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Transcript of 1 11 Fundamentals of the Nervous System and Nervous Tissue Part A.
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11Fundamentals of the Nervous System and
Nervous Tissue
Part A
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Nervous System
The master controlling and communicating system of the body
Functions
Sensory input – monitoring stimuli occurring inside and outside the body
Integration – interpretation of sensory input
Motor output – response to stimuli by activating effector organs
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Nervous System
Figure 11.1
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Organization of the Nervous System
Central nervous system (CNS)
Brain and spinal cord
Integration and command center
Peripheral nervous system (PNS)
Paired spinal and cranial nerves
Carries messages to and from the spinal cord and brain
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Sensory (afferent) division
Sensory afferent fibers – carry impulses from skin, skeletal muscles, and joints to the brain
Visceral afferent fibers – transmit impulses from visceral organs to the brain
Motor (efferent) division
Transmits impulses from the CNS to effector organs
Peripheral Nervous System (PNS): Two Functional Divisions
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Somatic nervous system
Conscious control of skeletal muscles
Autonomic nervous system (ANS)
Regulates smooth muscle, cardiac muscle, and glands
Divisions – sympathetic and parasympathetic
Motor Division: Two Main Parts
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The two principal cell types of the nervous system are:
Neurons – excitable cells that transmit electrical signals
Supporting cells – cells that surround and wrap neurons
Histology of Nerve Tissue
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The supporting cells (neuroglia or glial cells):
Provide a supportive scaffolding for neurons
Segregate and insulate neurons
Guide young neurons to the proper connections
Promote health and growth
Supporting Cells: Neuroglia
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Most abundant, versatile, and highly branched glial cells
They cling to neurons and their synaptic endings, and cover capillaries
Functionally, they:
Support and brace neurons
Anchor neurons to their nutrient supplies
Guide migration of young neurons
Control the chemical environment by buffering the potassium and recapturing neurotransmitters
Astrocytes
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Astrocytes
Figure 11.3a
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Microglia – small, ovoid cells with spiny processes
Phagocytes that monitor the health of neurons
Ependymal cells – range in shape from squamous to columnar
They line the central cavities of the brain and spinal column
They help circulate the cerebrospinal fluid
Microglia and Ependymal Cells
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Microglia and Ependymal Cells
Figure 11.3b, c
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Oligodendrocytes – branched cells that wrap CNS nerve fibers
Schwann cells (neurolemmocytes) – surround fibers of the PNS
Satellite cells surround neuron cell bodies with ganglia
Oligodendrocytes, Schwann Cells, and Satellite Cells
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14Figure 11.3d, e
Oligodendrocytes, Schwann Cells, and Satellite Cells
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Structural units of the nervous system
Composed of a body, axon, and dendrites
Long-lived, amitotic, and have a high metabolic rate
Their plasma membrane functions in:
Electrical signaling
Cell-to-cell signaling during development
Neurons (Nerve Cells)
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Neurons (Nerve Cells)
Figure 11.4b
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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
Nerve Cell Body (Perikaryon or Soma)
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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
Processes
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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)
Dendrites of Motor Neurons
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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
Axons: Structure
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Generate and transmit action potentials
Secrete neurotransmitters from the axonal terminals
Movement along axons occurs in two ways
Anterograde — toward axonal terminal
Retrograde — away from axonal terminal
Axons: Function
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Whitish, fatty (protein-lipoid), segmented sheath around most long axons
It functions to:
Protect the axon
Electrically insulate fibers from one another
Increase the speed of nerve impulse transmission
Myelin Sheath
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Formed by Schwann cells in the PNS
A Schwann cell:
Envelopes an axon in a trough
Encloses the axon with its plasma membrane
Has concentric layers of membrane that make up the myelin sheath
Neurilemma – remaining nucleus and cytoplasm of a Schwann cell
Myelin Sheath and Neurilemma: Formation
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Myelin Sheath and Neurilemma: Formation
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
Nodes of Ranvier (Neurofibral Nodes)
InterActive Physiology®: Nervous System I: Anatomy ReviewPLAYPLAY
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A Schwann cell surrounds nerve fibers but coiling does not take place
Schwann cells partially enclose 15 or more axons
Unmyelinated 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
Axons of the CNS
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White matter – dense collections of myelinated fibers
Gray matter – mostly soma and unmyelinated fibers
Regions of the Brain and Spinal Cord
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Structural:
Multipolar — three or more processes
Bipolar — two processes (axon and dendrite)
Unipolar — single, short process
Neuron Classification
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Functional:
Sensory (afferent) — transmit impulses toward the CNS
Motor (efferent) — carry impulses away from the CNS
Interneurons (association neurons) — shuttle signals through CNS pathways
Therefore, it connects to other neurons
Neuron Classification
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Comparison of Structural Classes of Neurons
Table 11.1.1
