & renew

membranes of the cell

Chromatophilic(Nissl) bodies

The Cell Body

• Neurofibrils bundles of intermediate filaments – Form a network between chromatophilic

bodies

Neurofibril

The Cell Body

• Most neuronal cell bodies are:– Located within the CNS!– Protected by bones of skull &

vertebral column

• Ganglia (“knot in a string”)

clusters of cell bodies– Lie along nerves in the PNS!

Structure of a Typical Large NeuronDendrites(receptiveregions)

Cell body(biosynthetic centerand receptive region)

Nucleolus

Nucleus

Nissl bodies

Axon(impulse generatingand conductingregion)

Axon hillock

NeurilemmaTerminalbranches

Node of Ranvier

Impulsedirection

Schwann cell(one inter-node)

Axon terminals(secretoryregion)

(b)

(a)

Neurofibril

Chromatophilic(Nissl) bodies

Nuclei ofneuroglialcells

Nucleus withnucleolus

Neuroncell body

Dendrites

Neuron Processes

• Dendrites – Extensively branching from the cell body– Transmit electrical signals toward cell body– Chromatophilic bodies only extend into

basal part of dendrites & to the base of axon hillock

– Function as receptive sites for receiving signals from other neurons

Neuron Processes

• Axon– Neuron has only one– Impulse generator & conductor– Transmits impulses away from the cell body– Impulses travel to axon terminals– Chromatophilic bodies are absent– No protein synthesis in axon

Neuron Processes

• Axons – Neurofilaments, actin

microfilaments, &

microtubules• Provide strength &

structure along

length of axon• Aid in the transport of

substances to & from

the cell body– Axonal transport

Neuron Processes

• Axons – Branches along length are infrequent

• Axon collaterals

– Multiple branches at end of axon• Terminal branches

– End in knobs called axon terminals (also called end bulbs or boutons)

Nerve Impulse

• Nerve impulse = action potential– Generated at the initial segment of the axon

(hillock)– Conducted along the axon– Releases neurotransmitters at axon terminals

• Neurotransmitters excite or inhibit neurons

– This is how the neuron

receives & sends signals

Synapses

• Site at which neurons communicate (by neurotransmitters)

• Signals pass across synapse in 1 direction

• Presynaptic neuron– Conducts signal toward a synapse

• Postsynaptic neuron– Transmits electrical signal away from a synapse

2 Neurons Communicating at a Synapse

Axon terminalat synapse

Axon

Presynapticneuron

Postsynapticneuron

Dendrite

(a) 2 neurons connected by synapses

Synapse

SynapsePLAYPLAY

Types of Synapses

• Axodendritic– Between axon terminals of one neuron &

dendrites of another– Most common type of synapse!

• Axosomatic – Between axons & cell bodies

Synapses

• Elaborate cell junctions

• Axodendritic synapses representative type

• Synaptic vesicles on presynaptic side– Membrane-bound sacs containing neurotransmitters– Neurotransmitter released & binds to receptor &

initiates depolarization of postsynaptic neuron– Mitochondria abundant in axon terminals

• Synaptic cleft– The gap between the plasma membranes of

the 2 neurons

The Synapse – Remember this at NMJ?

(b) Enlarged view of the synapse

Neurofilament

Postsynaptic dendrite

Axon terminal

Nerveimpulses

Presynaptic axonMicrotubule

Mitochondrion

Synapticcleft

Synapticvesicles

Vesicle releasingneurotransmitter

Classification of Neurons

• Structural classification – Multipolar have more than 2 processes

• Numerous dendrites & one axon

– Bipolar have 2 processes • Rare neurons• Found in some special sensory organs

– Unipolar (pseudounipolar) have 1 short, single process

Neurons Classified by Structure

Functional Classification of Neurons

• Functional classification is:– According to the direction the nerve

impulse travels relative to the CNS

• Types of neurons– Sensory Neurons– Motor Neurons– Interneurons


• Sensory (afferent) neurons– Transmit impulses toward the CNS

• Virtually all are unipolar neurons• Cell bodies in ganglia outside the CNS

– Short, single process divides into:

