Anap 6 (Nervous Tissue).pdf

Nervous Tissue Copyright © John Wiley & Sons, Inc. All rights reserved. Organization and Functions of the Nervous System Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly acc...

Nervous Tissue Copyright © John Wiley & Sons, Inc. All rights reserved. Organization and Functions of the Nervous System Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 2 The Functions of the Nervous System Sensation—receiving information Integration—combining sensory information with higher cognitive functions Association areas accomplish this function Response—motor functions carried out by effectors Both conscious and unconscious nervous pathways exist Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 3  The Central and Peripheral Nervous Systems (Figure 13.1) Central Nervous System (CNS) Brain and spinal cord Housed within cranial cavity and vertebral cavity Peripheral Nervous System (PNS) Nerves outside of brain and spinal cord Outside of bony protection CNS and PNS are made of nervous tissue Neurons = cells capable of communication Glial cells = cells that provide structure and support to neurons Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 4  Functional Divisions of the Nervous System (Figure 13.2) Sensory—sends information toward CNS Afferent (sensory) neurons Integration—occurs in brain and spinal cord Interneurons Response—communicates with effectors Effector = muscle or organ that responds Achieved via efferent (motor) neurons Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 5 Classification Based on Innervation Based on type of location nerve innervates Somatic nervous system (SNS) Responsible for conscious perception and voluntary motor responses Innervates skeletal muscle Autonomic nervous system (ANS) Responsible for involuntary control of body Helps maintain homeostasis Innervates smooth muscle, cardiac muscle, and glands Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 6  Nervous System Overview ❖ The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events. ▪ It is responsible for all our behaviors, memories, and movement. It is able to accomplish all these functions because of the excitable characteristic of nervous tissue, which allows for the generation of nerve impulses (called action potentials). Copyright © John Wiley & Sons, Inc. All rights reserved. Nervous System Overview Copyright © John Wiley & Sons, Inc. All rights reserved.  Nervous System Overview ❖ Over 100 billion neurons and 10–50 times that number of support cells (called neuroglia) are organized into two main subdivisions: ▪ The central nevous system (CNS) ▪ The peripheral nervous system (PNS) Copyright © John Wiley & Sons, Inc. All rights reserved. Nervous Tissue and Cells Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 10  Anatomy of Neurons (1 of 2) Responsible for communication within nervous system Cell body—houses organelles like nucleus, nucleolus, ribosomes, and endoplasmic reticulum Dendrites—receive signals from other neurons Axon—begins at axon hillock Sends signals to other neurons Each neuron has one axon Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 11  Anatomy of Neurons (2 of 2) Synapses – junctions where neurons communicate with other cells Axons may be wrapped in myelin Gaps in myelin create neurofibril nodes Multiple axonal branches (axon terminals) allows a single neuron to communicate with multiple cells Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 12 Anatomical Classification of Neurons Unipolar neuron—only one process from cell body that splits into an axon and dendrites Most sensory neurons Bipolar neuron—two processes, one dendrite and one axon, extend from cell body Sensory for smell and vision Multipolar neuron—many dendrites and one axon Majority of neurons in body Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 13 Structural Categories of Neurons (Table 13.1) Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 14 Functional Classification of Neurons Sensory neurons—collect and send information to CNS Interneurons—integrate and process information from sensory neurons Motor neurons—communicate with effectors to make them perform an action Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 15  Glial Cells Supportive cells found throughout nervous system Can multiply and divide Glial cells of the CNS Astrocytes, oligodendrocytes, microglia, ependymal cells Glial cells of the PNS Satellite cells, Schwann cells Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 16 Glial Cells of the CNS (Figure 13.3) Astrocytes—regulate extracellular environment Make up blood-brain barrier (BBB) Oligodendrocytes—myelination Microglia—immune defense and waste removal Ependymal cells—produce cerebrospinal fluid (CSF) Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 17 Glial Cells of the PNS (Figure 13.4) Satellite cells—regulate extracellular environment Cluster around cell bodies Similar in function to astrocytes of CNS Schwann (neurilemma) cells— myelination Similar in function to oligodendrocytes of CNS Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 18 Glial Cell Types by Location and Basic Function (Table 13.2) Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 19  Nervous System Overview ❖ A “typical neuron is shown in this graphic. ❖ The Schwann cell depicted in red/purple is one of 6 types of neuroglia that we will study shortly. blue: neuron red/purple: glial cell Photomicrograph showing neurons in a network. Copyright © John Wiley & Sons, Inc. All rights reserved.  