Chapter 12 - Nervous Tissue PDF

Chapter 12 Nervous Tissue 1  INTRODUCTION The nervous system, along with the endocrine system, helps to keep controlled conditions within limits that maintain health and helps to maintain homeostasis The nervous system is responsible for all our b...

Chapter 12 Nervous Tissue 1  INTRODUCTION The nervous system, along with the endocrine system, helps to keep controlled conditions within limits that maintain health and helps to maintain homeostasis The nervous system is responsible for all our behaviors, memories, and movements The branch of medical science that deals with the normal functioning and disorders of the nervous system is called neurology 2  Nervous System Three basic functions: – sensing changes with sensory receptors fullness of stomach or sun on your face – interpreting and remembering those changes – reacting to those changes with effectors muscular contractions glandular secretions 3 Major Structures of the Nervous System 4  Structures of the Nervous System - Overview Twelve pairs of cranial nerves emerge from the base of the brain through foramina of the skull. – A nerve is a bundle of hundreds or thousands of axons, each serving a specific region of the body. The spinal cord connects to the brain through the foramen magnum of the skull and is encircled by the bones of the vertebral column. – Thirty-one pairs of spinal nerves emerge from the spinal cord. Ganglia, located outside the brain and spinal cord, are small masses of nervous tissue, containing primarily cell bodies of neurons. Sensory receptors are either parts of neurons or specialized cells that monitor changes in the internal or external environment. 5  Functions of the Nervous Systems The sensory function of the nervous system is to sense changes in the internal and external environment through sensory receptors. – Sensory (afferent) neurons serve this function. The integrative function is to analyze the sensory information, store some aspects, and make decisions regarding appropriate behaviors. – Association or interneurons serve this function. The motor function is to respond to stimuli by initiating action. – Motor (efferent) neurons serve this function. 6  Nervous System Divisions Central nervous system (CNS) – consists of the brain and spinal cord Peripheral nervous system (PNS) – consists of cranial and spinal nerves that contain both sensory and motor fibers – connects CNS to muscles, glands & all sensory receptors 7  Subdivisions of the PNS Somatic (voluntary) nervous system (SNS) – neurons from cutaneous and special sensory receptors to the CNS – motor neurons to skeletal muscle tissue Autonomic (involuntary) nervous systems (ANS) – sensory neurons from visceral organs to CNS – motor neurons to smooth & cardiac muscle and glands sympathetic division (speeds up heart rate) parasympathetic division (slow down heart rate) Enteric nervous system (ENS) – involuntary sensory & motor neurons control GI tract – neurons function independently of ANS & CNS 8 Organization of the Nervous System CNS is brain and spinal cord PNS is everything else 9  Enteric NS The enteric nervous system (ENS) consists of neurons that extend the length of the GI tract. – Many neurons of the enteric plexuses function independently of the CNS. – Sensory neurons of the ENS monitor chemical changes within the GI tract and stretching of its walls – Motor neurons of the ENS govern contraction of GI tract organs, and activity of the GI tract endocrine cells. 10 Neuronal Structure & Function 11  Neurons Functional unit of nervous system Have capacity to produce action potentials – electrical excitability Cell body – single nucleus with prominent nucleolus – Nissl bodies (chromatophilic substance) rough ER & free ribosomes for protein synthesis – neurofilaments give cell shape and support Cell processes = dendrites & axons 12  Parts of a Neuron Neuroglial cells Nucleus with Nucleolus Axons or Dendrites Cell body 13 Dendrites Conducts impulses towards the cell body Typically short, highly branched & unmyelinated Surfaces specialized for contact with other neurons 14  Axons Conduct impulses away from cell body To another neuron or to an effector (muscle or gland) Long, thin cylindrical process of cell Arises at axon hillock Impulses arise from initial segment (trigger zone) Swollen tips called synaptic end bulbs contain vesicles filled with neurotransmitters 15  Axonal Transport Cell body is location for most protein synthesis – neurotransmitters & repair proteins Axonal transport system moves substances – slow axonal flow movement in one direction only -- away from cell body movement at 1-5 mm per day – fast axonal flow moves organelles & materials along surface of microtubules at 200-400 mm per day transports in either direction Fast axonal transport route by which toxins or pathogens reach neuron cell bodies – tetanus (Clostridium tetani bacteria) – disrupts motor neurons causing painful muscle spasms 16  Structural Classification of Neurons Based on number of processes found on cell body – multipolar = several dendrites & one axon most common cell type – bipolar neurons = one main dendrite & one axon found in retina, inner ear & olfactory – unipolar neurons = one process only are always sensory neurons 17  Functional Classification of Neurons Sensory (afferent) neurons – transport sensory information from