Nervous Tissue

Lecture Presentation by Lori Garrett

Learning Outcomes 11.1 Describe the anatomical and functional divisions of the nervous system. 11.2 Sketch and label the structure of a typical neuron, and describe the functions of each component. 11.3 Classify and describe neurons on the basis of their structure and function.

Learning Outcomes (continued) 11.5 Describe the locations and functions of Schwann cells and satellite cells. 11.4 Describe the locations and functions of neuroglia in the CNS.

Nervous system—three divisions 1. Central nervous system (CNS) • Brain and spinal cord • Information processing—integrates, processes, coordinates sensory and motor commands 2. Peripheral nervous system (PNS) • All nervous tissue outside CNS, excluding the ENS 3. Enteric nervous system (ENS) • Neural tissues in wall of gastrointestinal tract; helps control digestive function

Subdivisions of the peripheral nervous system . Sensory (afferent) division brings information to CNS from receptors in peripheral tissues and organs • Sensory receptors – Position, touch, pressure, pain, temperature • Special sensory organs – Smell, taste, sight, balance, hearing

. Motor (efferent) division carries motor commands from CNS • Somatic nervous system (SNS) – Voluntary nervous system—conscious control of movement – To skeletal muscles; conscious control of movement • Autonomic nervous system (ANS) – Involuntary nervous system—automatically regulates activities – To smooth muscle, cardiac muscle, glands, adipose tissue

General functions of the nervous system 1. Receptors detect changes in internal or external environment 2. Information is sent to the CNS by the sensory division of the PNS 3. Information processing (integration and distribution of information) occurs in the CNS 4. Motor commands are carried by the motor division of the PNS 5. Effectors respond to those commands and change their activities

A. Compare the central and peripheral nervous systems. B. Which division of the PNS brings information to the CNS? C. Name the effectors of the ANS.

Learning Outcome: Describe the anatomical and functional divisions of the nervous system.

Neurons . Three general regions 1. Dendrites receive stimuli from environment/other neurons 2. Cell body—contains nucleus, other organelles 3. Axon—carries information toward other cells

Dendrites . Highly branched, with dendritic spines . CNS neurons receive most information here Cell body . Perikaryon = cytoplasm; contains organelles that provide energy/synthesize neurotransmitters

Cell body (continued) . Cytoskeleton contains • Neurofilaments—similar to intermediate filaments • Neurofibrils—bundles of neurofilaments; extend into and support dendrites and axon

Axon components . Axon hillock—origin of axon from cell body . Initial segment—where action potential is initiated . Axolemma—axon’s plasma membrane . Axoplasm—axon’s cytoplasm; contains neurofibrils, neurotubules, vesicles, lysosomes, mitochondria, enzymes Telondendria . = Fine extensions; end at axon terminals (synaptic terminals)

Transport of materials . Most organelles are in cell body . Materials travel along axon via neurotubules (neuron microtubules) • Process is axoplasmic transport • Occurs in both directions • Retrograde flow = movement back toward cell body

Synapse = Where neuron (presynaptic cell) communicates with another cell (postsynaptic cell) . Most common type—neurotransmitter released from presynaptic membrane into synaptic cleft (narrow space between the cells); binds receptors on postsynaptic membrane . Neurotransmitters are packaged in synaptic vesicles in axon terminals . Collateral branches allow a single neuron to communicate with more than one other cell.

Neuron replacement . Most CNS neurons lack centrioles—cannot divide • If lost to injury or disease, seldom replaced . Some neural stem cells exist, but most are inactive • Exceptions – Olfactory epithelium (smell) – Retina of the eye (vision) – Hippocampus (memory)

A. Name the structural components of a typical neuron. B. Describe a synapse. C. Compare presynaptic and postsynaptic cells. D. Why is a CNS neuron not usually replaced after it is injured?

Learning Outcome: Sketch and label the structure of a typical neuron, and describe the functions of each component.

