29 - Collin College

29 - Collin College

Nervous Tissue Fundamentals of the Nervous P A R and T A System Nervous Tissue 1 Nervous System

The master controlling and communicating system of the body Functions Sensory input the stimuli goes to CNS Integration interpretation of sensory input Motor output response to stimuli coming from CNS 2 Nervous System 3

Figure 11.1 Organization of the Nervous System Central nervous system (CNS) Brain and spinal cord Integration and command center Peripheral nervous system (PNS) Paired spinal and cranial nerves

Carries messages to and from the spinal cord and brain 4 Peripheral Nervous System (PNS): Two Functional Divisions Sensory (afferent) division Somatic sensory fibers carry impulses from skin, skeletal muscles, and joints to the brain Visceral sensory fibers transmit impulses from visceral organs to the

brain 5 Motor Division: Two Main Parts Motor (efferent) division Somatic motor nervous system (voluntary)- carries impulses from the CNS to skeletal muscles. Conscious control 6

Motor Division: Two Main Parts Visceral motor nervous system or Autonomic nervous system (ANS) (involuntary) - carry impulses from the CNS to smooth muscle, cardiac muscle, and glands Sympathetic and Parasympathetic Enteric Nervous System (ENS) 7

Histology of Nerve Tissue The two principal cell types of the nervous system are: Neurons excitable cells that transmit electrical signals Neuroglias (glial) cells that surround and wrap neurons 8 Supporting Cells: Neuroglia

Neuroglia Provide a supportive scaffolding for neurons Segregate and insulate neurons Guide young neurons to the proper connections Promote health and growth 9 Neuroglia of the CNS

Astrocytes Most abundant, versatile, and highly branched glial cells Maintain blood-brain barrier They cling to neurons and their synaptic endings, They wrap around capillaries Regulate their permeability 10 Neuroglia of the CNS

Provide structural framework for the neuron Guide migration of young neurons Control the chemical environment Repair damaged neural tissue 11

Astrocytes 12 Figure 11.3a Neuroglia of the CNS Microglia small, ovoid cells with spiny processes Phagocytes that monitor the health of neurons

Ependymal cells range in shape from squamous to columnar They line the central cavities of the brain and spinal column 13 Microglia and Ependymal Cells 14 Figure 11.3b, c Neuroglia of the CNS

Oligodendrocytes branched cells Myelin Wraps of oligodendrocytes processes around nerve fibers Insulates the nerve fibers 15 Neuroglia of the PNS Schwann cells (neurolemmocytes)

Myelin Wraps itself around nerve fibers Insulates the nerve fibers Satellite cells surround neuron cell bodies located within the ganglia Regulate the environment around the neurons 16 Oligodendrocytes, Schwann Cells, and Satellite Cells 17 Figure 11.3d, e

Neurons (Nerve Cells) Structural units of the nervous system Composed of a body, axon, and dendrites Long-lived, amitotic, and have a high metabolic rate Their plasma membrane function in: Electrical signaling

Cell-to-cell signaling during development 18 Neurons (Nerve Cells) 19 Figure 11.4b Nerve Cell Body (Perikaryon or Soma)

Contains the nucleus and a nucleolus Is the major biosynthetic center Is the focal point for the outgrowth of neuronal processes Has no centrioles (hence its amitotic nature) Has well-developed Nissl bodies (rough ER) Contains an axon hillock coneshaped area from which axons arise20

Processes Arm like extensions from the soma There are two types of processes: axons and dendrites Myelinated axons are called tracts in the CNS and nerves in the PNS 21

Dendrites: Structure Short, tapering processes They are the receptive, or input, regions of the neuron Electrical signals are conveyed as graded potentials (not action potentials)

22 Neurons (Nerve Cells) 23 Figure 11.4b Axons: Structure

Slender processes of uniform diameter arising from the hillock Long axons are called nerve fibers Usually there is only one unbranched axon per neuron Axon collaterals Telodendria Axonal terminal or synaptic knobs 24