skin, muscles, joints, sense organs & viscera to CNS Motor (efferent) neurons – send motor nerve impulses to muscles & glands Interneurons (association) neurons – connect sensory to motor neurons – 90% of neurons in the body 18  Neuroglial Cells Half of the volume of the CNS Smaller cells than neurons 50X more numerous Cells can divide – rapid mitosis in tumor formation (gliomas) 4 cell types in CNS: – astrocytes, oligodendrocytes, microglia & ependymal 2 cell types in PNS: – schwann and satellite cells 19  Astrocytes Star-shaped cells Form blood-brain barrier by covering blood capillaries Metabolize neurotransmitters Regulate K+ balance Provide structural support 20 Microglia Small cells found near blood vessels Phagocytic role -- clear away dead cells Derived from cells that also gave rise to macrophages & monocytes 21 Ependymal cells Form epithelial membrane lining cerebral cavities & central canal Produce cerebrospinal fluid (CSF) 22  Satellite Cells Flat cells surrounding neuronal cell bodies in peripheral ganglia Support neurons in the PNS ganglia 23 Oligodendrocytes Most common glial cell type Each forms myelin sheath around multiple axons in CNS Analogous to Schwann cells of PNS 24  Myelination A multilayered lipid and protein covering called the myelin sheath and produced by Schwann cells and oligodendrocytes surrounds the axons of most neurons The sheath electrically insulates the axon and increases the speed of nerve impulse conduction. 25  Schwann Cell Cells encircling PNS axons Each cell produces part of the myelin sheath surrounding an axon in the PNS 26  Axon Coverings in PNS Neurolemma is cytoplasm & nucleus of Schwann cell – gaps called nodes of Ranvier Tube guides growing axons that are repairing themselves Myelinated fibers appear white – jelly-roll like wrappings made of lipoprotein = myelin – acts as electrical insulator – speeds conduction of nerve impulses Unmyelinated fibers – slow, small diameter fibers – only surrounded by neurilemma but no myelin sheath wrapping 27  Myelination in the CNS Oligodendrocytes myelinate axons in the CNS Broad, flat cell processes wrap about CNS axons, but the cell bodies do not surround the axons No neurilemma is formed Little regrowth after injury is possible due to the lack of a distinct tube or neurilemma 28  Gray and White Matter White matter = myelinated axons (white in color) Gray matter = nerve cell bodies, dendrites, axon terminals, bundles of unmyelinated axons and neuroglia (gray color) – In the spinal cord = gray matter forms an H-shaped inner core surrounded by white matter – In the brain = a thin outer shell of gray matter covers the surface & is found in clusters called nuclei inside the CNS A nucleus is a mass of nerve cell bodies and dendrites inside the CNS. 29  Electrical Signals in Neurons Neurons are electrically excitable due to the voltage difference across their membrane Communicate with 2 types of electric signals – action potentials that can travel long distances – graded potentials that are local membrane changes only 30  Ion Channels In living cells, a flow of ions occurs through ion channels in the cell membrane Gated channels open and close in response to a stimulus – results in neuron excitability 31  Ion Channels Gated ion channels : – Voltage-gated channels respond to a direct change in the membrane potential – Ligand-gated channels respond to a specific chemical stimulus – Mechanically gated ion channels respond to mechanical vibration or pressure 32 Gated Ion Channels 33  Resting Membrane Potential Negative ions along inside of cell membrane & positive ions along outside – potential energy difference at rest is -70 mV – cell is “polarized” Resting potential exists because – concentration of ions different inside & outside extracellular fluid rich in Na+ cytosol full of K+, organic phosphate & amino acids 34  Graded Potentials Small deviations from resting potential of -70mV – hyperpolarization = membrane has become more negative – depolarization = membrane has become more positive The signals are graded, meaning they vary in amplitude (size), depending on the strength of the stimulus and are localized. Graded potentials occur most often in the dendrites and cell body of a neuron. 35  How do Graded Potentials Arise? Source of stimuli – mechanical stimulation of membranes with mechanical gated ion channels (pressure) – chemical stimulation of membranes with ligand gated ion channels (neurotransmitter) Graded/postsynaptic/receptor or generator potential – ions flow through ion channels and change membrane potential locally – amount of change varies with strength of stimuli Flow of current (ions) is local change only 36  Generation of an Action Potential An action potential (AP) or impulse is a sequence of rapidly occurring events that decrease and eventually reverse the membrane potential (depolarization) and then restore it to the resting state (repolarization). – During an action potential, voltage-gated Na+ and K+ channels open in sequence 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. 