2/27/2019. Functions of the Nervous System. Nervous Tissue and Neuron Function. Fundamentals Of The Nervous System And Nervous Tissue

Transcription

1 Nervous Tissue and Neuron Function Fundamentals Of The Nervous System And Nervous Tissue Learn and Understand 1. Like muscle cells, neurons use membrane polarity upset (AP) as a signal therefore keeping their membranes constantly ready (RMP). 2. Neuroglia help create and maintain the environmental conditions necessary for optimal neuron functioning. 3. In order to carry their message, some neurons have axons greater than 1 m in length. 4. Increasing the frequency of action potentials, not its strength, is how the NS controls the intensity of its message. 5. Graded potentials may sum to threshold depolarization causing AP in the neuron. The source of graded potentials is the up to 10,000 synapses with other neurons. Functions of the Nervous System Master controlling and communicating system of body 1. Sensory: Receiving internal and external sensory input. 2. Integration: Process and evaluate, coordinate and control response 3. Motor: Generate response signals A. Controlling muscles and glands B. Maintaining homeostasis Rapid and specific - usually causes almost immediate responses Establishing and maintaining mental activity, consciousness, thinking, behavior, memory, emotion 1

2 Figure 11.1 The nervous system s functions. Sensory input Integration Motor output Anatomic Divisions of the Nervous System 100 Billion Neurons 100 Million Neurons CNS: Integration and control center. Interprets sensory input and dictates motor output PNS: Consists mainly of nerves that extend from brain and spinal cord. Cranial nerves to and from brain. Spinal nerves to and from spinal cord. Plexus network of sensory input, motor output and integration outside of the CNS Figure 11.2 Levels of organization in the nervous system. Central nervous system (CNS) Brain and spinal cord Integrative and control centers Peripheral nervous system (PNS) Cranial nerves and spinal nerves Communication lines between the CNS and the rest of the body Sensory (afferent) division Somatic and visceral sensory nerve fibers Conducts impulses from receptors to the CNS Motor (efferent) division Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands) Somatic sensory fiber Skin Somatic nervous system Somatic motor (voluntary) Autonomic nervous system (ANS) Visceral motor (involuntary) Visceral sensory fiber Stomach Conducts impulses from the CNS to skeletal muscles Skeletal muscle Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Motor fiber of somatic nervous system Sympathetic division Mobilizes body systems during activity Parasympathetic division Conserves energy Promotes housekeeping functions during rest Sympathetic motor fiber of ANS Heart Structure Function Sensory (afferent) division of PNS Motor (efferent) division of PNS Parasympathetic motor fiber of ANS Bladder 2

3 Histology of Nervous Tissue Highly cellular; little extracellular space Two principal cell types Neurons (nerve cells) excitable cells that transmit electrical signals Neuroglia small cells that surround and wrap delicate neurons CNS: Astrocytes Microglial cells Ependymal cells Oligodendrocytes Satellite cells (PNS) Schwann cells (PNS) Neurons Structural units of nervous system Large, highly specialized cells that conduct impulses Extreme longevity (100 years or more) Amitotic with few exceptions High metabolic rate requires continuous supply of oxygen and glucose All have cell body and one or more processes Nucleus Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Soma = Biosynthetic center of neuron Synthesizes proteins, membranes, and other chemicals Rough ER (chromatophilic substance or Nissl bodies) Most active and best developed in body Most neuron cell bodies in CNS Nuclei are clusters of neuron cell bodies in CNS Dendrites Convey incoming messages toward cell body as graded potentials Nucleolus Chromatophilic substance (rough endoplasmic reticulum) Axon hillock Axon (impulsegenerating and -conducting region) Impulse direction Schwann cell Terminal branches Myelin sheath gap (node of Ranvier) Axon terminals (secretory region) 3

4 Structure of a Motor Neuron The Axon: Structure One axon per cell arising from axon hillock Cone-shaped area of cell body In some, axon short or absent, in others most of length of cell Long axons called nerve fibers Occasional branches (axon collaterals) Branches profusely at end (terminus) Can be 10,000 terminal branches Distal endings called axon terminals or terminal boutons, axon bulbs, presynaptic terminals The Axon: Functional Characteristics Generates and conducts AP Transmits AP along axolemma to axon terminal Neurotransmitters released into extracellular space Synapsed with many other neurons at same time Lacks rough ER and Golgi apparatus Relies on cell body to renew proteins and membranes Quickly decay if cut or damaged 4

