Download Nerve Tissue

Nervous Tissue
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• overview of the nervous
system
• properties of neurons
• supportive cells
(neuroglia)
• electrophysiology of
neurons
• synapses
Neurofibrils
• neural integration
(d)
Figure 12.4d
Axon
12-1
Overview of Nervous System
• nervous system carries out its task in three basic steps:
1. Sense organs receive information
2. They transmit coded messages to the spinal cord and the
brain
3. The brain and spinal cord processes this information,
relates it to past experiences, and determine what response is
appropriate to the circumstances and issues commands to
muscles and gland cells to carry out such a response
12-2
Two Major Anatomical
Subdivisions of Nervous System
• central nervous system (CNS)
– brain and spinal cord enclosed in bony coverings
• enclosed by cranium and vertebral column
• peripheral nervous system (PNS)
– all the nervous system except the brain and spinal cord
– composed of nerves and ganglia
• nerve – a bundle of nerve fibers (axons) wrapped in fibrous
connective tissue
• ganglion – a knot-like swelling in a nerve where neuron cell
bodies are concentrated
12-3
Subdivisions of Nervous System
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Central nervous
system (CNS)
Peripheral nervous
system (PNS)
Brain
Spinal
cord
Nerves
Ganglia
Figure 12.1
12-4
Subdivisions of Nervous System
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Central nervous system
Brain
Peripheral nervous system
Spinal
cord
Visceral
sensory
division
Figure 12.2
Sensory
division
Somatic
sensory
division
Motor
division
Visceral
motor
division
Sympathetic
division
Somatic
motor
division
Parasympathetic
division
12-5
Functional Classes of Neurons
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Peripheral nervous system
Central nervous system
1 Sensory (afferent)
neurons conduct
signals from
receptors to the CNS.
3 Motor (efferent)
neurons conduct
signals from the CNS
to effectors such as
muscles and glands.
2 Interneurons
(association
neurons) are
confined to
the CNS.
Figure 12.3
12-6
Functional Types of Neurons
• sensory (afferent) neurons
– specialized to detect stimuli
– transmit information about them to the CNS
• begin in almost every organ in the body and end in CNS
• afferent – conducting signals toward CNS
• interneurons (association) neurons
– lie entirely within the CNS
– process, store, and retrieve information and ‘make decisions’ that determine how the
body will respond to stimuli
– 99% of all neurons are interneurons
• motor (efferent) neuron
– send signals out to muscles and gland cells (the effectors)
• motor because most of them lead to muscles
• efferent neurons conduct signals away from the CNS
12-7
Structure of a Neuron
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• soma – the control center of the
neuron
•
•
also called neurosoma, cell body, or perikaryon
Soma
Nucleus
dendrites – vast number of branches coming
from a few thick branches from the soma
•
•
Dendrites
Nucleolus
primary site for receiving signals from other neurons
Trigger zone:
Axon hillock
Initial segment
axon (nerve fiber) – originates from a mound on
one side of the soma called the axon hillock
•
•
Axon collateral
cylindrical, relatively unbranched for most of its length
distal end of the axon has the Telodendria– extensive
complex of fine branches
Axon
Direction of
signal transmission
Internodes
Node of Ranvier
Myelin sheath
Schwann cell
Figure 12.4a
Figure 12.4a
Synaptic knobs
(a)
12-8
Universal Properties of Neurons
• excitability (irritability)
– respond to environmental changes called stimuli
• conductivity
– neurons respond to stimuli by producing electrical
signals that are quickly conducted to other cells at
distant locations
• secretion