» The central process runs centrally into the CNS

» The peripheral process extends peripherally to the receptors


• Motor (efferent) neurons – Carry impulses away from the CNS to effector

organs– Most Motor neurons are Multipolar – Cell bodies are within the CNS– Form junctions with effector cells

• Interneurons (= association neurons) – Most are multipolar– Lie between motor & sensory neurons– Confined to the CNS

Neurons Classified by Function

Supporting Cells

• 6 types of supporting cells:– 4 in the CNS– 2 in the PNS

• Provide supportive functions for neurons

• Cover nonsynaptic regions of the neurons

Neuroglia in the CNS

• Neuroglia– Glial cells have branching processes & a

central cell body– Outnumber neurons 10 to 1!!!– Make up half the mass of the brain– Can divide throughout life!!!– Have neuroglial adult stem cells!!!


• Astrocytes most abundant glial cell type– Sense when neurons release glutamate– Extract blood sugar from capillaries for energy– Take up & release ions to control environment around neurons– Involved in synapse formation in developing neural

tissue– Produce molecules necessary for neuronal growth (BDTF)– Propagate calcium signals involved with memory


(a) Astrocytes are the most abundant CNS neuroglia

Capillary

Neuron

Astrocyte


• Microglia smallest & least abundant glial cell– Phagocytes the macrophages of the CNS– Engulf invading microorganisms & dead neurons– Derived from blood cells called monocytes


(b) Microglial cells are defensive cells in the CNS

Neuron

Microglialcell


• Ependymal cells• Line the central cavity of spinal cord & brain• Bear cilia help circulate the cerebrospinal fluid

• Oligodendrocytes have few branches• Wrap their cell processes around axons in CNS

– Produce myelin sheaths!!!


(d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers.

Nervefibers

Myelin sheath

Process ofoligodendrocyte

Brain orspinal cordtissue

Ependymalcells

Fluid-filled cavity

(c) Ependymal cells line cerebrospinal fluid–filled cavities.

Neuroglia in the PNS• Satellite cells surround neuron cell bodies

within ganglia• Schwann cells surround axons in the PNS

– Form myelin sheath around axons of the PNS

Satellite cells and Schwann cells (which form myelin) surround neurons in the PNS.

Schwann cells(forming myelin sheath)

Cell body of neuronSatellitecells

Nerve fiber

Myelin Sheaths

• Segmented structures composed of the lipoprotein myelin whitish in color

• Surround thicker axons

• Form an insulating layer – Prevent leakage of electrical current

• Increase the speed of impulse conduction!

• Like the casing surrounding an electrical wire

Myelin Sheaths in the PNS

• Formed by Schwann cells

• Develop during fetal period & in the first year of postnatal life

• Schwann cells wrap in concentric layers around axon– Cover axon in a tightly packed coil of

membranes

• Neurilemma– Material external to myelin layers

Myelin Sheaths

• Nodes of Ranvier gaps along axon – Gaps in between adjacent Schwann cells– Thick axons are myelinated– Thin axons are unmyelinated

• Conduct impulses

more slowly

Nodes of Ranvier

Myelin Sheaths in the PNS

Axon

A Schwann cellenvelops an axon.

The Schwann cell thenrotates around the axon, wrapping its plasma membrane loosely around it in successive layers.

The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath.

Schwann cellcytoplasm

Axon

NeurilemmaMyelinsheath

Schwann cellnucleus

Schwann cell plasma membrane

An axon wrapped with a fatty insulating sheathformed from Schwann cells

1

2

3

Myelin sheath

Schwann cellcytoplasm

Neurilemma

C section of a myelinated axon (TEM 30,000)

Unmyelinated Axons in the PNS

c s of unmyelinated axons (TEM 11,000)

A Schwanncell surrounds multiple axons.

Each axon isencircled by the Schwann cell plasma membrane.

