Chapter 10



Nerve Impulse

Speed on nerve impulse


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Principles of Coordination

Two main forms of coordination in mammals:

Nervous system- uses nerve cells that can pass electrical impulses along their length. Response is quick, but short lived and only acts on localised region of body

Hormonal system- chemical messengers transported in the blood plasma, to reach target cells. Responses due to secretion of hormones often act over a longer period of time, slower to act.

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Coordination- Chemical Mediators

Nervous and hormonal forms of communication are only useful at coordinating the activites of the whole organism. At a cellular level, they are complimented by chemical mediators.

Chemical medators are secreted by individual cells and affect otherr cells in the immediate vincity.

Common example- imflamation og certain tissues when they are damaged or exposed to foreign agents

Two examples of chemical ediators are:

Histamine- stored in wbc, and is secreted due to the presence of antigens. It causes the diliation of blood vessels, increased permability of capillaries and therefore swelling the infected area

Prostaglandins- found in cell membrane and cause dilation of small arteries and arterioles. They are released due to injuries and increase the permability of capilleries. They also affect bp and neurotransmitters. They relieve pain

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Coordination- Differences between hormonal and ner


Transmition and response is slow

Travel to all parts of body

Transmition by blood system

Response is widespread and long lasting

Effect may be permament and irrevesible


Transmition and response is rapid

Travel to specific parts of the body

Transmition by neurons

Response is localised and short lived

Effect is temporary and reversible

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Coordination- Plants

Plants respond to external stimuli by means of plant growth factors,

Plant growth factors: exert their influence by effecting growth, aren't produced by a particular organ so are produced by all cells, only affect the tissues that actually produce them.

Control of IAA

Used to ensure plants grow towards light source. Decreases root growth and increases shoot growth

1) IAA released by cells in tip of shoot

2) IAA transported to all sides as it begins to move down shoot

3) Light causes movement of IAA from light side to dark side of shoot

4) There is a greater concentration og IAA built up on shaded sidee of shoot

5) Causes shaded side to elongate more

6) Shaded side grows fasterm cuasing shoot to bend towards light

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Specialised cells adapted to rapidly carry elctrochemical changes from one part of the body to another

Cell body- Nucleus. Large amounts of rough endoplasmic reticulum to produce neurotransmitters.

Dendrons- Extentions of the cell body. Carry nervous impulses to cell body

Axon- A single long fibre that carries nerve impulses away from the cell body

Schwann cell- Surrounds axon.Protection/ insulation of myelin. Remove cell debris, nerve regeneration 

Myelin Sheath- Made up from Schwann membrane producing myelin. Unmyelinated= scarry slower impulses

Node of Ranvier- Gaps between myelinated areas. 2-3 micrometers long, occur every 1-3 mm

Sensory Neuron- Transmit impulses from receptor to intermediate/motor neuron.

Motor Neuron- Transmit impulses from sensory/intermediate to effector. Long axon many, short dendrites

Intermediate Neuron- Transmit impulses between neurons. Numerous short processes

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Nerve Impulse

A self-prpagating wave of electrical distrubance that travels along the surface of an axon membrane

The temporary reversal of the electrical potential difference across and axon membrane. Reversal between 2 states:

Resting potential- no nerve impluse transmitted

Action potential- nerve impulse transmitted

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Nerve Impulse- Resting Potential

Na/K not lipid soluble, and cant cross plasma membrane.

Transported via intrinsic protien ion channels

Some intrinsic protien actively transport K ions into axon and sodium inos out. (sodium potassium pump)

Soduim Potassium Pump

2 Na ions pumped out for every K ion pumped in

Most gated K channels reamin open- K ions move out of axon down chemical gradient

Most gated Na channels remain closed

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Nerve Impulse- Action Potential

Temporary reversal of charge of the membrane. When p.d is +40mV, axon said to be depolarised

Occurs because ion channels open/close depending on voltage across membrane.

When action potential is reached, Na ion channels open and K close, allowing Na to flood into axon. Na positively charged= axon becomes positively charged

Passage of Action Potential alon umyelinated axon

Stimulus causes some voltage gated Na channels to open, Na ions move down electrochemical gradient

Causes more Na channles to open

When action potential reaches +40mV, Na channels close

Valtage gated K channels open and begin repolarisation of axon.

