Transcript Ch12.Nervous.Tissue_1

THE NERVOUS SYSTEM &
NERVOUS TISSUE
Leonardo Da Vinci, 1508
Human Anatomy
Sonya Schuh-Huerta, Ph.D.
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
Integration
Motor output
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
CNS
Spinal
cord
Nerves
PNS
Ganglia
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 Information
Central nervous system (CNS)
Peripheral nervous system (PNS)
Brain and spinal cord
Integrative and control centers
Cranial nerves and spinal nerves
Communication lines between the CNS
and the rest of the body
Sensory (afferent) division
Motor (efferent) division
Somatic and visceral sensory
nerve fibers
Conducts impulses from
receptors to the CNS
Somatic sensory
fiber
Visceral sensory
fiber
Motor nerve fibers
Conducts impulses from the CNS
to effectors (muscles and glands)
Somatic nervous
system
Skin
Somatic motor
(voluntary)
Conducts impulses
from the CNS to
skeletal muscles
Stomach
Autonomic nervous
system (ANS)
Visceral motor
(involuntary)
Conducts impulses
from the CNS to
cardiac muscles,
smooth muscles,
and glands
Skeletal
muscle
Motor fiber of somatic nervous system
Sympathetic division
Mobilizes body systems
during activity
Sympathetic motor fiber
of ANS
Parasympathetic
division
Conserves energy
Promotes housekeeping functions
during rest
Heart
Structure
Function
Sensory (afferent)
division of PNS
Motor (efferent)
division of PNS
Parasympathetic motor fiber of ANS
Bladder
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” NS
• =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 their 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 & hippocampal 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
– Protein machinery
– 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 Neuron
Dendrites
(receptive
regions)
Cell body
(biosynthetic center
and receptive region)
Dendrites
Neuron
cell
body
Nucleus with
nucleolus
Neurofibril
Nucleus
Chromatophilic
(Nissl) bodies
(b)
Nucleolus
Nissl bodies
Axon
(impulse generating
and conducting
region)
Axon hillock
(a)
Neurilemma
Nuclei of
neuroglial
cells
Impulse
direction
Schwann cell
(one internode)
Node of Ranvier
Axon terminals
(secretory
region)
Terminal
branches
Neuron Processes
• Dendrites
– Extensively branch from the cell body
– Transmit electrical signals toward the 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  axon hillock
– 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 initial segment of the axon (hillock)
– Conducted along the axon  electrical signal
– Causes release of neurotransmitters at axon
terminals
• Neurotransmitters  chemical signals (excite or inhibit neurons)
– This is how the neuron
receives & sends signals
Synapses
• Site at which neurons communicate
• 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
Presynaptic
neuron
Axon
Axon terminal
at synapse
Synapse
Dendrite
(a) 2 neurons connected by synapses
Postsynaptic
neuron
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
• Synaptic vesicles in presynaptic neuron
– Membrane-bound sacs containing neurotransmitters
– Neurotransmitter released & binds to receptor &
initiates depolarization of postsynaptic neuron
– Mitochondria abundant in axon terminals
• Synaptic cleft
– Separates the plasma membrane of the 2
neurons
The Synapse – Remember this at NMJ?
Presynaptic axon
Nerve
impulses
Microtubule
Neurofilament
Axon terminal
Vesicle releasing
neurotransmitter
Mitochondrion
Synaptic
vesicles
Synaptic
cleft
Postsynaptic dendrite
(b) Enlarged view of the synapse
Classification of Neurons
• Structural classification
– Multipolar  possess more than 2 processes
• Numerous dendrites & one axon
– Bipolar  possess 2 processes (1 axon, 1
dendrite)
• Rare neurons
• Found in some special sensory organs
– Unipolar (pseudounipolar)  possess 1
short, single process
Neurons Classified by Structure
Functional Classification of
Neurons
• Types of neurons based on functional
classification:
– 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/connect 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
Capillary
Neuron
Astrocyte
(a) Astrocytes are the most abundant CNS neuroglia
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
Neuron
Microglial
cell
(b) Microglial cells are defensive cells in the CNS
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
Fluid-filled cavity
Ependymal
cells
Brain or
spinal cord
tissue
(c) Ependymal cells line cerebrospinal fluid–filled cavities.
Myelin sheath
Process of
oligodendrocyte
Nerve
fibers
(d) Oligodendrocytes have processes that form myelin
sheaths around CNS nerve fibers.
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
Cell body of neuron
Schwann cells
(forming myelin sheath)
Nerve fiber
Satellite cells and Schwann cells (which form myelin) surround neurons in the PNS.
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
Nodes of Ranvier
– Thin axons are unmyelinated
• Conduct impulses
more slowly
• What happens at
Nodes of Ranvier?