Schwann cell

Axons

Schwann cellnucleus

(b) Unmyelinated axons in PNS

Axons that are not covered with an insulating sheath

1

2

Schwann cell

Neurilemma

Axons

Myelin Sheaths in the CNS

• Oligodendrocytes form the myelin sheaths in the CNS– Have multiple processes – Coil around several different axons– Can also see coiled layers of myelin & Nodes

of Ranvier

Nerves

• Nerves cable-like organs in the PNS– Consist of numerous axons wrapped in

connective tissue– Axon is surrounded by Schwann cells, that

are then surrounded by connective tissue

• You’ll see many nerves in lab under the microscope & models

Nerves

• Endoneurium layer of delicate connective tissue surrounding the axon

• Perineurium connective tissue wrapping surrounding a nerve fascicle – Nerve fascicles groups of axons bound

into bundles (just like in skeletal muscle!)

• Epineurium whole nerve is surrounded by tough fibrous sheath

Structure of a Nerve

Blood vessels

Fascicle

Epineurium

Perineurium

Endoneurium

Axon

Myelin sheath

(a)

Blood vesselsFascicle

Perineurium

Endoneurium

(b)

Nervefibers

Myelin

Schwann cellnucleus

Node ofRanvier

Axon

(c)

Gray & White Matter in the CNS

• Gray matter– Is gray-colored & surrounds hollow central

cavities of the CNS– Forms butterfly-shaped region in spinal cord

• Dorsal half contains cell bodies of interneurons• Ventral half contains cell bodies of motor neurons

– Primarily composed of neuronal cell bodies, dendrites, unmyelinated axons gray

– Surrounds white matter of CNS in cerebral cortex & cerebellum


• White matter– Lies external to the gray matter of CNS– Composed of myelinated axons white– Consists of axons passing between specific

regions of the CNS– Tracts are bundles of axons traveling to

similar destinations


Spinalnerve

PNS CNS

Gray matter

White matter

Hollow central cavity

Sensory (afferent)fiber

Motor (efferent)fiber

Short unmyelinatedinterneuronsCell bodies of interneurons and motor neuronsNeuroglia

Fiber tracts of myelinated and unmyelinated axons

Integration Between the PNS & CNS

• CNS & PNS are functionally interrelated

• Nerves of the PNS– Information pathways to & from body periphery

• Afferent PNS fibers respond to sensory stimuli• Efferent PNS fibers transmit motor stimuli from CNS

to muscles & glands

• Nerves of the CNS– Composed of interneurons that:

• Process & receive sensory information• Direct information to specific CNS regions• Initiate appropriate motor responses• Transport information from one area of the CNS to

another

Integration Between the PNS & CNS

Reflex Arcs

• Reflex arcs simple chains of neurons– Explain reflex behaviors– Determine structural plan of the nervous

system– Responsible for reflexes

• Rapid, autonomic motor responses– Can be visceral or somatic

5 Essential Components to the Reflex Arc

• 1.) Receptor site where stimulus acts• 2.) Sensory neuron transmits afferent impulses to the

CNS• 3.) Integration center consists of one or more

synapses in the CNS

• 4.) Motor neuron conducts efferent impulses from

integration center to an effector • 5.) Effector muscle or gland cell

– Responds to efferent impulses• Contracts or secretes something

Components of a Reflex Arc

Receptor

Sensory neuron

Integration center

Motor neuron

Effector

Spinal cord(in cross section)

Interneuron

Stimulus

Skin

1

2

3

4

5

Types of Reflexes

• Monosynaptic reflex– Simplest of all reflexes– Just 1 synapse– The fastest of all reflexes!!!

• ie. Knee-jerk reflex

Monosynaptic Reflex

(a) Monosynaptic stretch reflex

Sensory (stretch) receptor

Sensory (afferent) neuron

Motor (efferent) neuron

Effector organ

1

2

4

5

3

Types of Reflexes

• Polysynaptic reflex– More common type of reflex– Most have a single interneuron between the

sensory & motor neuron (= 3 neurons)• Withdrawal reflexes

Polysynaptic Reflex

Interneuron

Effector organ

Sensory (afferent) neuron

Motor (efferent) neuron

Sensory receptor

(b) Polysynaptic withdrawal reflex

4

5

2

3

1

Simplified Design of the Nervous System

• 3-neuron reflex arcs – Basis of the structural plan of the nervous

system– Similar reflexes are associated with the brain


• Sensory neurons located dorsally– Cell bodies outside the CNS in sensory

ganglia– Central processes enter dorsal aspect of the

spinal cord

• Motor neurons located ventrally – Axons exit the ventral aspect of the spinal

cord


• Interneurons located centrally – Synapse with sensory neurons– Interneurons are confined to CNS– Long chains of interneurons between sensory

& motor neurons

Simplified Design of the NS

Withdrawal reflex. A painful stimulus triggers nerve impulses in a sensory neuron, which initiate the polysynaptic withdrawal reflex.

