Nervous Tissue Chapter 12. Nervous System Controls and integrates all body activities Basic functions: Sense change Interpret and remember change React.

Nervous Tissue

Chapter 12

Nervous System

Controls and integrates all body activities

Basic functions:Sense changeInterpret and remember changeReact to changes

Nervous vs Endocrine System

Nervous system

electrical

fast

local

Endocrine system

chemical

slow

general

Nervous System

Processing

Sensory Motor

input output

Integration

stimulus response

CNS

PNS

Major Structures of the Nervous System

Organization of Nervous System

Peripheral Nervous System - PNSsomatic (SNS)

sensorymotor

autonomic (ANS)sensorymotor parasympathetic

sympatheticenteric (ENS)

Central Nervous System – CNSbrain 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

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+

• 50-100x’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

Resting Membrane Potential

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

Graded Potential

Graded Potential

Graded Potential

Action 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

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 PropagationThe 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 & 12-130 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 (0.5-1.5 µm & 0.5-2 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

Summation

Summary

Neurotransmitters

Acetylcholine

Amino Acids glutamate and aspartateGABA and glycine

Biogenic amines norepinephrineepinephrinedopamineserotonin

ATP and Other Purines

Nitric oxide

Neuropeptidesendorphins

enkephalindynorphins

substance P

Neural Circuits

Regeneration