Transcript nerve-tissue-lecture

THE NERVOUS
SYSTEM: NEURAL
TISSUE
Two organ systems coordinate and
direct activities of body
• Nervous system
– Swift, brief responses to stimuli
• Endocrine system
– Adjusts metabolic operations
– Directs long-term changes
An Overview of the Nervous System
Divisions of the nervous system
Anatomical Classification of the
Nervous System
• Central Nervous System
– Brain and spinal cord
• Peripheral Nervous System
– All neural tissue outside CNS
Functional divisions of nervous
system
• Afferent
– Sensory information from receptors to CNS
• Efferent
– Motor commands to muscles and glands
– Somatic division
• Voluntary control over skeletal muscle
– Autonomic division
• Involuntary regulation of smooth and cardiac muscle, glands
Histology of Neural Tissue
Cells in Nervous Tissue
• Neurons
• Neuroglia
•(supporting cells)
Neuroglia (Glia)
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about half the volume of cells in the CNS
smaller than neurons
5 to 50 times more numerous
do NOT generate electrical impulses
divide by mitosis
two types in PNS
– Schwann cells
– Satellite cells
• Four types in the CNS
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Astrocytes
Oligodendrocytes
Microglia
Ependymal cells
Astrocytes
• Largest of glial cells
• Star shaped with many processes
projecting from the cell body
• Help form and maintain blood-brain barrier
• Provide structural support for neurons
• Maintain the appropriate chemical
environment for generation of nerve impulses/action potentials
• Regulate nutrient concentrations for neuron survival
• Regulate ion concentrations - generation of action potentials by neurons
• Take up excess neurotransmitters
• Assist in neuronal migration during brain development
• Perform repairs to stabilize tissue
Oligodendrocytes
• Most common glial cell
type
• Each forms myelin sheath
around the axons of
neurons in CNS
• Analogous to Schwann
cells of PNS
• Form a supportive
• fewer processes than astrocytes
network around CNS
• round or oval cell body
neurons
Microglia
• few processes
• derived from mesodermal cells
that also give rise to monocytes
and macrophages
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Small cells found near blood vessels
Phagocytic role - clear away dead cells
protect CNS from disease through phagocytosis of microbes
migrate to areas of injury where they clear away debris of
injured cells - may also kill healthy cells
Ependymal Cells
• epithelial cells arranged in a
single layer
• range in shape from cuboidal
to columnar
• Form epithelial membrane lining cerebral cavities (ventricles) &
central canal - that contain CSF
• Produce & circulate the cerebrospinal fluid (CSF) found in these
chambers
• CSF = colourless liquid that protects the brain and Spinal Cord
against chemical & physical injuries, carries oxygen, glucose and
other necessary chemicals from the blood to neurons and
neuroglia
PNS: Satellite Cells
• Flat cells surrounding PNS axons
• Support neurons in the PNS
PNS: Schwann Cells
• Each cell surrounds multiple unmyelinated PNS axons
with a single layer of its plasma membrane
• Each cell produces part of the myelin sheath
surrounding an axon in the PNS
• contributes regeneration of PNS axons
Neurons
•What is the main defining characteristic of neurons?
•Have the property of electrical excitability - ability to produce
action potentials or impulses in response to stimuli
Representative Neuron
1. cell body or soma
-single nucleus with prominent nucleolus
-Nissl bodies
-rough ER & free ribosomes for protein
synthesis
-proteins then replace neuronal cellular
components for growth
and repair of damaged axons in the PNS
-neurofilaments or neurofibrils
give cell shape and support bundles of
intermediate filaments
-microtubules move material
inside cell
-lipofuscin pigment clumps
(harmless aging) - yellowish
brown
Neurons
2. Cell processes =
dendrites (little trees)
- the receiving or input
portion of the neuron
-short, tapering and
highly branched
-surfaces specialized
for contact with other
neurons
-cytoplasm contains
Nissl bodies &
mitochondria
3. Cell processes = axons
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Conduct impulses away from cell bodypropagates nerve impulses to another neuron
Long, thin cylindrical process of cell
contains mitochondria, microtubules &
neurofibrils - NO ER/NO protein synth.
