12-1
Nervous Tissue
• overview of the nervous system
• properties of neurons
• supportive cells (neuroglia)
• electrophysiology of neurons
• synapses
• neural integration
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Neurofibrils
Axon(d)
Figure 12.4d
12-2
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
12-3
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
12-4
Subdivisions of Nervous SystemCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Brain
Nerves
Ganglia
Peripheral nervoussystem (PNS)
Central nervoussystem (CNS)
Spinalcord
Figure 12.1
12-5
Subdivisions of Nervous SystemCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Brain
Central nervous system Peripheral nervous system
Spinalcord
Sensorydivision
Motordivision
Visceralsensorydivision
Somaticsensorydivision
Visceralmotor
division
Somaticmotor
division
Sympatheticdivision
Parasympatheticdivision
Figure 12.2
12-6
Functional Classes of NeuronsCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1
2
3
Peripheral nervous system Central nervous system
Sensory (afferent)neurons conductsignals fromreceptors to the CNS.
Motor (efferent)neurons conductsignals from the CNSto effectors such asmuscles and glands.
Interneurons(associationneurons) areconfined tothe CNS.
Figure 12.3
12-7
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
Structure of a Neuron• soma – the control center of the
neuron• also called neurosoma, cell body, or perikaryon
• dendrites – vast number of branches coming from a few thick branches from the soma• primary site for receiving signals from other neurons
• axon (nerve fiber) – originates from a mound on one side of the soma called the axon hillock• cylindrical, relatively unbranched for most of its length• distal end of the axon has the Telodendria– extensive
complex of fine branches
Figure 12.4a
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Dendrites
Soma
Nucleus
Nucleolus
Axon
Node of Ranvier
Internodes
Synaptic knobs
Axon hillockInitial segment
Myelin sheath
Schwann cell
Axon collateral
(a)
Trigger zone:
Direction ofsignal transmission
Figure 12.4a12-8
12-9
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
12-10
Variation in Neuron Structure• multipolar neuron
– one axon and multiple dendrites – most common– most neurons in the brain and spinal
cord
• bipolar neuron– one axon and one dendrite– olfactory cells, retina, inner ear
• 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 Figure 12.5
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Dendrites
Dendrites
Dendrites
Axon
Axon
Dendrites
Axon
Unipolar neuron
Multipolar neurons
Bipolar neurons
Anaxonic neuron
12-11
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
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
12-13
Myelin Sheath in PNS
nodes of Ranvier and internodes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Myelin sheath
Axolemma
Axoplasm
Neurilemma
(c)
Schwann cellnucleus
Figure 12.4c
One schann cell covers one axon.
12-14
Myelination in CNSCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(b)
Oligodendrocyte
Nerve fiber
Myelin
Figure 12.7b
One oligodendrocytes covers multiple axons
12-15
Myelination in PNS
Figure 12.7a
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Axon
(a)
Myelin sheath
Schwann cell
Nucleus
Basal lamina
Neurilemma
Endoneurium
12-16
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
12-17
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 membrane2. plasma membrane is selectively permeable and
allows some ions to pass easier than others3. electrical attraction of cations and anions to
each other
12-18
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
12-19
• Na+ concentrated outside of cell (ECF) • K+ concentrated inside cell (ICF)
Figure 12.11
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ECF
ICF
Na+
channel
K+
channel
Na+ 145 mEq/L
K+ 4 mEq/L
Na+ 12 mEq/L
K+ 150 mEq/L
Large anionsthat cannotescape cell
Ionic Basis of Resting Membrane Potential
12-20
Excitation of a Neuron by a Chemical Stimulus
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Dendrites Soma Axon
Current
Na+
ECF
ICF
Triggerzone
Ligand
Plasmamembraneof dendrite
Receptor
Figure 12.12
12-21
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 voltage-regulated 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
12-22
Action Potentials
• only a thin layer of the cytoplasm next to the cell membrane is affected • in reality, very few ions are
involved• action potential is often called a
spike – happens so fast• characteristics of action potential
