11 Neural Tissue.

11 Neural Tissue

Section 1: Nervous System Components
Learning Outcomes 11.1 Sketch and label the structure of a typical neuron, and describe the functions of each component. 11.2 Classify and describe neurons on the basis of their structure and function. 11.3 Describe the locations and functions of neuroglia in the CNS. 11.4 Describe the locations and functions of Schwann cells and satellite cells.

Figure 11 Section 1 Neurons and Neuroglia
The major components and functions of the nervous system Central Nervous System The central nervous system (CNS) consists of the brain and spinal cord and is responsible for integrating, processing, and coordinating sensory data and motor commands. Information processing includes the integration and distribution of information in the CNS. Peripheral Nervous System The motor division of the PNS carries motor commands from the CNS to peripheral tissues and systems. The peripheral nervous system (PNS) includes all the neural tissue outside the CNS. includes The somatic nervous system (SNS) controls skeletal muscle contractions. The autonomic nervous system (ANS) provides automatic regulation of smooth muscle, cardiac muscle, glands, and adipose tissue. The sensory division of the PNS brings information to the CNS from receptors in peripheral tissues and organs. Figure 11 Section 1 Neurons and Neuroglia Somatic sensory receptors provide position, touch, pressure, pain, and temperature sensations. Special sensory receptors provide sensations of smell, taste, vision, balance, and hearing. • Smooth muscle • Cardiac muscle • Glands • Adipose tissue Skeletal muscle Visceral sensory receptors monitor internal organs. Receptors are sensory structures that detect changes in the internal or external environment. Effectors are target organs whose activities change in response to neural commands. Figure 11 Section 1 3

Module 11.1: Neurons Neuron components Dendrites Cell body
Highly branched, bearing spines 0.5–1 µm long (dendritic spines) CNS neurons receive most information here Neuron receives stimuli from environment or other neurons at dendrites Cell body Contains nucleus Organelles contained within perikaryon (peri, around + karyon, nucleus) Cytoskeleton comprised of neurofilaments and neurofibrils (extend into dendrites and axon)

Module 11.1: Neurons Neuron components (continued) Axon
Carries information toward other cells Transport of materials (enzymes and lysosomes) using neurotubules (= axoplasmic transport) Occurs in both directions Back toward cell body = retrograde flow Components Axon hillock (base or initial segment) Axolemma (plasma membrane of axon) Axoplasm (cytoplasm of axon with organelles, structural components, and transported materials) Collateral branches (communicate with other cells)

Module 11.1: Neurons Neuron components (continued)
Telodendria (telo-, end + dendron, tree) Axonal extensions at end of axon trunk Terminate at synaptic terminals Where neuron communicates with other cells Animation: Neurophysiology: Neuron Structure

A diagrammatic view of a representative neuron
Dendrites Axon Dendritic spines of dendrites Axon hillock Axolemma Axoplasm Nissl bodies (clusters of RER and free ribosomes) Mitochondrion Nucleus Nucleolus Figure Neurons are nerve cells specialized for intercellular communication Cell Body Telodendria Perikaryon Neurofilament Synaptic terminals Figure 7

Module 11.1: Neurons Synapse
Specialized site of communication between neuron and another cell Components Presynaptic cell (before synaptic cleft) Usually a neuron May have synaptic knob Has synaptic vesicles that contain neurotransmitters (chemical messengers synthesized in cell body) Presynaptic membrane (where neurotransmitters are released)

Module 11.1: Neurons Synapse (continued) Components (continued)
Postsynaptic cell (after synaptic cleft) Can be a neuron or other type of cell Postsynaptic membrane (bears receptors for neurotransmitters Synaptic cleft (narrow space between cells)

A representative synapse
Telodendrion of presynaptic cell Mitochondrion Synaptic knob Endoplasmic reticulum Presynaptic membrane Synaptic vesicles Figure Neurons are nerve cells specialized for intercellular communication Synaptic cleft Postsynaptic membrane Cytoplasm of postsynaptic cell Figure 10

Synapses with another neuron
The type of synapses Synapses with another neuron Synapses with another neuron Neuron 1 Neuron 2 Collateral branch Dendrites Axolemma Neuromuscular junctions Neuromuscular junctions Collateral branch Neuron Skeletal muscle fibers Telodendria Synaptic terminals Figure Neurons are nerve cells specialized for intercellular communication Neuroglandular synapses Neuroglandular synapses Neuron Gland cells Figure 11

Module 11.1: Neurons Most CNS neurons lack centrioles and cannot divide Neurons lost to injury or disease are seldom replaced Some neural stem cells exist but mostly inactive Exceptions: Olfactory epithelium (smell) Retina of eye Hippocampus (area of brain for memory storage)

Module 11.1 Review a. Name the structural components of a typical neuron. b. Describe a synapse. c. Why is a CNS neuron not usually replaced after it is injured?

