BIOL 3301 Chapter 11 Functional Organization of Nervous Tissue Lecture Fall 2024 PDF

11/1/24 1 Functional Organization of Nervous Tissue BIOL 3301 Kristen Winter, PhD 2 3 Learning Objectives Explain the functions of the nervous system 4 Nervous Tissue Cells of nervous tissue Neurons Electrically excitable cells of the nervous system...

11/1/24 1 Functional Organization of Nervous Tissue BIOL 3301 Kristen Winter, PhD 2 3 Learning Objectives Explain the functions of the nervous system 4 Nervous Tissue Cells of nervous tissue Neurons Electrically excitable cells of the nervous system Glial cells Supportive cells 5 Functions of the Nervous System Maintaining homeostasis Receiving sensory input Integrating information Controlling muscles and glands Establishing and maintain mental activity 6 Check Point Questions List and give examples of the general functions of the nervous system. 7 Learning Objectives List the divisions of the nervous system and describe the characteristics of each Differentiate between the somatic and autonomic nervous systems Contrast the general functions of the CNS and the PNS 8 Divisions of the Nervous System Central nervous system (CNS) Receives information from and sends information to the body Decision maker Consists of the brain and spinal cord Peripheral nervous system (PNS) Detects stimuli in and around the body 9 1  Peripheral nervous system (PNS) Detects stimuli in and around the body Carries information to the CNS and from the CNS to the body Consists of nerves, ganglia, and sensory receptors 9 Structures of the CNS Brain Housed in the skull Spinal cord Housed in the vertebral column 10 Structures of the PNS Nerves Collection of axons outside the brain and spinal cord Can carry electrical signals away from or towards the CNS Cranial nerves – 12 pairs of nerves originating from the brain Spinal nerves – 31 pairs of nerves originating from the spinal cord Plexus – bundle of nerves outside the brain and spinal cord Ganglia Group of neuron cell bodies outside the brain and spinal cord Sensory receptors Cells that respond to a specific stimuli Can be neurons or specialized cells Distributed throughout the body 11 Divisions of the PNS Sensory (afferent) division Transmits electrical signals from receptors to the CNS Motor (efferent) division Transmits electrical signals from the CNS to effector organs Effector organs include muscle (skeletal, cardiac, smooth) and glands 12 Divisions of the Motor Nervous System Somatic nervous system Voluntary division Regulates movement of skeletal muscles Autonomic nervous system Involuntary division Regulates contract of cardiac and smooth muscles and secretions 13  Involuntary division Regulates contract of cardiac and smooth muscles and secretions of glands 13 Divisions of the Autonomic Nervous System Sympathetic nervous system Prepares the body for physical activity “fight or flight” Parasympathetic nervous system Regulates resting functions (digesting food) “rest and digest” Enteric nervous system Neuronal networks in the wall of digestive tract 14 Information Flow in the Nervous System 15 Check Point Questions Name the components of the CNS and PNS. What are the following: sensory receptor, nerve? Based on the direction they transmit action potentials, what are the two divisions of the PNS? Based on the structures they supply, what are the two subcategories of the motor division? What are the subcategories of the ANS? Compare the general functions of the CNS and PNS. 16 Learning Objectives Describe the structure of neurons Describe the functions of the components of a neuron Classify neurons based on structure Classify neurons based on function Describe the location, structure, and function of CNS glial cells Describe the location, structure, and functions of PNS glial cells Discuss the function of the myelin sheath Describe the formation of myelin sheaths in the CNS and PNS 17 Cells of Nervous Tissue Neurons Electrically excitable cells of the nervous system ~100 billion neurons in the body 18 17 Electrically excitable cells of the nervous system ~100 billion neurons in the body Glial cells Supportive cells 50% of the brain’s weight 10-50 times more glial cells than neurons in various brain regions 18 Neuron Structure Cell body (Soma) Single, centrally located nucleus with nucleolus Nissl bodies – extensive rough endoplasmic reticulum Abundant intermediate filaments (neurofilaments) and microtubules forming bundles in the cytoplasm Dendrites Processes off the cell body Short, often highly branched Tapered from base to tip Receive input from other neurons and other sensory receptors Dendritic spines – small extension on the surface where synapses are formed 19 Neuron Structure Axons Single process off the cell body Constant diameter, varied length Axoplasm – cytoplasm of the axon Axolemma – plasma membrane of the axon Axon hillock – cone shaped area coming off cell body Initial segment – formed by the narrowing of the axon hillock Trigger zone – axon hillock