Axons: Function Generate and transmit action potentials Secrete neurotransmitters from the axonal terminals Movement along axons occurs in two ways Anterograde toward the axon

terminal Retrograde toward the cell body 25 Myelin Sheath Whitish, fatty (protein-lipoid), segmented sheath around most long axons It functions to:

Protect the axon Electrically insulate fibers from one another Increase the speed of nerve impulse transmission 26 Myelin Sheath and Neurilemma: Formation Formed by Schwann cells in the PNS A Schwann cell:

Envelopes an axon Encloses the axon with its plasma membrane Has concentric layers of membrane that make up the myelin sheath 27 Myelin Sheath and Neurilemma: Formation 28 Figure 11.5ac

Nodes of Ranvier (Neurofibral Nodes) Gaps in the myelin sheath between adjacent Schwann cells They are the sites where axon collaterals can emerge 29 Unmyelinated Axons

A Schwann cell surrounds nerve fibers but coiling does not take place Schwann cells partially enclose 15 or more axons Conduct nerve impulse slowly 30

Axons of the CNS Both myelinated and unmyelinated fibers are present Myelin sheaths are formed by oligodendrocytes Nodes of Ranvier are widely spaced

31 Regions of the Brain and Spinal Cord White matter dense collections of myelinated fibers Gray matter mostly soma, dendrites, glial cells and unmyelinated fibers

32 Neuron Classification Structural: Multipolar three or more processes Bipolar two processes (axon and dendrite) Unipolar single, short process 33

A Structural Classification of Neurons 34 Figure 12.4 Neuron Classification Functional: Sensory (afferent) transmit impulses toward the CNS Motor (efferent) carry impulses toward the body surface

Interneurons (association neurons) any neurons between a sensory and a motor neuron 35 Comparison of Structural Classes of Neurons 36 Table 11.1.1 Comparison of Structural Classes of Neurons

37 Table 11.1.2 Neurophysiology Neurons are highly irritable Action potentials, or nerve impulses, are: Electrical impulses carried along the length of axons Always the same regardless of stimulus

38 Electricity Definitions Voltage (V) measure of potential energy generated by separated

charge Potential difference voltage measured between two points Current (I) the flow of electrical charge between two points Resistance (R) hindrance to charge flow 39 Electricity Definitions

Insulator substance with high electrical resistance Conductor substance with low electrical resistance 40 Electrical Current and the Body Reflects the flow of ions rather than electrons

There is a potential on either side of membranes when: The number of ions is different across the membrane The membrane provides a resistance to ion flow 41 Role of Ion Channels Types of plasma membrane ion channels: Nongated, or leakage channels

always open Chemically gated channels open with binding of a specific neurotransmitter 42 Role of Ion Channels Voltage-gated channels open and close in response to membrane potential. 2 gates

Mechanically gated channels open and close in response to physical deformation of receptors 43 Gated Channels 44 Figure 12.13 Operation of a Chemically Gated Channel

Example: Na+-K+ gated channel Closed when a neurotransmitter is not bound to the extracellular receptor Na+ cannot enter the cell and K + cannot exit the cell Open when a neurotransmitter is attached to the receptor Na+ enters the cell and K+ exits the cell

45 Operation of a Voltage-Gated Channel Example: Na+ channel Closed when the intracellular environment is negative Na+ cannot enter the cell

Open when the intracellular environment is positive Na+ can enter the cell 46 Gated Channels When gated channels are open: Ions move quickly across the membrane Movement is along their electrochemical gradients

An electrical current is created Voltage changes across the membrane 47 Electrochemical Gradient Ions flow along their chemical gradient when they move from an

area of high concentration to an area of low concentration Ions flow along their electrical gradient when they move toward an area of opposite charge Electrochemical gradient the electrical and chemical gradients taken together 48 Resting Membrane Potential (Vr)