Module 11.2: Classification of neurons
Four major anatomical classes of neurons Anaxonic neurons All cell processes look alike (dendrites vs. axons) Located in brain and special sense organs Functions are poorly understood Bipolar neurons Two distinct processes One with branching dendritic processes One axon Rare, but occur in special sense organs Small (30 µm in length)

Four major anatomical classes of neurons (continued) Unipolar neurons Dendrites and axon are continuous (fused) Cell body lies off to one side Initial segment where dendrites converge Most sensory neurons of peripheral nervous system May extend 1 meter or more Multipolar neurons Two or more dendrites and one axon Most common neurons in CNS Can be as long as unipolar (voluntary motor neurons)

The four major anatomical classes of neurons
Dendrites Dendritic process An anaxonic neuron A bipolar neuron Cell body Axon Synaptic terminals Dendrites Initial segment Dendrites Axon Figure Neurons may be classified on the basis of structure or function A unipolar neuron A multipolar neuron Axon Axon Synaptic terminals Synaptic terminals Figure – 4 16

Three major functional classes 1. Sensory neurons (~10 million in body) 2. Interneurons (~20 billion in body) 3. Motor neurons (~500,000 in body)

Functional relationships of neurons Sensory receptors (relay stimuli to sensory neurons) Interoceptors (intero-, inside) Monitor sensations inside body from various systems Proprioceptors Monitor body position and movement of joints and muscles Exteroceptors (extero, outside) Monitor sensations from external environment

Functional relationships of neurons (continued) Afferent nerve fibers (axons from receptor to CNS) Sensory ganglia Contain cell bodies of unipolar sensory neurons Somatic sensory neurons (outside world) Visceral sensory neurons (internal conditions) Central Nervous System Interneurons Usually between sensory and motor neurons Also responsible for higher functions (memory, etc.)

Functional relationships of neurons (continued) Central Nervous System and Peripheral Nervous System Motor neurons (originate in CNS and transmit impulses to effectors through PNS) Somatic motor neurons (skeletal muscles) Visceral motor neurons (smooth and cardiac muscle, glands, and adipose tissue) Synapse with 2nd set of neurons at autonomic ganglia

Module 11.2 Review a. Classify neurons according to their structure.
b. Classify neurons according to their function. c. Are unipolar neurons in a tissue sample of the PNS more likely to function as sensory neurons or motor neurons?

Module 11.3: CNS neuroglia Neuroglia (or glial cells)
Cells that support and protect neurons Are abundant and diverse ~Half the volume of nervous system

Module 11.3: CNS neuroglia CNS neuroglia Ependymal cells Microglia
Form epithelia (ependyma) lining fluid-filled passageway in brain and spinal cord Fluid = cerebrospinal fluid (CSF) Also surrounds brain and spinal cord Assist in producing, circulating, and monitoring CSF Microglia Embryologically related to monocytes and macrophages Migrate into CNS Persist as mobile phagocytic cells Remove cellular debris, waste products, and pathogens

Module 11.3: CNS neuroglia CNS neuroglia Astrocytes
Maintain blood–brain barrer Isolates CNS from chemicals and hormones in the blood Provide structural support Regulate ion, nutrient, and dissolved gas concentrations in interstitial fluid Absorb and recycle neurotransmitters Form scar tissue after CNS injury

Module 11.3: CNS neuroglia CNS neuroglia
Oligodendrocytes (oligo-, few) Provide CNS framework by stabilizing axons Produce myelin Coats axons and increases speed of neural impulse transmission Cell process wraps axon with layers of myelin and plasma membrane creating myelin sheath One oligodendrocyte wraps axonal segments of many neurons Myelin sheath is incomplete Myelin-wrapped areas = internodes Gaps between internodes = nodes

Module 11.3: CNS neuroglia CNS neuroglia Oligodendrocytes (continued)
Axons that have myelin sheath = myelinated Appear white due to lipid content Constitute white matter of the CNS Axons that lack myelin sheath = unmyelinated Contribute to gray matter of the CNS Along with neuron cell bodies and dendrites

Astrocytes Oligodendrocyte Myelin sheath in internode Section of spinal cord Capillary Ependymal cell Unmyelinated axon Microglial cell Axon Neurons Myelinated axons Figure 11.3 Oligodendrites, astrocytes, ependimal cells, and microglia are neuroglia of the CNS Myelin (cut) Nodes Gray matter White matter Figure 11.3 27

Module 11.3 Review a. Identify the neuroglia of the central nervous system. b. Which glial cell protects the CNS from chemicals and hormones circulating in the blood? c. Which type of neuroglia would occur in increased numbers in the brain tissue of a person with a CNS infection?