Nervous System Overview ❖ As the “thinking” cells of the brain, each neuron does, in miniature, what the entire nervous system does as an organ: Receive, process and transmit information by manipulating the flow of charge across their membranes. ❖ Neuroglia (glial cells) play a major role in support and nutrition of the brain, but they do not manipulate information. ▪ They maintain the internal environment so that neurons can do their jobs. Copyright © John Wiley & Sons, Inc. All rights reserved.  Divisions of the Nervous System ❖ The central nervous system (CNS) consists of the brain and spinal cord. ❖ The peripheral nervous system (PNS) consists of all nervous tissue outside the CNS, including nerves, ganglia, enteric plexuses, and sensory receptors. Copyright © John Wiley & Sons, Inc. All rights reserved.  Divisions of the Nervous System ❖ Most signals that stimulate muscles to contract and glands to secrete originate in the CNS. ❖ The PNS is further divided into: ▪ A somatic nervous system (SNS) ▪ An autonomic nervous system (ANS) ▪ An enteric nervous system (ENS) Copyright © John Wiley & Sons, Inc. All rights reserved.  Divisions of the Nervous System ❖ The SNS consists of: ▪ Somatic sensory (afferent) neurons that convey information from sensory receptors in the head, body wall and limbs towards the CNS. ▪ Somatic motor (efferent) neurons that conduct impulses away from the CNS towards the skeletal muscles under voluntary control in the periphery. ▪ Interneurons are any neurons that conduct impulses between afferent and efferent neurons within the CNS. Copyright © John Wiley & Sons, Inc. All rights reserved.  Divisions of the Nervous System ❖ The ANS consists of: 1. Sensory neurons that convey information from autonomic sensory receptors located primarily in visceral organs like the stomach or lungs to the CNS. 2. Motor neurons under involuntary control conduct nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands. The motor part of the ANS consists of two branches which usually have opposing actions: the sympathetic division the parasympathetic division Copyright © John Wiley & Sons, Inc. All rights reserved.  Divisions of the Nervous System ❖ The operation of the ENS, the “brain of the gut”, involuntarily controls GI propulsion, and acid and hormonal secretions. ▪ Once considered part of the ANS, the ENS consists of over 100 million neurons in enteric plexuses that extend most of the length of the GI tract. Copyright © John Wiley & Sons, Inc. All rights reserved. Divisions of the Nervous System Copyright © John Wiley & Sons, Inc. All rights reserved.  Divisions of the Nervous System ❖ Ganglia are small masses of neuronal cell bodies located outside the brain and spinal cord, usually closely associated with cranial and spinal nerves. ▪ There are ganglia which are somatic, autonomic, and enteric (that is, they contain those types of neurons.) Copyright © John Wiley & Sons, Inc. All rights reserved.  Neurons and Neuroglia ❖ Neurons and neuroglia combine in a variety of ways in different regions of the nervous system. ▪ Neurons are the real “functional unit” of the nervous system, forming complex processing networks within the brain and spinal cord that bring all regions of the body under CNS control. ▪ Neuroglia, though smaller than neurons, greatly outnumber them. They are the “glue” that supports and maintains the neuronal networks. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neurons ❖ Though there are several different types of neurons, most have: ▪ A cell body ▪ An axon ▪ Dendrites ▪ Axon terminals Copyright © John Wiley & Sons, Inc. All rights reserved.  Neurons ▪ Neurons gather information at dendrites and process it in the dendritic tree and cell body. ▪ Then they transmit the information down their axon to the axon terminals. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neurons ❖ Dendrites (little trees) are the receiving end of the neuron. ▪ They are short, highly branched structures that conduct impulses toward the cell body. ▪ They also contain organelles. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neurons ❖ The cell body has a nucleus surrounded by cytoplasm. ▪ Like all cells, neurons contain organelles such as lysosomes, mitochondria, Golgi complexes, and rough ER for protein production (in neurons, RER is called Nissl bodies) – it imparts a striped “tiger appearance”. ▪ No mitotic apparatus is present. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neurons ❖ Axons conduct impulses away from the cell body toward another neuron or effector cell. ▪ The “axon hillock” is where the axon joins the cell body. ▪ The “initial segment” is the beginning of the axon. ▪ The “trigger zone” is the junction between the axon hillock and the initial segment. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neurons ❖ The axon and its collaterals end by dividing into many fine processes called axon terminals (telodendria). Like the dendrites, telodendria may also be highly branched as they interact with the dendritic tree of neurons “downstream”. ▪ The tips of some axon terminals swell into bulb-shaped structures called synaptic end bulbs. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neurons ❖ The site of communication between two neurons or between a neuron and another effector cell is called a synapse. ▪ The synaptic cleft is the gap between the pre and post-synaptic cells. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neurons ❖ Synaptic end bulbs and other varicosities on the axon terminals of presynaptic neurons contain many tiny membrane-enclosed sacs called synaptic vesicles that store packets of neurotransmitter chemicals. ▪ Many neurons contain two or even three types of neuro- transmitters, each with different effects on the postsynaptic cell. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neurons ❖ Electrical impulses or action potentials (AP) cannot propagate across a synaptic cleft. Instead, neurotransmitters are used to communicate at the synapse, and re-establish the AP in the postsynaptic cell. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neurons ❖ Substances synthesized or recycled in the neuron cell body are needed in the axon or at the axon terminals. Two types of transport systems carry materials from the cell body to the axon terminals and back. ▪ Slow axonal transport conveys axoplasm in one direction only – from the cell body toward the axon terminals. ▪ Fast axonal transport moves materials in both directions. Copyright © John Wiley & Sons, Inc. All rights reserved.  Classifying Neurons ❖ Structural classification is based on the number of processes (axons or dendrites) extending from the cell body. Copyright © John Wiley & Sons, Inc. All rights reserved.  Classifying Neurons ❖ Multipolar neurons have several dendrites and only one axon and are located throughout the brain and spinal cord. ▪ The vast majority of the neurons in the human body are multipolar. Copyright © John Wiley & Sons, Inc. All rights reserved.  Classifying Neurons ❖ Bipolar neurons have one main dendrite and one axon. ▪ They are used to convey the special senses of sight, smell, hearing and balance. As such, they are found in the retina of the eye, the inner ear, and the olfactory (olfact = to smell) area of the brain. Copyright © John Wiley & Sons, Inc. All rights reserved.  Classifying Neurons ❖ Unipolar (pseudounipolar) neurons contain one process which extends from the body and divides into a central branch that functions as an axon and as a dendritic root. ▪ Unipolar structure is often employed for sensory neurons that convey touch and stretching information from the extremities. Copyright © John Wiley & Sons, Inc. All rights reserved.  Classifying Neurons ❖ The functional classification of neurons is based on electrophysiological properties (excitatory or inhibitory) and the direction in which the AP is conveyed with respect to the CNS. ▪ Sensory or afferent neurons convey APs into the CNS through cranial or spinal nerves. Most are unipolar. ▪ Motor or efferent neurons convey APs away from the CNS to effectors (muscles and glands) in the periphery through cranial or spinal nerves. Most are multipolar. Copyright © John Wiley & Sons, Inc. All rights reserved.  Classifying Neurons ❖ The functional classification continued… ▪ Interneurons or association neurons are mainly located within the CNS between sensory and motor neurons. Interneurons integrate (process) incoming sensory information from sensory neurons and then elicit a motor response by activating the appropriate motor neurons. Most interneurons are multipolar in structure. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neuroglia ❖ Neuroglia do not generate or conduct nerve impulses. ▪ They support neurons by: Forming the Blood Brain Barrier (BBB) Forming the myelin sheath (nerve insulation) around neuronal axons Making the CSF that circulates around the brain and spinal cord Participating in phagocytosis Copyright © John Wiley & Sons, Inc. All rights reserved.  Neuroglia ❖ There are 4 types of neuroglia in the CNS: ▪ Astrocytes - support neurons in the CNS Maintain the chemical environment (Ca2+ & K+) ▪ Oligodendrocytes - produce myelin in CNS ▪ Microglia - participate in phagocytosis ▪ Ependymal cells - form and circulate CSF ❖ There are 2 types of neuroglia in the PNS: ▪ Satellite cells - support neurons in PNS ▪ Schwann cells - produce myelin in PNS Copyright © John Wiley & Sons, Inc. All rights reserved. Neuroglia Copyright © John Wiley & Sons, Inc. All rights reserved.  Neuroglia ❖ In the CNS: Copyright © John Wiley & Sons, Inc. All rights reserved.  Neuroglia ❖ In the PNS: Copyright © John Wiley & Sons, Inc. All rights reserved.  Neuroglia ❖ Myelination is the process of forming a myelin sheath which insulates and increases nerve impulse speed. ▪It is formed by Oligodendrocytes in the CNS and by Schwann cells in the PNS. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neuroglia ❖ Nodes of Ranvier are the gaps in the myelin sheath. ▪ Each Schwann cell wraps one axon segment between two nodes of Ranvier. Myelinated nodes are about 1 mm in length and have up to 100 layers. ❖ The amount of myelin increases from birth to maturity, and its presence greatly increases the speed of nerve conduction. ▪ Diseases like Multiple Sclerosis result from autoimmune destruction of myelin. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neuronal Regeneration ❖ The cell bodies of neurons lose their mitotic features at birth and can only be repaired through regeneration after an injury (they are never replaced by daughter cells as occurs with epithelial tissues.) ❖ Nerve tissue regeneration is largely dependent on the Schwann cells in the PNS and essentially doesn’t occur at all in the CNS where astrocytes just form scar tissue. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neuronal Regeneration ❖ The outer nucleated cytoplasmic layer of the Schwann cell, which encloses the myelin sheath, is the neurolemma (sheath of Schwann). ▪ When an axon is injured, the neurolemma aids regeneration by forming a regeneration tube that guides and stimulates regrowth of the axon. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neuronal Regeneration ❖ To do any regeneration, neurons must be located in the PNS, have an intact cell body, and be myelinated by functional Schwann cells having a neurolemma. ▪ Demyelination refers to the loss or destruction of myelin sheaths around axons. It may result from disease, or from medical treatments such as radiation therapy and chemotherapy. Any single episode of demyelination may cause deterioration of affected nerves. Copyright © John Wiley & Sons, Inc. All rights reserved.  Gray and White Matter ❖ White matter of the brain and spinal cord is formed from aggregations of myelinated axons from many neurons. ▪ The lipid part of myelin imparts the white appearance. ❖ Gray matter (gray because it lacks myelin) of the brain and spinal cord is formed from neuronal cell bodies and dendrites. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neurophysiology Section 13.3 Learning Objectives 13.3.1–13.3.12 Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 57 Communication Within the Nervous System Once a stimulus is detected, communication depends on electrical signaling Occurs due to the movement of ions Ion movement generates action potentials Action potentials lead to release of neurotransmitters Chemicals that relay messages from neurons Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 58  Membrane Potentials (Figure 13.6) The cell membrane is a barrier to ionic movement Different charges can build up inside and outside of neurons Sodium/potassium (Na+/K+) pumps play a key role Pumps three sodium ions out of cell and two potassium ions into cell Creates a relatively negative internal environment of neuron Membrane becomes polarized Inside and outside of cell have different charges Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 59  Resting Membrane Potential Resting membrane potential of neuron is −70 mV Established by: Unequal distribution of Na+ and K+ ions across cell membrane From activities of sodium/potassium pumps Negatively charged proteins inside of cell Make interior of neuron more negative Exit of K+ ions due to leak channels Further loss of positive charges from interior of cell Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 60  The Na+/K+ Pump (Figure 13.7) Plays critical role in resting membrane potential of neurons Pumps 3 Na+ ions out of cell and 2 K+ ions into cell using energy from ATP Builds chemical and electrical gradient across membrane Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 61  Changes in Resting Membrane Potential As ions flow into and out of neurons, membrane potential changes Changes that cause the charge difference to decrease are called depolarizing Changes that cause the charge difference to increase are called hyperpolarizing Repolarization occurs if depolarization is followed by a return to a polarized state Changes can be caused by ions channels that allow ions to move Some channels are open and allow ions to freely move Some channels open and close in response to various stimuli Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 62  Classes of Membrane Channels Ligand-gated channels—open and close due to binding of a molecule (ligand) Mechanically-gated channels—open and close in response to pressure Voltage-gated channels—open and close in response to changes in electrical potential Leak channels—always open or randomly open and close No single stimulus influences their activity Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 63 Ligand-Gated Channels (Figure 13.8) Ligand-gated channels— open and close due to binding of an extracellular messenger molecule (ligand) Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 64  Mechanically-Gated Ion Channels (Figure 13.9) Mechanically-gated channels—open and close in response to pressure Detected by distortions in cell membrane Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 65 Voltage-Gated Channels (Figure 13.10) Voltage-gated channels— open and close in response to changes in electrical potential A change in