– when electrical signal reaches end of nerve fiber, a
chemical neurotransmitter is secreted that crosses
the gap and stimulates the next cell
12-9
Variation in Neuron Structure
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• multipolar neuron
– one axon and multiple dendrites
– most common
– most neurons in the brain and spinal
cord
Dendrites
Axon
Multipolar neurons
• bipolar neuron
Dendrites
– one axon and one dendrite
– olfactory cells, retina, inner ear
Axon
• unipolar neuron
– single process leading away from the
soma
– sensory from skin and organs to
spinal cord
• anaxonic neuron
– many dendrites but no axon
– help in visual processes
Bipolar neurons
Dendrites
Axon
Unipolar neuron
Dendrites
Anaxonic neuron
Figure 12.5
12-10
Myelin
• in PNS, Schwann cell spirals repeatedly around a single
nerve fiber
– neurilemma – thick outermost coil of myelin sheath
• contains nucleus and most of its cytoplasm
• external to neurilemma is basal lamina and a thin layer of fibrous
connective tissue – endoneurium
• in CNS – oligodendrocytes reaches out to myelinate
several nerve fibers in its immediate vicinity
– nerve fibers in CNS have no neurilemma or endoneurium
12-11
Myelin
• many Schwann cells or oligodendrocytes are needed to
cover one nerve fiber
• myelin sheath is segmented
– nodes of Ranvier – gap between segments
– internodes – myelin covered segments from one gap to the next
– initial segment – short section of nerve fiber between the axon
hillock and the first glial cell
– trigger zone – the axon hillock and the initial segment
• play an important role in initiating a nerve signal
12-12
Myelin Sheath in PNS
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
One schann
cell covers
one axon.
Axoplasm
Axolemma
Schwann cell
nucleus
Neurilemma
Figure 12.4c
(c)
Myelin sheath
nodes of Ranvier and internodes
12-13
Myelination in CNS
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
One
oligodendrocyte
s covers multiple
axons
Oligodendrocyte
Myelin
Nerve fiber
Figure 12.7b
(b)
12-14
Myelination in PNS
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Schwann cell
Axon
Basal lamina
Endoneurium
Nucleus
(a)
Neurilemma
Myelin sheath
Figure 12.7a
12-15
Electrical Potentials and Currents
• electrical potential – a difference in the concentration of charged
particles between one point and another
• electrical current – a flow of charged particles from one point to
another
– in the body, currents are movement of ions, such as Na+ or K+ through
gated channels in the plasma membrane
– gated channels enable cells to turn electrical currents on and off
• resting membrane potential (RMP) – is the charge difference
across the plasma membrane of a cell
– -70 mV in a resting, unstimulated neuron
– negative value means there are more negatively charged particles
on the inside of the membrane than on the outside
12-16
Resting Membrane Potential
• RMP exists because of unequal electrolyte
distribution between extracellular fluid
(ECF) and intracellular fluid (ICF)
• RMP results from the combined effect of
three factors:
1. ions diffuse down their concentration gradient
through the membrane
2. plasma membrane is selectively permeable
and allows some ions to pass easier than
others
3. electrical attraction of cations and anions to
each other
12-17
Creation of Resting Membrane Potential
• potassium ions (K+) found inside the cell
•
K+ is about 40 times as concentrated in the ICF as in the ECF
–
have the greatest influence on RMP
– leaks out until electrical charge of cytoplasmic anions attracts it back in
and equilibrium is reached and net diffusion of K+ stops
• Sodium ions (Na+) found outside the cell
– Na+ is about 12 times as concentrated in the ECF as in the ICF
– resting membrane is much less permeable to Na+ than K+
• Large anions in the ICF that keep the RMP negative.