37  Action Potential Series of rapidly occurring events that change and then restore the membrane potential of a cell to its resting state Ion channels open, Na+ rushes in (depolarization), K+ rushes out (repolarization) All-or-none principal = with stimulation, either happens one specific way or not at all (lasts 1/1000 of a second) Travels (spreads) over surface of cell without dying out 38 Depolarizing Phase of Action Potential Chemical or mechanical stimulus caused a graded potential to reach at least (-55mV or threshold) Voltage-gated Na+ channels open & Na+ rushes into cell – in resting membrane, inactivation gate of sodium channel is open & activation gate is closed (Na+ can not get in) – when threshold (-55mV) is reached, both open & Na+ enters – inactivation gate closes again in few ten-thousandths of second – only a total of 20,000 Na+ actually enter the cell, but they change the membrane potential considerably (up to +30mV) Positive feedback process 39 Repolarizing Phase of Action Potential When threshold potential of -55mV is reached, voltage-gated K+ channels open K+ channel opening is much slower than Na+ channel opening which caused depolarization When K+ channels finally do open, the Na+ channels have already closed (Na+ inflow stops) K+ outflow returns membrane potential to -70mV If enough K+ leaves the cell, it will reach a -90mV membrane potential and enter the after-hyperpolarizing phase K+ channels close and the membrane potential returns to the resting potential of -70mV 40 Refractory Period of Action Potential Period of time during which neuron can not generate another action potential Absolute refractory period – even very strong stimulus will not begin another AP – inactivated Na+ channels must return to the resting state before they can be reopened – large fibers have absolute refractory period of 0.4 msec and up to 1000 impulses per second are possible Relative refractory period – a suprathreshold stimulus will be able to start an AP – K+ channels are still open, but Na+ channels have closed 41  The Action Potential: Summarized Resting membrane potential is -70mV Depolarization is the change from -70mV to +30 mV Repolarization is the reversal from +30 mV back to -70 mV) 42  Local Anesthetics Local anesthetics and certain neurotoxins – Prevent opening of voltage-gated Na+ channels – Nerve impulses cannot pass the anesthetized region Examples: – Novocaine and lidocaine 43  Propagation of Action Potential An action potential spreads (propagates) over the surface of the axon membrane – as Na+ flows into the cell during depolarization, the voltage of adjacent areas is affected and their voltage- gated Na+ channels open – self-propagating along the membrane The traveling action potential is called a nerve impulse 44  Continuous vs. Saltatory Conduction Continuous conduction (unmyelinated fibers) – step-by-step depolarization of each portion of the length of the axolemma Saltatory conduction (myelinated fibers) – depolarization only at nodes of Ranvier where there is a high density of voltage-gated ion channels – current carried by ions flows through extracellular fluid from node to node 45  Saltatory Conduction Nerve impulse conduction in which the impulse jumps from node to node 46  Speed of Impulse Propagation The propagation speed of a nerve impulse is not related to stimulus strength. – larger, myelinated fibers conduct impulses faster due to size & saltatory conduction Fiber types – A fibers largest (5-20 microns & 130 m/sec) myelinated somatic sensory & motor to skeletal muscle – B fibers medium (2-3 microns & 15 m/sec) myelinated visceral sensory & autonomic preganglionic – C fibers smallest (.5-1.5 microns & 2 m/sec) unmyelinated sensory & autonomic motor 47  Encoding of Stimulus Intensity How do we differentiate a light touch from a firmer touch? – frequency of impulses firm pressure generates impulses at a higher frequency – number of sensory neurons activated firm pressure stimulates more neurons than does a light touch 48  SIGNAL TRANSMISSION AT SYNAPSES A synapse is the functional junction between one neuron and another or between a neuron and an effector such as a muscle or gland 49  Signal Transmission at Synapses 2 Types of synapses – electrical ionic current spreads to next cell through gap junctions faster, two-way transmission & capable of synchronizing groups of neurons – chemical one-way information transfer from a presynaptic neuron to a postsynaptic neuron – axodendritic -- from axon to dendrite – axosomatic -- from axon to cell body – axoaxonic -- from axon to axon 50 51  Chemical Synapses Action potential reaches end bulb and voltage-gated Ca+2 channels open Ca+2 flows inward triggering release of neurotransmitter Neurotransmitter crosses synaptic cleft & binding to ligand-gated receptors – the more neurotransmitter released the greater the change in potential of the postsynaptic cell Synaptic delay is 0.5 msec One-way information transfer 52  Excitatory & Inhibitory Potentials The effect of a neurotransmitter can be either excitatory or inhibitory – a depolarizing postsynaptic potential is called an EPSP it results from the opening of ligand-gated Na+ channels the postsynaptic cell is more likely to reach threshold – an inhibitory postsynaptic potential is called an IPSP it results from the opening of ligand-gated Cl- or K+ channels it causes the postsynaptic cell to become more negative or hyperpolarized the postsynaptic cell is less likely to reach threshold 53  Removal of Neurotransmitter Diffusion – away from synaptic cleft Enzymatic degradation – acetylcholinesterase Uptake by neurons or glia cells – neurotransmitter transporters – Prozac = serotonin reuptake inhibitor 54 Comparison of Graded & Action Potentials Origin – GPs arise on dendrites and cell bodies – APs arise only at trigger zone on axon hillock Types of Channels – GP is produced by ligand or mechanically-gated channels – AP is produced by voltage-gated ion channels Conduction – GPs are localized (not propagated) – APs conduct over the surface of the axon 55 Comparison of Graded & Action Potentials Amplitude – GPs vary depending upon stimulus – amplitude of the AP is constant (all-or-none) Duration – The duration of the GP is as long as the stimulus lasts Refractory period – The AP has a refractory period and the GP has none 56  Summation If several presynaptic end bulbs release their neurotransmitter at about the same time, the combined effect may generate a nerve impulse due to summation Summation may be spatial or temporal 57  Spatial Summation Summation of effects of neurotransmitters released from several end bulbs onto one neuron 58 Temporal Summation Summation of effect of neurotransmitters released from 2 or more firings of the same end bulb in rapid succession onto a second neuron 59  Neurotransmitters Both excitatory and inhibitory neurotransmitters are present in the CNS and PNS; the same neurotransmitter may be excitatory in some locations and inhibitory in others. Important neurotransmitters include acetylcholine, glutamate, aspartate, gamma aminobutyric acid, glycine, norepinephrine, epinephrine, serotonin and dopamine. 60  Neurotransmitter Effects Neurotransmitter effects can be modified – synthesis can be stimulated or inhibited – release can be blocked or enhanced – removal can be stimulated or blocked – receptor site can be blocked or activated Agonist – anything that enhances a transmitters effects Antagonist – anything that blocks the action of a neurotranmitter 61  Small-Molecule Neurotransmitters Acetylcholine (ACh) – released by many PNS neurons & some CNS – excitatory on NMJ but inhibitory at others – inactivated by acetylcholinesterase Amino Acids – glutamate released by nearly all excitatory neurons in the brain – GABA is inhibitory neurotransmitter for 1/3 of all brain synapses (Valium is a GABA agonist -- enhancing its inhibitory effect) 62  Small-Molecule Neurotransmitters Biogenic Amines – modified amino acids (tyrosine) norepinephrine -- regulates mood, dreaming, awakening from deep sleep dopamine -- regulating skeletal muscle tone serotonin -- control of mood, temperature regulation & induction of sleep 63  Small-Molecule Neurotransmitters ATP and other purines (ADP, AMP & adenosine) – excitatory in both CNS & PNS Gases (nitric oxide or NO) – formed from amino acid arginine by an enzyme – formed on demand and acts immediately diffuses out of cell that produced it to affect neighboring cells may play a role in memory & learning – first recognized as vasodilator that helps lower blood pressure 64  Neuropeptides 3-40 amino acids linked by peptide bonds Substance P -- enhances our perception of pain Pain relief – enkephalins -- pain-relieving effect by blocking the release of substance P – acupuncture may produce loss of pain sensation because of release of opioid-like substances such as endorphins or dynorphins 65  Strychnine Poisoning In spinal cord, an inhibitory neurotransmitter (glycine) is normally released onto motor neurons preventing excessive muscle contraction Strychnine binds to and blocks glycine receptors in the spinal cord Massive tetanic contractions of all skeletal muscles are produced – when the diaphragm contracts & remains contracted, breathing can not occur 66  Regeneration & Repair Plasticity maintained throughout life – sprouting of new dendrites – synthesis of new proteins – changes in synaptic contacts with other neurons Limited ability for regeneration (repair) – PNS can repair damaged axons – CNS no repairs are possible 67  Repair within the PNS Axons may be repaired if: – neuron cell body remains intact – Schwann cells remain active and form a tube – scar tissue does not form too rapidly Within several months, regeneration occurs – Schwann cells on each side of injury repair tube – axonal buds grow down the tube to reconnect (1.5 mm per day) 68  Neurogenesis in the CNS Formation of new neurons from stem cells was not thought to occur in humans – 1992 -- a growth factor was found that stimulates adult mice brain cells to multiply – 1998 -- new neurons found to form within adult human hippocampus (area important for learning) There is a lack of neurogenesis in other regions of the brain and spinal cord 69  Multiple Sclerosis (MS) Autoimmune disorder causing destruction of myelin sheaths in CNS – sheaths becomes scars or plaques – 1/2 million people in the United States – appears between ages 20 and 40 – females twice as often as males Symptoms include muscular weakness, abnormal sensations or double vision Remissions & relapses result in progressive, cumulative loss of function 70  Epilepsy The second most common neurological disorder – affects 1% of population Characterized by short, recurrent attacks initiated by electrical discharges in the brain – lights, noise, or smells may be sensed – skeletal muscles may contract involuntarily – loss of consciousness Epilepsy has many causes, including; – brain damage at birth, metabolic disturbances, infections, toxins, vascular disturbances, head injuries, and tumors 71