Module 11.4: PNS neuroglia PNS neuroglia Schwann cells Satellite cells
Form sheath around peripheral axons Outer surface of Schwann cell is called neurilemma Cover both myelinated and unmyelinated axons A single cell myelinates an axon A single cell can wrap many unmyelinated neurons Satellite cells Surround neuron cell bodies in ganglia Regulate intercellular environment (much like astrocytes)

Nucleus Axon hillock Internode (myelinated) Cell body Initial segment (unmyelinated) Dendrite Node Schwann cell Figure Schwann cells and satellite cells are the neuroglia of the PNS Neurilemma Myelin covering internode Axon Axolemma A Schwann cell Figure 30

The steps in the myelination of an axon in the PNS
Schwann cell nucleus Axon Neurilemma Myelin covering internode Figure Schwann cells and satellite cells are the neuroglia of the PNS The steps in the myelination of an axon in the PNS Figure 31

A single Schwann cell forming the internode of many unmyelinated axons
Satellite cells Schwann cell #1 Schwann cell Schwann cell #2 Schwann cell nucleus Neurilemma Axons Figure Schwann cells and satellite cells are the neuroglia of the PNS Schwann cell #3 nucleus Axons Figure 32

Module 11.4: PNS neuroglia Repair of damaged nerves in PNS
Axon and myelin degenerate distal to injury Schwann cells proliferate along original axon path Macrophages move in and remove cellular debris Axon grows along original path created by Schwann cells Schwann cells wrap around elongating axon If axon makes normal synaptic contacts, normal function may be regained If axon stops growing or wanders off, normal function may not return Repair that does not restore full function = Wallerian degeneration

The process of repair of damaged PNS nerves, or Wallerian degeneration
Site of injury Step 1: Distal to the injury site, the axon and myelin degenerate and fragment. Axon Myelin Proximal stump Distal stump Step 2: The Schwann cells do not degenerate; instead, they proliferate along the path of the original axon. Over this period, macrophages move into the area and remove the degenerating debris distal to the injury site. Macrophage Cord of proliferating Schwann cells Step 3: As the neuron recovers, its axon grows into the site of injury and then distally, along the path created by the Schwann cells. Figure Schwann cells and satellite cells are the neuroglia of the PNS Step 4: As the axon elongates, the Schwann cells wrap around it. If the axon reestablishes its normal synaptic contacts, normal function may be regained. However, if it stops growing or wanders off in some new direction, normal function will not return. Figure 34

Module 11.4: PNS neuroglia Only limited repair can occur in CNS due to: Many more axons involved Astrocytes produce scar tissue that can prevent axon growth Astrocytes release chemicals that block axon regrowth

Module 11.4 Review a. Identify the neuroglia of the peripheral nervous system. b. Describe the neurilemma. c. In which part of the nervous system does Wallerian degeneration occur?

Section 2: Neurophysiology
Learning Outcomes 11.5 Explain how the resting potential is created and maintained. 11.6 Describe the functions of gated channels with respect to the permeability of the plasma membrane. 11.7 Describe graded potentials. 11.8 Describe the events involved in the generation and propagation of an action potential. 11.9 Describe continuous propagation and saltatory propagation, and discuss the factors that affect the speed with which action potentials are propagated.

Learning Outcomes Describe the general structure of synapses in the CNS and PNS, and discuss the events that occur at a chemical synapse. Discuss the significance of postsynaptic potentials, including the roles of excitatory postsynaptic potentials and inhibitory postsynaptic potentials. Discuss the interactions that make the processing of information in neural tissue possible.

Transmembrane potential An unequal distribution of charge across a cell membrane Inside membrane is slightly negative Due to slight excess of negatively charged ions and proteins Outside membrane is slightly positive Due to slight excess of positively charged ions Results from differences in membrane permeability to various ions and active transport Is characteristic of all cells Animation: Transmembrane Potentials

The unequal distribution of charges inside and outside the
plasma membrane, which produces a transmembrane potential Extracellular fluid Plasma membrane Cytosol Protein Protein Figure 11 Section 2 Neurophysiology Protein Figure 11 Section 2 1 40

Neural activity and transmembrane potential Changes in transmembrane potential can cause muscle contraction, gland secretion, or transfer of information Resting potential Transmembrane potential of a cell at rest All neural activities begin with a change from resting potential Graded potential Temporary, localized change in resting potential due to typical stimulus Decreases with distance from stimulus