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Comparison of Structural Classes of Neurons
Table 11.1.2
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Comparison of Structural Classes of Neurons
Table 11.1.3
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Neurons are highly irritable
Action potentials, or nerve impulses, are:
Electrical impulses carried along the length of axons
Always the same regardless of stimulus
The underlying functional feature of the nervous system
Neurophysiology
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11Fundamentals of the Nervous System and
Nervous Tissue
Part B
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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
Electricity Definitions
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Reflects the flow of ions rather than electrons
There is a potential on either side of membranes when:
The number of ions is different across the membrane
The membrane provides a resistance to ion flow
Electrical Current and the Body
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Types of plasma membrane ion channels:
Passive, or leakage, channels – always open
Chemically gated channels – open with binding of a specific neurotransmitter
Voltage-gated channels – open and close in response to membrane potential
Mechanically gated channels – open and close in response to physical deformation of receptors
Role of Ion Channels
InterActive Physiology®: Nervous System I: Ion ChannelsPLAYPLAY
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Example: Na+-K+ gated channel
Closed when a neurotransmitter is not bound to the extracellular receptor
Na+ cannot enter the cell and K+ cannot exit the cell
Open when a neurotransmitter is attached to the receptor
Na+ enters the cell and K+ exits the cell
Operation of a Gated Channel
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Operation of a Gated Channel
Figure 11.6a
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Example: Na+ channel
Closed when the intracellular environment is negative
Na+ cannot enter the cell
Open when the intracellular environment is positive
Na+ can enter the cell
Operation of a Voltage-Gated Channel
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Operation of a Voltage-Gated Channel
Figure 11.6b
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When gated channels are open:
Ions move quickly across the membrane
Movement is along their electrochemical gradients
An electrical current is created
Voltage changes across the membrane
Gated Channels
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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
Electrochemical Gradient
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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:
Differential permeability of the neurilemma to Na+ and K+
Operation of the sodium-potassium pump
Resting Membrane Potential (Vr)
InterActive Physiology®: Nervous System I: Membrane Potential
PLAYPLAY
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Resting Membrane Potential (Vr)
Figure 11.8
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Used to integrate, send, and receive information
Membrane potential changes are produced by:
Changes in membrane permeability to ions
Alterations of ion concentrations across the membrane
Types of signals – graded potentials and action potentials
Membrane Potentials: Signals
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Changes are caused by three events
Depolarization – the inside of the membrane becomes less negative
Repolarization – the membrane returns to its resting membrane potential
Hyperpolarization – the inside of the membrane becomes more negative than the resting potential
Changes in Membrane Potential
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Changes in Membrane Potential
Figure 11.9
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Short-lived, local changes in membrane potential
Decrease in intensity with distance
Their magnitude varies directly with the strength of the stimulus
Sufficiently strong graded potentials can initiate action potentials
Graded Potentials
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Graded Potentials
Figure 11.10
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Graded Potentials
Voltage changes in graded potentials are decremental
Current is quickly dissipated due to the leaky plasma membrane
Can only travel over short distances
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Graded Potentials
Figure 11.11
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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
Action Potentials (APs)
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Na+ and K+ channels are closed
Leakage accounts for small movements of Na+ and K+
Each Na+ channel has two voltage-regulated gates
Activation gates – closed in the resting state
Inactivation gates – open in the resting state
Action Potential: 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
Action Potential: Depolarization Phase
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
Action Potential: Repolarization Phase
Figure 11.12.3
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Action Potential: Hyperpolarization
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
Restores the resting electrical conditions of the neuron
Does not restore the resting ionic conditions
Ionic redistribution back to resting conditions is restored by the sodium-potassium pump
Action Potential: Role of the Sodium-Potassium Pump
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Phases of the Action Potential
1 – resting state
2 – depolarization phase
3 – repolarization phase
4 – hyperpolarization
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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
Propagation of an Action Potential (Time = 0ms)
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Propagation of an Action Potential (Time = 0ms)
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
Propagation of an Action Potential (Time = 1ms)
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Propagation of an Action Potential (Time = 1ms)
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
Propagation of an Action Potential (Time = 2ms)
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Propagation of an Action Potential (Time = 2ms)
Figure 11.13c
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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
Threshold and Action Potentials
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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