5 Schwann cell plasma membrane Schwann cell cytoplasm Axon 1 Schwann cell nucleus 2 Segmented sheath around most long or large-diameter axons Myelinated fibers Function of myelin Protects and electrically insulates axon Increases speed of nerve impulse transmission Myelin sheath Schwann cell cytoplasm Myelination of a nerve fiber (axon) 3 Nonmyelinated fibers conduct impulses more slowly Figure 11.5a Nerve fiber myelination by Schwann cells in the PNS. Figure 11.5b Nerve fiber myelination by Schwann cells in the PNS. Myelin sheath Outer collar of perinuclear cytoplasm (of Schwann cell) Axon Cross-sectional view of a myelinated axon (electron micrograph 24,000x) 5

6 Functional Classifications: Sensory Transmit impulses from sensory receptors toward CNS Cell bodies in ganglia in PNS ganglion is a grouping of NCBs outside of the CNS Motor Carry impulses from CNS to effectors Most cell bodies in CNS (except some autonomic neurons) Interneuron (association neuron) Lie between motor and sensory neurons Shuttle signals through CNS pathways; most are entirely within CNS 99% of body's neurons Functional Classification of Neurons Sensory Transmit impulses from sensory receptors toward CNS Almost all are Unipolar Cell bodies in ganglia in PNS ganglion is a grouping of NCBs outside of the CNS Motor Carry impulses from CNS to effectors Multipolar Most cell bodies in CNS (except some autonomic neurons) Interneurons (association neurons) Lie between motor and sensory neurons Shuttle signals through CNS pathways; most are entirely within CNS 99% of body's neurons 6

7 The Resting Membrane Potential Potential difference across membrane of resting cell Approximately 70 mv in neurons Actual voltage difference varies from -40 mv to -90 mv Membrane termed polarized Generated by: Differences in ionic makeup of ICF and ECF ECF has higher concentration of Na+ than ICF Balanced chiefly by chloride ions (Cl-) ICF has higher concentration of K+ than ECF Balanced by negatively charged proteins K+ plays most important role in membrane potential Differential permeability of the plasma membrane Measuring Membrane Potential in Neurons Figure 11.6 Operation of gated channels. Open and close to change which ions move across membrane and when. One stimulated by messenger; one stimulated by electrical charge Chemically gated ion channels Open in response to binding of the appropriate neurotransmitter Voltage-gated ion channels Open in response to changes in membrane potential Receptor Neurotransmitter chemical attached to receptor Chemical binds Membrane voltage changes Closed Open Closed Open Each Na+ channel has two voltage-sensitive gates Activation gates Closed at rest; open with depolarization allowing Na+ to enter cell Inactivation gates Open at rest; block channel once it is open to prevent more Na+ from entering cell 7

8 Differences in Plasma Membrane Permeability Impermeable to large anionic proteins Slightly permeable to Na + (through leakage channels) Sodium diffuses into cell down concentration gradient 25 times more permeable to K + than sodium (more leakage channels) Potassium diffuses out of cell down concentration gradient Quite permeable to Cl Membrane Potential Changes Used as Communication Signals Membrane potential changes when Concentrations of ions across membrane change Membrane permeability to ions changes Changes produce two types signals Graded potentials Incoming signals operating over short distances Mostly arrive at axodendritic and axosomatic synapses Collectively control the post-synaptic neuron Action potentials Long-distance signals of axons Action Potentials (AP) Principle way neurons send signals Principal means of long-distance neural communication Occur only in muscle cells and axons of neurons Brief reversal of membrane potential with a change in voltage of ~100 mv Do not decay over distance as graded potentials do 8

