Nerves

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Roles of sensory receptors

  • Light sensitive cells in the retina detect light intensity and range of wavelengths (colour)
  • Olefactory cells in the nasal cavity detect the presence of volatile chemicals
  • Tastebuds detect the presence of soluble chemicals
  • Pressure receptors in the skin detect pressure on the skin
  • Sound receptors in the cochlea detect vibrations in the air
  • Muscle spindles detect the length of muscle fibres

These are all transductors and convert the stimulus to a nerve impulse

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Structure and functions of sensory and motor neuro

Motor neurones: 

  • A cell body at the end of the neurone, in the central nervous system, CNS
  • Many short dendrites that carry impulses towards the cell body
  • A long axon which carries an impulse away from the cell body and which ends in a motor end plate

Sensory neurones:

  • A cell body in the centre of the neurone, in the peripheral nervous system, PNS
  • A dendron carrying nerve impulses from a receptor towards the cell body. There are dendrites at the end of the dendron
  • An axon (shorter than the motor one) carrying an impulse from the cell body to the central nervous system
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Maintaining resting potential

When not conducting an impulse, the potential difference across the membrane is -60mV.
Sodium-Potassium pumps actively transport 3 Na+ ions out for every 2 K+ ions in. The axon contains organic anions, which the membrane is impermeable to. Slight loss of K+ ions through the permeable memrabne. 
Membrane is impermeable to Na+ ions.

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Generating and transmitting an action potential

1. The membrane is at resting state; -60mV inside compared to outside. Polarised.
2. Na+ ion channels open and some Na+ ions diffuse in.
3. The membrane depolarises - it becomes less negative with respect to the outside and reaches the threshold potential of -50mV. 
4. Voltage-gated sodium ion channels open and there is a rapid influx of Na+ ions. As more Na+ ions enter, the more positively charged the cell becomes compared to the outside.
5. The potential difference across the membrane reaches +40mV. The inside is now positive compared to the outside.
6. The Na+ ion channels close and the K+ ion channels open.
7. K+ ions diffuse out of the cell, bringing the potential difference back to negative compared with the outside. The cell is repolarised.
8. The potential difference overshoots slightly, making the cell hyperpolarised.
9. The original potential different is restored, so the cell returns to its resting state, -60mV.

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Frequency of impulse

A neurone will either conduct an action potential or not; this is described as the all or nothing law. A stimulus at a higher intensity does not cause a larger impule, they will cause the sensory neurones to produce more generator potentials so more frequent action potentials in the sensory neurone. This means more vesicles released at the synapse, a higher frequency of action potentials in the postsynaptic neurone, and a higher frequency of impulses to the brain

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Myelinated and non-myelinated neurones

The myelin sheath is an insulating layer of fatty material secreted by the Schwann cells. Na and K ions cannot pass through this layer so it prevents the movement of ions into and out of the axon, preventing depolarisation.
It speeds up the transmission of the impulse (action potential) as the action potentials can only occur at the nodes of Ranvier.
The action potential 'jumps' from one node to the next by causing larger local currents in the axon - saltatory condution

Myelinated neurones: 100-120ms-1 transmission speed, up to 1m transmission distance, fast response time, used in movement, 1/3 of all neurones, one neurone is surrounded by one Schwann cell, wrapped round many times.

Non-myelinated neurones: 2-20ms-1 transmission speed, mm or cm transmission distance, slow response time, used in breathing and digestion, 2/3 of all neurones.

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Structure of cholinergic synapse

The synaptic knob contains:

  • Many mitochondria
  • A large amount of smooth endoplasmic reticulum
  • Vesicles containing acetylcholine
  • There are also voltage-gated sodium ion channels in the membrane

The postsynaptic membrane contains:

  • Specialised sodium ion channels that will only open when acetylcholine binds to them
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Role of neurotransmitters

A neurotransmitter is a chemical that diffuses across the cleft of the cynapse to transmit a signal to the postsynaptic neurone. They cause the generation of a new action potential in the postsynaptic neurone. In cholinergic synapses the neurotransmitter is acetylcholine. It is stored in vesicles in the synaptic knob. When the action potential arrives:

  • Calcium channels open, and calcium ions diffuse into the synapse
  • Vesicles containing acetylcholine move towards the presynaptic membrane, and fuse with it
  • The vesicles release the acetylcholine by exocytosis into the synaptic cleft
  • Acetylcholine diffuses across the synaptic cleft, and binds to specifc receptors on the postsynaptic membrane
  • This causes sodium ion channels to open on the postsynaptic membrane
  • Sodium ions diffuse into the postsynaptic neurone and depolarise it

Synapses only allow transmission of the impulse in one direction because:

  • Only the presynaptic neurone produces acetylecholine
  • Only the presynaptic membrane has calcium ion channels
  • Only the postsyanptic membrane has acetylcholine (ACh) receptors
  • ACh is broken down at the postsynaptic membrane
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Role of synapses

  • Several presynaptic neurones may converge together to allow signals from different parts of the nervous system to create the same response
  • One presynaptic neurone may diverge to several postsynaptic neurones to allow one signal to be transmitted to several parts of the nervous system - one may elicit a response, and one may inform the brain
  • They ensure that signals are transferred in only one direction
  • They can filter out unwanted low-level signals, possible created by a low level stimulus. Several vescibles of acetylcholine must be released for an action potential to be created in the postsynaptic neurone
  • Low level signals can be amplified by summation (when several small potential charges combine to produce one larger charge in the potential membrane). It a low-level stimulus is persistant, it can generate several successive action potentials in the presynaptic neurone. The release of many vesicles of acetylcholine in a short space of time will enable the postsynaptic generator potentials to combine together to produce an action potential
  • Acclimatisation - after repeated stimulation, a synapse may run out of vesicles containing the transmitter substance. The synapse is said to be fatigued. This helps avoid overstimulation of an effector, which could damage it
  • The creation of specific pathways in the nervous system is thought to be the basis of conscious thought and memory.
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