and initial segment, where action potentials are generated Presynaptic terminal – region at the end of the axon that house synaptic vesicles storing neurotransmitters Synapse – point of contact between the axon ending and its effector 20 Functional Classes of Neurons Based on the direction of action potential conduction Types: Sensory (afferent) neurons – conduct action potentials toward CNS 21 20 Types: Sensory (afferent) neurons – conduct action potentials toward CNS Motor (efferent) neurons – conduct action potentials away from the CNS toward muscles or glands Interneurons – conduct action potentials within the CNS 21 Structural Classes of Neurons Based on the number of dendrites Types Multipolar neurons Many dendrites and a single axon Dendrite number vary with varying branching Motor neurons of the PNS and most neurons within the CNS Bipolar neurons One dendrite and one axon Dendrites are often specialized to receive stimulus Axons conduct action potentials Located in sensory organs (retina of the eye, nasal cavity) Pseudo-unipolar neurons Single process exits the cell body and divides into two branches that function as a single axon Peripheral process – extends to the periphery and has dendrites that act as sensory receptors or communicate with sensory receptors Central process – extend to the CNS Most sensory neurons Anaxonic neurons Do not have axons, only dendrites Found within the brain and retina Communicate using only graded potentials 22 Glial Cells of the CNS Astrocytes Ependymal cells Microglia Oligodendrocytes 23 Glial Cells of the CNS: Astrocytes Cytoplasmic processes extending from cell body Foot processes cover blood vessels, neurons, and pia mater 23 Cytoplasmic processes extending from cell body Foot processes cover blood vessels, neurons, and pia mater Regulate the composition of extracellular brain fluid Produce chemicals that promote formation of tight junctions between endothelial cells of capillaries to form the blood-brain barrier Blood-brain barrier Controls substances that pass from blood into brain and spinal cord Protects neurons from toxins Allows nutrients and waste products to be exchanged Prevents fluctuations in blood composition 24 Glial Cells of the CNS: Astrocytes Play a role in response to tissue damage in CNS Reactive astrocytosis Caused by injuries in the CNS Astrocytes wall off injury site Limit spread of inflammation Limit regeneration of axons of injured neurons Promote development of synapses and help regulate synaptic activity by synthesizing, absorbing, and recycling neurotransmitters 25 Glial Cells of the CNS: Ependymal Cells Line ventricles of the brain and central canal of the spinal cord Choroid plexuses Specialized ependymal cells and blood vessels located in regions of the ventricles Secretes cerebrospinal fluid May of cilia to move CSF Extension of the basal surface extend deep into brain and spinal cord 26 Glial Cells of the CNS: Microglia CNS specific immune cells Become mobile and phagocytic in response to inflammation Phagocytize necrotic tissue, microorganisms, and other foreign substances 27 Glial Cells of the CNS: Oligodendrocytes Form the myelin sheath 28 27 Glial Cells of the CNS: Oligodendrocytes Form the myelin sheath Cytoplasmic extensions wrap around multiple axons Insulate axons 28 Glial Cells of the PNS Schwann cells Satellite cells 29 Glial Cells of the PNS: Schwann Cells Form the myelin sheath Schwann cell wraps around only one axon Neurilemma Outermost layer of each Schwann cell Contains majority of Schwann cell cytoplasm, nucleus, and organelles 30 Glial Cells of the PNS: Satellite Cells Surround neuron cell bodies in sensory and autonomic ganglia Provide support and nutrition Protect neurons from heavy metal poisons 31 Myelinated Axons Schwann cells (PNS) or oligodendrocytes (CNS) wrap around axons Forms layers of phospholipids with small amounts of cytoplasm Give myelinated axons a white appearance Nodes of Ranvier – gaps in the myelin sheath Schwann cells or oligodendrocytes extend across and connect Protect and electrically insulate the axons 32 Unmyelinated Axons Not devoid of myelin Axons rest in invaginations of Schwann cells or oligodendrocytes Protects axons 33 Development of Myelin Sheath Begins in late fetal development Continues rapidly until end of first year after birth Slows and continues after 34 35 33 Slows and continues after 34 Multiple Sclerosis Chronic disease of the CNS Gradual loss of myelin sheath Slows action potential transmission Impairs control of skeletal and smooth muscle 35 Check Point Questions Describe and give the function of a neuron cell body, a dendrite, and an axon. What is the function of the trigger zone? What is the role of a neurotransmitter? Where is it stored? Describe the three types of neurons based on function. Explain the four types of neurons based on structure, and give an example of where each type is found. What characteristic makes glial cells different from neurons? Which glial cells are found in the CNS? In the PNS? Which type of glial cell supports neurons and blood vessels and promotes formation of the blood-brain barrier? What is the blood- brain barrier, and what is its function? Name the different kinds of glial cells that are responsible for the following functions: production of cerebrospinal fluid, phagocytosis, production of myelin sheaths in the CNS, production of myelin sheaths in the PNS, support of neuron cell bodies in the PNS. What is a myelin sheath? How is it formed in the CNS? In the PNS? How do myelinated axons differ from unmyelinated axons? 