Four major anatomical classes of neurons 1. Anaxonic neurons • Small neurons, lacking features distinguishing axons from dendrites (all cell processes look alike) • Located in brain and special sense organs • Functions are poorly understood

2. Bipolar neurons • Two distinct processes – Dendritic process that branches – Axon • Rare, but occur in special sense organs • Small

3. Unipolar neurons • Dendrites and axons continuous (fused) • Cell body off to one side • Initial segment is where dendrites converge – Remaining part of process is considered “axon” • Includes most sensory neurons in peripheral nervous system – Axons may extend a meter or more—longest carry sensations from toes to spinal cord

4. Multipolar neurons • Two or more dendrites and single axon • Most common neurons in CNS • All motor neurons to skeletal muscles are multipolar • Can be as long as unipolar neurons— longest carry motor signals from spinal cord to muscles that move toes

Neuron function within the CNS and PNS Three major functional classes of neurons 1. Sensory neurons ~10 million 2. Interneurons ~20 billion 3. Motor neurons ~500,000

Classification of sensory receptors . Sensory receptors—Detect stimuli; are processes of sensory neurons or cells monitored by sensory neurons • Interoceptors (intero-, inside) – Monitor internal organs/systems – Detect distension (stretch), deep pressure, pain • Proprioceptors – Monitor position/movement of skeletal muscles/joints • Exteroceptors (extero, outside) – Monitor external environment (touch, temperature, pressure, input for special senses)

Afferent fibers carry sensory information to CNS . Ganglion = collection of neuron cell bodies in PNS Sensory neurons = mostly unipolar neurons with cell bodies in sensory ganglia • Somatic sensory neurons—monitor outside world and body position/awareness • Visceral sensory neurons—monitor internal conditions and organ systems

Interneurons—located in CNS . Usually between sensory and motor neurons . Receive information from PNS and CNS . Also responsible for higher functions (e.g., memory, learning)

Somatic motor neurons innervate skeletal muscles; provide conscious control . Cell body lies in CNS; axon extends within a peripheral nerve (nerve = a bundle of axons in the PNS) Visceral motor neurons—part of the autonomic nervous system (parasympathetic and sympathetic divisions); innervate all other effectors (smooth muscle, cardiac muscle, glands, adipose tissue) . Located in CNS and PNS . Autonomic ganglia—location of cell bodies for visceral motor neurons going to peripheral receptors. © 2018 Pearson Education, Inc. Module 11.3: Neuron structure/function

Efferent fibers carry information from CNS to effectors (somatic effectors—skeletal muscles; visceral effectors—cardiac or smooth muscle and glands)

A. Classify neurons based on their structure. B. Classify neurons based on their function. C. Are unipolar neurons in a tissue sample of the PNS more likely to have a sensory or a motor function?

Learning Outcome: Classify and describe neurons on the basis of their structure and function.

© 2018 Pearson Education, Inc. Module 11.4: Oligodendrocytes, astrocytes, ependymal cells, and microglia are neuroglia of the CNS Neuroglia (or glial cells) . Cells that support/protect neurons . Comprise ~ half the total volume of the nervous system Four types of CNS glial cells 1. Ependymal cells 2. Microglia 3. Astrocytes 4. Oligodendrocytes

Ependymal cells . Form simple cuboidal to columnar epithelium = ependyma . Lines central canal (spinal cord) and ventricles (brain) . Assist in producing, circulating, monitoring cerebrospinal fluid (CSF) that fills these spaces/passages

Three types of ependymal cells 1. Ependymocytes—cilia circulate CSF 2. Tanycytes—only in one brain ventricle; nonciliated with microvilli; may transport substances between CSF and brain 3. CSF-producing ependymal cells

Microglia . Developmentally related to monocytes and macrophages . Mobile phagocytic cells that remove cellular debris, waste products, and pathogens

Astrocytes . Maintain the blood–brain barrier (isolates CNS from chemicals/hormones in blood) . Structural support . Regulate ion, nutrient, and gas concentrations in interstitial fluid around neurons . Absorb/recycle neurotransmitters . Form scar tissue after injury

Oligodendrocytes . Provide CNS framework by stabilizing axons . Produce myelin . Cell process wraps axon with layers of myelin and plasma membrane, creating a myelin sheath—speeds up nerve impulse transmission

