a2 edexcel biology session 5.8 nervous system and sense organs 1-3.pptx

Session 5.8

Unit 5: Energy, Exercise and CoordinationTopic 8: Grey matter

Session 5.8 The nervous system and sense organs 1/3Introduction

Neurones – structure and functionReflex arcs and reflex actionTransmission of an impulse

- Resting potential- Action potential

A2 Edexcel Biology Xavier DANIEL, Ph.D.

Introduction


Sensitivity =

ability to detect changes in environment (external/internal) and respond appropriately

Individual cells (prokaryotes, unicellular animals/plants)

Entire organisms (prokaryotes , Animals, Plants)

Changes = stimulusReceptor detects stimulusResponse done by effector

Sensitivity

Receptor and effector often not close

need internal communication Animals’ internal communicationNervous system and Endocrine system

Session 5.8






Neurones – structure and function


All neurons have cell body: Nucleus + cytoplasm

From cell body, very fine and long cytoplasmic fibres = Axon

CF specialized for transmission of information = impulses

Speed of impulses = 30-120 meters per second !!!

Nervous coordination extremely FAST

Response to stimuli almost instantaneous

motor neuron



Relay neuron

Stimulus

Response

unipolar

unipolar

Multi-polar

Receptor12

3

45 Effector



Motor neurones

Reception of impulses from several precedent neuronesTransmit information to cell body

Conduction of impulse from cell bodyTo motor plate on muscle fibers

Judges if worth it sending impulse itselfIntegrationDown the axon

1

2

3

Transmission of impulse from motor plateMakes muscle fibers contract

4

MN have many dendrites



Motor neurones

5 Wrap themselves around axon to protect itSuccession of SC = myelin sheath for electric insulation

6 Between two consecutive Schwann cellsIncreases speed of impulse conduction



Myelin sheath and Nodes of Ranvier

Myelin is a lipid that prevents ions to go in/out of axonSpeeds up conduction of nerve impulse along axon



Review

motor neuron

Session 5.8






Reflex arcs and reflex action


Reflex arcs and reflex actionThe pathwayConnects sense cell/organ (receptor) with muscle or gland (effector)Via neurones

The responseRapid, automatic, short-lived response to stimulusInvoluntary (reflex)Does not involve central nervous system

One given stimulus always triggers the same response

Session 5.8






Transmission of an impulse- Resting potential


Transmission of an impulse- Resting potential


“Resting potential”Cell not conducting impulse

Na+/K+ pump: Active transport Na+ and K+ across cell membrane

Na+ OUT 3 Na+ cannot re-enter cell by simple diffusion

K+ IN 2 K+ can diffuse out of the cell through K+ channels

out

in

Na+

Na+ K+

K+

Overall: more negative inside cell

(more positive outside cell)Hence the RMP

-70mV

Session 5.8






Transmission of an impulse- Action potential


Transmission of impulse

Impulse transmitted along nerve fibers

NOT electrical current

Rather momentary reversal in electrical potential difference across membrane of nerve fibers

=Change in amounts of anions/cations inside/outside membrane of nerve fibers

Reversal travels along axon in milliseconds



Stimulus

If transmitted along the axon: Action potential

= hyperpolarization


- Generation of action potential


VGSC

The actors


Generation of action potentialVGKC


- Generation of action potential The actors


Let us see this in 8 steps…


- Generation of action potential The process


Threshold potential

1 At resting potential-70mV

Resting potential

VGSC closed

VGKC closed

out

in

Na+

Na+ K+

K+






Threshold potential

2 Depolarizationby stimulusreachesmembranethreshold potential

VGSC open

Action potentialgenerated

VGSC open

VGKC closed

Resting potential

out

in

Na+

Na+ K+

K+




3 Na+ enters the celldown gradient

Depolarizationnear VGSC

VGSC opened

VGKC closed

Threshold potential

Resting potential

out

in

Na+

Na+ K+

K+




4 Na+ enters the celldown gradient

Depolarizationincreases

out

in

VGKC closed

Threshold potential

Resting potential

K+

K+

Na+

Na+

VGSC opened




VGSC close

VGKC open

Threshold potential

Resting potential

out

in

5 Action potentialPeaks+30mV

VGSC close

VGSC cannot respond to stimulusRefractory period

VGKC open

K+Na+

K+Na+

Xavier DANIEL, Ph.D.A Edexcel

Generation of action potential The process

VGSC closed

VGKC opened

out

in

6 K+ leaves the celldown gradient

Repolarization starts

Threshold potential

Resting potential

K+Na+

K+Na+




Threshold potential

Resting potential

out

in

VGSC closed

VGKC opened

7 K+ leaves the celldown gradient

Repolarization untilHyperpolarization K+

Na+ K+

Na+




VGSC closed

VGKC close

Threshold potential

Resting potential Resting potential

out

in

8 Na+/K+ pumpagainst gradients

Membrane potentialreturns to Resting potential

VGKC close

VGSC responsive againReady for new stimulus

Na+

Na+ K+

K+



Transmission of impulse

Impulse transmitted along nerve fibers

NOT electrical current

Rather momentary reversal in electrical potential difference across membrane of nerve fibers

=Change in amounts of anions/cations inside/outside membrane of nerve fibers

Reversal travels along axon in milliseconds



- Generation of action potential Review

a2 edexcel biology session 5.8 nervous system and sense organs 1-3.pptx

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