Neural activity and transmembrane potential (continued) Action potential Electrical event involving one location of axonal membrane Can be triggered by sufficiently large graded potential Is propagated along axon surface toward synaptic terminals Synaptic activity Typically involves release of neurotransmitters (like ACh) by presynaptic cell Compounds bind to receptors on postsynaptic cell, changing its permeability producing a graded potential

Neural activity and transmembrane potential (continued) Information processing Integration of stimuli at individual cell level Response of postsynaptic cell to stimulated receptors and other stimuli

An overview of the role of the transmembrane
potential in neural activity Graded potential Action potential may produce Resting potential stimulus produces triggers Synaptic activity Information processing Figure 11 Section 2 Neurophysiology Presynaptic neuron Postsynaptic cell Figure 11 Section 2 2 44

Module 11.5: Resting potential
Extracellular fluid (ECF) has high concentrations of Na+ and Cl– Cytosol has high concentrations of K+ and negatively charged proteins (Pr–) These proteins cannot cross plasma membrane Neuron resting potential is usually near 0.07 volts (V) or –70 millivolts (mV) (slightly negative inside) Charged ions cannot freely cross plasma membrane Can move across membrane only through membrane channels or active transport mechanisms

Leak channels Always open Size, shape, and structure determine which ions will pass Potassium ions diffuse out of cell through K+ leak channels Sodium ions diffuse into cell through Na+ leak channels Primarily causes the transmembrane potential Animation: Neurophysiology: Ion Movement

Passive leak channels, which are primarily
Plasma membrane Figure The resting potential is the transmembrane potential of an undisturbed cell Passive leak channels, which are primarily responsible for the transmembrane potential Figure 47

Active transport Sodium–potassium exchange pump Ejects 3 Na+ for 2 K+ recovered from ECF Maintains stable resting potential Animation: Neurophysiology: Sodium Potassium Exchange Pump

The sodium–potassium exchange
pump ejects 3 Na+ for every 2 K+ recovered from the extracellular fluid. At a transmembrane potential of –70 mV, the rate of Na+ entry versus K+ loss is 3:2, and the exchange pump maintains a stable resting potential. Potassium ions can diffuse out of the cell through potassium leak channels. Sodium ions can diffuse into the cell through sodium leak channels. EXTRACELLULAR FLUID The unit of measurement of potential difference is the volt (V), and the transmembrane potential of a neuron is usually near 0.07 V. Such a value is usually expressed as –70 mV (or –70 millivolts—thousandths of a volt) with the minus sign indicating that the interior is negatively charged. Sodium– potassium exchange pump Plasma membrane CYTOSOL Figure The resting potential is the transmembrane potential of an undisturbed cell Protein The cytosol contains an abundance of negatively charged proteins, whereas the extracellular fluid contains relatively few. These proteins cannot cross the plasma membrane. Protein Protein An overview of the events responsible for the normal resting potential of a neuron Figure 49

Electrochemical gradients Chemical gradient Concentration gradient for an ion across plasma membrane Electrical gradient Attraction between opposite charges or repulsion between like charges (+/+ or –/–) Equilibrium potential When electrical and chemical gradients are equal and opposite, resulting in no net movement across membrane In most cells, the gradients for Na+ and K+ are most important

Potassium ion gradients At normal resting potential, the electrical and chemical gradients are in opposition, but not equal The net electrochemical gradient for K+ is out of the cell If the plasma membrane were freely permeable to potassium ions, K+ would continue to leave the cell until an equilibrium potential of –90 mV

Potassium Ion Gradients
The dynamics of potassium ion gradients Potassium Ion Gradients At normal resting potential, an electrical gradient opposes the chemical gradient for potassium ions (K+). The net electrochemical gradient tends to force potassium ions out of the cell. If the plasma membrane were freely permeable to potassium ions, the outflow of K+ would continue until the equilibrium potential (–90 mV) was reached. Potassium chemical gradient Potassium electrical gradient Net potassium electrochemical gradient Potassium chemical gradient Potassium electrical gradient Resting potential Equilibrium potential Plasma membrane Plasma membrane Figure The resting potential is the transmembrane potential of an undisturbed cell Protein Protein Cytosol Cytosol Figure 52

Sodium ion gradients At normal resting potential, both the chemical and electrical gradients cause Na+ to move into the cell If the plasma membrane were freely permeable to sodium, Na+ would continue to enter the cell until an equilibrium potential of +66 mV was reached A&P Flix: Resting Membrane Potential

The dynamics of sodium ion gradients
At the normal resting potential, chemical and electrical gradients combine to drive sodium ions (Na+) into the cell. If the plasma membrane were freely permeable to sodium ions, the influx of Na+ would continue until the equilibrium potential (+66 mV) was reached. Sodium chemical gradient Sodium electrical gradient Net sodium electrochemical gradient Sodium chemical gradient Sodium electrical gradient Resting potential Equilibrium potential Plasma membrane Plasma membrane Figure The resting potential is the transmembrane potential of an undisturbed cell Protein Protein Cytosol Cytosol Figure 54

Module 11.5 Review a. Define resting potential.
b. What effect would decreasing the concentration of extracellular potassium ions have on the transmembrane potential of a neuron? c. What happens at the sodium–potassium exchange pump?

