Biology

Nervous system

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  • Created by: steph
  • Created on: 03-06-10 20:03

The nervous system

Function of the Nervous System : to detect a change or stimuli in the body or its surroundings, process the information and initiate a response.

Receptors: sensory organs e.g. ears or sensory cells e.g. skin

Chemoreceptors - changes in chemicals

Photoreceptors - changes in light

Thermoreceptors - changes in temperature

Effectors: glands or muscles

Central Nervous System: consists of the brain and spinal cord

- it processes information and initiates a response

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Spinal Cord

Spinal Cord : protected by the vertebral column where most peripheral neurones originate

Grey Matter: consists of nerve cell bodies

White Matter: consists of nerve fibres

Sensory neurones: enter the spinal cord via the dorsal route

Motor neurones: leave the spinal cord via the ventral route

Function of the spinal cord

- relay information in and out of any point along the spinal cord

- relay information up and down the body including to and from the brain

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Reflex Actions

Reflex Action - an involuntary response (it does not involve the brain) resulting form a nerve impulse initiated by a stimulus that is usually a protextive function

Non protective reflex actions : coughing, swallowing, blinking

Reflex action due to hot surface

  • stimulus - hot surface
  • detected by pain and heat receptors in the sking
  • sensory neurone passes this impulse to the relay neurone
  • relay neurone passes the impulse to the motor neurone
  • motor neurone passes impulse to the effector
  • response - muscle contracts removing hand from surface

Inhibitory nerve fibres - allow the brain to override reflex actions

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Nerve Nets

Nerve nets - simple cells with extensions branching in different directions

They connect sensory photoreceptors to touch sensitive nerve cells

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Neurones

Neurones - highly specialised cells that generate and transmit nerve impulses

Sensory neurons - relay impulses from the the receptors to the central nervous system

Relay neurones - relay information from sensory neurones and other intermediate neurones to motor neurones and other intermediate neurones

Motor neurones - pass information from the CNS to the effector

Dendrites - thin extensions that pass information from other neurones to the cell body

Axon - carries impulses away from the nerve cell body and branches into extensions which form synapses

Schwann cells - grow on the axon to form a myelin sheath that has intervals known as nodes of ranvier

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Cathode Ray Oscilloscope

Cathode Ray Oscilloscope

- measures the magnitude and speed of nerve transmission

- analyses the pattern of impulse in different parts of the nervous system

Voltage: the difference in charge inside and outside

Electricity : the movement of charged particles

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Action Potential & Resting Potential

Resting Potential - the difference in charge between the inside and outside of a membrane when there is no impulse being transmitted

Action Potential - the potential difference across a membrane from a negative value of -70mv to a positive value of +40mv

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How Action Potential Travels along a membrane

  • the impulse creates a small electrical current along the membrane
  • as depolorisation of one portion occurs the depolorisation of the next portion is initiated
  • sodium pumps restore the resting potential
  • once the resting potential is restored another impulse can be transmitted
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Action Potential

  • impulse increases permeability of the membrane
  • potassium and sodium ions flood in
  • this allows the electrical impulse to move along the membrane
  • the impulse must be greater than -60mv for an action potential to be initiated
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Repolorisation

  • permeablitiy decreases
  • sodium and potassium are pumped out
  • there is a potential difference of -90mv
  • there is a refractory period where the potential difference is restored to -70mv
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All or Nothing Law

size of the impulse is independent of the size of the stimulus

the impulse has to reach above a certain threshold or no action potential is initiated

the stronger the stimulus the greater the frequency of action potential

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Factors Affecting Speed of Transmission

Myelination

- acts as an insulator so action potentials and depoloristation can only occur at the nodes of ranvier

- this means the impulse jumps from node to node speeding up transmission

Diameter of the Axon

- the larger the axon the greater the speed of transmission

- giant axons are found in squid and are associated with rapid escape responses

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