The potential difference (70 mV) across the membrane of a resting neuron It is generated by different concentrations of Na+, K+, Cl, and protein anions (A) 49 Resting Membrane Potential (Vr) Ionic differences are the

consequence of: Differential permeability of the neurilemma to Na+ and K+ Operation of the sodiumpotassium pump 50 Measuring Membrane Potential 51 Figure 11.7 Resting Membrane Potential (Vr)

52 Figure 11.8 Nervous Tissue Fundamentals P ANervous RT B of the System and Nervous Tissue 53 Membrane Potentials: Signals

Used to integrate, send, and receive information Membrane potential changes are produced by: Changes in membrane permeability to ions Alterations of ion concentrations across the membrane

Types of signals graded potentials and action potentials 54 Changes in Membrane Potential Changes are caused by three events Depolarization the inside of the membrane becomes less negative Repolarization the membrane returns to its resting membrane potential

Hyperpolarization the inside of the membrane becomes more negative than the resting 55 potential Depolarization, Repolarization and Hyperpolarization 56 Figure 12.15 Graded Potentials

Short-lived, local changes in membrane potential Decrease in intensity with distance Magnitude varies directly with the strength of the stimulus Depolarization or hyperpolarization Sufficiently strong graded potentials

can initiate action potentials 57 Graded Potentials 58 Figure 11.10 Graded Potentials

Voltage changes are decremental Current is quickly dissipated due to the leaky plasma membrane Only travel over short distances 59 Action Potentials (APs)

A brief reversal of membrane potential with a total amplitude of 100 mV Action potentials are only generated by muscle cells and neurons They do not decrease in strength over distance They are the principal means of neural communication An action potential in the axon of a neuron is a nerve impulse 60

Action Potential: Resting State Na+ and K+ voltage gated channels are closed Leakage accounts for small movements of Na+ and K+ Each Na+ channel has two voltageregulated gates Activation gates Inactivation gates

61 Figure 11.12.1 Action Potential: Resting State Na channel is closed, but capable of opening 62 Action Potential: Depolarization Phase

Na+ permeability increases; membrane potential reverses Na+ gates are opened; K+ gates are closed Threshold a critical level of depolarization (-55 to -50 mV) At threshold, depolarization becomes self-generating 63 Figure 11.12.2

Action Potential: Depolarization Phase Both Na channels are opened 64 Action Potential: Repolarization Phase

Sodium inactivation gates close Membrane permeability to Na+ declines to resting levels As sodium gates close, voltagesensitive K+ gates open K+ exits the cell and internal negativity of the resting neuron is restored 65 Figure 11.12.3 Action Potential: Repolarization Phase

Na activation channel is opened and inactivation is closed. 66 Action Potential: Hyperpolarization Potassium gates remain open, causing an excessive efflux of K+ This efflux causes hyperpolarization

of the membrane (undershoot) 67 Figure 11.12.4 Action Potential: Hyperpolarization 68 Action Potential: Role of the Sodium-Potassium Pump

Repolarization Restores the resting electrical conditions of the neuron Does not restore the resting ionic conditions Ionic redistribution back to resting conditions is restored by the sodiumpotassium pump 69 Phases of the Action Potential 1 resting state

2 depolarization phase 3 repolarization phase 4 hyperpolarization 70 Figure 11.12 Action Potential 71 Propagation of an Action Potential

along an Unmyelinated Axon 72 Propagation of an action potential Continuous propagation Unmyelinated axon Saltatory propagation Myelinated axon

73 Propagation of an Action Potential (Time = 0ms)- continuous propagation Na gates open Na+ influx causes a patch of the axonal membrane to depolarize Positive Na ions in the axoplasm move

toward the polarized (negative) portion of the membrane 74 Propagation of an Action Potential (Time = 2ms)- continuous propagation A local current is created that depolarizes

the adjacent membrane in a forward direction Local voltage-gated Na channels are opened The action potential moves away from the stimulus 75 Propagation of an Action Potential (Time = 4ms)- continuous propagation

Sodium gates close Potassium gates open Repolarization Hyperpolarization Sluggish K channels are kept opened Resting membrane potential is restored in this region 76