Module 11.6: Gated channels
Resting potential remains stable until the cell is disturbed or stimulated Changes in transmembrane potential primarily occur due to gated channels opening or closing in response to stimuli Three different gated channel classes Chemically gated channels Voltage-gated channels Mechanically gated channels

Figure 11.6.1 Gated channels can change the permeability of the plasma membrane
57

Chemically gated channels Open when they bind specific chemicals Example: neuromuscular junction receptors that bind ACh Most abundant on dendrites and cell body of neurons Where most synaptic communication occurs

Resting state Arrival of ACh Gated channel opens
The function of chemically gated channels Extracellular fluid Binding site ACh ACh Plasma membrane Cytosol Gated channel (closed) Figure Gated channels can change the permeability of the plasma membrane Resting state Arrival of ACh Gated channel opens Figure 59

Voltage-gated channels Characteristic of excitable membranes (capable of generating and conducting an action potential) Open or close in response to changes in transmembrane potential Most important for neurons Voltage-gated potassium channels Voltage-gated calcium channels Voltage-gated sodium channels Have two gates that function independently Activation gates (open on stimulation) Inactivation gates (close to stop sodium entry)

Channel closed Channel open Channel inactivated
The function of voltage-gated channels Activation gate Figure Gated channels can change the permeability of the plasma membrane Inactivation gate Channel closed Channel open Channel inactivated Figure 61

Mechanically gated channels Open in response to physical distortion of membrane Important in sensory receptors Examples: touch, pressure, vibration

Channel closed Channel open Channel closed
The function of mechanically gated channels Applied pressure Pressure removed Figure Gated channels can change the permeability of the plasma membrane Channel closed Channel open Channel closed Figure 63

Module 11.6 Review a. Define gated channels.
b. Identify the three types of gated channels, and state the conditions under which each operates. c. What effect would a chemical that blocks voltage-gated sodium channels in a neuron’s plasma membrane have on its transmembrane potential?

Module 11.7: Graded potentials
Also known as local potentials Changes in transmembrane potential that cannot spread far from stimulation site Example: effects of chemically gated sodium channels

Graded potentials produced by chemically gated Na+ channels At resting potential, chemically gated sodium channels are closed Binding of chemical, opens channels allowing sodium influx Positively charged ions entering the cell cause depolarization (shift from resting potential to more positive) Intracellular Na+ spread out, attracted to negative charges lining membrane (= local current) Extracellular Na+ moves to replace

Initial segment The events in the propagation of a graded potential Extracellular Fluid Cytoplasm A neuron plasma membrane at normal resting potential Figure Graded potentials are localized changes in the transmembrane potential A chemical stimulus opens the chemically gated sodium channels, producing a depolarization. Local current Local current Movement of positive charges causes a local current. Figure – 3 67

Degree of depolarization decreases with distance from stimulation site Ions enter at one location Spread occurs in all directions Change in transmembrane potential proportional to stimulus intensity Greater stimulus = more open channels = more ion flow

The effect of distance from the stimulation site
on the degree of depolarization Transmembrane potential Figure Graded potentials are localized changes in the transmembrane potential Figure 69

Graded potentials produced by chemically gated Na+ channels (continued) With removal of chemical stimulus, membrane returns to resting potential Na+ pumped out of cell = Repolarizition

Effects of chemically gated potassium channels Some chemicals open K+ channels Potassium ions leave cytoplasm Results in more negative transmembrane potential = Hyperpolarization

The changes in transmembrane potential over time when different
A chemical stimulus opens chemically gated sodium ion channels. Removal of the chemical stimulus leads to repolarization. A different chemical stimulus opens chemically gated potassium channels, causing hyperpolarization. Chemical stimulus removed Repolarization Transmembrane potential (mV) Resting potential Depolarization Hyperpolarization Return to resting potential Figure Graded potentials are localized changes in the transmembrane potential Time The changes in transmembrane potential over time when different chemical stimuli are applied to the axon hillock Figure 72

Figure 11.7.6 Graded potentials are localized changes in the transmembrane potential
73

Module 11.7 Review a. Define graded potential.
b. Describe depolarization, repolarization, and hyperpolarization. c. What factors account for the local currents associated with graded potentials?