Axons with myelin sheaths are myelinated; appear white because of lipid content of myelin . CNS white matter—many myelinated (whitish) axons • Myelin-wrapped areas = internodes • Gaps between internodes = nodes (nodes of Ranvier)

Axons without myelin sheath = unmyelinated . CNS gray matter—areas with mostly cell bodies, dendrites, unmyelinated axons; lack of myelin makes them appear gray

A. Name the neuroglia of the CNS. B. Which neuroglia appear in increased numbers in a person with a brain infection? C. Which glial cell protects the CNS from chemicals and hormones circulating in the blood? D. Contrast the white matter and gray matter in the CNS.

Learning Outcome: Describe the locations and functions of neuroglia in the CNS.

Two types of neuroglia in the PNS 1. Schwann cells (neurolemmocytes) cover peripheral axons • Participate in axon repair 2. Satellite cells surround peripheral cell bodies • Regulate environment around neurons, similar to astrocyte role in CNS

Myelinated axons . Formed by myelinating Schwann cells . Outer surface of Schwann cells = neurilemma . A single Schwann cell wraps a single internode

This illustration shows the steps in the myelination of an axon in the PNS. A single myelinating Schwann cell myelinates one internode of only one axon.

Unmyelinated axons . Single nonmyelinating Schwann cell surrounds segments of a group of axons . Axons lie in membrane folds at edge of cell— stabilizes their positions; isolates them from chemicals in interstitial fluid . No nodes between adjacent nonmyelinating Schwann cells

Axon injury and repair in the PNS 1. Axon and myelin degenerate distal to injury (= Wallerian degeneration)

Axon injury and repair in the PNS (continued) 2. Schwann cells proliferate along original axon path • Macrophages move in and remove cellular debris distal to injury site

Axon injury and repair in the PNS (continued) 3. Axon grows along path created by Schwann cells

Axon injury and repair in the PNS (continued) 4. Schwann cells wrap around the elongating axon • If normal synaptic contacts are reestablished, normal function may return • If axon stops growing or grows in wrong direction, normal function will not return

Axon injury and repair in the PNS (continued)

Axon injury and repair in the CNS . In the CNS, regeneration is limited because: • Many more axons are involved • Astrocytes produce scar tissue that can block axon growth in damaged area • Astrocytes release chemicals that block axon regrowth

A. Identify the neuroglia of the PNS. B. In which part of a neuron does Wallerian degeneration occur? C. Describe the neurilemma of a myelinating and nonmyelinating Schwann cell.

Learning Outcome: Describe the locations and functions of Schwann cells and satellite cells.

Learning Outcomes 11.6 Describe the general role of membrane potential changes in neuronal activity. 11.7 Explain how the resting membrane potential is created and maintained. 11.8 Describe the functions of gated ion channels with respect to the permeability of the plasma membrane. 11.9 Describe graded potentials.

Learning Outcomes (continued) 11.10 Describe the events involved in the generation of an action potential. 11.11 Describe continuous propagation and saltatory propagation, and discuss the factors that affect the speed with which action potentials are propagated. 11.12 Describe the general structure of synapses in the CNS and PNS, and discuss the events that occur at a chemical synapse.

Learning Outcomes (continued) 11.13 Discuss the significance of postsynaptic potentials, including the roles of excitatory postsynaptic potentials and inhibitory postsynaptic potentials. 11.14 Discuss the interactions that make information processing in nervous tissue possible.

Membrane potential = potential difference across a membrane due to unequal charge distribution across the membrane . Inside of membrane slightly negative compared to outside • Results from difference in permeability of cell membrane to various ions and from active transport mechanisms

Neural activity 1. Resting membrane potential (or resting potential) = Membrane potential of an undisturbed cell; starting point • Inside slightly negative compared to outside 2. Graded potential = temporary, localized change in resting potential, produced by a typical stimulus • Decreases with distance from stimulus (graded potential)

3. Action potential = electrical event that involves one location on the axon membrane but then spreads along axon surface toward axon terminals • Triggered by sufficiently large graded potential

4. Synaptic activity • Presynaptic cell releases neurotransmitters • Neurotransmitter binds to receptors on postsynaptic cell membrane; changes permeability and produces graded potentials in postsynaptic cell membrane • Response of postsynaptic cell depends on action of stimulated receptors and other stimuli acting on cell at same time – Simplest form of information processing in nervous system

A. Define membrane potential. B. Compare a graded potential with an action potential. C. Define information processing.

Learning Outcome: Describe the general role of membrane potential changes in neuronal activity.