Coding for Stimulus Intensity
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Coding for Stimulus Intensity
Upward arrows – stimulus applied
Downward arrows – stimulus stopped
Figure 11.14
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Coding for Stimulus Intensity
Length of arrows – strength of stimulus
Action potentials – vertical lines
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:
Prevents the neuron from generating an action potential
Ensures that each action potential is separate
Enforces one-way transmission of nerve impulses
Absolute Refractory Period
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Absolute Refractory Period
Figure 11.15
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The interval following the absolute refractory period when:
Sodium gates are closed
Potassium gates are open
Repolarization is occurring
The threshold level is elevated, allowing strong stimuli to increase the frequency of action potential events
Relative Refractory Period
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Conduction velocities vary widely among neurons
Rate of impulse propagation is determined by:
Axon diameter – the larger the diameter, the faster the impulse
Presence of a myelin sheath – myelination dramatically increases impulse speed
Conduction Velocities of Axons
InterActive Physiology®: Nervous System I: Action Potential
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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
Saltatory Conduction
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Saltatory Conduction
Figure 11.16
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An autoimmune disease that mainly affects young adults
Symptoms include 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
Multiple Sclerosis (MS)
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The advent of disease-modifying drugs including interferon beta-1a and -1b, Avonex, Betaseran, and Copazone:
Hold symptoms at bay
Reduce complications
Reduce disability
Multiple Sclerosis: Treatment
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11Fundamentals of the Nervous System and
Nervous Tissue
Part C
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Nerve fibers are classified according to:
Diameter
Degree of myelination
Speed of conduction
Nerve Fiber Classification
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A junction that mediates information transfer from one neuron:
To another neuron
To an effector cell
Presynaptic neuron – conducts impulses toward the synapse
Postsynaptic neuron – transmits impulses away from the synapse
Synapses
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Synapses
Figure 11.17
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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:
Axoaxonic (axon to axon)
Dendrodendritic (dendrite to dendrite)
Dendrosomatic (dendrites to soma)
Types of Synapses
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Electrical synapses:
Are less common than chemical synapses
Correspond to gap junctions found in other cell types
Are important in the CNS in:
Arousal from sleep
Mental attention
Emotions and memory
Ion and water homeostasis
Electrical Synapses
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Specialized for the release and reception of neurotransmitters
Typically composed of two parts:
Axonal terminal of the presynaptic neuron, which contains synaptic vesicles
Receptor region on the dendrite(s) or soma of the postsynaptic neuron
Chemical Synapses
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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:
Is a chemical event (as opposed to an electrical one)
Ensures unidirectional communication between neurons
Synaptic Cleft
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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
Synaptic Cleft: Information Transfer
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Synaptic Cleft: Information Transfer
Figure 11.19
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Neurotransmitter bound to a postsynaptic neuron:
Produces a continuous postsynaptic effect
Blocks reception of additional “messages”
Must be removed from its receptor
Removal of neurotransmitters occurs when they:
Are degraded by enzymes
Are reabsorbed by astrocytes or the presynaptic terminals
Diffuse from the synaptic cleft
Termination of Neurotransmitter Effects
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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
Synaptic Delay
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Neurotransmitter receptors mediate changes in membrane potential according to:
The amount of neurotransmitter released
The amount of time the neurotransmitter is bound to receptors
The two types of postsynaptic potentials are:
EPSP – excitatory postsynaptic potentials
IPSP – inhibitory postsynaptic potentials
Postsynaptic Potentials
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EPSPs are graded potentials that can initiate an action potential in an axon
Use only chemically gated channels
Na+ and K+ flow in opposite directions at the same time
Postsynaptic membranes do not generate action potentials
Excitatory Postsynaptic Potentials
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Excitatory Postsynaptic Potentials
Figure 11.20a
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Neurotransmitter binding to a receptor at inhibitory synapses:
Causes the membrane to become more permeable to potassium and chloride ions
Leaves the charge on the inner surface negative
Reduces the postsynaptic neuron’s ability to produce an action potential
Inhibitory Synapses and IPSPs
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Inhibitory Synapses and IPSPs
Figure 11.20b
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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
Summation
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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
Summation
InterActive Physiology®: Nervous System II: Synaptic Potentials
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Summation
Figure 11.21
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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
Neurotransmitters
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Acetylcholine (ACh)
Biogenic amines
Amino acids
Peptides
Novel messengers: ATP and dissolved gases NO and CO
Chemical Neurotransmitters
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First neurotransmitter identified, and best understood