Sensoryneuron

Motorneuron

Interneuron

Parallel processing.Simultaneously, the nerve impulses travel on an axon branch that extends into the white matter. This ascending fiber carries the nerve impulses to the brain.

White matter

Gray matter

Brain stem

Cervicalspinal cord

Cerebrum

Disorders of the NS

• There are many!

• Multiple sclerosis– Common cause of neural disability– Symptoms: fatigue, numbness, tingling, pain,

blurred vision, lack of coordination, & paralysis – Cause is incompletely understood– An autoimmune disease:

• Immune system attacks myelin around axons in CNS • Varies widely in intensity• More women than men affected• When men are affected, disease develops quicker &

is more devastating

Neuronal Regeneration in the PNS

• Neural injuries may cause permanent dysfunction

• If axons alone are destroyed, cell bodies often survive & axons may regenerate– PNS macrophages invade & destroy axon

distal to the injury• Schwann cells form regeneration tube• Axon filaments grow peripherally from injured site• Partial recovery is sometimes possible

Regeneration of the Peripheral Nerve Fiber

The axonbecomes fragmented at the injury site.

Endoneurium

Dropletsof myelin

Fragmentedaxon

Schwann cells

Site of nerve damage

1

Macrophagesclean out the dead axon distal to the injury.

Schwann cell Macrophage2

Axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells.

Fine axon sproutsor filaments

Aligning Schwann cellsform regeneration tube

3

The axon regenerates, and a new myelin sheath forms.

Schwann cell

Single enlargingaxon filament

Site of new myelinsheath formation

4

Neuronal Regeneration

In CNS neuroglia don’t form tubes to guide axon growth & may hinder axon growth with growth-inhibiting chemicals

– Very limited regeneration after injury to the spinal cord & brain!!!

– This is why injuries to

the CNS are so damaging

growthfactors

Culture stem cellsin vitro

Grow & characterize cells

Neurons

transplantationinto patient

BUT…. Current stem cell research has great potential for generating functional neurons & glia and treating many diseases of the nervous system

Adult cells can be transformed into embryonic-like cells =Induced Pluripotent Stem Cells (iPSCs)& this could prevent immune rejection

Stem cells

Clinical Trials on Stem Cells for Spinal Cord Injuries Already Underway

Nervous Tissue Throughout Life

• Nervous system develops from:

the dorsal ectoderm– Invaginates to form the neural tube & neural

crest• These cells divide & become neuroblasts

Embryonic Development of NS

Ectoderm

Neuralcrest

Neuroepithelial cells

Neuraltube

(a) 28 days.Neural tube andneural crest formfrom invaginatingectoderm.

Neuroepithelial cells

Neuroblasts

(b) Week 5.Neuroepithelial cells of the neuraltube divide andmigrate externallyto becomeneuroblasts andneuroglia.

Axons formwhite matter

Neuroepithelial cells Centralcavity

Alar plate:interneurons

Sensory neuronsfrom neural crest

Basal plate:motor neurons

(c) Week 6.Neural crest cellsform the sensory neurons.

Dorsal neuroblasts form the alar plate (future interneurons). Long axons extending from the interneurons form the white matter.

Ventral neuroblasts form the basal plate (future motor neurons).

Questions…?

What’s Next?Lab: Nervous Tissue & CNSMon Lecture: CNS Mon Lab: CNS Wed Lab: Brain dissection!

S. Schuh-Huerta

THE NERVOUS SYSTEM & NERVOUS TISSUE Human Anatomy Sonya Schuh-Huerta, Ph.D.

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Transcript of THE NERVOUS SYSTEM & NERVOUS TISSUE Human Anatomy Sonya Schuh-Huerta, Ph.D.