© 2018 Pearson Education, Inc. Module 11.7: Differences in electrochemical gradients determine the resting membrane potential Contributors to resting membrane potential . Extracellular fluid (ECF) has high concentrations of sodium ions (Na+) and chloride ions (Cl–) . Cytosol has high concentrations of potassium ions (K+) and negatively charged proteins (Pr–) . Sodium and potassium ions are main factors influencing membrane potential . Proteins and ions cannot move freely across cell membrane—enter/leave only by membrane channels or active transport

The resting membrane potential = form of potential energy . Measured in millivolts (1 mV = 1/1000 volt) . In body cells, ranges from –5 mV to –100 mV . Resting membrane potential of a neuron is near –70 mV; negative value means inside is more negative than the outside

. Maintaining the resting membrane potential involves: • Passive forces – Diffusion of Na+ and K+ through leak channels, driven by chemical and electrical gradients – Abundance of negatively charged proteins inside cell that cannot diffuse out • Active process – Sodium–potassium exchange pump

Leak channels = passive membrane channels that are always open . Main reason membrane potential exists . Size/shape/structure determine which ions can diffuse through • Potassium ions leave cell through K+ leak channels • Sodium ions enter cell through Na+ leak channels

Sodium–potassium exchange pump . Ejects 3 Na+ ions for every 2 K+ recovered from the extracellular fluid (ratio of 3:2) . Maintains stable resting potential

Gradients and potentials . Chemical gradient = Concentration gradient for an ion across the plasma membrane . Electrical gradient—Created by attraction between opposite charges (+/–) or repulsion between like charges (+/+ or –/–) . Electrochemical gradient = Form of potential energy determined by the combination of an ion’s chemical and electrical gradients

Equilibrium potential = Membrane potential at which an ion’s electrical and chemical gradients are in balance; no net movement of ions across the membrane . Indicates an ion’s contribution to resting potential • Equilibrium potential for potassium is –90 mV • Equilibrium potential for sodium is +66 mV

Potassium ion gradients at resting membrane potential . Smaller electrical gradient into cell . Larger chemical gradient out of cell . K+ net electrochemical gradient is out of the cell

Sodium ion gradients at resting membrane potential . Electrical gradient directed into the cell . Chemical gradient directed into the cell . Na+—strong, net electrochemical gradient into the cell

A. Define resting membrane potential. B. What happens at the sodium–potassium exchange pump? C. What effect would decreasing the concentration of extracellular potassium ions have on the resting membrane potential of a neuron?

Learning Outcome: Explain how the resting membrane potential is created and maintained.

© 2018 Pearson Education, Inc. Module 11.8: Three types of gated ion channels change the permeability of the plasma membrane Gated channels . Permeability changes are due to gated ion channels in plasma membrane that open/close in response to stimuli Three types of gated ion channels 1. Chemically gated channels 2. Voltage-gated channels 3. Mechanically gated channels

Chemically gated ion channels . Also called ligand-gated ion channels . Open when they bind specific chemicals . Example: Receptors that bind acetylcholine (ACh) at the neuromuscular junction . Most abundant on dendrites and cell body of a neuron, where most synaptic communication occurs

Chemically gated ion channels (continued) . Also called ligand-gated ion channels . Open when they bind specific chemicals . Example: Receptors that bind acetylcholine (ACh) at the neuromuscular junction . Most abundant on dendrites and cell body of a neuron, where most synaptic communication occurs