• Na+/K+ pumps out 3 Na+ for every 2 K+ it brings in
– works continuously to compensate for Na+ and K+ leakage, and
requires great deal of ATP
• 70% of the energy requirement of the nervous system
12-18
Ionic Basis of Resting Membrane
Potential
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ECF
Na+ 145 mEq/L
K+
Na+
channel
K+
channel
4 mEq/L
Figure 12.11
Na+ 12 mEq/L
ICF
K+ 150 mEq/L
Large anions
that cannot
escape cell
• Na+ concentrated outside of cell (ECF)
• K+ concentrated inside cell (ICF)
12-19
Excitation of a Neuron by a
Chemical Stimulus
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Dendrites
Soma Trigger
zone
Axon
Current
ECF
Ligand
Receptor
Plasma
membrane
of dendrite
Na+
ICF
Figure 12.12
12-20
Action Potentials
• action potential – dramatic change produced by voltage-regulated ion
gates in the plasma membrane
– trigger zone where action potential is generated
– threshold – critical voltage to which local potentials must rise to open the voltageregulated gates
• Between -55mV and -50mV
– when threshold is reached, neuron ‘fires’ and produces an action potential
– more and more Na+ channels open in in the trigger zone in a positive feedback cycle
creating a rapid rise in membrane voltage – spike
– when rising membrane potential passes 0 mV, Na+ gates are inactivated
• when all closed, the voltage peaks at about +30 mV
• membrane now positive on the inside and negative on the outside
• polarity reversed from RMP - depolarization
– by the time the voltage peaks, the slow K+ gates are fully open
• K+ repelled by the positive intracellular fluid now exit the cell
• their outflow repolarizes the membrane
– shifts the voltage back to negative numbers returning toward RMP
– K+ gates stay open longer than the Na+ gates
• slightly more K+ leaves the cell than Na+ entering
• negative overshoot – hyperpolarization or afterpotential
12-21
Action Potentials
• only a thin layer of the cytoplasm
next to the cell membrane is
affected
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4
+35
• in reality, very few ions are
involved
– follows an all-or-none law
• if threshold is reached, neuron
fires at its maximum voltage
• if threshold is not reached it does
not fire
– nondecremental - do not get
weaker with distance
– irreversible - once started goes
to completion and can not be
stopped
5
0
Depolarization
mV
• action potential is often called a
spike – happens so fast
• characteristics of action potential
versus a local potential
3
Repolarization
Action
potential
Threshold
2
–55
Local
potential
1
7
–70
Resting membrane
potential
6
Hyperpolarization
Time
(a)
Figure 12.13a
12-22
Sodium and Potassium Gates
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
K+
Na+
K+
gate
1 Na+ and K+ gates closed
35
0
–70
Resting membrane
potential
mV
mV
Na+
gate
2 Na+ gates open, Na+
enters cell, K+ gates
beginning to open
35
0
–70
Depolarization begins
3 Na+ gates closed, K+ gates
fully open, K+ leaves cell
35
0
35
0
–70
Depolarization ends,
repolarization begins
4 Na+ gates closed,
K+ gates closing
mV
mV
Figure 12.14
–70
Repolarization complete
12-23
The Refractory Period
during an action potential and for a few
milliseconds after, it is difficult or impossible
to stimulate that region of a neuron to fire
again.
•
refractory period – the period of resistance
to stimulation
•
two phases of the refractory period
– absolute refractory period
• no stimulus of any strength will
trigger AP
– relative refractory period
• only especially strong
stimulus will trigger new AP
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Absolute
refractory
period
Relative
refractory
period
+35
0
mV
•
Threshold
–55
Resting membrane
potential
–70
•
•
refractory period is occurring only at a small
patch of the neuron’s membrane at one time
other parts of the neuron can be stimulated
while the small part is in refractory period
Time
Figure 12.15
12-24
Nerve Signal Conduction
Unmyelinated Fibers
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
unmyelinated
fiber has
voltageregulated ion
gates along its
entire length
and Na+ and K+
gates open and
close producing
a new action
potential all the
way down the
axon. This
slows down the
signal being
sent.