Saltatory Conduction Current passes through a myelinated axon only at the nodes of Ranvier Voltage-gated Na+ channels are concentrated at these nodes

Action potentials are triggered only at the nodes and jump from one node to the next Much faster than conduction along unmyelinated axons 77 Saltatory Conduction 78 Figure 11.16 Types of stimuli

Threshold stimulus Stimulus strong enough to bring the membrane potential to a threshold voltage causing an action potential Subthreshold stimulus weak stimuli that cause depolarization (graded potentials) but not action potentials 79

Coding for Stimulus Intensity Action potential are All-or-none phenomenon Strong stimuli can generate an action potential more often than weaker stimuli The CNS determines stimulus

intensity by the frequency of impulse transmission 80 Stimulus Strength and AP Frequency 81 Figure 11.14 Absolute Refractory Period

Time from the opening of the Na+ activation gates until the closing of inactivation gates The absolute refractory period: Prevents the neuron from generating an action potential Ensures that each action potential is separate Enforces one-way transmission of nerve impulses 82

Absolute and Relative Refractory Periods 83 Figure 11.15 Relative Refractory Period The interval following the absolute refractory period when: Sodium gates are closed

Potassium gates are open Repolarization is occurring Stimulus stronger than the original one cause a new action potential 84 Conduction Velocities of Axons Conduction velocities vary widely among neurons

Rate of impulse propagation is determined by: Axon diameter the larger the diameter, the faster the impulse Presence of a myelin sheath myelination dramatically increases impulse speed 85 Nerve Fiber Classification Nerve fibers are classified according to:

Diameter Degree of myelination Speed of conduction 86 Axon classification Type A fibers

Diameter of 4 to 20 m and myelinated Type B fibers Diameter of 2 to 4 m and myelinated Type C fibers Diameter less than 2 m and unmyelinated 87 Multiple Sclerosis (MS)

An autoimmune disease that mainly affects young adults Symptoms: visual disturbances, weakness, loss of muscular control, and urinary incontinence Nerve fibers are severed and myelin sheaths in the CNS become nonfunctional scleroses Shunting and short-circuiting of nerve impulses occurs

88 Synapses A junction that mediates information transfer from one neuron: To another neuron To an effector cell Presynaptic neuron conducts

impulses toward the synapse Postsynaptic neuron transmits impulses away from the synapse 89 Synapses 90 Figure 11.17 Types of Synapses

Axodendritic synapses between the axon of one neuron and the dendrite of another Axosomatic synapses between the axon of one neuron and the soma of another Others: Axoaxonic, etc 91 Electrical Synapses

Electrical synapses: Are less common than chemical synapses Correspond to gap junctions found in other cell types Very fast propagation of action potentials 92 Electrical Synapses Are

important in the CNS in: Arousal from sleep Mental attention Emotions and memory Ion and water homeostasis Light transmitting (eye) 93 Chemical Synapses

Specialized for the release and reception of neurotransmitters Typically composed of: Presynaptic neuron Contains synaptic vesicles Postsynaptic neuron The receptors are located typically on dendrites and soma 94 Chemical Synapses

Synaptic Cleft Fluid-filled space separating the presynaptic and postsynaptic neurons Transmission across the synaptic cleft: Is a chemical event (as opposed to an electrical one) Ensures unidirectional communication between neurons

95 Synaptic Cleft: Information Transfer Nerve impulses reach the axonal terminal of the presynaptic neuron and open Ca2+ channels Neurotransmitter is released into the synaptic cleft via exocytosis

96 Synaptic Cleft: Information Transfer Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron Postsynaptic membrane permeability changes, causing an excitatory or inhibitory effect

97 Synaptic Cleft: Information Transfer n tio ial Ac tent po Ca2+ 1 Neurotransmitter

Axon terminal of presynaptic neuron Postsynaptic membrane Mitochondrion Axon of presynaptic neuron Na+ Receptor

Postsynaptic membrane Ion channel open Synaptic vesicles containing neurotransmitter molecules 5 Degraded neurotransmitter

