Transcript Nervous Tissue

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The master controlling and communicating system of the
body
Functions
o Sensory input – monitoring stimuli
o Integration – interpretation of sensory input
o Motor output – response to stimuli
Figure 11.1
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Central nervous system (CNS)
o Brain and spinal cord
o Integration and command center
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Peripheral nervous system (PNS)
o Paired spinal and cranial nerves
o Carries messages to and from the spinal cord and brain
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Sensory (afferent) division
o Sensory afferent fibers – carry impulses from skin, skeletal muscles,
and joints to the brain
o Visceral afferent fibers – transmit impulses from visceral organs to
the brain
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Motor (efferent) division
o Transmits impulses from the CNS to effector organs
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Somatic nervous system
o Conscious control of skeletal muscles
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Autonomic nervous system (ANS)
o Regulates smooth muscle, cardiac muscle, and glands
o Divisions – sympathetic and parasympathetic
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The two principal cell types of the nervous system are:
o Neurons – excitable cells that transmit electrical signals
o Supporting cells – cells that surround and wrap neurons
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The supporting cells (neuroglia or glial cells):
o Provide a supportive scaffolding for neurons
o Segregate and insulate neurons
o Guide young neurons to the proper connections
o Promote health and growth
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Most abundant, versatile, and highly branched glial cells
They cling to neurons and their synaptic endings, and cover
capillaries
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Functionally, they:
o Support and brace neurons
o Anchor neurons to their nutrient supplies
o Guide migration of young neurons
o Control the chemical environment
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Microglia – small, oval cells with spiny processes
o Phagocytes that monitor the health of neurons
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Ependymal cells – range in shape from squamous to
columnar
o They line the central cavities of the brain and spinal column
Microglia
Ependymal Cells
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Oligodendrocytes – branched cells that wrap CNS nerve
fibers with myelin
Schwann cells (neurolemmocytes) – surround fibers of the
PNS with myelin
Satellite cells surround neuron cell bodies with ganglia
Oligodendrocytes
Schwann Cells
Satellite Cells
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Structural units of the nervous system
o Composed of a body, axon, and dendrites
o Long-lived, amitotic, and have a high metabolic rate
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Their plasma membrane function in:
o Electrical signaling
o Cell-to-cell signaling during development
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Contains the nucleus and a nucleolus
Is the major biosynthetic center
Is the focal point for the outgrowth of neuronal processes
Has no centrioles (hence its amitotic nature)
Has well-developed Nissl bodies (rough ER)
Contains an axon hillock – cone-shaped area from which
axons arise
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Armlike extensions from the soma
Called tracts in the CNS and nerves in the PNS
There are two types: axons and dendrites
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Short, tapering, and diffusely branched processes
They are the receptive, or input, regions of the neuron
Electrical signals are conveyed as graded potentials (not
action potentials)
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Slender processes of uniform diameter arising from the
hillock
Long axons are called nerve fibers
Usually there is only one unbranched axon per neuron
Rare branches, if present, are called axon collaterals
Axonal terminal – branched terminus of an axon
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Generate and transmit action potentials
Secrete neurotransmitters from the axonal terminals
Movement along axons occurs in two ways
o Anterograde — toward axonal terminal
o Retrograde — away from axonal terminal
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Whitish, fatty (protein-lipoid), segmented sheath around
most long axons
It functions to:
o Protect the axon
o Electrically insulate fibers from one another
o Increase the speed of nerve impulse transmission
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Formed by Schwann cells in the PNS
A Schwann cell:
o Envelopes an axon in a trough
o Encloses the axon with its plasma membrane
o Has concentric layers of membrane that make up the myelin sheath
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Neurilemma – remaining visible nucleus and cytoplasm of a
Schwann cell
Figure 11.5a–c
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Gaps in the myelin sheath between adjacent Schwann cells
They are the sites where axon collaterals can emerge
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A Schwann cell surrounds nerve fibers but coiling does not
