Chapter 14: Response to Stimuli

Action Potential

Change that occurs in the elctrical charge across the membrane of an axon when it is stimulated and a nerve impulse passes

Axoplasm

The cytoplasm of a neurone

Depolarisation

The temporary reversal of charges on the cell-surface membrane of a neurone that takes place when a nerve impulse is transmitted

Hyperpolarisation

When the membrane potential becomes more negative than the resting potential following an action potential or as a result of an inhibitory impulse

Impulse

a series of action potentials. Transmitted by saltatory conduction in myelinated axons

Ion Channel

a passage across a cell-surface membrane made up of a protein that spans the membrane and opens and closes to allow ions to pass in and out of the cell

Membrane Potential

The potential difference across a membrane

Refractory period

The p;eriod during which the membrane of the axon of a neurone cannot be depolarised and no new action potential can be initiated

Repolarisation

Return to the resting potential in the axon of a neurone after an action potential

Resting potential

the difference in electrical charge maintained across the membrane of the axon of a neurone when not stimulated

Saltatory conduction

Propagation of a nerve impulse along a myelinated dendron or axon in which the action potential jumps from one not of Ranvier to another

Sodium-Potassium pump

Protein channels across cell-surface membranes that use ATP to move sodium ions out of the cell in exchange for potassium ions that move in

Threshold potential

The minimum intensity that a stimulus must reach in order to trigger an action potential in a neurlne

Tissue fluid

Fluid that surrounds the cells of the body. Its composition is similar to that of blood plasma except that it lacks some of the larger proteins, in particular those that cause the blood to clot (Platelets) It supplies nutrients to the cells and removes waste products

Voltage-gated channel

a protein channel across a cellsurface membrane that opens and closes according to changes in the electrical potential across the membrane

The reversal of charge of the membrane

From -70mV to +40mV

Factors that affect the speed of an impulse

• The presence of a myelin sheath

• The diameter of the axon

• The temperature

In what way does the presence of a myelin sheath change the speed of an impulse

Increases speed by preventing action potential forming in myelinated areas of the axon forcing the action potential to jump from one node of Ranvier to another (this is saltatory conduction)

Therefore less action potentials need to occur

In what way does the diameter of an axon change the speed of an impulse

The greater the diameter the greater the speed of c onductance due to less leakage of ions from the axon

In what way does temperature change the speed of an impulse

a higher temperature results in a faster nerve impulse:

• Increase in the speed of diffusion

• Increases energy the energy for active transport (Energy comes from respiration which is controlled by enzymes as there is an optimum temperature, too hot and the proteins can become denatured)

All or nothing principle

• A stimulus must exceed a certain threshold value to trigger an action potential.

• The strength of the action potential will always be of the same strength regardless of the strength of the stimulus

• A stimulus can only produce one type of action potential

• A larger stimulus can result in a higher frequency of impulses per second and therefore the likelihood of a greater response

How many Sodium vs Potassium enter and leave the axon

Na⁺ = 3 (exit)

K⁺ = 2 (enter)

The membrane can only exist in two states.

What are they?

The resting potential - no nerve impulse is transmitted

The action potential - nerve impulse is transmitted

what is another term for an unmyelinated axon

a naked axon

What are the steps of passage of Action Potential (AP) in an unmyelinated axon

• One region of the axon produces an AP and becomes depolarised, the influx of Na⁺ acts as a stimulus for the depolarisation of the next region of the axon

• APs are propagated along each small region of the axon membrane

• The previous region of the membrane returns to its RP (Resting potential), undergoing repolarisation

Which neurone is unmyelinated

relay neurones

Why are relay neurones unmyelinated

Relay neurones are so short that it is not worth the energy cost

What are the steps of passage of Action potential (AP) in a myelinated axon

• myelin sheath around the axon acts as an electrical insulator, preventing action potentials forming

• Localised circuits arise between adjacent nodes of Ranvier and the action potential jumps from one node to the next (This is saltatory conduction)

• As a result, an AP passes along a myelinated neurone faster than an unmyelinated neurone

What is the time of AP across a myelinated neurone

90ms^-1

What is the time of AP across an unmyelinated neurone

30ms^-1

How is the size of a stimulus detected

The frequency of impulses

the larger the stimulus the higher the frequency of impulses generated

What is the purpose of the refractory period

`time is needed to restore the proteins of voltage sensitive ion channels to their original conditions

What does the refractory period ensure

• AP travels in one direction

• Discrete impulses

• Limit to the frequency of impulses

Discrete impulses

Separate packets of AP where there is a clear beginning and end

What is the potential difference of resting potential

-70mV

What is the potential difference of action potential

+40mV

What is the potential difference of the period of hyperpolarisation

-80mV

How is the resting potential sustained

• Sodium ion channels are closed

• Potassium ion channels are closed but leaky

• Sodium-Potassium pump is active

• This results in:

  • a high concentration of sodium ions outside the axon (they are being pumped out)

  • An even concentration of potassium ions inside and outside of the axon (Pumped in and diffusing out)

  • The presence of anions in the axoplasm

How is the action potential achieved

• A stimulus from a receptor causes some voltage-gated ion channels to open

• This allows sodium ions to move in down the electrochemical gradient and the axon is depolarised

• When the threshold potential is reached (55mV) more sodium ion channels open and potassium channels close

• This allows sodium to flood into the axon meaning that the inside of the axon is more positive in charge

• When the action potential reached +40mV sodium channels close and Potassiu channels open

• This allows potassium ions to diffuse out of the axon down the electrochemical gradient and begin repolarisation

• The inside of the axon becomes more negative than usual as a result of an overshoot in potassium diffusing out of the axon

• Potassium channels close but remain leaky

• The sodium-potassium pump reestablishes the -70mV resting potential

Autonomic nervous system

part of the nervous system, controlling the muscles and glands, that is not under voluntary control

Pressure receptor (Baroreceptor)

receptor cell capable of detecting changes in pressure

Central nervous system (CNS)

The complex of nerve tissues that controls the activities of the body

In vertebrates what does the CNS comprise of

the brain and spinal chord

Chemoreceptor

Receptor cell capable of detecting changes in the chemical composition of the solution surrounding it

Where are chemoreceptors found

Walls of the aorta, carotid artery and medulla oblongata detecting blood pH

Medulla oblongata

region of the brain responsible for modifying heart rate and breathing rate

Noradrenaline

Neurotransmitter that is responsible for increasing heart rate

Parasympathetic nervous system

part of the autonomic nervous system responsible for decreasing heart rate

Reflex action

Response to a stimulus as a result of impulses sent along a reflex arc

Reflex arc

the nerve pathway in the body taken in by an action potential that leads to a rapid involuntary response to a stimulus

Sinoatrial node

An area of heart muscle in the right atrium that controls and coordinates the contraction of the heart rate (Pacemaker)

Sympathetic nervous system

Part of the autonomic nervous system responsible for increasing heart rate

What are the 7 steps of a reflex arc

1. Stimulus

2. Receptor

3. Sensory neurone

4. Relay neurone

5. Motor neurone

6. Effector

7. response