BIO 201 Chapter 12, Part 1 Lecture

8/14/2019 BIO 201 Chapter 12, Part 1 Lecture

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Chapter 12, Part 1Nervous Tissue



Overview of the NervousS stem

The nervous system, along with theendocrine system, helps to keep

controlled conditions within limitsthat maintain health and helps tomaintain homeostasis.

The nervous system is responsible for

all our behaviors, memories, andmovements.

The branch of medical science that



Major Structures of theNervous System



Overview of MajorStructures Twelve pairs of cranial nerves. 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, containingprimarily cell bodies of neurons.

Enteric plexuses help regulate the digestivesystem.

Sensory receptors are either parts of neurons or

specialized cells that monitor changes in theinternal or external environment.



Functions of NervousS stem

Sensory function: to sense changes in theinternal and external environment throughsensory receptors.

Sensory (afferent) neurons serve this function. Integrative function: to analyze the sensory

information, store some aspects, and makedecisions regarding appropriate behaviors.

Association or interneurons serve this function. Motor function is to respond to stimuli by

initiating action.

Motor(efferent) neurons serve this function.



Nervous SystemDivisions

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



Structure of a MultipolarNeuron



Histology of the NervousSystem: 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

microtubules move material inside cell lipofuscin pigment clumps (harmless aging)

Cell processes = dendrites & axons



Structural Classification of Neurons



Sensory receptors that are



CNS Neurons



Axonal Transport

Cell body is location for most proteinsynthesis 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 flowmoves organelles & materials along surface of microtubules

at 200-400 mm per day

transports in either direction



Neuroglia of the CNS

Most commonglial cell type

oligodendrocyte

forms myelinsheath aroundmore than oneaxons in CNS

Analogous toSchwann cells of PNS



Neuroglia of the PNS

Schwann cells encircling PNS axons Each cell produces part of the myelin

sheath surrounding an axon in the PNS



Myelinated andunmyelinated axons



Organization of theNervous System



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 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



Electrical Signals inNeurons

Neurons are electrically excitable dueto the voltage difference across their

membraneCommunicate with 2 types of electricsignals action potentials that can travel long

distances graded potentials that are local

membrane changes only



Overview of Nervous SystemFunctions



Types of Ion Channels

Leakage (nongated) channels are alwaysopen nerve cells have more K+ than Na+ leakage

channels as a result, membrane permeability to K+ is higher explains resting membrane potential of -70mV in

nerve tissue Ligand-gated channels open and close in

response to a stimulus results in neuron excitability

Voltage-gated channels respond to a directchange in the membrane potential.

Mechanically gated ion channels respond

to mechanical vibration or pressure.



Ion channels in plasmamembrane



Resting MembranePotential 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+ and Cl

cytosol full of K+, organic phosphate & amino

acids membrane permeability differs for Na+ and K+

50-100 greater permeability for K+

inward flow of Na+ can’t keep up withoutward flow of K+



Resting MembranePotential



Factors that contribute toresting membrane potential



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 inamplitude (size), depending on the strengthof the stimulus and localized .

Graded potentials occur most often in thedendrites and cell body of a neuron.



Hyperpolarized/DepolarizedGraded Potential

G d d t ti l i t



Graded potentials in response toopening mechanically-gated



Stimulus strength andgraded potentials



Summation



Generation of ActionPotentials

An action potential (AP) or impulse is asequence of rapidly occurring events thatdecrease and eventually reverse the

membrane potential (depolarization) and thenrestore 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 stimulusreaches threshold, the action potential is always thesame.

A stronger stimulus will not cause a larger impulse.



Action Potentials



Stimulus strength and ActionPotential generation

Ch i i fl d i



Changes in ion flow duringdepolarizing and repolarizing



Refractory period

Period of time during whichneuron can not generateanother 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 arepossible Relative refractory period

a suprathreshold stimulus will be able to start an AP K+ channels are still open, but Na+ channels have

closed

E d f Ch 12 P



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BIO 201 Chapter 12, Part 1 Lecture