joins the soma at a cone-shaped elevation =
axon hillock
first part of the axon = initial segment
most impulses arise at the junction of the
axon hillock and initial segment = trigger
zone
cytoplasm = axoplasm
plasma membrane = axolemma
Side branches = collaterals arise from the
axon
axon and collaterals end in fine processes
called axon terminals
Swollen tips called synaptic end bulbs
contain vesicles filled with neurotransmitters
Structural Classification of Neurons
• Based on number of processes found on cell body
– multipolar = several dendrites & one axon
• most common cell type in the brain and SC
– bipolar neurons = one main dendrite & one axon
• found in retina, inner ear & olfactory
– unipolar neurons = one process only, sensory only (touch, stretch)
• develops from a bipolar neuron in the embryo - axon and dendrite fuse and
then branch into 2 branches near the soma - both have the structure of axons
(propagate APs) - the axon that projects toward the periphery = dendrites
Structural Classification of Neurons
• Named for histologist that first described them or
their appearance
•Purkinje = cerebellum
•Renshaw = spinal cord
• others are named for shapes
e.g. pyramidal cells
Functional Classification of Neurons
• Sensory (afferent) neurons
– transport sensory information from skin, muscles,
joints, sense organs & viscera to CNS
• Motor (efferent) neurons
– send motor nerve impulses to muscles & glands
• Interneurons (association) neurons
– connect sensory to motor neurons
– 90% of neurons in the CNS
The Nerve Impulse
Terms to know
• membrane potential = electrical voltage difference measured across the
membrane of a cell
• resting membrane potential = membrane potential of a neuron
measured when it is unstimulated
– results from the build-up of negative ions in the cytosol along the inside of
the neuron’s PM
– the outside of the PM becomes more positive
– this difference in charge can be measured as potential energy – measured
in millivolts
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polarization
depolarization
repolarization
hyperpolarization
The electric potential across an axonal
membrane can be measured
• the differences in positive and
negative charges in and out
of the neuron can be measured by
electrodes = resting membrane potential
-ranges from -40 to -90 mV
Ion
Channels
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ion channels in the PM of neurons and muscles contributes
to their excitability
when open - ions move down their concentration gradients
channels possess gates to open and close them
two types: gated and non-gated
1. Leakage (non-gated) or Resting channels: are always open, contribute to the resting potential
-nerve cells have more K+ than Na+ leakage channels
-as a result, membrane permeability to K+ is higher
-K+ leaks out of cell - inside becomes more negative
-K+ is then pumped back in
2. Gated channels: open and close in response to a stimulus
A. voltage-gated: open in response to change in voltage - participate in the AP
B. ligand-gated: open & close in response to particular chemical stimuli (hormone,
neurotransmitter, ion)
C. mechanically-gated: open with mechanical stimulation
The resting potential, generated mainly by
open “resting”, non-gated K+ channels
-the number of K+ channels
dramatically outnumbers that
of Na+
-however, there are a few Na
leak channels along the axonal
membrane
AXON
ECF
Action Potential
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Resting membrane potential is 70mV
triggered when the membrane
potential reaches a threshold
usually -55 MV
if the graded potential change
exceeds that of threshold – Action
Potential
Depolarization is the change from 70mV to +30 mV
Repolarization is the reversal from
+30 mV back to -70 mV)
• action potential = nerve impulse
• takes place in two stages: depolarizing phase (more positive) and repolarizing
phase (more negative - back toward resting potential)
• followed by a hyperpolarizing phase or refractory period in which no new AP
http://www.blackwellpublish
can be generated
ing.com/matthews/channel.