Dendrites
Cell body
Axon
Signal
Action potential
in progress
Refractory
membrane
Excitable
membrane
++++–––+++++++++++
––––+++–––––––––––
–––– +++–––––––––––
++++–––+++++++++++
+++++++++–––++++++
–––––––––+++––––––
–––––––––+++––––––
+++++++++–––++++++
+++++++++++++–––++
–––––––––––––+++––
–––––––––––––+++––
+++++++++++++–––++
Figure 12.16
12-25
Saltatory Conduction
Myelinated Fibers
• voltage-gated channels needed for APs
• fast Na+ diffusion occurs between nodes
– signal weakens under myelin sheath, but still strong enough to stimulate an
action potential at next node
• saltatory conduction – the nerve signal seems to jump from node to
node increasing speed of signal being sent.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 12.17a
(a)
Na+ inflow at node
generates action potential
(slow but nondecremental)
Na+ diffuses along inside
of axolemma to next node
(fast but decremental)
Excitation of voltageregulated gates will
generate next action
potential here
12-26
Saltatory Conduction
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
+
–
–
+
+
–
–
+
–
+
+
–
–
+
+
–
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
–
+
+
–
–
+
+
–
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
–
+
+
–
–
+
+
–
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
(b)
Action potential
in progress
Refractory
membrane
Excitable
membrane
Figure 12.17b
• much faster than conduction in unmyelinated
fibers
12-27
Synapses
• a nerve signal can go no further when it reaches the end of the axon
– triggers the release of a neurotransmitter
– stimulates a new wave of electrical activity in the next cell across the
synapse
– electrical synapses do exist
•
•
•
• advantage of quick transmission
– no delay for release and binding of neurotransmitter
– Gap junctions in cardiac and smooth muscle and some neurons
• disadvantage is they cannot integrate information and make decisions
– ability reserved for chemical synapses in which neurons communicate
by releasing neurotransmitters
synapse between two neurons
– 1st neuron in the signal path is the presynaptic neuron
• releases neurotransmitter
– 2nd neuron is postsynaptic neuron
• responds to neurotransmitter
presynaptic neuron may synapse with a dendrite, soma, or axon of postsynaptic
neuron
neuron can have an enormous number of synapses
12-28
– in cerebellum of brain, one neuron can have as many as 100,000 synapses
Synaptic Relationships Between
Neurons
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Soma
Synapse
Axon
Presynaptic
neuron
Direction of
signal
transmission
Postsynaptic
neuron
(a)
Axodendritic synapse
Axosomatic
synapse
(b)
Figure 12.18
Axoaxonic synapse
12-29
Synaptic Knobs
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Axon of
presynaptic
neuron
Synaptic
knob
Soma of
postsynaptic
neuron
© Omikron/Science Source/Photo Researchers, Inc
Figure 12.19
12-30
Structure of a Chemical Synapse
• synaptic knob of presynaptic neuron contains synaptic
vesicles containing neurotransmitter
• ready to release neurotransmitter on demand
• postsynaptic neuron membrane contains proteins that
function as receptors and ligand-regulated ion gates
12-31
Structure of a Chemical Synapse
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Microtubules
of cytoskeleton
Axon of presynaptic neuron
Mitochondria
Postsynaptic neuron
Synaptic knob
Synaptic vesicles
containing neurotransmitter
Synaptic cleft
Neurotransmitter
receptor
Postsynaptic neuron
Neurotransmitter
release
Figure 12.20
• presynaptic neurons have synaptic vesicles with
neurotransmitter and postsynaptic have receptors and
ligand-regulated ion channels
12-32
Neurotransmitters and Related
Messengers
• more than 100 neurotransmitters have been identified
• fall into four major categories according to chemical
composition
– acetylcholine
• in a class by itself
• formed from acetic acid and choline
– amino acid neurotransmitters
• include glycine, glutamate, aspartate, and -aminobutyric acid (GABA)
– monoamines
• synthesized from amino acids by removal of the –COOH group
• retaining the –NH2 (amino) group
• major monoamines are:
– epinephrine, norepinephrine, dopamine (catecholamines)
– histamine and serotonin
– neuropeptides
12-33
Function of Neurotransmitters
at Synapse
• they are synthesized by the presynaptic
neuron
