• Created by: ava.scott
  • Created on: 23-03-15 17:22

The nervous system


  • Detects changes or stimuli inside and around the body.
  • Processes information
  • Inititates responses.

Central Nervous System

  • The brain and spinal cord. 
  • Processes and initates a response.

Peripheral Nervous system

Pairs of nerves that originate from the brain or spinal cord e.g. sensory neurones and motor neurones.

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Responses to all stimuli involve the reception of information and its transfer from the receptor to an effector via the nervous system.

Stimuli- a detectable change in the internal or external environment.

Receptor cells detect these changes e.g. specialised sensory neurones (in the skin) or complex organs (the eye).

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Effectors, Reflex Arc and Hydra

Effectors are either muscles or glands.

The Reflex Arc

  • basis for protective involuntary actions
  • Related to nerve pathways involved in voluntary actions via ascending and descending tracts in the spinal cord.


  • Simple organism
  • Limited number of stimuli, so small amount of effectors
  • Nerve net system consists of simple nerve cells joined to eachother and branching in various directions.
  • Draw synaptic clefts (synapses) 
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Physiology of Spinal Cord 12 POINTS


  • central canal- fluid filled space in centre
  • grey matter- contains darkly staining cell bodies
  • white matter- contains myleinated axons
  • dorsal root- upper neurone (like fin!)
  • ventral root- lower neurone
  • sensory neurone- comes in through dorsal root
  • dorsal root ganglion- relays sensor information to spinal cord
  • connector/relay neurone- within the grey matter
  • motor neurone - leaves cord via ventral root
  • effector- muscle or gland that brings about response
  • meninges - outer layers of spinal cord
  • Spinal Nerve- big bundles of nerves coming in and out of spinal column.


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Mammalian Neurones SMR + 6 POINTS

  • Sensory neurones
  • Motor neurones
  • Relay (connector) neurones

Myleinated motor neurone.

  • Dendrites- carry impulse towards the cell body
  • Cell body- contains nucleus and granular cytoplasm. Ribosomes are grouped to form Nissl granules, concerned with formation of neurotransmitters substances.
  • Nucleus- contains genetic information, allowing protein synthesis.
  • Axon- carries impulse away from cell body.
  • Myelin Sheath- insulates action potential of neurone, and allows faster transmission via nodes of Ranvier. made from phospholipids
  • Nodes of Ranvier- gaps in the myelin sheath which the transmission jumps between.
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RESTING POTENTIAL + how to measure

Resting Potential- the potential difference between the inside and the outside of a membrane when a nerve impulse is not being conducted.

  • Resting potentials are typically minus values;
  • The inside of the membrane is negative.
  • The membrane is said to be polarised

We use microelectrodes and cathode rays oscilloscopes to measure the potential difference across the membrane of giant axons. e.g. SQUID

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Maintaining resting potential


  • The sodium-potassium pumps maintain the ineven distribution of sodium and potassium ions. These are transmembrane proteins (like the channels).
  • 2 Potassium ions are pumped into the membrane, for every 3 Sodium ions pumped out.


  • Potassium ion channels are open, so K+ ions diffuse out.
  • Sodium ion channels are closed, so cannot move in.


  • COO- and soluble negative proteins remain in the axon.


  • Axon membrane is 100 times more permeable to potassium ions than sodium ions.
  • Potassium ions diffuse out fast, as they are high in concentration.
  • Outside of the membrane is more positive than the inside.
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NERVE IMPULSE + Depolarisation

Impulse occurs from changes of Na+ and K+ permeability of the nerve cell membrane, leading to potential difference and an action potential.


  • Suitable stimulation of an axon results in change of potential across the membrane from a negative value to a postive value inside the membrane.
  • The membrane is said to be depolarised.
  • Membrane has the action potential of+40mV

Results from:

  • A sudden increase of permeability to Na+,
  • allowing a sudden influx of Na+ ions INTO the membrane.
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Repolarisation, Ultrapolarisation Refractory perio


  • K+ diffuse out quickly.
  • This is called repolarisation.