html
The action potential
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1. neuron is at resting membrane potential (resting MP)
2. neuron receives a signal
– Neurotransmitter (NT)
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3. NT binds ligand-gated sodium channel
4. LGNa channel opens
5. Na flows into neuron = depolarization
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6. if neuron depolarizes enough to Threshold = Action Potential (AP)
7. 1st stage of AP – opening of voltage-gated Na channels
8. even more Na flows in through VGNa channels = BIG depolarization
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Membrane potential goes from negative to positive
9. closing of VGNa channels & opening of voltage-gated K channels
10. BIG outflow of potassium through VGK channels = repolarization
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Inside of neuron (i.e. MP) becomes more positive
Inside of neuron (MP) becomes more negative
11. neuron repolarizes so much – it goes past resting and hyperpolarizes
12. closing of VGK channels
13. all voltage-gated channels closed, Na/K pump “resets” ion distribution to
resting situation
Action Potential
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Continuous versus Saltatory Conduction
• Continuous conduction
(unmyelinated fibers)
– An action potential spreads
(propagates) over the surface of
the axolemma
– as Na+ flows into the cell
during depolarization, the
voltage of adjacent areas is
effected and their voltage-gated
Na+ channels open
– step-by-step depolarization of
each portion of the length of
the axolemma
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter45/animations.html#
Saltatory Conduction
• Saltatory conduction
-depolarization only at nodes of
Ranvier - areas along the axon
that are unmyelinated and
where there is a high density of
voltage-gated ion channels
-current carried by ions flows
through extracellular fluid from
node to node
http://www.blackwellpublishing.com/matthews/actionp.html
Rate of Impulse Conduction
• Properties of axon
• Presence or absence of myelin sheath
• Diameter of axon
Action Potentials in Nerve and Muscle
• Entire muscle cell membrane versus only the
axon of the neuron is involved
• Resting membrane potential
– nerve is -70mV
– skeletal & cardiac muscle is closer to -90mV
• Duration
– nerve impulse is 1/2 to 2 msec
– muscle action potential lasts 1-5 msec for skeletal &
10-300msec for cardiac & smooth
• Fastest nerve conduction velocity is 18 times
faster than velocity over skeletal muscle fiber
Synaptic Communication
Synapses
Synapse: Site of intercellular communication between 2
neurons or between a neuron and an effector (e.g.
muscle – neuromuscular junction)
• Permits communication between neurons and other cells
– Initiating neuron = presynaptic neuron
– Receiving neuron = postsynaptic neuron
• You can classify a synapse according to:
– 1. the action they produce on the post-synaptic neuron –
excitatory or inhibitory
– 2. the mode of communication – chemical vs.
electrical
Synapses – Excitatory vs. Inhibitory
• If the NT depolarizes the postsynaptic neuron =
excitatory
– The depolarization event is often called an excitatory
postsynaptic potential (EPSP)
– Opening of sodium channels or other cation channels (inward)
• Some NTs will cause hyperpolarization = inhibitory
– The hyperpolarization event is often called an inhibitory
postsynaptic potential (IPSP)
– Opening of chloride channels (inward) or potassium channels
(outward)
• Neural activity depends on summation of all synaptic
activity
– Excitatory and inhibitory
Synapses
• Electrical
– Direct physical contact between cells required
– Conducted through gap junctions
– Two advantages over chemical synapses
• 1. faster communication – almost instantaneous
• 2. synchronization between neurons or muscle fibers
– e.g. heart beat
Chemical Synapse
• Synapse
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Most are axodendritic axon -> dendrite
Some are axoaxonic – axon > axon
http://www.lifesci.ucsb.edu/~mcdougal/neurobehavior/modules_homework/lect3.dcr
Synapses – Chemical vs. Electrical
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Chemical - Most common type of synapse
– Membranes of pre and postsynaptic neurons do not touch
– Space = Synaptic cleft
– the AP cannot travel across the cleft – release of
neurotransmitters
– The neurotransmitter induces a postsynaptic potential in the
PS neuron
• if the potential is an EPSP – excitatory and an AP results
• If the potential is an IPSP – inhibitory and NO AP results (e.g.