9 Membrane potential (mv) Membrane potential (mv) Membrane potential (mv) Relative membrane permeability Closed Opened Figure The action potential (AP) is a brief change in membrane potential in a patch of membrane that is depolarized by local currents. The big picture 1 Resting state 2 Depolarization The key players Voltage-gated Na + channels Outside cell Voltage-gated K + channels Outside cell Action potential 3 Repolarization 4 Hyperpolarization Inside Activation Inactivation cell gate gate Closed Opened Inactivated Inside cell The events Threshold 1 Sodium channel Potassium channel Time (ms) The AP is caused by permeability changes in the plasma membrane: Activation gates Inactivation gate 1 Resting state Action potential 2 Na + permeability K + permeability 4 Hyperpolarization 2 Depolarization Time (ms) 3 Repolarization At threshold ( 55 to 50 mv) positive feedback causes opening of all Na + channels a reversal of membrane polarity to +30mV Figure The action potential (AP) is a brief change in membrane potential in a patch of membrane that is depolarized by local currents. (1 of 3) 1 Resting state. No 2 Depolarization ions move through is caused by Na + voltage-gated flowing into the cell. channels. 3 Repolarization is caused by K + flowing out of the cell Action potential 4 Hyperpolarization is caused by K + continuing to leave the cell Threshold Time (ms) Each K+ channel has one voltage-sensitive gate Closed at rest; Opens slowly with depolarization Repolarization and hyperpolarization: Slow voltage-gated K + channels open K + exits the cell and internal negativity is restored 4 1 Role of the Sodium-Potassium Pump Repolarization resets electrical conditions, not ionic conditions After repolarization Na + /K + pumps (thousands of them in an axon) restore ionic conditions 9

10 Membrane potential (mv) Membrane potential (mv) Membrane potential (mv) Figure 11.12a Propagation of an action potential (AP) Voltage at 0 ms Recording electrode Na+ influx causes local currents Local currents cause depolarization of adjacent membrane areas in direction away from AP origin (toward axon's terminals) Time = 0 ms. Action potential has not yet reached the recording electrode. Resting potential Peak of action potential Hyperpolarization Figure 11.12b Propagation of an action potential (AP). +30 Voltage at 2 ms 70 Since Na+ channels closer to AP origin are inactivated no new AP is generated there Once initiated an AP is selfpropagating Time = 2 ms. Action potential peak reaches the recording electrode. Resting potential Peak of action potential Hyperpolarization Figure 11.12c Propagation of an action potential (AP) Voltage at 4 ms Time = 4 ms. Action potential peak has passed the recording electrode. Membrane at the recording electrode is still hyperpolarized. Resting potential AP to propagates AWAY from the AP Peak of action potential origin Hyperpolarization 10

11 Absolute and Relative Refractory Periods a period when a neuron is unable to respond to a new stimulus or is less responsive to stimulus Absolute refractory period Time from opening of Na + channels until resetting of the channels Ensures that each AP is an all-or-none event Enforces one-way transmission of nerve impulses Relative refractory period Follows absolute refractory period Most Na+ channels have returned to their resting state Some K+ channels still open Repolarization is occurring Threshold for AP generation is elevated Inside of membrane more negative than resting state Figure Action potential propagation in nonmyelinated and myelinated axons. Stimulus Size of voltage In bare plasma membranes, voltage decays. Without voltage-gated channels, as on a dendrite, voltage decays because current leaks across the membrane. receptive zone graded potentials Stimulus Voltage-gated ion channel In nonmyelinated axons, conduction is slow (continuous conduction). Voltage-gated Na + and K + channels regenerate the action potential at each point along the axon, so voltage does not decay. Conduction is slow because it takes time for ions and for gates of channel proteins to move, and this must occur before voltage can be regenerated. Group C fibers Group A & B fibers Myelin sheath Stimulus Myelin sheath Myelin sheath gap 1 mm In myelinated axons, conduction is fast (saltatory conduction). Myelin keeps current in axons (voltage doesn t decay much). APs are generated only in the myelin sheath gaps and appear to jump rapidly from gap to gap. Saltatory conduction is about 30 times faster 11