36 Learning Objectives Distinguish between gray matter and white matter Describe the components of gray matter in the CNS and PNS Describe the components of white matter in the CNS and PNS 37 Gray Matter & White Matter Gray matter Contains neuron cell bodies, dendrites CNS Cortex – surface of the brain Nuclei – clusters deep within the brain PNS Ganglia – neuron cell bodies  PNS Ganglia – neuron cell bodies White matter Bundles of myelinated axons CNS Nerve tracts – carry action potentials from one area of the CNS to another PNS Nerves – bundles of axons and their connective tissue coverings 38 Check Point Questions What makes up gray matter and white matter? Describe and state the location of the following: nerve tracts, nerves, the brain cortex, nuclei, ganglia. 39 Learning Objectives Define resting membrane potential Explain how resting membrane potential is created and maintained Explain the processes that can change the resting membrane potential List the three phases of neuron communication Describe the characteristics of a graded potential Describe the creation of an action potential Explain how an action potential is propagated Discuss the all or none principle as it applies to action potentials Explain the characteristics and purpose of the refractory period Explain the factors that determine action potential frequency Explain the five levels of stimulation Describe the effect of myelination on the speed of action potential propagation Describe other factors that affect the speed of action potential conduction 40 Action Potentials & Membrane Potential Action potentials - electrical signals produced by the nervous system Membrane potential – measure of electrical properties of the plasma membrane due to Ionic concentration differences across the plasma membrane Permeability characteristics of the plasma membrane 41 42  Permeability characteristics of the plasma membrane 41 Ionic Concentration Difference 42 Permeability of the Plasma Membrane Determined by ion channels and pumps Sodium-potassium pump – contribute to the maintaining the differences in cytoplasmic and extracellular concentrations of ions Leak channels Gated channels 43 Leak Channels Always open Responsible for permeability of the plasma membrane when it is at rest Determine permeability of resting membrane More permeable to K+ and Cl- than to Na+ Specific for one type of ion 44 Gated Ion Channels Open and close due to a specific signal Ligand-gated ion channels Opened by binding of a specific molecule (ligand) on the extracellular side Channel crosses the membrane Voltage-gated ion channels Open and close in response to specific voltage changes across the plasma membrane Required for action potentials Mechanically-gated ion channels Open in response to mechanical stimulation Thermoreceptors Respond to temperature changes 45 Establishing Resting Membrane Potential Cytoplasm and extracellular fluid are electrically neutral Charge difference across the plasma membrane Plasma membrane is polarized Potential difference – electrical charge difference across the plasma 46 47 45 Plasma membrane is polarized Potential difference – electrical charge difference across the plasma membrane Resting membrane potential – potential difference in a resting cell 46 Resting Membrane Potential 47 Changes in Resting Membrane Potential Ions diffuse down their concentration gradients Movement results in electrical current and changes in resting membrane potential Two types of changes: Depolarization Hyperpolarization 48 Depolarization Inside of the cell becomes more positive Excitatory – always moves the membrane potential closer to the point of action potential generation Several factors can lead to depolarization of neurons Na+ entry Ca2+ entry Changes in extracellular K+ concentration 49 Sodium Ions Na+ entry is the most common cause of depolarization Limited Na+ leak channels Entry of Na+ is typically regulated Ligand-gated Na+ or voltage-gated Na+ channels 50 Calcium Ions Ca2+ enters the cell causing depolarization Important for some cardiac muscle cells to generate action potentials Plays significant roles in action potentials Regulation of voltage-gated Na2+ channels 50 Plays significant roles in action potentials Regulation of voltage-gated Na2+ channels Regulation of neurotransmitter secretion at the presynaptic terminal Hypocalcemia – lower levels of Ca2+ in the blood Symptoms include nervousness and