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Coordinating: The Reflex Arc


Stimuli are changes in the internal or external environment.


Receptor cells detect a stimulus.


These are the interneurones that connect to the sensory and motor systems.


These are the cells that effect a response, this includes both muscles and the glands


This is the product of a reflex arc, they aid survival.

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Nerve Cells

Humans have three different types of nerve cells which are actually called neurones.

Sensory Neurone

  • Long dendrons
  • The cell body is in the middle of the axon
  • Tranmits nerve impulse from sensory recpetors to CNS

Intermediate Neurone

  • A small cell with many interconnections
  • Link together a sensory neurone and a motor neurone
  • 99.9% of all neurones in the body are interneurones

Motor Neurones

  • Have long axons
  • Cell body is at the head of the cell
  • Transmit nerve impulses from the CNS to effectors
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Adaptions of Nerve Cells

Numerous Dendrites

  • Provides a large surface area
  • Better connection to other neurones

Myelin Sheath

  • Electrical Insulation
  • Physical protection of the axon

Nodes of Ranvier

  • Allows Saltory Conduction
  • Impulses must jump from node to node making them faster
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Resting Potential

1. Phospholipid bilayer prevents sodium and potassium ions from diffusing across it

2. A sodium potassium pump, pumps 3 sodium ions out of the membrane and 2 potassium ions into the axon via active transport

3. Potassium ion channel proteins allows potassium ions to diffuse out of the axon, showing the membrane is more permeable to potassium ions and not sodium ions

4. The movement of potasssium ions is greater than sodium ions, which then causes the outside of the membrane to be more positive than the inside of the membrane

5. A resting potential of -65 mV is set up inside the membrane, causing it to become depolarised

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Action Potential

1. Energy from the stimulus arrives at the synapse

2. This energy causes some of the sodium voltage gated channels to open, allowing sodium ions into the axons, since these ions are positively charged the charge of the membrane changes

3. As sodium ions diffuse in more of the sodium voltage gates channles open allowing even more sodium ions to entre via diffusion

4. An action potential of +40mV is established which then causes the voltage gated sodium ion channels to close and voltage gated potassium ion channels to open

5. As the electrochemical gradient is reversed more potassium ion channels open and a greater influx of potassioum ions diffuse out causing repolarisation

6. Hyperpolarisation occurs as the inside becomes more negative than the outside. All of the gates close but the sodium potassioum pump begins to pump out sodium ions and pump in potassium ions which then allows the resting potential to be established once more.

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Features That Help Nerve Impulses Work Effectively

Refactory Period

  • Rest period for each ion channel
  • Action potential can only travel downstream as upstream channels are resting
  • Therefore an action potential can only travel in one direction
  • It also means that action potentials are seperated from each other, one cannot be formed directly after another.
  • Also limits the number of action potentials that occur within a given time period

All Or Nothing

  • Ion channels are either open or closed there is no in between
  • This means an action potential always reach +40mV as it moves along and is never reduced while travelling down a long axon
  • Stimulus must overcome a certain threshold value to create an action potential
  • Even if the size of the action potential is larger than necessary only one impulse will be sent
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How Can Nerve Impulses Convey Strength & How They

Since nerve impulses are all-or-nothing, they don't vary in size. Therefore the strength of a stimulus is determined by frequency

Low Frequency

A low frequency would be a weak stimulus, examples of this include low vibrations and quiet sounds

High Frequency

A high frequency would be a strong stimulus, examples are high vibrations and loud sounds.

Propagating A Nerve Impulse

  • Once an action potential has started it is moved along the axon automatically
  • The reversal of voltage is detected by surrounding voltage gated channels, that open when the potential changes enough, for example when it's -30mV the sodium voltage gated channels open
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Factors That Affect Speed of Nerve Impulses


  • The higher the temperature the faster the impulse
  • Diffusion and enzymes affected by temp
  • Enzymes needed for respiration which provides ATP for active transport of ions

Axon Diameter

  • Larger the diamter the faster the impulse
  • Less leakage of ions, which make it hard to maintain membrane potentials

Myelin Sheath

  • Voltage gated channels only found at the nodes of ranvier. Myelin Sheath conducts the rest of axon
  • Saltory Propagation happens in which impulses jump from node to node
  • Increase speed because they have less distance to cover
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1. When the action potential arrives at the pre-synpatic neurone, voltage gated calcium ions open allowing calcium ions to enter the synpatic knob

2. The calcium ions cause the synpatic vesicles to fuse with the presynaptic membrane, the neurotransmitter (acetylcholine) is released by exocytosis

3. The neurotransmitter (acetylcholine) diffuses across the synaptic cleft

4. The neurotransmitter binds to neuroreceptors on sodium ion channels on the post-synaptic membrane this causes the channels to open

5. The post-synaptic membrane becomes depolarised which then causes an action potential to be created

6. The neurotransmitter is then broken down by a specific enzyme, to stop an action potential from being continously generated. The break down products (e.g ethanoic acid and choline) are absorbed by the presynaptic neurone and using ATP from the mitochondria is used to resynthesise  the neurotransmitter. Sodium channels close without the neurotransmitter

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Types Of Synapses

Excitatory Ion-Channel Synapses

  • Have neuroreceptors that are sodium channels
  • When channels open positive ions diffsused causing depolarisation
  • Action potentials are more likely

Inhibitory Ion-Channel Synapses

  • Neuroreceptors that are Cl- channels
  • When channels open negative ions diffuse in causing hyperpolarisation
  • Action potentials are less likely (Inhibit impulses from passing on)

Non-Channel Synapses

  • Neuroreceptors are membrane bound enzymes
  • When activated by neurotransmitters they catalyse Ca2+ in cell
  • Can alter number and sensitivity of ion channel receptors
  • Involved in slow and long lasting responses such as memory
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Types of Synapses Continued

Neuromuscular Junction

  • Synapses between motor neurones and muscle cells
  • Always uses the neurotransmitter acetylcholine
  • Is alsways excitatory

Electrical Synapses

  • The membranes of the cells touch and share proteins
  • Action potentials pass directly from one synapses to the next without neurotransmitter
  • Very fast and also rare in the human body
  • Occur in the heart and eye
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Synaptic Integration

Synapses are unidirectional and also recieve many inputs. Each nerve cell can only have one output and in order to make sure that they are able to overcome a threshold value they have two special features that enable this to happen

Spatial Summation

This is when a number of different synapses work together to release enough neurotransmitter in order to overcome threshold value of the post-synaptic neurone

Temporal Summation

This is when a single pre-synaptic neurone releases neurotransmitters many times over a short period of time. If the total amount of neurotransmitter exceeds the threshold value then an action potential is triggered.

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Pacinian Corpuscle

  • The Pacinian Corpuscle is a mechanoreceptor that detects pressure
  • It is found deep in the skin, so it requires intense pressure not just a light touch
  • When pressure is applied the stretch-mediated sodium channels open
  • The sodium ions causes local depolarisation and a generator potential is created

The greater the pressure the better chances of theshold value being overcome and an action potential being triggered

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Rod & Cone Cells


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