Voltage-gated ion channels . Open or close in response to changes in membrane potential . Characteristic of excitable membranes, which can generate and/or spread action potentials . Examples: Na+, K+, and Ca2+ channels . Sodium channels have 2 independent gates— activation gate opens on stimulation to let sodium in; inactivation gate closes to block entry of sodium ions

Voltage-gated ion channels (continued) . Open or close in response to changes in membrane potential . Characteristic of excitable membranes, which can generate and/or spread action potentials . Examples: Na+, K+, and Ca2+ channels . Sodium channels have 2 independent gates— activation gate opens on stimulation to let sodium in; inactivation gate closes to block entry of sodium ions

Voltage-gated ion channels (continued) . Open or close in response to changes in membrane potential . Characteristic of excitable membranes, which can generate and/or spread action potentials . Examples: Na+, K+, and Ca2+ channels . Sodium channels have 2 independent gates— activation gate opens on stimulation to let sodium in, inactivation gate closes to block entry of sodium ions

Mechanically gated channels . Open in response to physical distortion of membrane surface . Important in sensory receptors responding to stretch, pressure, or vibration and for sense of touch and hearing

Mechanically gated channels (continued) . Open in response to physical distortion of membrane surface . Important in sensory receptors responding to stretch, pressure, or vibration and for sense of touch and hearing

Distribution of gated channels on a neuron . Chemically gated channels—neuron cell body and dendrites . Voltage-gated Na+ and K+ channels—along axon . Voltage-gated Ca2+ channels—axon terminals Most gated ion channels are closed at resting membrane potential; opening changes rate of ion movement across membrane, which changes the membrane potential

A. Define gated ion channels. B. Identify the three types of gated ion channels, and state the conditions under which each operates. C. What effect would a chemical that blocks voltage-gated sodium ion channels in a neuron’s plasma membrane have on its membrane potential?

Learning Outcome: Describe the functions of gated ion channels with respect to the permeability of the plasma membrane.

Graded potentials (also called local potentials) . Changes in membrane potential that cannot spread far from site of stimulation; caused by stimuli that open gated ion channels Example: chemically gated sodium ion channels 1. At resting membrane potential, chemically gated sodium channels are all closed

Graded potentials (continued) 2. Membrane exposed to chemical that opens the chemically gated sodium channels • When gates open, Na+ enters cell • Influx of positive charge reduces membrane potential • Shift from resting membrane potential (–70 mV) toward more positive value (less negative) = depolarization

Graded potentials (continued) 3. Sodium ions entering cell move away from channels (attracted to negative charges along inner surface of membrane) • Extracellular sodium ions move along outer surface of membrane toward channels to replace those that entered • Movement of positive charges along inner and outer surface of membrane = local current

Graded potentials (continued) . Degree of depolarization decreases with distance from stimulus • Ions enter at only one location but spread in all directions • Change of membrane potential proportional to stimulus size – Larger stimulus opens more channels • More sodium enters; greater depolarization

Changes in membrane potential 1. Opening chemically gated Na+ channels depolarizes the membrane as positive sodium ions enter the cell

Changes in membrane potential (continued) 2. Membrane returns to normal resting membrane potential (repolarization) as excess Na+ is transported out of cell

Changes in membrane potential (continued) 3. Opening chemically gated K+ channels causes membrane potential to shift past resting membrane potential (more negative) = hyperpolarization

Changes in membrane potential (continued)

A. Define graded potential. B. Describe depolarization, repolarization, and hyperpolarization. C. What factors account for the local currents associated with graded potentials?

Learning Outcome: Describe graded potentials.

© 2018 Pearson Education, Inc. Module 11.10: Action potentials are all-or-none events for communication that begin with membrane potential reversal Action potentials = propagated changes in the membrane potential that affect the entire excitable membrane . A neuron receives graded potentials at its dendrites/cell body . Action potentials at axon terminals release neurotransmitter . Action potentials allow long-range communication between cell body and axon terminals

© 2018 Pearson Education, Inc. Module 11.10: Action potentials are all-or-none events for communication that begin with membrane potential reversal Channel types and potentials . Resting membrane potential depends on leak channels . Graded potentials depend on chemically gated ion channels . Action potentials depend on voltage-gated ion channels