uncontrolled skeletal muscle contraction Caused by lack of dietary Ca2+ or vitamin D or insufficient PTH 51 Potassium Ions Changes in extracellular K+ concentration can affect resting membrane potential Increases can cause cytoplasmic K+ to stay inside the cell When K+ stays inside the cell it can cause depolarization 52 Hyperpolarization Inside of the cell becomes even more negative Inhibitory - makes the cell less likely to generate an action potential Two major ways to hyperpolarize neurons K+ exits Cl- enters 53 Potassium Ions Exit of K+ is primary cause of hyperpolarization after action potential Voltage-gated K+ channels Ligand-gated K+ channels – mechanism for some inhibitory neurotransmitters Hypokalemia – lower blood K+ concentration Decrease in extracellular K+ can cause more K+ to exit the cell through leak channels Symptoms include muscular weakness, abnormal heart function, sluggish reflexes Caused by starvation, alkalosis, some kidney disease 54 Chloride Ions Cl- concentration is higher outside the cell Opening of ligand-gated Cl- channels allows Cl- to diffuse into the cell Some inhibitory neurotransmitters use this mechanism 55 56 54 Some inhibitory neurotransmitters use this mechanism 55 Neuron Communication 1. Generation of action potentials 2. Action potential propagation along the axon 3. Communication with a target cell at the synapse 56 Graded Potentials Relatively small change in membrane potential localized to one area of the plasma membrane Vary in size depending on strength of the stimulus Caused by several types of stimuli Chemicals binding to ligand-gated ion channels Changes in voltage triggering opening or closing of voltage-gated ion channels Mechanical stimuli opening mechanically gated ion channels Temperature changes affecting specific temperature receptors Can be Hyperpolarizing – inhibitory Depolarizing – excitatory 57 Graded Potentials Summation Combining/adding graded potentials Large enough (reaches threshold) will result in an action potential Spread in decremental fashion 58 Graded Potentials 59 Action Potentials Used by neurons for communication Result from summation of graded potentials Large change in membrane potential Propagates (travels) without changing in magnitude over long distances Phases of an action potential Depolarization phase Repolarization phase Afterpotential Return to resting membrane potential 60 61  Afterpotential Return to resting membrane potential 60 Action Potentials 61 All-or-None Principle If graded potential reaches threshold, action potential is generated Voltage-gated channels open altering membrane permeability If graded potential does not reach threshold, action potential is not generated Membrane potential returns to resting membrane potential 62 Voltage-Gated Ion Channels & Action Potentials Voltage-gated ion channels are required for generation of action potentials Phases of an action potential Depolarization phase Repolarization phase Afterpotential Return to resting membrane potential 63 Refractory Period Plasma membrane becomes less sensitive to further stimulation Absolute refractory period First part of the refractory period Membrane is completely insensitive to stimulus From beginning of action potential until near the end of repolarization Allows depolarization and repolarization phases to be completed before another action potential can begin Prevents a strong stimulus from causing prolonged depolarization of plasma membrane Relative refractory period Stronger than threshold stimulus required to initiate another action potential Membrane is more permeable to K+ Ends when voltage-gated K+ channels close and membrane potential returns to rest 64  potential returns to rest 64 Action Potential Frequency Number of action potentials per unit of time in response to a stimulus Directly proportional to stimulus strength and to the size of the graded potential Subthreshold stimulus – stimulus not strong enough to reach threshold, does not generate an action potential Threshold stimulus – graded potential just reaches threshold and causes a single action potential Submaximal stimulus – stimuli between threshold and maximal stimulus strength Maximal stimulus – strong enough to produce a maximum frequency of action potential Supramaximal stimulus – stimulus stronger than maximal stimulus, does not increase action potential frequency 65 Propagation of Action Potentials Involves the generation of a new action potential in adjacent region of the plasma membrane Action potentials are generated in the trigger zone and travel in one direction down the axon Types Continuous conduction Saltatory conduction 66 Continuous Conduction Occurs in unmyelinated axons Generates an action potential in each section of the plasma membrane An action potential in one section of membrane allows for Na+ to diffuse to adjacent areas (local current) causing depolarization A new identical action potential is generated in response to the depolarization 67 Saltatory Conduction Occurs