membrane potential can open or close voltage-gated channels Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 66  Graded Potentials (1 of 2) Small changes in resting membrane potential Can vary in size Caused by mechanically-gated and ligand-gated membrane channels Occur along dendrites and cell body Membrane channels open to allow Na+ ions to enter neuron Depolarization occurs as inside of neuron becomes more positively charged Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 67  Graded Potentials (2 of 2) Graded potentials are additive—smaller graded potentials can add together; this is called summation Depolarization of graded potentials—move membrane toward threshold Na+ or Ca++ usually enters neuron Hyperpolarization graded potentials—move membrane away from threshold K+ may exit or Cl− enters neuron If threshold is reached, action potential is guaranteed to move down axon Threshold value is −55 mV Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 68  Types of Graded Potentials Postsynaptic potential (PSP)—graded potentials occurring in neuron that received signals Excitatory (EPSP)—moves membrane toward threshold Depolarizes membrane Inhibitory (IPSP)—moves membrane away from threshold Hyperpolarizes membrane Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 69  Summation (Figure 13.12) Spatial summation—graded potentials occurring at several different synapses over short timeframe Temporal summation— graded potentials occur at one synapse over short timeframe Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 70  The Action Potential (1 of 3) Begins at axon hillock and travels toward axon terminals Membrane channels in dendrites and cell body respond to stimuli Produce graded potentials Depolarizing graded potentials allow positively charged sodium ions (Na+) to enter neuron Depolarization occurs as sodium ions make interior of neuron more positively charged Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 71  The Action Potential (2 of 3) If graded potentials depolarize the cell body to threshold, action potential is generated Threshold is −55 mV Once threshold is reached, voltage-gated sodium and potassium channels open Sodium enters rapidly and depolarizes neuron At +30mV, sodium-voltage gated channels close and potassium-voltage gated channels fully open Sodium no longer enters neuron and potassium begins to exit This begins repolarization Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 72  The Action Potential (3 of 3) Repolarization reestablishes resting membrane potential Potassium exits via potassium-voltage gated channels Membrane potential becomes more negative as a result of loss of K+ Membrane may hyperpolarize as excess potassium exits Na+/K+ pumps will bring membrane potential back to −70 mV, by removing sodium from cell and pumping K+ back into cell Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 73  Conformations of the Na+ Voltage-Gated Channels (Figure 13.11) Prior to threshold, Na+ voltage- gated channels are closed If threshold is reached, they quickly open allowing Na+ to quickly enter cell leading to depolarization Inactivation gates on Na+ voltage- gated channels allow them to quickly close aiding in repolarization Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 74 Anatomy of an Action Potential Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 75  Refractory Periods (Figure 13.13) The period after an action potential is generated and before another can begin Absolute refractory period—no action potential possible Relative refractory period— second action potential possible with strong stimulus Membrane potential must be between −55 mV and −70 mV Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 76  Inactivation of Na+ Voltage-Gated Channels (Figure 13.14) Na voltage-gated channels close when the membrane potential reaches +30 mV Inactivation gates stop inflow of Na+ Stops depolarization Inactivation gates not required on K+ voltage-gated channels K+ exits cell leading to repolarization Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 77  Propagation of an Action Potential down an Unmyelinated Axon (Figure 13.15) “All or nothing” event Propagation in unmyelinated axon is continuous conduction Na+ ions gather at axon hillock Na+ voltage-gated channels open and Na+ ions flow into neuron Each section of axon depolarizes in sequence Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 78  Action Potential Propagation down a Myelinated Axon (Figure 13.16) Propagation in myelinated axons is saltatory conduction Faster than continuous conduction Na+ ions gather at axon hillock Na+ ions move toward axon terminals Only depolarize neurofibril nodes Myelinated areas lack voltage-gated channels Myelin insulates sections of axon, preventing loss of Na+ ions Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 79  Speed of Action Potential Propagation (Figure 13.17) Speed of action potential movement is influenced by Myelination Faster conduction in myelinated versus unmyelinated axons Size of electrochemical gradient Diameter of axon Faster in larger axons Larger axons have less resistance to ion movement Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 80  Electrical Signals in Neurons ❖ Like muscle fibers, neurons are electrically excitable. They communicate with one another using two types of electrical signals: ▪ Graded potentials are used for short-distance communication only. ▪ Action potentials allow communication over long distances within the body. Copyright © John Wiley & Sons, Inc. All rights reserved.  