Module 11.8: Action potential generation
Information transfer in neurons Reception of information as graded potentials on dendrites and cell bodies At synaptic terminals, graded potentials cause release of neurotransmitters Distance between cell body and synaptic terminals can be large Graded potentials only travel short distances Action potentials can travel longer distances Are propagated changes in transmembrane potential that affect entire excitable membrane Animation: Neurophysiology: Action Potential

Channel types and transmembrane potential Leak channels are responsible mainly for resting potential Chemically gated channels often produce graded potentials Voltage-gated channels produce action potentials

Prior to action potential generation Transmembrane potentials are at resting levels Sodium channels are closed but capable of opening Activation gate closed Inactivation gate open Potassium channels are closed but capable of opening Single gate closed A&P Flix: Generation of an Action Potential

Steps of action potential generation and propagation Depolarization to threshold Graded depolarization large enough to open voltage-gated sodium channels = Threshold Approximate transmembrane potential of –60 mV Activation of Na+ channels and rapid depolarization Sodium ions rush into cell through open channels Causes rapid depolarization From –60 mV to a positive value

Steps of action potential generation and propagation (continued) Inactivation of Na+ channels and activation of K+ channels At ~+30 mV, sodium inactivation gates close = Sodium channel inactivation Voltage-gated potassium channels open Potassium ions leave the cell Begins repolarization Potassium channels close As membrane reaches resting potential (–70 mV) K+ ions continue to leave cell until all channels are closed Produces brief hyperpolarization

transmembrane potential at one location during the generation of an
The changes in the transmembrane potential at one location during the generation of an action potential A graded depolarization brings an area of excitable membrane to threshold (–60 mV). Voltage-gated sodium channels open and sodium ions move into the cell. The transmembrane potential rises to +30 mV. DEPOLARIZATION REPOLARIZATION Sodium channels close, voltage-gated potassium channels open, and potassium ions move out of the cell. Repolarization begins. Transmembrane potential (mV) Threshold Potassium channels close, and both sodium and potassium channels return to their normal states. ABSOLUTE REFRACTORY PERIOD During the absolute refractory period, the membrane cannot respond to further stimulation. Resting potential Figure An action potential begins with the opening of voltage-gated sodium ion channels RELATIVE REFRACTORY PERIOD During the relative refractory period, the membrane can respond only to a larger-than-normal stimulus. Time (msec) Figure 80

Graded and action potential analogy: gun firing Graded potential Pulling trigger of gun Enough pressure will cause gun to fire Action potential Firing of gun Enough pressure on trigger will cause gun to fire same way every time Stimulus triggers action potential or not at all = All-or-none principle

Module 11.8 Review a. Define action potential.
b. List the events involved in the generation and propagation of an action potential. c. Compare the absolute refractory period with the relative refractory period.

Module 11.9: Action potential propagation
A generated action potential does not itself move along the axon Once generated at the initial segment, the action potential is regenerated at each adjacent axonal segment = Propagation (not conduction) Two types of action potential propagation Continuous propagation Saltatory propagation Animation: Neurophysiology: Continuous and Saltatory Propagation

Continuous propagation Occurs along unmyelinated axons Appears to move in a series of tiny steps Each step takes ~1 msec = Propagation speed of ~1 m/s

Steps of continuous propagation Action potential develops at initial segment Transmembrane potential = +30 mV Entering sodium spreads away from voltage-gated channels to depolarize adjacent segment to threshold Action potential occurs in adjacent segment while initial segment begins repolarizing Sodium enters new segment, spreads, and causes depolarization of next adjacent axonal segment Action potential can only move forward because last axonal segment is in absolute refractory period

Animation: Neurophysiology: Positive Potential A&P Flix: Propagation of an Action Potential

Initial segment Axon hillock The events that occur in continuous propagation Step 1: As an action potential develops at the initial segment 1 , the transmembrane potential at this site depolarizes to +30 mV. Action potential Extracellular fluid Cell membrane Cytosol Step 2: As the sodium ions entering at 1 spread away from the open voltage-gated channels, a graded depolarization quickly brings the membrane in segment 2 to threshold. Graded depolarization Local current Step 3: An action potential now occurs in segment 2 while segment 1 begins repolarization. Repolarization (refractory) Figure Action potentials may affect adjacent portions of the plasma membrane through continuous propagation or saltatory propogation Step 4: As the sodium ions entering at Segment 2 spread laterally, a graded depolarization quickly brings the membrane in Segment 3 to threshold. The action potential can only move forward, not backward, because the membrane at segment 1 is in the absolute refractory period of repolarization. Local current Figure 87

Saltatory propagation (saltere, leaping) Occurs in myelinated axons Only exposed nodes can respond to depolarizing stimulus Internodes covered with myelin prevent ion flow across membrane Prevents continuous propagation Much faster than continuous propagation Speed varies with axon diameter Larger axon = faster current