Myelin Sheaths in the PNS
An axon wrapped with a fatty insulating sheath
formed from Schwann cells
Myelin sheath
Schwann cell
plasma membrane
Schwann cell
cytoplasm
Axon
1 A Schwann cell
envelops an axon.
Schwann cell
nucleus
Schwann cell
cytoplasm
Neurilemma
2 The Schwann cell then
rotates around the axon,
wrapping its plasma
membrane loosely around
it in successive layers.
Neurilemma
Myelin
sheath
Axon
3 The Schwann cell
cytoplasm is forced from
between the membranes. The
tight membrane wrappings
surrounding the axon form
the myelin sheath.
C section of a myelinated axon (TEM 30,000)
Unmyelinated Axons in the PNS
(b) Unmyelinated axons in PNS
Axons that are not covered with an insulating sheath
Schwann cell
Schwann cell
Axons
Schwann cell
nucleus
1 A Schwann
cell surrounds
multiple axons.
Neurilemma
Axons
c s of unmyelinated axons (TEM 11,000)
2 Each axon is
encircled by the
Schwann cell
plasma membrane.
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
Axon
Myelin sheath
Endoneurium
Perineurium
Blood vessels
Endoneurium
Perineurium
Fascicle
Blood vessels
Fascicle
(b)
Nerve
fibers
Epineurium
Schwann cell
nucleus
Axon
Myelin
(a)
Node of
Ranvier
Like an electrical wire/cable
(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
PNS
Sensory (afferent)
fiber
Spinal
nerve
Motor (efferent)
fiber
CNS
Gray matter
Short unmyelinated
interneurons
Cell bodies of
interneurons and
motor neurons
Neuroglia
White matter
Fiber tracts of
myelinated and
unmyelinated axons
Hollow central cavity
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
Integration Between the PNS & CNS
• 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
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 of 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
Stimulus
Skin
1 Receptor
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 Effector
Spinal cord
(in cross section)
Types of Reflexes
• Monosynaptic reflex
– Simplest of all reflexes
– Just 1 synapse
– The fastest of all reflexes!!!
• ie. Knee-jerk reflex
Monosynaptic Reflex
1 Sensory (stretch) receptor
2 Sensory (afferent) neuron
3
4 Motor (efferent) neuron
5 Effector organ
(a) Monosynaptic stretch reflex
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
1 Sensory receptor
2 Sensory (afferent) neuron
3 Interneuron
4 Motor (efferent) neuron
5 Effector organ
(b) Polysynaptic withdrawal reflex
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
Gray matter
White matter
Withdrawal reflex. A
painful stimulus triggers
nerve impulses in a
sensory neuron, which
initiate the polysynaptic
withdrawal reflex.
Cerebrum
Brain stem
Sensory
neuron
Cervical
spinal cord
Motor
neuron
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.
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
Endoneurium
Schwann cells
Droplets
of myelin
1 The axon
becomes
fragmented at
the injury site.
Fragmented
axon
Site of nerve damage
Schwann cell
Macrophage
Aligning Schwann cells
form regeneration tube
3 Axon sprouts,
or filaments,
grow through a
regeneration
tube formed by
Schwann cells.
Fine axon sprouts
or filaments
2 Macrophages
clean out the dead
axon distal to the
injury.
Schwann cell
Single enlarging
axon filament
Site of new myelin
sheath formation
4 The axon
regenerates, and
a new myelin
sheath forms.
Neuronal Regeneration
In CNS  neuroglia never form tubes to guide
axon growth & may hinder axon growth with
growth-inhibiting chemicals
– No effective regeneration after injury to the
spinal cord & brain!!!
– This is why injuries to
the CNS are so damaging
 BUT…. Current stem cell research has great
potential for generating functional neurons & glia and
treating many diseases of the nervous system
Stem cells
Neurons
growth
factors
Culture stem cells
in vitro
transplantation
into patient
Grow & characterize cells
Adult cells can be transformed
into embryonic-like cells =
Induced Pluripotent Stem
Cells (iPSCs)
& this could prevent immune rejection
Clinical Trials on Stem Cells for Spinal
Cord Injuries 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
(a) 28 days.
Neural tube and Neural
neural crest form tube
from invaginating
ectoderm.
Neural
crest
(b) Week 5.
Neuroepithelial
cells of the neural
tube divide and
migrate externally
to become
neuroblasts and
neuroglia.
Neuroblast
s
Neuroepithelial cells
Neuroepithelial cells
Sensory neurons
from neural crest
Alar plate:
interneurons
Axons form
white matter
Neuroepithelial cells
Basal plate:
motor neurons
Central
cavity
(c) Week 6.
Neural crest cells
form 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…?
S. Schuh-Huerta
What’s Next?
Lab: Nervous Tissue & CNS
Mon Lecture: Finish NS tissue/CNS
Mon Lab: CNS
Wed Lecture & Lab:  CNS