12 Stimulus voltage Membrane potential (mv) Nerve Fiber Classification Group A fibers Large diameter, myelinated somatic sensory and motor fibers of skin, skeletal muscles, joints Transmit at 150 m/s Group B fibers Intermediate diameter, lightly myelinated fibers Transmit at 15 m/s Group C fibers Smallest diameter, unmyelinated ANS fibers Transmit at 1 m/s Coding for Stimulus Intensity All action potentials are alike and are independent of stimulus intensity How does CNS tell difference between a weak stimulus and a strong one? Strong stimuli cause action potentials to occur more frequently # Of impulses per second or frequency of APs CNS determines stimulus intensity by the frequency of impulses Higher frequency means stronger stimulus Figure Relationship between stimulus strength and action potential frequency. +30 Action potentials 70 0 Threshold Stimulus Time (ms) 12

13 Synapses Synapse Classification Axodendritic between axon terminals of one neuron and dendrites of others Axosomatic between axon terminals of one neuron and soma of others Less common types: Axoaxonal (axon to axon) Dendrodendritic (dendrite to dendrite) Somatodendritic (dendrite to soma) Important Terminology Presynaptic neuron Neuron conducting impulses toward synapse Sends the information Postsynaptic neuron (in PNS may be a neuron, muscle cell, or gland cell) Neuron transmitting electrical signal away from synapse Receives the information Most function as both Figure Synapses. Axodendritic synapses Axosomatic synapses Dendrites Cell body Axoaxonal synapses Axon Axon Axosomatic synapses Cell body (soma) of postsynaptic neuron 13

14 Varieties of Synapses: Electrical Synapses Less common than chemical synapses Neurons electrically coupled (joined by gap junctions that connect cytoplasm of adjacent neurons) Communication very rapid May be unidirectional or bidirectional Synchronize activity More abundant in: Embryonic nervous tissue Cardiac muscle Nerve impulse remains electrical Varieties of Synapses: Chemical Synapses Specialized for release and reception of chemical neurotransmitters Typically composed of two parts Axon terminal of presynaptic neuron Contains synaptic vesicles filled with neurotransmitter Neurotransmitter receptor region on postsynaptic neuron's membrane Usually on dendrite or cell body Two parts separated by synaptic cleft Fluid-filled space Electrical impulse changed to chemical across synapse, then back into electrical Figure Chemical synapses transmit signals from one neuron to another using neurotransmitters. Reuptake Enzymatic degradation Presynaptic neuron Diffusion away from synapse Postsynaptic neuron Presynaptic neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca 2+ channels open and Ca 2+ enters the axon terminal. Mitochondrion 3 Ca 2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis Axon terminal Synaptic cleft Synaptic vesicles 4 Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. Postsynaptic neuron 14

15 Figure Chemical synapses transmit signals from one neuron to another using neurotransmitters. Ion movement Graded potential 5 Binding of neurotransmitter opens ion channels, resulting in graded potentials. Synaptic Delay Time needed for neurotransmitter to be released, diffuse across synapse, and bind to receptors ms Synaptic delay is rate-limiting step of neural transmission Neurotransmitters Language of nervous system 50 or more neurotransmitters have been identified Most neurons make two or more neurotransmitters Neurons can exert several influences Usually released at different stimulation frequencies Classified by chemical structure and by function 15

16 Classification of Neurotransmitters: Function Effects - excitatory versus inhibitory Neurotransmitter effects can be excitatory (depolarizing) and/or inhibitory (hyperpolarizing) Effect determined by receptor to which it binds Acetylcholine and NE bind to at least two receptor types with opposite effects ACh excitatory at neuromuscular junctions in skeletal muscle ACh inhibitory in cardiac muscle Figure Direct neurotransmitter receptor mechanism: Channel-linked receptors. Ion flow blocked Ligand Ions flow Closed ion channel Open ion channel Direct action Neurotransmitter binds to and opens ion channels Promotes rapid responses by altering membrane potential Examples: ACh and amino acids Graded Potentials Short-lived, localized changes in membrane potential Magnitude varies with stimulus strength Stronger stimulus more voltage changes; farther current flows Either depolarization or hyperpolarization Triggered by stimulus that opens gated ion channels Current flows but dissipates quickly and decays Graded potentials are signals only over short distances 16