Steps of saltatory propagation Action potential occurs at initial segment Local current produces graded depolarization to threshold at next node Action potential develops at node Local current flow produces graded depolarization to threshold at next node

The events that occur in saltatory propagation
Step 1: Extracellular fluid An action potential has occurred at the initial segment 1 . Myelinated internode Myelinated internode Myelinated internode Cell membrane Cytosol Step 2: A local current produces a graded depolarization that brings the axolemma at the next node to threshold. Local current Step 3: An action potential develops at node 2 . Repolarization (refractory) Figure Action potentials may affect adjacent portions of the plasma membrane through continuous propagation or saltatory propogation Step 4: A local current produces a graded depolarization that brings the 3 axolemma at node to threshold. Local current Figure 90

Module 11.9 Review a. Define continuous propagation and saltatory propagation. b. What is the relationship between myelin and the propagation speed of action potentials?

Module 11.10: Synaptic events
Transmission of a message or “nerve impulse” within a neuron = Action potential generation and propagation Transfer of a message between cells (from a neuron to another neuron or effector cell) = Must be relayed across a synapse Types of synapses Chemical synapses Electrical synapses Animation: Neurophysiology: Synapse

Chemical synapses Rely on neurotransmitter release Those that release acetylcholine (ACh) are cholinergic synapses Most abundant synapse type Most of those between neurons All synapses between neurons and other cells

Events at a cholinergic synapse Depolarization of synaptic knob by arriving action potential Opening of voltage-gated calcium channels Influx of Ca2+ causes exocytosis of ACh from synaptic vesicles Ca2+ quickly removed to end release of ACh

Events at a cholinergic synapse (continued) ACh diffuses across synaptic cleft and binds to chemically gated Na+ channels Na+ diffuses into postsynaptic cell and depolarizes membrane More ACh bound = larger depolarization Acetylcholinesterase (AChE, an enzyme) breaks down ACh Makes effects on postsynaptic cell temporary Occurs within 20 msec

The events that occur at a cholinergic synapse
Events Occurring at Synapse Mitochondrion 1 An arriving action potential depolarizes the synaptic knob. 2 Calcium ions enter the cytoplasm, and after a brief delay, ACh is released through the exocytosis of synaptic vesicles. Acetyl-CoA CoA 3 ACh binds to sodium channel receptors on the postsynaptic membrane, producing a graded depolarization. Acetylcholine Synaptic vesicle 4 Depolarization ends as ACh is broken down into acetate and choline by AChE. SYNAPTIC KNOB Figure At a synapse, information travels from the presynaptic cell to the postsynaptic cell 5 The synaptic knob reabsorbs choline from the synaptic cleft and uses it to synthesize new molecules of ACh. SYNAPTIC CLEFT Choline Acetylcholinesterase Acetate (AChE) POSTSYNAPTIC MEMBRANE ACh receptor Figure 96

Chemical synapse physiology Synaptic fatigue After extended stimulation, the recycling of neurotransmitter unable to keep up with demand Synapse weakens until neurotransmitter can be replenished Synaptic delay Release and binding of neurotransmitters takes ~0.2–0.5 msec With many neurons, cumulative delay may be significant Rapid reflexes involve few synapses

Electrical synapses Presynaptic and postsynaptic membranes are locked together by gap junctions Changes in transmembrane potential are transferred directly between cells through local current flow Occur in CNS and PNS but extremely rare Some areas of brain, eye, ciliary ganglia of PNS Less adaptable and complex compared to chemical synapses Example: changes in chemical environment or multiple neurotransmitter affecting postsynaptic cell response

The structure of an electrical synapse
Gap junctions connecting presynaptic and postsynaptic neurons Presynaptic neuron Figure At a synapse, information travels from the presynaptic cell to the postsynaptic cell Postsynaptic neuron Figure 99

Module 11.10 Review a. Describe the parts of a chemical synapse.
b. Contrast an electrical synapse with a chemical synapse. c. What is synaptic fatigue, and how does the synapse recover?