take place
Schwann cells partially enclose 15 or more axons
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Both myelinated and unmyelinated fibers are present
Myelin sheaths are formed by oligodendrocytes
Nodes of Ranvier are widely spaced
There is no neurilemma
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White matter – dense collections of myelinated fibers
Gray matter – mostly soma and unmyelinated fibers
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Structural:
o Multipolar — three or more processes
o Bipolar — two processes (axon and dendrite)
o Unipolar — single, short process
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Functional:
o Sensory (afferent) — transmit impulses toward the CNS
o Motor (efferent) — carry impulses away from the CNS
o Interneurons (association neurons) — shuttle signals through CNS
pathways
Table 11.1.1
Table 11.1.2
Table 11.1.3
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Neurons are highly irritable
Action potentials, or nerve impulses, are:
o Electrical impulses carried along the length of axons
o Always the same regardless of stimulus
o The underlying functional feature of the nervous system
 Voltage
(V) – measure of potential energy
generated by separated charge
 Potential difference – voltage measured between
two points
 Current (I) – the flow of electrical charge between
two points
 Resistance (R) – hindrance to charge flow
 Insulator – substance with high electrical
resistance
 Conductor – substance with low electrical
resistance
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Reflects the flow of ions rather than electrons
There is a potential on either side of membranes when:
o The number of ions is different across the membrane
o The membrane provides a resistance to ion flow
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Types of plasma membrane ion channels:
o Passive, or leakage, channels – always open
o Chemically gated channels – open with binding of a specific
neurotransmitter
o Voltage-gated channels – open and close in response to membrane
potential
o Mechanically gated channels – open and close in response to
physical deformation of receptors
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Example: Na+-K+ gated channel
Closed when a neurotransmitter is not bound to the
extracellular receptor
o Na+ cannot enter the cell and K+ cannot exit the cell
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Open when a neurotransmitter is attached to the receptor
o Na+ enters the cell and K+ exits the cell
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Example: Na+ channel
Closed when the intracellular environment is negative
o Na+ cannot enter the cell
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Open when the intracellular environment is positive
o Na+ can enter the cell
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When gated channels are open:
o Ions move quickly across the membrane
o Movement is along their electrochemical gradients
o An electrical current is created
o Voltage changes across the membrane
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Ions flow along their chemical gradient when they move
from an area of high concentration to an area of low
concentration
Ions flow along their electrical gradient when they move
toward an area of opposite charge
Electrochemical gradient – the electrical and chemical
gradients taken together
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The potential difference (–70 mV) across the membrane of
a resting neuron
It is generated by different concentrations of Na+, K+, Cl,
and protein anions (A)
Ionic differences are the consequence of:
o Differential permeability of the neurilemma to Na+ and K+
o Operation of the sodium-potassium pump
Figure 11.8
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Used to integrate, send, and receive information
Membrane potential changes are produced by:
o Changes in membrane permeability to ions
o Alterations of ion concentrations across the membrane
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Types of signals – graded potentials and action potentials
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Changes are caused by three events
o Depolarization – the inside of the membrane becomes less negative
o Repolarization – the membrane returns to its resting membrane
potential
o Hyperpolarization – the inside of the membrane becomes more
negative than the resting potential
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Short-lived, local changes in membrane potential
Decrease in intensity with distance
Magnitude varies directly with the strength of the stimulus
Sufficiently strong graded potentials can initiate action
potentials
Figure 11.10
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Voltage changes are decremental
Current is quickly dissipated due to the leaky plasma
membrane
Only travel over short distances
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A brief reversal of membrane potential with a total
amplitude of 100 mV
Action potentials are only generated by muscle cells and
neurons
They do not decrease in strength over distance
They are the principal means of neural communication
An action potential in the axon of a neuron is a nerve
impulse
 Na+
and K+ channels are closed
 Leakage accounts for small movements of Na+ and
K+
 Each Na+ channel has two voltage-regulated gates
