OCR F214 Nerves

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F214 Module 1: Communication and Homeostasis
4.1.2 Nerves
(a) outline the roles of sensory receptors in mammals in converting different forms of energy into nerve
impulses
Sensory receptors ­ specialised cells that can detect changes in our surroundings. They are energy transducers
that convert one form of energy to another . A transducer is adapted to detect changes in a particular form of
energy.
Stimulus a change in energy levels in the environment. Whatever the stimulus the sensory receptors convert
the energy into a form of electrical energy called nerve impulse.
Stimulus Receptor Energy Change
Light intensity Rods and cones in retina of the eye Light Electrical
Length of muscles Proprioceptors muscle spindles Kinetic Electrical
Pressure on skin Pressure receptors (Pacinian corpuscles ) Kinetic Electrical
on skin
Soluble chemicals Taste buds in the tongue , hard palate, Chemical Electrical
epiglottis and the first part of the
oesophagus
Volatile chemicals Olfactory cells lining the inner surface in Chemical Electrical
the nasal cavity
Vibrations in the air Sound receptors in the inner ear (cochlea ) Sound Electrical
(b) describe, with the aid of diagrams, the structure and functions of sensory and motor neurones
There is a number of different neurones, including:
Sensory neurone carry action potentials from a sensory receptor to the central nervous system.
Relay neurone connect sensory and motor neurones.
Motor neurone carry action potentials from the central nervous system to an effector, e.g. muscle or
gland.
Sensory Neurone Motor Neurone
Similarities Long transmit action potentials over a long distance.
Cell surface (plasma) membrane has many gated ion
channels that control the entry and exit of sodium,
potassium or calcium ions.
Cell surface (plasma) membrane has many
sodiumpotassium ion pumps that use ATP to actively
transport sodium ions out the cell and potassium ions
into the cell maintain a potential difference.
They are surrounded by the myelin sheath (fatty sheath)
made up of a series of Schwann cells that insulates the
neurone from electrical activity . Gaps between the
Schwann cells meet are called nodes of Ranvier .
They have a cell body that contains the nucleus , many
mitochondria and ribosomes .
Both have numerous dendrites connected to other
neurones.

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F214 Module 1: Communication and Homeostasis
4. This all results in a potential difference of 60mV between the inside and outside of the cell. The
difference is called the resting potential . The axon membrane is said to be polarised when it is in this
state.
(d) describe and explain how an action potential is generated
Action potential ­ the depolarisation of the cell membrane so that the inside is more positive than the outside,
with a potential difference across the membrane of +40mV .…read more

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F214 Module 1: Communication and Homeostasis
When a stimulus is at a higher intensity the sensory receptor will produce more generator potentials . This will
cause more frequent action potentials in the sensory neurone. When these arrive at the synapse they will cause
more vesicles to be released. Therefore this creates a higher frequency of action potentials in the postsynaptic
neurone . Our brain can determine the intensity of the stimulus from the frequency of signals arriving.…read more

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F214 Module 1: Communication and Homeostasis
Acclimatisation after repeated stimulation a synapse may run out of vesicles containing the transmitter
substance. The synapse is said to be fatigued , which explains why we soon get used to a smell or a
background noise . It may also help to avoid overstimulation of an effector, which could damage it.
Creation of specific pathways thought to be the basis of conscious thought and memory .
Allows temporal and spatial summation .…read more

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