versus a local potential– 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
Figure 12.13a
Time
–70
Depolarization Repolarization
Hyperpolarization
ThresholdmV
+35
0
–55
(a)
7
2
6
3
4
5
1
Localpotential
Resting membranepotential
Actionpotential
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
12-23
Sodium and Potassium GatesCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
–70
350
Repolarization complete
mV
Na+
Na+ gates closed,K+ gates closing
K+
4
Na+
gate
K+
gate
–70
350
mV
1 Na+ and K+ gates closed
Resting membranepotential
–70
35
0
Depolarization begins
mV
2 Na+ gates open, Na+
enters cell, K+ gatesbeginning to open
–70
35
0
mV
Na+ gates closed, K+ gatesfully open, K+ leaves cell
3
Depolarization ends,repolarization begins
Figure 12.14
12-24
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
• 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 Figure 12.15
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Threshold
mV
Time
+35
–55
–70
0
Absoluterefractoryperiod
Relativerefractoryperiod
Resting membranepotential
12-25
Nerve Signal Conduction Unmyelinated Fibers
Figure 12.16
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
+ + + + + + + + + – – – + + + + + +
+ + + + + + + + + – – – + + + + + +
– – – – – – – – – + + + – – – – – –
+ + + + – – – + + + + + + + + + + +
+ + + + – – – + + + + + + + + + + +
– – – – + + + – – – – – – – – – – –
– – – – + + + – – – – – – – – – – –
– – – – – – – – – + + + – – – – – –
+ + + + + + + + + + + + + – – – + +
+ + + + + + + + + + + + + – – – + +
– – – – – – – – – – – – – + + + – –
– – – – – – – – – – – – – + + + – –
DendritesCell body Axon
Signal
Action potentialin progress
RefractorymembraneExcitablemembrane
unmyelinated fiber has voltage-regulated 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.
12-26
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.
(a)
Na+ inflow at nodegenerates action potential(slow but nondecremental)
Na+ diffuses along insideof axolemma to next node(fast but decremental)
Excitation of voltage-regulated gates willgenerate next actionpotential here
Figure 12.17a
12-27
Saltatory Conduction
• much faster than conduction in unmyelinated fibers
+ ++ +
+ +
+ +
+ +
+ +
+ ++ +
+ +
+ +
+ +
+ +
+ ++ +
+ +
+ +
+ +
+ +
– –– –
– –– –
– –– –
– –– –
– –– –
+ +
+ +
– –– –
+ +
+ +
– –– –
– –– –
+ +
+ +
– –– –
– –
– –
– –
– –
– –
– –
(b)
+ +
+ +
– –– –
+ +
+ +
– –– –
Action potentialin progress
Refractorymembrane
Excitablemembrane
+ +
+ +
– –– –
+ +
+ +
– –– –
+ +
+ +
– –– –
+ +
+ +
– –– –
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 12.17b
12-28
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– in cerebellum of brain, one neuron can have as many as 100,000 synapses
12-29
Synaptic Relationships Between Neurons
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Soma
Axon
Axodendritic synapse
(a)
(b) Axoaxonic synapse
Synapse
Presynapticneuron
Direction ofsignaltransmission
Postsynapticneuron
Axosomaticsynapse
Figure 12.18
12-30
Synaptic Knobs
Figure 12.19
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Axon ofpresynapticneuron
Synapticknob
Soma ofpostsynapticneuron
© Omikron/Science Source/Photo Researchers, Inc
12-31
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
12-32
Structure of a Chemical Synapse
• presynaptic neurons have synaptic vesicles with neurotransmitter and postsynaptic have receptors and ligand-regulated ion channels
Figure 12.20
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Axon of presynaptic neuron
Postsynaptic neuron
Postsynaptic neuron
Mitochondria
Synaptic cleft
Synaptic knob
Microtubulesof cytoskeleton
Synaptic vesiclescontaining neurotransmitter
Neurotransmitterreceptor
Neurotransmitterrelease
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
12-34
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
12-35
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
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
12-37
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– causing the postsynaptic neuron to fire
12-38
Excitatory Cholinergic SynapseCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Presynaptic neuron
Postsynaptic neuron
Presynaptic neuron
Ca2+
Na+
ACh
K+
+ +
– –
+
–
+ +
– –
+
–
+
–
2
3
4
5
1
Figure 12.22
12-39
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– choline reabsorbed by synaptic knob
12-40
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
12-41
Neural CircuitsCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Diverging
Output
Output
Reverberating Parallel after-discharge
Input
Input
Converging
Output
OutputInput
Input
Figure 12.30
Top Related