glycine or GABA)
– Communication in one direction only
http://www.blackwellpublishing.com/matthews/nmj.html
The Neuromuscular Junction
• end of neuron (synaptic terminal or
axon bulb) is in very close association
with the muscle fiber
• distance between the bulb and the folded
sarcolemma = synaptic cleft
• nerve impulse leads to release of
neurotransmitter (acetylcholine)
• N.T. binds to receptors on myofibril
surface
• binding leads to influx of sodium,
potassium ions (via channels)
• eventual release of calcium by
sarcoplasmic recticulum = contraction
• Acetylcholinesterase breaks down ACh
• Limits duration of contraction
The Events in Muscle Contraction
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AP generated at trigger zone in
pre-synaptic neuron
AP arrives in end bulb – causes entry
of calcium into end-bulb – release
of Ach
Binding of Ach to ligand-gated Na
channels on muscle PM (Ach receptors)
Na enters muscle cell – depolarization
Muscle membrane potential reaches
threshold = Action Potential
AP travels through PM of muscle cell into
T-tubules
AP “passes by” sarcoplasmic reticulum –
release of calcium into muscle cell
Ca binds troponin-tropomyosin complex &
“shifts” it off myosin binding site
Cross-bridging between actin and myosin,
pivoting of myosin head = Contraction
(ATP dependent)
Neurotransmitters
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More than 100 identified
Some bind receptors and cause channels to open
Others bind receptors and result in a second messenger system
Results in either excitation or inhibition of the target
1. small molecules: Acetylcholine (ACh)
-All neuromuscular junctions use ACh
-ACh also released at chemical synapses in the PNS and by
some CNS neurons
-Can be excitatory at some synapses and inhibitory at others
-Inactivated by an enzyme acetylcholinesterase
Neurotransmitters
2. Amino acids: glutamate & aspartate & GABA
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Powerful excitatory effects
Glutamate is the main excitatory neurotransmitter in the CNS
Stimulate most excitatory neurons in the CNS (about ½ the neurons in the brain)
Binding of glutamate to receptors opens calcium channels = EPSP
GABA (gamma amino-butyric acid) is an inhibitory neurotransmitter for 1/3 of
all brain synapses
Neurotransmitters
3. Biogenic amines: modified amino acids
– catecholamines: norepinephrine (NE), epinephrine, dopamine (tyrosine)
– serotonin - concentrated in neurons found in the brain region = raphe
nucleus
• derived from tryptophan
• sensory perception, temperature regulation, mood control, appetite, sleep
induction
• feeling of well being
– NE - role in arousal, awakening, deep sleep, regulating mood
– epinephrine (adrenaline) - flight or fight response
– dopamine - emotional responses and pleasure, decreases skeletal muscle
tone
Removal of Neurotransmitter
• Enzymatic degradation
– acetylcholinesterase
• Uptake by neurons or glia cells
– neurotransmitter transporters
• NE, dopamine, serotonin
Neuropeptides
• widespread in both CNS and PNS
• excitatory and inhibitory
• act as hormones elsewhere in the body
-Substance P -- enhances our perception of pain
-opioid peptides: endorphins - released during stress, exercise
-breaks down bradykinins (pain chemicals), boosts
the immune system and slows the growth of cancer
cells
**acupuncture
-binds to mu-opioid receptors
may produce loss
-released by the neurons of the Hypothalamus and by
of pain sensation
the cells of the pituitary
because of release
of opioid-like
enkephalins - analgesics
substances such as
-breaks down bradykinins (200x stronger than morph
endorphins or
-pain-relieving effect by blocking the release of
dynorphins
substance P
dynorphins - regulates pain and emotions