Released at the neuromuscular junction
Synthesized and enclosed in synaptic vesicles
Degraded by the enzyme acetylcholinesterase (AChE)
Released by:
All neurons that stimulate skeletal muscle
Some neurons in the autonomic nervous system
Neurotransmitters: Acetylcholine
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Include:
Catecholamines – dopamine, norepinephrine (NE), and epinephrine
Indolamines – serotonin and histamine
Broadly distributed in the brain
Play roles in emotional behaviors and our biological clock
Neurotransmitters: Biogenic Amines
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Synthesis of Catecholamines
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 11.22
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Include:
GABA – Gamma ()-aminobutyric acid
Glycine
Aspartate
Glutamate
Found only in the CNS
Neurotransmitters: Amino Acids
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Include:
Substance P – mediator of pain signals
Beta endorphin, dynorphin, and enkephalins
Act as natural opiates, reducing our perception of pain
Bind to the same receptors as opiates and morphine
Gut-brain peptides – somatostatin, and cholecystokinin
Neurotransmitters: Peptides
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ATP
Is found in both the CNS and PNS
Produces excitatory or inhibitory responses depending on receptor type
Induces Ca2+ wave propagation in astrocytes
Provokes pain sensation
Neurotransmitters: Novel Messengers
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Nitric oxide (NO)
Activates the intracellular receptor guanylyl cyclase
Is involved in learning and memory
Carbon monoxide (CO) is a main regulator of cGMP in the brain
Neurotransmitters: Novel Messengers
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Two classifications: excitatory and inhibitory
Excitatory neurotransmitters cause depolarizations (e.g., glutamate)
Inhibitory neurotransmitters cause hyperpolarizations (e.g., GABA and glycine)
Functional Classification of Neurotransmitters
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Some neurotransmitters have both excitatory and inhibitory effects
Determined by the receptor type of the postsynaptic neuron
Example: acetylcholine
Excitatory at neuromuscular junctions with skeletal muscle
Inhibitory in cardiac muscle
Functional Classification of Neurotransmitters
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Direct: neurotransmitters that open ion channels
Promote rapid responses
Examples: ACh and amino acids
Indirect: neurotransmitters that act through second messengers
Promote long-lasting effects
Examples: biogenic amines, peptides, and dissolved gases
Neurotransmitter Receptor Mechanisms
InterActive Physiology®: Nervous System II: Synaptic TransmissionPLAYPLAY
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Composed of integral membrane protein
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
Channel-Linked Receptors
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Channel-Linked Receptors
Figure 11.23a
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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
G Protein-Linked Receptors
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Neurotransmitter binds to G protein-linked receptor
G protein is activated and GTP is hydrolyzed to GDP
The activated G protein complex activates adenylate cyclase
Adenylate cyclase catalyzes the formation of cAMP from ATP
cAMP, a second messenger, brings about various cellular responses
G Protein-Linked Receptors: Mechanism
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G Protein-Linked Receptors: Mechanism
Figure 11.23b
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G protein-linked receptors activate intracellular second messengers including Ca2+, cGMP, diacylglycerol, as well as cAMP
Second messengers:
Open or close ion channels
Activate kinase enzymes
Phosphorylate channel proteins
Activate genes and induce protein synthesis
G Protein-Linked Receptors: Effects
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Functional groups of neurons that:
Integrate incoming information
Forward the processed information to its appropriate destination
Neural Integration: Neuronal Pools
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Simple neuronal pool
Input fiber – presynaptic fiber
Discharge zone – neurons most closely associated with the incoming fiber
Facilitated zone – neurons farther away from incoming fiber
Neural Integration: Neuronal Pools
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Neural Integration: Neuronal Pools
Figure 11.24
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Divergent – one incoming fiber stimulates ever increasing number of fibers, often amplifying circuits
Types of Circuits in Neuronal Pools
Figure 11.25a, b
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Convergent – opposite of divergent circuits, resulting in either strong stimulation or inhibition
Types of Circuits in Neuronal Pools
Figure 11.25c, d
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Reverberating – chain of neurons containing collateral synapses with previous neurons in the chain
Types of Circuits in Neuronal Pools
Figure 11.25e
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Parallel after-discharge – incoming neurons stimulate several neurons in parallel arrays
Types of Circuits in Neuronal Pools
Figure 11.25f
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Serial Processing
Input travels along one pathway to a specific destination
Works in an all-or-none manner
Example: spinal reflexes (rapid, automatic responses to stimuli)
Patterns of Neural Processing
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Parallel Processing
Input travels along several pathways
Pathways are integrated in different CNS systems
One stimulus promotes numerous responses
Example: a smell may remind one of the odor and associated experiences
Patterns of Neural Processing
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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:
Proliferation of cells needed for development
Migration – cells become amitotic and move externally
Differentiation into neuroblasts
Development of Neurons
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Guided by:
Scaffold laid down by older neurons
Orienting glial fibers
Release of nerve growth factor by astrocytes
Neurotropins released by other neurons
Repulsion guiding molecules
Attractants released by target cells
Axonal Growth
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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
N-CAMs