  • There is an overshoot of K+
  • Membrane is around -90mv

Refractory Period

  • The membrane returns to resting potential.
  • The sodium-potassium pump restores ionic balance.
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Refractory period effects

  • Another action potential cannot be generated.
  • This ensures a undirectional impulse and a limiting frequency.
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All or nothing law

  • Changes in the intensity of the stimulus do not affect the size of the action potential.
  • If the intensity is below the threshold, no action potential occurs.

Fails to react threshold potential, as too few sodium gates are opened.

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Transmission and myelination


The propagation of nerve impulse involves the stimulation of the next part of the membrane by local electric currents, causing depolarisation.


The depolarisation passes along the axon of the neurone, with repolarisation occurring just behind it. The next section of the axon is depolarised by local electric currents behind it.

MYELINATION and speed of conduction

  • Speeds up rate of transmission of impulses by increasing the distance over which the local currents can bring about depolarisation.
  • Speed of conduction also relies on diameter of the axon.
  • Greater the diameter, the greater the velocity of conduction.

Saltatory conduction- Transmission of a nerve impulse along a myelinated axon in which the action potential jumps from one node of Ranvier to another.

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Physiology of Synapse 6 points


  • Synaptic knob
  • synaptic vesicle
  • mitochondria
  • pre-synaptic membrane
  • synaptic cleft
  • post-synaptic membrane.
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Transmission is chemical rather than electrical.--- Synapse is between two neurones.

  • The chemical/neurotransmitter is enclosed in a synaptic vesicle at the end of the axon.
  • The arrival of the action potential depolarises the presynaptic membrane and causes Calcium ions to rush in.
  • Influx of calcium causes the vesicles to fuse with the presynaptic membrane, and release contents into the cleft.
  • by exocytosis
  • Neurotransmitter diffuses across the cleft, depolarising the post-synaptic membrane.
  • It does this by attaching to receptor sites on the gated sodium channels.
  • When both receptor sites are filled, the gate opens, and sodium ions flood into the post-synaptic membrane.
  • This propogates the impulse into the axon of the adjacent neurone.
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Transmitter substances

Most common

  • acetyl choline
  • noradrenaline


Breaks down acetylcholine in the synaptic cleft.

  • it breaks it into choline and ethanoic acid, which diffuse back into the pre-synaptic membrane.
  • ATP is required to reform acetlycholine and package them into vesicles.

This prevents:

  • merging of impulses
  • limits the effect of the action potential
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Functions of synapse 5 points

Conveys action potentials between neurones

  • Transmit information between neurones
  • passes impulse in one direction
  • acts as a junction- vesicles are only present in the presynaptic knob so only 1 direction.
  • filter out low level stimuli
  • protect from over stimulation.
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Organophoshorus insecticides 5 points


  • Mimic the action of neurotransmitters
  • They effect the post-synaptic membrane in the same way, so an action potential fires.
  • They could also prevent the breakdown of a transmitter, by inhibiting an enzyme.
  • Organophosphorus insecticides prolongs the effect of neurotransmitters by inhibiting the enzyme. 
  • If at a neuromuscular junction-  this can cause repeated firings, and therefore, a twitch.
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  • Chemicals that affect the CNS, altering brain function e.g. perception, conciousness and behaviour.
  • Excitory drugs stimulate the nervous system by creating more action potentials.
  • Inhibitory durgs inhibit the nervous system by creating fewer action potentials.


  • Drugs may block receptor sites to cause the effect.
  • The body then creates more receptor sites.
  • This is known as tolerance to a drug.
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Excitory and Inhibtory action

Excitory drugs

  • inhibit enzyme, preventing breakdown of neurotransmitter
  • keep sodium-gate channels open by mimcking action of neurotransmitter

Inhbitory drugs

  • Prevent release of neurotransmitter via vesicles.
  • Keep sodium-gated channels closed by mimicking shape of neurotransmitter
  • Stops Ca+ from entering synaptic knob
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Relay neuron 

  • Unmyelinated
  • In grey matter

Sensory neuron

  • Cell body in middle

Motor neuron

  • Cell body with dendrites 
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