Module 11.11: Information processing within a neuron
Postsynaptic potentials Graded potentials in postsynaptic cell in response to a neurotransmitter Two types Excitatory postsynaptic potential (EPSP) Graded depolarization caused by neurotransmitter arrival Shifts transmembrane potential closer to threshold (= facilitated) More facilitation, the less additional stimulus needed to reach threshold

Postsynaptic potentials (continued) Two types (continued) Inhibitory postsynaptic potential (IPSP) Graded hyperpolarization Example: opening of chemically gated K+ channels Shifts transmembrane potential farther from threshold (= inhibited) More inhibition, larger-than-usual stimulus needed to reach threshold

Postsynaptic potentials, graded potentials that develop in the postsynaptic membrane in response to
a neurostransmitter Summation: the integration of the effects of graded potentials on a segment of the plasma membrane An excitatory postsynaptic potential, or EPSP, a graded depolarization An inhibitory postsynaptic potential, or IPSP, a graded hyperpolarization Time 2: Hyperpolarizing stimulus applied Time 3: Hyperpolarizing stimulus applied Stimulus removed EPSP Resting potential Resting potential EPSP IPSP IPSP Time 1: Depolarizing stimulus applied Stimulus removed Time 3: Depolarizing stimulus applied Stimuli removed Figure Postsynaptic potentials are responsible for information processing in a neuron Time Figure 103

Integration of information at postsynaptic cell Single postsynaptic cell may receive information from thousands of synapses Some excitatory, some inhibitory Net effect at axon hillock determines cell response Axon hillock is closest to initial segment where action potential is generated Threshold at axon hillock is lowest of cell body Is the simplest information processing in the nervous system Allows neurons to respond to changes in oxygen, nutrients, or abnormal chemicals

The axon hillock, the site at which a single neuron
Figure Postsynaptic potentials are responsible for information processing in a neuron Initial segment Glial cell processes Dendrite Synaptic knobs The axon hillock, the site at which a single neuron integrates the excitatory and inhibitory stimuli it receives across thousands of synapses Figure 105

Summation Integration of graded potential effects on plasma membrane segment May be combining opposite stimulations (EPSP + IPSP) or similar stimulations (EPSP + EPSP or IPSP + IPSP) Individual EPSP or IPSP has small effect on transmembrane potential (~0.5 mV) Summation of EPSPs can lead to action potential Threshold commonly ~10 mV

Two types of summation Temporal summation (tempus, time) A single synapse stimulated repeatedly Example: before effects of one EPSP can dissipate, another arrives = More ACh release = more postsynaptic cell depolarization Possibly to threshold at initial segment

Temporal summation, in which a single synapse is active repeatedly
ACTION POTENTIAL PROPAGATION FIRST STIMULUS SECOND STIMULUS Threshold reached Initial segment Figure Postsynaptic potentials are responsible for information processing in a neuron Figure 108

Two types of summation (continued) Spatial summation Involves multiple synapses activated simultaneously Example: EPSPs at multiple sites allowing Na+ channels to open May lead to action potential at initial segment Degree of depolarization dependent on Number of synapses are active at a particular moment Distance from initial segment

Spatial summation, in which multiple synapses are active simultaneously
TWO SIMULTANEOUS STIMULI ACTION POTENTIAL PROPAGATION Figure Postsynaptic potentials are responsible for information processing in a neuron Threshold reached Figure 110

Module Review a. Define excitatory postsynaptic potential (EPSP) and inhibitory postsynaptic potential (IPSP). b. Compare temporal summation with spatial summation. c. If a single EPSP depolarizes the initial segment from a resting potential of –70 mV to –65 mV, and threshold is at –60 mV, will an action potential be generated?

Module 11.12: Higher-level information processing
Involves regulatory neurons Facilitate or inhibit presynaptic neurons by: Affecting cell body membrane Altering sensitivity of synaptic knobs

The positions of regulatory neurons,
which facilitate or inhibit the activities of presynaptic neurons Regulatory neurons Postsynaptic neuron Presynaptic neuron Figure Information processing involves interacting groups of neurons, and information is encoded in the frequency and pattern of action potentials Figure 113

Involves different neurotransmitters More than 100 exist and work in different ways May have direct or indirect effects on ion channels Indirect effects usually involve G proteins Trigger formation or release of second messengers to alter postsynaptic cell activity

Figure Information processing involves interacting groups of neurons, and information is encoded in the frequency and pattern of action potentials Figure 115

In nervous system, complex information is translated to action potentials Solely on frequency of action potentials Example: muscle contraction changes in response to increasing action potential frequency

Frequency of action potentials
How the rate of action potentials arriving at a neuromuscular junction determines the nature of the resulting muscle contraction Time Maximum tension (in tetanus) Muscle tension KEY Arrival of action potential = Twitch contractions Incomplete tetanus Tetanus Figure Information processing involves interacting groups of neurons, and information is encoded in the frequency and pattern of action potentials Muscle tension Frequency of action potentials (per second) Figure 117

Figure Information processing involves interacting groups of neurons, and information is encoded in the frequency and pattern of action potentials Figure 118

Module 11.12 Review a. Describe the role of regulatory neurons.
b. What determines the frequency of action potential generation? c. The greater the degree of sustained depolarization at the axon hillock, the __________ (higher or lower) the frequency of generation of action potentials.

11 Neural Tissue.

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