in myelinated axons Action potential is conducted from one node of Ranvier to the next 68 67 68 Speed of Propagation Depends on: Myelination Thickness of myelin sheath Diameter of the axon 69 Check Point Questions Describe the concentration differences for sodium ions and potassium ions that exist across the plasma membrane Explain how the sodium-potassium pump works to move ions. Describe leak ion channels and gate ion channels. How are they responsible for permeability of a resting versus a stimulated plasma membrane? Define ligand, receptor, and receptor site. What kinds of stimuli cause gated ion channels to open or close? What is the resting membrane potential? What does it result from? Is the outside of the plasma membrane positively or negatively charged relative to the inside? Which ion is the major influence on the resting membrane potential? Explain its role. How does the resting membrane establish an equilibrium? What happens to cause depolarization and hyperpolarization? 70 Check Point Questions What is a graded potential and what events can cause it? What does it mean to say a graded potential can summate and then spread in a decremental fashion? How does an action potential differ from a graded potential? Explain “all or None” principle of action potentials. What happens during the depolarization and repolarization phases of an action potential? What happens when the activation gates in voltage-gated sodium channels open and the inactivation gate close? Describe the afterpotential and its cause. Describe the absolute and relative refractory period.  Describe the afterpotential and its cause. Describe the absolute and relative refractory period. What is action potential frequency and what factors determine it? Describe subthreshold, threshold, maximal, submaximal. And supramaximal stimuli. What is local current and how does it contribute to propagation of action potentials? Compare and contrast continuous and saltatory conduction. 71 Learning Objectives Describe the general structure and function of a synapse Distinguish between electrical and chemical synapses as to mode of operation and types of tissues where they are found Describe the release of a neurotransmitter in a chemical synapse Describe the removal of a neurotransmitter from a synapse Explain the effects of neurotransmitter binding to receptors in a chemical synapse Discuss the effects of neuromodulators in a chemical synapse Contrast excitatory and inhibitory postsynaptic potentials Explain the role of presynaptic inhibition Define facilitation Describe the process of spatial summation Describe the process of temporal summation 72 Synapse Composed of Presynaptic cell Postsynaptic cell Types Electrical synapses Chemical synapses 73 Electrical Synapses Occur between cells connected by gap junctions Allow ions to flow from one cell to the next Composed of connexons 6 tubular proteins (connexin) Not common in the nervous system 74 73 6 tubular proteins (connexin) Not common in the nervous system Found in cardiac muscle and some smooth muscle 74 Chemical Synapses Chemical messenger (neurotransmitter) is used to communicate between the cells Composed of Presynaptic terminal – axon terminal of the presynaptic cell that houses synaptic vesicles containing neurotransmitters Synaptic cleft – space separating the cells Postsynaptic membrane – membrane of the postsynaptic cell (neuron, muscle cell, gland cell) Release of neurotransmitter occurs due to action potential in the presynaptic terminal Voltage-gated Ca2+ channels open and Ca2+ entering the axon terminal triggers exocytosis of the neurotransmitter 75 Neurotransmitter Removal Neurotransmitter and receptor equilibrium: High concentration of neurotransmitter in synaptic cleft results in more receptor binding Rapid removal or destruction of neurotransmitter results in short term effects of neurotransmitter 76 Receptors in Synapses Located on the postsynaptic cell Can also be found on some presynaptic cells Highly specific Determine the affect the neurotransmitter has on a cell Neurotransmitters can stimulate some cells and inhibit other 77 Neuron Communication Graded potential Action potential Depolarization 78 77 Action potential Depolarization Repolarization Action potential propagation Synaptic communication 78 Neurotransmitters Chemical messengers released from neurons Some neurons can secrete more than one type of neurotransmitter Characteristics of neurotransmitters Must be synthesized by the neuron and stored in synaptic vesicles in presynaptic terminal Action potential must stimulate its exocytosis into the synaptic cleft Must bind to a specific receptor on the post synaptic membrane Must evoke a response in the postsynaptic cell 79 Neurotransmitters Classified based on Chemical structure Effect on postsynaptic membrane Mechanism of action at their target 80 Chemical Classification of Neurotransmitters Acetylcholine Synthesized from precursors acetic acid and choline Biogenic amines Catecholamines – derived from amino acid tyrosine, includes dopamine, norepinephrine, epinephrine Indoleamines – derived from histidine and tryptophan, includes histamine and serotonin Amino acids Includes GABA, glycine, glutamate Purines Nitrogen containing compounds Includes adenosine and ATP Neuropeptides 10-40 amino acids Includes substance P and endorphins Gases and lipids Gases (gasotransmitters) – nitric oxide (NO) and carbon monoxide 81  Gases and lipids Gases (gasotransmitters) – nitric oxide (NO) and carbon monoxide (CO) Lipids - endocannabinoids 81 Effect of Neurotransmitter on Postsynaptic Cell Excitatory Causes depolarization Makes cell more likely to generate an action potential Inhibitory Causes hyperpolarization Makes cell less likely to generate an action potential 82 Neurotransmitter Mechanisms of Action Ionotropic effect – binding to ion channels Metabotropic effect – binding to G protein-linked receptors 83 Postsynaptic Potentials Excitatory postsynaptic potential (EPSP) Depolarization Could generate an action potential Typically results from increase permeability of membrane to Na+ Inhibitory postsynaptic potential (IPSP) Hyperpolarization Do not generate action potentials Typically results from increase in the plasma membrane’s permeability to Cl- or K+ 84 Neuromodulators Substance released by neurons that influence the likelihood of an action potential being generated in the postsynaptic cell Axoaxonic synapses – axon of neuron synapses on the presynaptic terminal (axon) of another Allows for the release of a neuromodulator to influence the action of another neuron 85 Neuromodulation Presynaptic inhibition Amount of neurotransmitter released from presynaptic terminal is reduced Enkephalins and endorphins released by inhibitory axoaxonic 85 reduced Enkephalins and endorphins released by inhibitory axoaxonic synapses to reduce or eliminate pain sensation by blocking release of neurotransmitter from sensory neurons Presynaptic facilitation Amount of neurotransmitter released from presynaptic terminal is elevated Serotonin released from axoaxonic synapses increases release of neurotransmitter 86 Summation of Graded Potentials Generation of an action potential is determined by the sum of all graded potentials generated by stimulation of the neuron IPSPs EPSPs Spatial summation Multiple action potentials arrive at the same time from separate neurons Temporal summation Two or more action potentials arrive very close together from the same neuron 87 Check Point Questions What are the components of a synapses? What is the purpose of a synapse? What is an electrical synapse? Describe the release of neurotransmitter in a chemical synapse. Why does a give type of neurotransmitter affect only certain types of cells? How can a neurotransmitter stimulate one type of cell but inhibit another type? Explain the production of EPSPs and IPSPs. Why are they important? What is a neuromodulator? What are axoaxonic synapses? Give an example of presynaptic inhibition. Describe presynaptic facilitation. Distinguish between spatial summation and temporal summation. How do EPSPs and IPSPs affect the likelihood that summation will result in an action potential? 88 89  result in an action potential? 88 Learning Objectives Contrast convergent and divergent neuron pathways Describe a reverberating circuit Explain a parallel after-discharge circuit 89 Neural Pathways & Circuits Serial pathway Simplest organization Input travels along only one pathway Parallel pathway Most pathways More complex Input travels along several pathways Patterns of parallel pathways Convergent pathways Divergent pathways Reverberating circuits Parallel after-discharge circuits 90 Convergent Pathways Multiple neurons converge upon and synapse with a smaller number of neurons Allows different parts of the nervous system to activate or inhibit the activity of neurons 91 Divergent Pathways Smaller number of presynaptic neurons synapse with a larger number of postsynaptic neurons Allows information transmitted in one neuronal pathway to diverge into two or more pathways 92 Reverberating Circuits Chain of neurons with synapses with previous neurons in the chain Makes positive-feedback loop Allows action potentials entering the circuit to cause a neuron farther along in the circuit to produce an action potential more than once (after-discharge) to prolong response to stimulus Circuit will continue to discharge until the synapses are fatigued or 93  (after-discharge) to prolong response to stimulus Circuit will continue to discharge until the synapses are fatigued or inhibited by other neurons Control rhythmic activities 93 Parallel After-Discharge Circuits Neurons that stimulate several neurons in parallel organization All converge upon a common output cell Involved in complex neuronal processes 94 Check Point Question Diagram a convergent pathway, a divergent pathway, a reverberating circuit, and a parallel after-discharge circuit, and describe what is accomplished in each.