o Activation gates –
closed in the resting
state
o Inactivation gates –
open in the resting
state
Figure 11.12.1
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Na+ permeability increases; membrane potential reverses
Na+ gates are opened; K+ gates are closed
Threshold – a critical level of depolarization
(-55 to -50 mV)
At threshold,
depolarization
becomes
self-generating
Figure 11.12.2
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Sodium inactivation gates close
Membrane permeability to Na+ declines to resting levels
As sodium gates close, voltage-sensitive K+ gates open
K+ exits the cell and
internal negativity
of the resting neuron
is restored
Figure 11.12.3
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Potassium gates remain open, causing an excessive efflux
of K+
This efflux causes hyperpolarization of the membrane
(undershoot)
The neuron is
insensitive to
stimulus and
depolarization
during this time
Figure 11.12.4
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Repolarization
o Restores the resting electrical conditions of the neuron
o Does not restore the resting ionic conditions
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Ionic redistribution back to resting conditions is restored by
the sodium-potassium pump
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1 – resting state
2 – depolarization phase
3 – repolarization phase
4 – hyperpolarization
Figure 11.12
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Na+ influx causes a patch of the axonal membrane to
depolarize
Positive ions in the axoplasm move toward the polarized
(negative) portion of the membrane
Sodium gates are shown as closing, open, or closed
Figure 11.13a
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Ions of the extracellular fluid move toward the area of
greatest negative charge
A current is created that depolarizes the adjacent
membrane in a forward direction
The impulse propagates away from its point of origin
Figure 11.13b
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The action potential moves away from the stimulus
Where sodium gates are closing, potassium gates are open
and create a current flow
Figure 11.13c
 Threshold
– membrane is depolarized by 15 to 20
mV
 Established by the total amount of current flowing
through the membrane
 Weak (subthreshold) stimuli are not relayed into
action potentials
 Strong (threshold) stimuli are relayed into action
potentials
 All-or-none phenomenon – action potentials either
happen completely, or not at all
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All action potentials are alike and are independent of
stimulus intensity
Strong stimuli can generate an action potential more often
than weaker stimuli
The CNS determines stimulus intensity by the frequency of
impulse transmission
Figure 11.14
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Time from the opening of the Na+ activation gates until the
closing of inactivation gates
The absolute refractory period:
o Prevents the neuron from generating an action potential
o Ensures that each action potential is separate
o Enforces one-way transmission of nerve impulses
Figure 11.15
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The interval following the absolute refractory period when:
o Sodium gates are closed
o Potassium gates are open
o Repolarization is occurring
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The threshold level is elevated, allowing strong stimuli to
increase the frequency of action potential events
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Conduction velocities vary widely among neurons
Rate of impulse propagation is determined by:
o Axon diameter – the larger the diameter, the faster the impulse
o Presence of a myelin sheath – myelination dramatically increases
impulse speed
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Current passes through a myelinated axon only at the nodes
of Ranvier
Voltage-gated Na+ channels are concentrated at these
nodes
Action potentials are triggered only at the nodes and jump
from one node to the next
Much faster than conduction along unmyelinated axons
Figure 11.16
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An autoimmune disease that mainly affects young adults
Symptoms: visual disturbances, weakness, loss of muscular
control, and urinary incontinence
Nerve fibers are severed and myelin sheaths in the CNS
become nonfunctional scleroses
Shunting and short-circuiting of nerve impulses occurs
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The advent of disease-modifying drugs including interferon
beta-1a and -1b, Avonex, Betaseran, and Copazone:
o Hold symptoms at bay
o Reduce complications
o Reduce disability
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Nerve fibers are classified according to:
o Diameter
o Degree of myelination
o Speed of conduction
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A junction that mediates information transfer from one
neuron:
o To another neuron
o To an effector cell
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Presynaptic neuron – conducts impulses toward the
synapse
Postsynaptic neuron – transmits impulses away from the
synapse
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Axodendritic – synapses between the axon of one neuron
and the dendrite of another
Axosomatic – synapses between the axon of one neuron
and the soma of another