Steps in action potential generation . Resting membrane potential— voltage-gated sodium and potassium channels are both closed

Steps in action potential generation (continued) 1. Depolarization to threshold • Initial stimulus—graded depolarization large enough to open voltage-gated sodium channels • Threshold = membrane potential at which channels open – About –60 mV

Steps in action potential generation (continued) 2. Activation of sodium channels—rapid depolarization • Sodium channel activation gates open; sodium ions rush in • Rapid depolarization— membrane potential goes from –60 mV to a positive value

All-or-none principle: . Any stimulus either triggers an action potential or does not . All stimuli that bring a membrane to threshold result in identical action potentials

Steps in action potential generation (continued) 3. Inactivation of sodium ion channels; activation of potassium ion channels • As membrane potential approaches +30 mV, inactivation gates of sodium channels close (sodium channel inactivation) • At same time, voltage-gated potassium channels open – Potassium ions leave cell, and membrane potential shifts back towards resting membrane potential (repolarization starts)

Steps in action potential generation (continued) 4. Potassium ion channels close • Sodium channels shift from inactivated to closed (but capable of opening) as membrane potential returns to near threshold • As membrane potential approaches normal resting potential (–70 mV), voltage-gated potassium channels begin closing – Potassium leaves until all gates close; brief hyperpolarization

Refractory periods of an action potential . Absolute refractory period • Time during which the membrane cannot respond to any further stimulation . Relative refractory period • Time during which the membrane can respond, but only to a stimulus that is stronger than normal

A. Define action potential. B. Describe the all-or-none principle. C. Compare the absolute refractory period with the relative refractory period. D. List the events involved in the generation of an action potential.

Learning Outcome: Describe the events involved in the generation of an action potential.

© 2018 Pearson Education, Inc. Module 11.11: Action potentials may affect adjacent portions of the plasma membrane through continuous propagation or saltatory propagation Action potential propagation . Action potentials are generated at initial segment of axon . Action potential at one site triggers action potential at adjacent site; process continues to end of axon (propagation)

Two types of propagation 1. Continuous propagation—action potential appears to move step by step through entire axon • Occurs in unmyelinated axons • Slower—depolarization moves at about 1 m/sec 2. Saltatory propagation—in myelinated axons; depolarizes only at nodes • Skips internodes because ions can’t cross membrane where there is myelin • Faster than continuous; speed varies with axon diameter – Larger diameter has less resistance to ion movement; faster propagation

A. Define continuous propagation and saltatory propagation. B. What is the relationship between myelin and the propagation speed of action potentials?

Learning Outcome: Describe continuous propagation and saltatory propagation, and discuss the factors that affect the speed with which action potentials are propagated.

© 2018 Pearson Education, Inc. Module 11.12: At a synapse, information travels from the presynaptic cell to the postsynaptic cell Synapse = location where information is transferred from a neuron to another neuron or to effector cell . If neurons are communicating, synapse involves two neurons—presynaptic neuron and postsynaptic neuron Two types of synapses 1. Chemical synapse 2. Electrical synapse

Chemical synapses . Most abundant type of synapse • All synapses between neurons and other types of cells • Most synapses between neurons . Rely on neurotransmitter release . Synapses that release acetylcholine (ACh) are categorized as cholinergic synapses (most common type)

Events at a cholinergic synapse . Step 1. Axon terminal depolarized by arriving action potential

Events at a cholinergic synapse (continued) . Step 2. Depolarization opens voltage-gated calcium channels • Calcium ions rush into axon terminal • Triggers synaptic vesicles to release ACh (exocytosis) into synaptic cleft • Calcium ions are quickly removed, ending release of ACh

Events at a cholinergic synapse (continued) . Step 3. ACh diffuses across synaptic cleft; binds to chemically gated Na+ channel receptors on postsynaptic membrane • More ACh = more channels open • More Na+ enters = greater depolarization • If threshold met, initiates action potential

Events at a cholinergic synapse (continued) . Step 4. Effects on the postsynaptic membrane are temporary • Acetylcholinesterase (AChE) in synaptic cleft breaks down bound ACh within 20 msec • Other ACh molecules diffuse away from binding sites