17 Membrane potential (voltage, mv) Membrane potential (voltage, mv) Figure 11.9a Depolarization and hyperpolarization of the membrane. Depolarizing stimulus Inside positive Inside negative Depolarization Decrease in membrane potential (toward zero and above) Inside of membrane becomes less negative than resting membrane potential Increases probability of producing a nerve impulse Resting potential Time (ms) Depolarization: The membrane potential moves toward 0 mv, the inside becoming less negative (more positive). Figure 11.9b Depolarization and hyperpolarization of the membrane. +50 Hyperpolarizing stimulus 0 An increase in membrane potential (away from zero) Inside of cell more negative than resting membrane potential) Resting potential Hyperpolarization Time (ms) Hyperpolarization: The membrane potential increases, the inside becoming more negative. Reduces probability of producing a nerve impulse 17

18 Membrane potential (mv) Excitatory Synapses and EPSPs Neurotransmitter binding opens chemically gated channels Allows simultaneous flow of Na + and K + in opposite directions Na + influx greater than K + efflux net depolarization called EPSP (not AP) EPSP help trigger AP if EPSP is of threshold strength Can spread to axon hillock, trigger opening of voltage-gated channels, and cause AP to be generated Figure 11.18a Postsynaptic potentials can be excitatory or inhibitory Stimulus Threshold Time (ms) An EPSP is a local depolarization of the postsynaptic membrane that brings the neuron closer to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing Na + and K + to pass through simultaneously. Excitatory postsynaptic potential (EPSP) Inhibitory Synapses and IPSPs Reduces postsynaptic neuron's ability to produce an action potential Makes membrane more permeable to K + or Cl If K + channels open, it moves out of cell If Cl - channels open, it moves into cell Therefore neurotransmitter hyperpolarizes cell Inner surface of membrane becomes more negative AP less likely to be generated 18

19 Membrane potential (mv) Figure 11.18b Postsynaptic potentials can be excitatory or inhibitory Stimulus Threshold Time (ms) Inhibitory postsynaptic potential (IPSP) An IPSP is a local hyperpolarization of the postsynaptic membrane that drives the neuron away from AP threshold. Neurotransmitter binding opens K + or Cl channels. Synaptic Integration: Summation A single EPSP cannot induce an AP EPSPs can summate to influence postsynaptic neuron IPSPs can also summate Temporal summation Spatial summation Most neurons receive both excitatory and inhibitory inputs from thousands of other neurons Only if EPSP's predominate and bring to threshold AP Postsynaptic Potentials and Their Summation 19

20 Temporal Summation Spatial Summation Integration of EPSPs and IPSPs 20

21 Integration: Presynaptic Inhibition Excitatory neurotransmitter release by one neuron inhibited by another neuron via an axoaxonal synapse Less neurotransmitter released Smaller EPSPs formed Additional Slides May not be shown on screen in class Capillary Neuron Astrocyte Astrocytes are the most abundant CNS neuroglia. 1. Support and brace neurons 2. Play role in exchanges between capillaries and neurons 3. Guide migration of young neurons 4. Control chemical environment around neurons 21

22 Neuron Microglial cell Microglial cells are defensive cells in the CNS. 1. Migrate toward injured neurons 2. Can transform to phagocytize microorganisms and neuronal debris Fluid-filled cavity Cilia Ependymal cells Brain or spinal cord tissue Ependymal cells line cerebrospinal fluid filled cavities. 1. Range in shape from squamous to columnar 2. May be ciliated - Cilia beat to circulate CSF 3. Line the central cavities of the brain and spinal column 4. Form permeable barrier between CSF in cavities and tissue fluid bathing CNS cells Myelin sheath Process of oligodendrocyte Nerve fibers Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers. 1. Branched cells 2. Processes wrap CNS nerve fibers, forming insulating myelin sheaths thicker nerve fibers 22

23 Satellite cells Cell body of neuron Schwann cells (forming myelin sheath) Nerve fiber Satellite cells and Schwann cells (which form myelin) surround neurons in the PNS. Satellite cells Surround neuron cell bodies in PNS Function similar to astrocytes of CNS Schwann cells (neurolemmocytes) Surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers Similar function as oligodendrocytes Vital to regeneration of damaged peripheral nerve fibers 23