Other types of synapses include:
o Axoaxonic (axon to axon)
o Dendrodendritic (dendrite to dendrite)
o Dendrosomatic (dendrites to soma)
 Electrical
synapses:
o Are less common than chemical synapses
o Correspond to gap junctions found in other cell types
o Are important in the CNS in:
• Arousal from sleep
• Mental attention
• Emotions and memory
• Ion and water homeostasis
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Specialized for the release and reception of
neurotransmitters
Typically composed of two parts:
o Axonal terminal of the presynaptic neuron, which contains synaptic
vesicles
o Receptor region on the dendrite(s) or soma of the postsynaptic
neuron
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Fluid-filled space separating the presynaptic and
postsynaptic neurons
Prevents nerve impulses from directly passing from one
neuron to the next
Transmission across the synaptic cleft:
o Is a chemical event (as opposed to an electrical one)
o Ensures unidirectional communication between neurons
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Nerve impulses reach the axonal terminal of the
presynaptic neuron and open Ca2+ channels
Neurotransmitter is released into the synaptic cleft via
exocytosis in response to synaptotagmin
Neurotransmitter crosses the synaptic cleft and binds to
receptors on the postsynaptic neuron
Postsynaptic membrane permeability changes, causing an
excitatory or inhibitory effect
Ca2+
1
Neurotransmitter
Axon terminal of
presynaptic neuron
Postsynaptic
membrane
Mitochondrion
Axon of
presynaptic
neuron
Na+
Receptor
Postsynaptic
membrane
Ion channel open
Synaptic vesicles
containing
neurotransmitter
molecules
5
Degraded
neurotransmitter
2
Synaptic
cleft
Ion channel
(closed)
3
4
Ion channel closed
Ion channel (open)
Figure 11.18
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Neurotransmitter bound to a postsynaptic neuron:
o Produces a continuous postsynaptic effect
o Blocks reception of additional “messages”
o Must be removed from its receptor
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Removal of neurotransmitters occurs when they:
o Are degraded by enzymes
o Are reabsorbed by astrocytes or the presynaptic terminals
o Diffuse from the synaptic cleft
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Neurotransmitter must be released, diffuse across the
synapse, and bind to receptors
Synaptic delay – time needed to do this (0.3-5.0 ms)
Synaptic delay is the rate-limiting step of neural
transmission
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Neurotransmitter receptors mediate changes in membrane
potential according to:
o The amount of neurotransmitter released
o The amount of time the neurotransmitter is bound to receptors
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The two types of postsynaptic potentials are:
o EPSP – excitatory postsynaptic potentials
o IPSP – inhibitory postsynaptic potentials
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EPSPs are graded potentials that can initiate an action
potential in an axon
o Use only chemically gated channels
o Na+ and K+ flow in opposite directions at the same time
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Postsynaptic membranes do not generate action potentials
Figure 11.19a
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Neurotransmitter binding to a receptor at inhibitory
synapses:
o Causes the membrane to become more permeable to potassium
and chloride ions
o Leaves the charge on the inner surface negative
o Reduces the postsynaptic neuron’s ability to produce an action
potential
Figure 11.19b
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A single EPSP cannot induce an action potential
EPSPs must summate temporally or spatially to induce an
action potential
Temporal summation – presynaptic neurons transmit
impulses in rapid-fire order
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Spatial summation – postsynaptic neuron is stimulated by a
large number of terminals at the same time
IPSPs can also summate with EPSPs, canceling each other
out
Figure 11.20
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Chemicals used for neuronal communication with the body
and the brain
50 different neurotransmitters have been identified
Classified chemically and functionally
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Acetylcholine (ACh)
Biogenic amines
Amino acids
Peptides
Novel messengers: ATP and dissolved gases NO and CO
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First neurotransmitter identified, and best understood
Released at the neuromuscular junction
Synthesized and enclosed in synaptic vesicles
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Degraded by the enzyme acetylcholinesterase (AChE)
Released by:
o All neurons that stimulate skeletal muscle
o Some neurons in the autonomic nervous system
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Include:
o Catecholamines – dopamine, norepinephrine (NE), and epinephrine
o Indolamines – serotonin and histamine
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Broadly distributed in the brain
Play roles in emotional behaviors and our biological clock
 Enzymes
present in the
cell determine length of
biosynthetic pathway
 Norepinephrine and
dopamine are
synthesized in axonal
terminals
 Epinephrine is released
by the adrenal medulla
Figure 11.21