Electrical Signals in Neurons ❖ Producing electrical signals in neurons depends on the existence of a resting membrane potential (RMP) - similar to the electrical potential of this 9 v battery which has a gradient of 9 volts from one terminal to another. ▪ A cell’s RMP is created using ion gradients and a variety of ion channels that open or close in response to specific stimuli. ▪ Because the lipid bilayer of the plasma membrane is a good insulator, ions must flow through these channels. Copyright © John Wiley & Sons, Inc. All rights reserved.  Electrical Signals in Neurons ❖ Ion channels are present in the plasma membrane of all cells in the body, but they are an especially prominent component of the nervous system. ❖ Much of the energy expended by neurons, and really all cells of the body, is used to create a net negative charge in the inside of the cell as compared to the outside of the cell. A cell’s RMP is created using ion channels to set-up transmembrane ion gradients. Copyright © John Wiley & Sons, Inc. All rights reserved. Copyright © John Wiley & Sons, Inc. All rights reserved.  Electrical Signals in Neurons ❖ When ion channels are open, they allow specific ions to move across the plasma membrane, down their electrochemical gradient. ▪ Ions move from areas of higher concentration to areas of lower concentration - the “chemical” (concentration) part of the gradient. ▪ Positively charged cations move toward a negatively charged area, and negatively charged anions move toward a positively charged area - the electrical aspect of the gradient. Copyright © John Wiley & Sons, Inc. All rights reserved.  Electrical Signals in Neurons ❖ Active channels open in response to a stimulus (they are “gated”). There are 3 types of active, gated channels: ▪ Ligand-gated channels respond to a neurotransmitter and are mainly concentrated at the synapse. ▪ Voltage-gated channels respond to changes in the transmembrane electrical potential and are mainly located along the neuronal axon. ▪ Mechanically-gated channels respond to mechanical deformation (applying pressure to a receptor). ❖ “Leakage” channels are also gated but they are not active, and they open and close randomly. Copyright © John Wiley & Sons, Inc. All rights reserved.  Action Potentials ❖ An AP has two main phases: ▪ a depolarizing phase and ▪ a repolarizing phase Copyright © John Wiley & Sons, Inc. All rights reserved. Copyright © John Wiley & Sons, Inc. All rights reserved.  Action Potentials ❖ Graded potentials that result in depolarization of the neuron from –70mV to threshold (about –55 mV in many neurons) will cause a sequence of events to rapidly unfold. ▪ Voltage-gated Na+ channels open during the steep depolarization phase allowing Na+ to rush into the cell and making the inside of the cell progressively more positive. Copyright © John Wiley & Sons, Inc. All rights reserved.  Action Potentials ❖ Only a total of 20,000 Na+ actually enter the cell in each little area of the membrane, but they change the potential considerably (up to +30mV). ❖ During the repolarization, phase K+ channels open and K+ rushes outward. ▪ The cell returns to a progressively more negative state until the RMP of –70mV is once again restored. Copyright © John Wiley & Sons, Inc. All rights reserved. Copyright © John Wiley & Sons, Inc. All rights reserved. Copyright © John Wiley & Sons, Inc. All rights reserved.  Action Potentials ❖ While the voltage-gated K+ channels are open, outflow of K+ may be large enough to cause an after- hyperpolarizing phase of the action potential. ▪ During this phase, the voltage-gated K+ channels remain open and the membrane potential becomes even more negative (about –90 mV). As the voltage-gated K+ channels close, the membrane potential returns to the resting level of –70 mV. Copyright © John Wiley & Sons, Inc. All rights reserved.  Action Potentials ❖ According to the all-or-none principle, if a stimulus reaches threshold, the action potential is always the same. ▪ A stronger stimulus will not cause a larger impulse. Copyright © John Wiley & Sons, Inc. All rights reserved.  Action Potentials ❖ After initiating an action potential, there is a period of time called the absolute refractory period during which a cell cannot generate another AP, no matter how strong the stimulus. ▪ This period coincides with the period of Na+ channel activation and inactivation (inactivated Na+ channels must first return to the resting state.) This places an upper limit of 10–1000 nerve impulses per second, depending on the neuron. Copyright © John Wiley & Sons, Inc. All rights reserved.  