Synaptic fatigue . Neurotransmitters usually reabsorbed and recycled . After extended stimulation, supply of neurotransmitter may not keep up with demand . Synapse unable to function until ACh replenished . Inability to function = synaptic fatigue

Synaptic delay = time lag between arrival of action potential at axon terminal and effect on postsynaptic membrane . Usually 0.2 to 0.5 msec . Cumulative delays may be considerable . Reflexes involve few synapses in order to minimize delay

Electrical synapses . Presynaptic and postsynaptic membranes joined by gap junctions . Changes in membrane potential of one cell produce local currents in adjacent cell, as if sharing common membrane . Rare . No variability in response of postsynaptic cell

A. Describe the components of a chemical synapse. B. What event must occur in the postsynaptic cell before it generates an action potential? C. Describe synaptic fatigue. D. Contrast an electrical synapse with a chemical synapse.

Learning Outcome: Describe the general structure of synapses in the CNS and PNS, and discuss the events that occur at a chemical synapse.

© 2018 Pearson Education, Inc. Module 11.13: Postsynaptic potentials are responsible for information processing in a neuron Postsynaptic potentials . Graded potentials in postsynaptic membrane in response to a neurotransmitter Two types 1. Excitatory postsynaptic potential (EPSP) 2. Inhibitory postsynaptic potential (IPSP)

1. Excitatory postsynaptic potential (EPSP) • Graded depolarization • Shifts membrane potential closer to threshold – Membrane is facilitated—more facilitation means less additional stimulus is needed to trigger action potential

2. Inhibitory postsynaptic potential (IPSP) • Graded hyperpolarization • Shifts membrane potential farther away from threshold – Membrane is inhibited—larger-than-normal stimulus is needed to trigger action potential

Summation = integration of effects of graded potentials . Collective effects of both EPSPs and IPSPs Example: Arrival of two different neurotransmitters at same time opens two different sets of ion channels • Net effect may be no change in membrane potential

Integration of information . Single neuron may receive information from thousands of synapses (excitatory/inhibitory) . Axon hillock integrates all stimuli; determines rate of action potential generation at initial segment • Is closest to initial segment where action potential starts • Threshold at axon hillock lower than elsewhere on cell body

Types of summation . Individual EPSP or IPSP has small effect on membrane potential • Single EPSPs will not cause an action potential, but they can combine through summation . Two types 1. Temporal summation 2. Spatial summation

Types of summation (continued) 1. Temporal summation (tempus, time) • When single synapse is stimulated repeatedly • Under maximum stimulation, action potential can reach synapse each millisecond • Each new action potential causes release of more ACh into the synaptic cleft • More ACh = more depolarization, possibly to threshold

Types of summation (continued) 2. Spatial summation • Multiple synapses active at same time • Degree of depolarization depends on: 1. Number of active excitatory synapses 2. Their distance from initial segment • Action potential is generated if membrane reaches threshold

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? Explain your answer. Learning Outcome: Discuss the significance of postsynaptic potentials, including the roles of excitatory postsynaptic potentials and inhibitory postsynaptic potentials.

© 2018 Pearson Education, Inc. Module 11.14: Information processing involves interacting groups of neurons and various neurotransmitters that encode information as action potential frequency Involve regulatory neurons . Facilitate or inhibit activities of presynaptic neurons • Affect cell body membrane • Alter sensitivity of axon terminals

Involve neurotransmitters . Over 100 different neurotransmitters exist, each working differently . May have direct effects on ion channels . May have indirect effects, usually binding to G proteins • Diverse family of enzyme complexes – Neurotransmitters bound to G-coupled membrane receptors are first messengers – Trigger formation or release of second messengers to alter conditions or activity of postsynaptic cell

Interpretation of complex information . Information is translated into action potentials . Messages are often interpreted solely on the frequency of arriving action potentials . Example: Muscle contraction changes in response to increasing frequency of action potentials

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 action potentials generated. D. Compare the effects of the neurotransmitters acetylcholine and serotonin on ion channels.