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Nerve Impulse- Hyper Polarisation

Inside of axon becomes more negative than usal due to an overshoot of K moving out of axon

K channels close

Sodium-Potassium Pumps re-established the -65mV resting potential

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Speed of Impulse- Factors Affecting

Myelin Sheath: prevents the action potential forming in myelinated areas of axon. Action potential jumps from one node of Ranvier to another (saltatory conduction). This increases tha speed of impulse as less action potentials need to occur

The greater the diameter the greater the speed of conductance: due to less leakage of ions from the axon

Tempuature: higher= faster nerve impulse. Energy for active traansport comes from respiration which is controlled by enzymes

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Speed of Impulse- Refractory Period

After and action potential, sodium voltage gated channels are closed, and sodium cant move into the axon. It is therefore impossible during this time for a furthur action potential to be generated. This time is known as the refractory peroid.

It ensures that an action potential can only be propagated iin one direction

It produced discrete impulses: ensures action potentials are seperated from one another

It limits the number of action potentials

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Speed of Impulse- All or Nothing

A stimulus must exceed a certain threshold value to trigger an action potential. A stimulus exceeds that value by a significant amount will produce the same strength of  action potential as if it had only just overcome the threshold. A stimulus can therefore only produce one action potential

An organism can percieve diifferent types of simulus in 2 ways:

The number of ipulses in a given time (larger stimulus=more impluses per second)

Having neurons with different threshold values- depending on which neurones are sending impulses and how frequently impulses are sent, the brain can interpret the strength of the stimulus

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Structure of Synapse

A synapase occurs when a dendrite of one neurone connects to the axon of another

Synapeses use neurotransmitters ti send impulses between neurones, The gap between 2 neurones is called the synaptic cleft. The neurone that produces neurotransmitters is called the pre-synaptic neuron. The axon of the presynaptic neuron ends at the pre-synaptic knob.

The presynaptic knobs consists of many mitochondria and endoplasmic reticulum. These organells are required to produce neurotransmitters which are store in synaptic vessicles.

These then flow with the pre-synaptic membrane to release the neurotransmitter.

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Functions of Synapse

  • A single impulse from neurone can be transmitted to several other neurones at a synapse. This means that one impulse can create a number of simultaneous responses.
  • A number of different impulses can be combined at a synapse. This means that several responses can be combined to give one single response.

Neurotransmitters are made only in the synaptic cleft

When an action potential reaches the presynaptic knob, it causes vesicles containing the neurotransmitter to flow with the presynaptic membrane.

The neurotransmitter will then diffuse across the synaptic cleft

The neurotransmitter then binds with receptor on the spost synaptic membrane. In doing so, generating a new action potential in the post synaptic neurone

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Features of Synapses


Impulses can be sent from the pre-synaptic membrane to the post-synaptic membrane.


Spatial summation- different pre-synaptic neurones togehter will release enough neurotransmitter to exceed the threshold value to form an action potential

Temporal summation- one neurone releasing neurotraansmitter many times over a short period. Eventually the neurotransmitter will accumulate so as to overcome the threshold value of the post-synaptic mebrane, therfore creating a new action potential.


Some post-synaptic membranes have protien channels that can allow chloride ions to diffuse into axon making it more negative than usual resting potential. This type of hyperpolarisation inhibits the postsynaptic neurone from generating a new action potential.

The importance of these inhibitory synapses is that it allows for nervous impulses to be controlled and stopped if necessary.

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Transmition Across Synapse

When the neurotransmitter across a synapse is acetylcholine, it is called a cholinergic synapse, which are most common in vertebrates. they occur in the CNS and neuromuscular junctions.

  • When the action potential reaches the presynaptic knob, calcium channels open, allowing calcium to diffuse into the presynaptic knob.
  • The influx of calcium ions cause synaptic vesicles to fuse with the presynaptic membrane, so releasing acetylcholine into synapptic cleft.
  • Acetylcholine diffuse across the cleft and fuses with receptor sites on sodium channeks found on the presynaptic membrane
  • When they do so, the sodium channles open, allowing sodium ions to diffuse along their concentration gradient into the post-synaptic knob. The influx of sodium ions generates a new action potential in the post-synaptic neuron
  • Acetylcholinesterase hydrolyses acetylcholineinto choline and ethnoic acid prevents it from continuosly generating a new action potential in post-synaptic membrane
  • ATP released by mitochondria, providing energy to combine acetyl and choline. Sodium channels on the postsynaptic membrane are now closed adue to the absense of acetylcholine attached to receptor sites
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