2 Synaptic cleft Ion channel (closed) 3 4 Ion channel closed Ion channel (open)

98 Figure 11.18 Nervous Tissue Fundamentals ART C of thePNervous System and Nervous Tissue 99 Termination of Neurotransmitter Effects

Neurotransmitter bound to a postsynaptic neuron: Produces a continuous postsynaptic effect Blocks reception of additional messages Must be removed from its receptor 100 Termination of Neurotransmitter Effects

Removal of neurotransmitters occurs when they: Are degraded by enzymes Are reabsorbed by astrocytes or the presynaptic terminals Diffuse from the synaptic cleft 101 Synaptic Delay

Neurotransmitter must be released, diffuse across the synapse, and bind to receptors Synaptic delay time needed to do this Synaptic delay is the rate-limiting step of neural transmission 102

Postsynaptic Potentials Neurotransmitter receptors mediate changes in membrane potential according to: The amount of neurotransmitter released The amount of time the neurotransmitter is bound to receptors 103 Postsynaptic Potentials

The two types of postsynaptic potentials are: EPSP excitatory postsynaptic potentials IPSP inhibitory postsynaptic potentials 104 Excitatory Postsynaptic Potentials

EPSPs are graded potentials that can initiate an action potential in an axon Use only chemically gated channels Na+ and K+ flow in opposite directions at the same time Postsynaptic membranes do not generate action potentials 105 Excitatory Postsynaptic

Potential (EPSP) 106 Figure 11.19a Inhibitory Synapses and IPSPs Neurotransmitter binding to a receptor at inhibitory synapses: Causes the membrane to become more permeable to potassium and chloride ions Leaves the charge on the inner

surface negative Reduces the postsynaptic neurons ability to produce an action potential 107 Inhibitory Postsynaptic (IPSP) 108 Figure 11.19b Summation

A single EPSP cannot induce an action potential EPSPs must summate temporally or spatially to induce an action potential Temporal summation presynaptic neurons transmit impulses in rapid-fire order 109

Temporal Summation 110 Summation Spatial summation postsynaptic neuron is stimulated by a large number of terminals at the same time

IPSPs can also summate with EPSPs, canceling each other out 111 Spatial Summation 112 EPSP IPSP Interactions 113 Neurotransmitters

Chemicals used for neuronal communication with the body and the brain 50 different neurotransmitters have been identified Classified chemically and functionally

114 Chemical Neurotransmitters Acetylcholine (ACh) Biogenic amines Amino acids Peptides Novel messengers: ATP and

dissolved gases NO and CO 115 Neurotransmitters: Acetylcholine First neurotransmitter identified, and best understood Released at the neuromuscular

junction Synthesized and enclosed in synaptic vesicles 116 Neurotransmitters: Acetylcholine Degraded by the enzyme acetylcholinesterase (AChE) Released by:

All neurons that stimulate skeletal muscle Some neurons in the autonomic nervous system 117 Neurotransmitters: Biogenic Amines

Include: Catecholamines dopamine, norepinephrine (NE), and epinephrine Indolamines serotonin and histamine Broadly distributed in the brain Play roles in emotional behaviors and our biological clock 118 Synthesis of Catecholamines

Enzymes present in the cell determine length of biosynthetic pathway Norepinephrine and dopamine are synthesized in axonal terminals Epinephrine is released by the adrenal medulla 119 Figure

11.21 Synthesis of Catecholamines 120 Neurotransmitters: Amino Acids Include: GABA Gamma ()-aminobutyric acid

Glycine Aspartate Glutamate Found only in the CNS 121 Neurotransmitters: Peptides

Include: Substance P mediator of pain signals Beta endorphin, dynorphin, and enkephalins Act as natural opiates; reduce pain perception Bind to the same receptors as opiates and morphine Gut-brain peptides somatostatin, 122 and cholecystokinin

Neurotransmitters: Novel Messengers ATP Is found in both the CNS and PNS Produces excitatory or inhibitory responses depending on receptor type Induces Ca2+ wave propagation in astrocytes Provokes pain sensation 123