Action Potentials ❖ The relative refractory period is the period of time during which a second action potential can be initiated, but only by a larger-than-normal stimulus. ▪ It coincides with the period when the voltage-gated K+ channels are still open after inactivated Na+ channels have returned to their resting state. ❖ In contrast to action potentials, graded potentials do not exhibit a refractory period. Copyright © John Wiley & Sons, Inc. All rights reserved.  Action Potentials ❖ Propagation of the AP down the length of the axon begins at the trigger zone near the axon hillock. ▪ By passive spread, the current proceeds by (a) continuous conduction in unmyelinated axons, or by the much faster process of (b) saltatory conduction in myelinated axons (as the AP jumps from one node to the next as shown in this graphic). Copyright © John Wiley & Sons, Inc. All rights reserved.  Action Potentials ❖ In addition to the nodes of Ranvier that allow saltatory conduction, the speed of an AP is also affected by: ▪ The axon diameter ▪ The amount of myelination ▪ The temperature ❖ The frequency of AP plays a crucial role in determining the perception of a stimulus, or the extent of our response. ▪ In addition to this “frequency code,” a second important factor is the number of neurons recruited (activated) to the cause. Copyright © John Wiley & Sons, Inc. All rights reserved.  Fiber Types ❖ The characteristics of the neuronal axon define the “fiber types” ▪ A fibers are large, fast (130 m/sec), myelinated neurons that carry touch and pressure sensations; many motor neurons are also of this type. ▪ B fibers are of medium size and speed (15 m/sec) and comprise myelinated visceral sensory & autonomic preganglionic neurons. ▪ C fibers are the smallest and slowest (2 m/sec) and comprise unmyelinated sensory and autonomic motor neurons. Copyright © John Wiley & Sons, Inc. All rights reserved.  Synaptic Transmission ❖ Signal transmission at the synapse is a one-way transfer from a presynaptic neuron to a postsynaptic neuron. ▪ When an AP reaches the end bulb of axon terminals, voltage-gated Ca2+ channels open and Ca2+ flows inward, triggering release of the neurotransmitter. ▪ The neurotransmitter crosses the synaptic cleft and binds to ligand-gated receptors on the postsynaptic membrane. The more neurotransmitter released, the greater the number and intensity of graded potentials in the postsynaptic cell. Copyright © John Wiley & Sons, Inc. All rights reserved.  Synaptic Transmission ❖ In this way, the presynaptic neuron converts an electrical signal (nerve impulse) into a chemical signal (released neurotransmitter). The postsynaptic neuron receives the chemical signal and in turn generates an electrical signal (postsynaptic potential). ❖ The time required for these processes at a chemical synapse produces a synaptic delay of about 0.5 msec. Copyright © John Wiley & Sons, Inc. All rights reserved.  Synaptic Transmission The events that occur at a synapse are outlined above. Copyright © John Wiley & Sons, Inc. All rights reserved.  Communication between Neurons Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 103  Synapses (Figure 13.18) Areas where neurons communicate Chemical synapses—release neurotransmitters Electrical synapses—direct connections where ions move from one cell to another Less common in the human nervous system Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 104 Copyright © John Wiley & Sons, Inc. All rights reserved. Components of a Chemical Synapse Presynaptic cell Neurotransmitter Synaptic cleft Receptors for neurotransmitter Postsynaptic cell A system for clearing neurotransmitter from synapse Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 106 Synaptic Events (1 of 2) (Figure 13.19) Action potential reaches axon terminal Calcium voltage-gated channels open Ca2+ ions enter synaptic end bulb Ca2+ causes synaptic vesicles to fuse with synaptic end bulb Neurotransmitter is released Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 107 Synaptic Events (2 of 2) (Figure 13.19) Neurotransmitter binds to receptors on postsynaptic neuron Causes graded potential Neurotransmitter eliminated from synapse by: 1. Diffusion 2. Reuptake 3. Breakdown Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 108 Copyright © John Wiley & Sons, Inc. All rights reserved.  Anatomy of a Synapse Events of chemical synaptic transmission Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 110  Neurotransmitters (1 of 2) Each neuron only releases one neurotransmitter Categories of Neurotransmitters: Cholinergic Acetylcholine Amino acids Glutamate, GABA (inhibitory), glycine Biogenic amines Serotonin, dopamine, epinephrine, norepinephrine Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 111  Neurotransmitters (2 of 2) Effect depends on receptor Same neurotransmitter can have different effects on different cells Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 112  Cholinergic Neurotransmitters Acetylcholine is released by cholinergic neurons Acts on two types of receptors: Nicotinic receptors – found at NMJ, adrenal medulla, and some autonomic synapses Muscarinic receptors – found at autonomic synapses Elimination by acetylcholinesterase Enzyme that breaks down acetylcholine Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 113 Biogenic Amine Neurotransmitters Made from amino acids Serotonin, dopamine, norepinephrine