Nervous Tissue Chapter 12

Nervous System Controls and integrates all body activities Basic functions: Sense change Interpret and remember change React to changes

Nervous vs Endocrine System Nervous system electrical fast local Endocrine system chemical slow general

Nervous System Processing Sensory Motor inputoutput Integration stimulusresponse CNS PNS

Major Structures of the Nervous System

Organization of Nervous System Peripheral Nervous System - PNS somatic (SNS) sensory motor autonomic (ANS) sensory motor parasympathetic sympathetic enteric (ENS) Central Nervous System – CNS brain spinal cord

Organization of Nervous System

Transmit electrical impulses (action potentials) Neurons Functional unit of the Nervous System

Structural Classes of Neurons

Functional Classes of Neurons Afferent

Efferent Functional Classes of Neurons

Interneurons Functional Classes of Neurons

Supportive and protective role CNS Astrocytes Oligodendrocyte Microglia Ependymal cells PNS Schwann cells Neuroglia

Schwann Cells Node of Ranvier: exposed axon between Schwann cells

Myelination and Schwann Cells

Gray and White Matter

Overview of Nervous Function

Ion Channels Leakage channels Voltage-gated channels Ligand-gated channels Mechanically gated channels

Ion Channels

Resting Membrane Potential -70 mV difference between outside and inside of the membrane, maintained by active transport High concentration of Na + on outside High concentration of K + on inside Cannot diffuse freely because gates of their channels are closed

Negative ions along inside of cell membrane & positive ions along outside –potential energy difference at rest is -70 mV Resting potential exists because –concentration of ions different inside & outside extracellular fluid rich in Na + and Cl - cytosol full of K +, organic phosphate & proteins –membrane permeability differs for Na + and K x’s greater permeability for K+ inward flow of Na + can’t keep up with outward flow of K + Na + /K + pump removes Na + as fast as it leaks in Resting Membrane Potential

Graded Potential Typically on dendrites or cell body Graded means that potential varies in amplitude. Stronger the stimulus, greater the amplitude. Stronger the stimulus the farther it will travel. Decreases as it gets farther away from the stimulus point.

Graded Potential

Action Potential

Action Potential Summary

Origin –GPs arise on dendrites and cell bodies –APs arise only at trigger zone on axon hillock Types of Channels –AP is produced by voltage-gated ion channels –GP is produced by ligand or mechanically- gated channels Conduction –GPs are localized (not propagated) –APs conduct over the surface of the axon Comparison of Graded & Action Potentials

Amplitude –amplitude of the AP is constant (all-or-none) –graded potentials vary depending upon stimulus Duration –The duration of the GP is as long as the stimulus lasts Refractory period –The AP has a refractory period due to the nature of the voltage-gated channels, and the GP has none.

Comparison of Graded & Action Potentials

Continuous vs Saltatory Conduction

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 µm & m/sec) myelinated somatic sensory & motor to skeletal muscle –B fibers: medium (2-3 µm & 15 m/sec) myelinated visceral sensory & autonomic preganglionic –C fibers: smallest ( µm & m/sec) unmyelinated sensory & autonomic motor

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

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

Chemical Synapse

Postsynaptic Potential Excitatory postsynaptic potential (EPSP) Na + and K + gates open at the same time, Na + diffuses faster results in a depolarizing potential Inhibitory postsynaptic potential (IPSP) Membrane made more permeable to K + and Cl -, Na + not affected results in a hyperpolarization

Postsynaptic potentials

Removal of Neurotransmitter Neurotransmitter must be removed from the synapse for normal synaptic function. - Diffusion - Enzymatic degradation - Uptake by cell

Summation

Summary

Neurotransmitters Acetylcholine Amino Acids glutamate and aspartate GABA and glycine Biogenic amines norepinephrine epinephrine dopamine serotonin ATP and Other Purines Nitric oxide Neuropeptides endorphins enkephalin dynorphins substance P

Neural Circuits

Regeneration