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THE NERVOUS SYSTEM & NERVOUS TISSUE Human Anatomy 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 - PowerPoint PPT Presentation
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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
Basic Divisions of the Nervous System
• Somatic sensory – General somatic senses receptors are
widely spread • Touch• Pain• Vibration• Pressure• Temperature
(receptors discussed later in Ch 14)
Basic Divisions of the Nervous System
• 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
Basic Divisions of the Nervous System
• 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
Basic Divisions of the Nervous System
• Somatic motor– General somatic motor signals contraction
of skeletal muscles• Under our voluntary control • Often called “voluntary nervous system”
Basic Divisions of the 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
Functional Classification of Neurons
• 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
Functional Classification of Neurons
• 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!!!
Neuroglia in the CNS
• 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
Neuroglia in the CNS
(a) Astrocytes are the most abundant CNS neuroglia
Capillary
Neuron
Astrocyte
Neuroglia in the CNS
• Microglia smallest & least abundant glial cell– Phagocytes the macrophages of the CNS– Engulf invading microorganisms & dead neurons– Derived from blood cells called monocytes
Neuroglia in the CNS
(b) Microglial cells are defensive cells in the CNS
Neuron
Microglialcell
Neuroglia in the CNS
• 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!!!
Neuroglia in the CNS
(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
Gray & White Matter in the CNS
• 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
Gray & White Matter in the CNS
Gray & White Matter in the CNS
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
Simplified Design of the Nervous System
• 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
Simplified Design of the Nervous System
• 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