• they are released in response to
stimulation
• they bind to specific receptors on the
postsynaptic cell
• they alter the physiology of that cell
12-34
Effects of Neurotransmitters
• a given neurotransmitter does not have
the same effect everywhere in the body
• multiple receptor types exist for a
particular neurotransmitter
– 14 receptor types for serotonin
• receptor governs the effect the
neurotransmitter has on the target cell
12-35
Synaptic Transmission
• neurotransmitters are diverse in their action
– some excitatory
– some inhibitory
– some the effect depends on what kind of receptor the
postsynaptic cell has
– some open ligand-regulated ion gates
– some act through second-messenger systems
• three kinds of synapses with different modes of action
– excitatory cholinergic synapse
– inhibitory GABA-ergic synapse
– excitatory adrenergic synapse
• synaptic delay – time from the arrival of a signal at the
axon terminal of a presynaptic cell to the beginning of an
action potential in the postsynaptic cell
– 0.5 msec for all the complex sequence of events to occur
12-36
Excitatory Cholinergic Synapse
• cholinergic synapse – employs acetylcholine (ACh) as its
neurotransmitter
– ACh excites some postsynaptic cells
• skeletal muscle
– inhibits others
•
describing excitatory action
– nerve signal approaching the synapse, opens the voltage-regulated calcium gates
in synaptic knob
– Ca2+ enters the knob
– triggers exocytosis of synaptic vesicles releasing ACh
– empty vesicles drop back into the cytoplasm to be refilled with ACh
– reserve pool of synaptic vesicles move to the active sites and release their ACh
– ACh diffuses across the synaptic cleft
– binds to ligand-regulated gates on the postsynaptic neuron
– gates open
– allowing Na+ to enter cell and K+ to leave
• pass in opposite directions through same gate
– as Na+ enters the cell it spreads out along the inside of the plasma membrane and
depolarizes it producing a local potential called the postsynaptic potential
– if it is strong enough and persistent enough
– it opens voltage-regulated ion gates in the trigger zone
12-37
– causing the postsynaptic neuron to fire
Excitatory Cholinergic Synapse
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Presynaptic neuron
Presynaptic neuron
3
Ca2+
1
2
ACh
Na+
4
K+
5
Figure 12.22
Postsynaptic neuron
12-38
Cessation of the Signal
• mechanisms to turn off stimulation to keep postsynaptic neuron from
firing indefinitely
– neurotransmitter molecule binds to its receptor for only 1 msec or so
• then dissociates from it
– if presynaptic cell continues to release neurotransmitter
• one molecule is quickly replaced by another and the neuron is restimulated
• stop adding neurotransmitter and get rid of that which is already there
– stop signals in the presynaptic nerve fiber
– getting rid of neurotransmitter by:
• diffusion
– neurotransmitter escapes the synapse into the nearby ECF
– astrocytes in CNS absorb it and return it to neurons
• reuptake
– synaptic knob reabsorbs amino acids and monoamines by endocytosis
– break neurotransmitters down with monoamine oxidase (MAO) enzyme
– some antidepressant drugs work by inhibiting MAO
• degradation in the synaptic cleft
– enzyme acetylcholinesterase (AChE) in synaptic cleft degrades ACh into
acetate and choline
12-39
– choline reabsorbed by synaptic knob
Kinds of Neural Circuits
• diverging circuit
– one nerve fiber branches and synapses with several postsynaptic cells
– one neuron may produce output through hundreds of neurons
• converging circuit
– input from many different nerve fibers can be funneled to one neuron or
neural pool
– opposite of diverging circuit
• reverberating circuits
– neurons stimulate each other in linear sequence but one cell
restimulates the first cell to start the process all over
– diaphragm and intercostal muscles
• parallel after-discharge circuits
– input neuron diverges to stimulate several chains of neurons
• each chain has a different number of synapses
• eventually they all reconverge on a single output neuron
• after-discharge – continued firing after the stimulus stops
12-40
Neural Circuits
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Diverging
Converging
Input
Output
Output
Input
Reverberating
Parallel after-discharge
Figure 12.30
Input
Input
Output
Output
12-41