Neurotransmitters: Novel Messengers Nitric oxide (NO) Activates the intracellular receptor guanylyl cyclase Is involved in learning and memory Carbon monoxide (CO) is a main regulator of cGMP in the brain

124 Functional Classification of Neurotransmitters Two classifications: excitatory and inhibitory Excitatory neurotransmitters cause depolarizations (e.g., glutamate) Inhibitory neurotransmitters cause hyperpolarizations (e.g.,

GABA and glycine) 125 Functional Classification of Neurotransmitters Some neurotransmitters have both excitatory and inhibitory effects Determined by the receptor type of the postsynaptic neuron Example: acetylcholine Excitatory at neuromuscular junctions with skeletal muscle

Inhibitory in cardiac muscle 126 Neurotransmitter Receptor Mechanisms Direct: neurotransmitters that open ion channels Promote rapid responses Examples: ACh and amino acids Indirect: neurotransmitters that act

through second messengers Promote long-lasting effects Examples: biogenic amines, peptides, and dissolved gases 127 Channel-Linked Receptors Composed of integral membrane

protein Mediate direct neurotransmitter action Action is immediate, brief, simple, and highly localized 128 Channel-Linked Receptors

Ligand binds the receptor, and ions enter the cells Excitatory receptors depolarize membranes Inhibitory receptors hyperpolarize membranes 129 Channel-Linked Receptors 130 Figure 11.22a

G Protein-Linked Receptors Responses are indirect, slow, complex, prolonged, and often diffuse These receptors are transmembrane protein complexes First and Second messengers.

131 Neurotransmitter Receptor Mechanism Ions flow Blocked ion flow (a) Channel closed Adenylate

cyclase Channel open Neurotransmitter (ligand) released from axon terminal of presynaptic neuron 3 1 PPi 4

GTP ATP 5 cAMP 5 3 Changes in membrane permeability and potential

GTP 2 GDP Protein synthesis Enzyme activation GTP

Receptor G protein (b) Nucleus Activation of specific genes 132 Figure 11.22b Neural Integration: Neuronal Pools

Functional groups of neurons that: Integrate incoming information Forward the processed information to its appropriate destination 133 Neural Integration: Neuronal Pools Simple neuronal pool Input fiber presynaptic fiber

Discharge zone neurons most closely associated with the incoming fiber Facilitated zone neurons farther away from incoming fiber 134 Simple Neuronal Pool 135 Figure 11.23 Types of Neuronal Pools

136 Types of Circuits in Neuronal Pools Divergent information spreads from one neuron to several neurons or from one pool to several pools. One motor neuron of the brain stimulates many muscle fibers 137 b Figure 11.24a,

Types of Circuits in Neuronal Pools Convergent input from many neurons is funneled to one neuron or neuronal pools Motor neurons of the diaphragm is controlled by different areas of the brain for subconscious and conscious functioning 138 d Figure 11.24c,

Types of Circuits in Neuronal Pools Reverberating collateral branches of axons extend back toward the source of the impulse Involved in rhythmic activities Breathing, sleep-wake cycle, etc Figure139 11.24e

Types of Circuits in Neuronal Pools Parallel - incoming neurons stimulate several neurons in parallel arrays Divergence must take place before the parallel processing Response to a painfull stimuli causes Withdrawal of the limb

Shift of the weight Fell pain Shout ouch! Figure140 11.24f Patterns of Neural Processing Serial Transmitting of the impulse from one neuron to another or from one pool to another Spinal reflexes

141 Development of Neurons The nervous system originates from the neural tube and neural crest The neural tube becomes the CNS There is a three-phase process of differentiation:

Proliferation of cells needed for development Migration neuroblasts become amitotic and move externally Differentiation into neurons 142 Axonal Growth Guided by: Scaffold laid down by older neurons Orienting glial fibers Release of nerve growth factor by

astrocytes Neurotropins released by other neurons Repulsion guiding molecules Attractants released by target cells 143

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