and epinephrine Each has its own membrane receptors Elimination from synapse by reuptake Serotonin reuptake can be blocked by selective serotonin reuptake inhibitors (SSRIs) Used in treatment of depression and anxiety Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 114  Amino Acid Neurotransmitters Include glutamate, GABA (gamma-aminobutyric acid) and glycine Released by glutamatergic, GABAergic, and glycinergic neurons Each has its own receptors Eliminated from synapse by reuptake GABA leads to IPSPs Receptors are Cl− channels that hyperpolarize membrane Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 115 Characteristics of Neurotransmitter Systems (Table 13.3) Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 116 Copyright © John Wiley & Sons, Inc. All rights reserved. Copyright © John Wiley & Sons, Inc. All rights reserved. Copyright © John Wiley & Sons, Inc. All rights reserved. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neurotransmitters ❖ Many amino acids act as neurotransmitters: ▪ Glutamate is released by nearly all excitatory neurons in the brain. ▪ GABA is an inhibitory neuro- transmitter for 1/3 of all brain synapses. Valium is a GABA agonist that enhances GABA’s depressive effects (causes sedation). ▪ Other important small- molecule neurotransmitters are listed. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neurotransmitters ❖ Neurotransmitter effects can be modified in many ways: ▪ Synthesis can be stimulated or inhibited. ▪ Release can be blocked or enhanced. ▪ Removal can be stimulated or blocked. ▪ The receptor site can be blocked or activated. An agonist is any chemical that enhances or stimulates the effects at a given receptor. An antagonist is a chemical that blocks or diminishes the effects at a given receptor. Copyright © John Wiley & Sons, Inc. All rights reserved.  Postsynaptic Potentials ❖ A neurotransmitter causes either an excitatory or an inhibitory graded potential: ▪ Excitatory postsynaptic potential (EPSP) causes a depolarization of the postsynaptic cell, bringing it closer to threshold. Although a single EPSP normally does not initiate a nerve impulse, the postsynaptic cell does become more excitable. ▪ Inhibitory postsynaptic potential (IPSP) hyperpolarizes the postsynaptic cell taking it farther from threshold. Copyright © John Wiley & Sons, Inc. All rights reserved.  Postsynaptic Potentials ❖ Spatial summation occurs when postsynaptic potentials arrive near the same location. Temporal summation occurs when postsynaptic potentials arrive close to the same time. ❖ Whether or not the postsynaptic cell reaches threshold depends on the net effect after Summation of all the postsynaptic potentials. Copyright © John Wiley & Sons, Inc. All rights reserved. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neurotransmitter Clearance ❖ If a neurotransmitter could linger in the synaptic cleft, it would influence the postsynaptic neuron, muscle fiber, or gland cell indefinitely – removal of the neurotransmitter is essential for normal function. ▪ Removal is accomplished by diffusion out of the synaptic cleft, enzymatic degradation, and re-uptake by cells. An example of a common neurotransmitter inactivated through enzymatic degradation is acetylcholine. The enzyme acetylcholinesterase breaks down acetylcholine in the synaptic cleft. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neural Circuits ❖ Neurons process information when changes occur at the trigger zone through spatial and temporal summation of IPSPs & EPSPs. ▪ An “average” neuron receives 10,000 synaptic inputs - multiply this by the number of neurons involved in any single process, and you can start to comprehend the exquisite level of information processing afforded by this system. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neural Circuits ❖ Integration is the process accomplished by the post- synaptic neuron when it combines all excitatory and inhibitory inputs and responds accordingly. ❖ This process occurs over and over as interneurons are activated in higher parts of the brain (such as the thalamus and cerebral cortex). Copyright © John Wiley & Sons, Inc. All rights reserved.  Neural Circuits ❖ A neuronal network may contain thousands or even millions of neurons. ▪ Types of circuits include diverging, converging, reverberating, and parallel after-discharge. Copyright © John Wiley & Sons, Inc. All rights reserved.  Neural Circuits ❖ In a diverging circuit, a small number of neurons in the brain stimulate a much larger number of neurons in the spinal cord. A converging circuit is the opposite. ❖ In a reverberating circuit, impulses are sent back through the circuit time and time again – used in breathing, coordinated muscular activities, waking up, and short-term memory. ❖ Parallel after-discharge circuits involve a single presynaptic cell that stimulates a group of neurons, which then synapse with a common postsynaptic cell – used in precise activities such as mathematical calculations. Copyright © John Wiley & Sons, Inc. All rights reserved.  Thank you Elizabeth Co, Anatomy and Physiology, 1st Edition. © 2023 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 131