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Include:
o GABA – Gamma ()-aminobutyric acid
o Glycine
o Aspartate
o Glutamate
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Found only in the CNS
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Include:
o Substance P – mediator of pain signals
o Beta endorphin, dynorphin, and enkephalins
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Act as natural opiates; reduce pain perception
Bind to the same receptors as opiates and morphine
Gut-brain peptides – somatostatin, and cholecystokinin
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ATP
o Is found in both the CNS and PNS
o Produces excitatory or inhibitory responses depending on receptor
type
o Induces Ca2+ wave propagation in astrocytes
o Provokes pain sensation
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Nitric oxide (NO)
o Activates the intracellular receptor guanylyl cyclase
o Is involved in learning and memory
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Carbon monoxide (CO) is a main regulator of cGMP in the
brain
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Two classifications: excitatory and inhibitory
o Excitatory neurotransmitters cause depolarizations
(e.g., glutamate)
o Inhibitory neurotransmitters cause hyperpolarizations (e.g., GABA
and glycine)
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Some neurotransmitters have both excitatory and inhibitory
effects
o Determined by the receptor type of the postsynaptic neuron
o Example: acetylcholine
• Excitatory at neuromuscular junctions with skeletal muscle
• Inhibitory in cardiac muscle
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Direct: neurotransmitters that open ion channels
o Promote rapid responses
o Examples: ACh and amino acids
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Indirect: neurotransmitters that act through second
messengers
o Promote long-lasting effects
o Examples: biogenic amines, peptides, and dissolved gases
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Composed of integral membrane protein- a protein
permanently attached to the cell membrane
Mediate direct neurotransmitter action
Action is immediate, brief, simple, and highly localized
Ligand binds the receptor, and ions enter the cells
Excitatory receptors depolarize membranes
Inhibitory receptors hyperpolarize membranes
Figure 11.22a
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Responses are indirect, slow, complex, prolonged, and often
diffuse
These receptors are transmembrane protein complexes
Examples: muscarinic ACh receptors, neuropeptides, and
those that bind biogenic amines
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G protein-linked receptors activate intracellular second
messengers including Ca2+, cGMP, diacylglycerol, as well as
cAMP
Second messengers:
o Open or close ion channels
o Activate kinase enzymes
o Phosphorylate channel proteins
o Activate genes and induce protein synthesis
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Functional groups of neurons that:
o Integrate incoming information
o Forward the processed information to its appropriate destination
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Simple neuronal pool
o Input fiber – presynaptic fiber
o Discharge zone – neurons most closely associated with the
incoming fiber
o Facilitated zone – neurons farther away from incoming fiber
Figure 11.23
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Divergent – one incoming fiber stimulates ever increasing
number of fibers, often amplifying circuits
Figure 11.24a, b
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Convergent – opposite of
divergent circuits, resulting
in either strong stimulation
or inhibition
Figure 11.24c, d
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Reverberating – chain of neurons containing collateral
synapses with previous neurons in the chain
Figure 11.24e
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Parallel after-discharge – incoming neurons stimulate
several neurons in parallel arrays
Figure 11.24f
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Serial Processing
o Input travels along one pathway to a specific destination
o Works in an all-or-none manner
o Example: spinal reflexes
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Parallel Processing
o Input travels along several pathways
o Pathways are integrated in different CNS systems
o One stimulus promotes numerous responses
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Example: a smell may remind one of the odor and
associated experiences
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The nervous system originates from the neural tube and
neural crest
The neural tube becomes the CNS
There is a three-phase process of differentiation:
o Proliferation of cells needed for development
o Migration – cells become amitotic and move externally
o Differentiation into neuroblasts
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Guided by:
o Scaffold laid down by older neurons
o Orienting glial fibers
o Release of nerve growth factor by astrocytes
o Neurotropins released by other neurons
o Repulsion guiding molecules
o Attractants released by target cells
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N-CAM – nerve cell adhesion molecule
Important in establishing neural pathways
Without N-CAM, neural function is impaired
Found in the membrane of the growth cone