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B1.2 Nerves and hormones
The nervous system enables humans to react to their surroundings and coordinate their behaviour.
o Any change in your surroundings e.g. temperature, visual, sound etc. is potentially a detectable stimulus to one
of you sensory organs e.g. skin, eyes, ears etc. The stimulus might be chemical, light, pain, position, pressure,
sound, temperature, touch etc.
You have five different sense organs which contain receptors that are sensitive to particular stimuli (changes in the
environment outside the organism). In the receptor cells the stimulus input is converted into an electrical signal - an
electrical impulse which is sent to the brain.
Sensory organ Receptor sensitive to
Ears Sound vibrations in the air
Changes in position enabling us to keep our balance
Skin Touch, pressure, pain and temperature changes.
Light receptor cells, like most animal cells, have a nucleus, cytoplasm and cell membrane.
Information in the form of an electrical signal, from receptors, passes along cells in nerves (neurones) to the
brain through the central nervous system (spinal cord brain)
The brain then coordinates the response
Reflex actions are automatic and rapid
They often involve sensory, relay and motor neurones.
Receptors Groups of cells that respond to a particular stimulus
Sensory neurones The nerve cells that carry the electrical impulse signal from the receptors in the sense organs to
the spinal cord and brain (central nervous system).
Relay neurones The nerve cells that carry the electrical signals from sensory neurones to the motor neurones
Synapse A connection between two neurones eg the junction between a sensory neurone and a relay
neurone, it enables the electrical impulse signal to reach the spinal cord and brain. Between the
end of one neurone, and the start of another, chemicals are released in the gap that transfers
the electrical signal.
Motor neurones The nerve cells that carry the electrical signals through the central nervous system from the
brain via the spinal cord to the effecter cells of the muscles or glands
Effectors The muscles or glands that respond in a variety of ways to the electrical signal from the brain
Reflex actions Automatic responses to stimuli detected by the receptors in the organs of the body.
They are an important defence mechanism of our body:
If in danger your body releases the hormone adrenaline to heighten your mental and physical response.
If the intensity of light impacting on your eye is too great, your pupil automatically gets smaller to allow less
light. In a dimly lit room, the opposite response occurs and your pupil widens to let more light in.
If something hot touches your skin, on feeling pain you immediately try to recoil from the heat source eg on
burning your hand, the muscles rapidly contract to take your hand away.
In a simple reflex action from a receptor to an effecter (via spinal cord and an unconscious part of the brain):
o stimulus detected by receptors causes impulses from a receptor to pass along a sensory neurone (nerve
cell) to the central nervous system,
o at a nerve junction (synapse) between a sensory neurone and a relay neurone in the central nervous system,
a chemical is released that causes an impulse to be sent along a relay neurone,
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The effector is either a muscle or a gland, a muscle responds by contracting and a gland responds by releasing
(secreting) chemical substances, examples:
Muscles in your arm may contract to withdraw your hand from a heat source or sharp point.
Glands may secrete a particular hormone in response to a particular stimulus eg adrenalin in a 'flight response'
from a dangerous situation.
The pupils in your eyes respond by decreasing/increasing in size if the light level is too high/low.…read more
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Hormones regulate the functions of many organs and cells
Several hormones are involved in the menstrual cycle of a woman and hormones are involved in promoting the release
of an egg:
The follicle stimulating hormone (FSH) is secreted by the pituitary gland and causes eggs to mature in the
ovaries and it also stimulates the ovaries to produce hormones including oestrogen,
The luteinising hormone (LH, also from the pituitary gland) stimulates the release of eggs from the ovary.…read more
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Plants are sensitive to light, moisture and gravity:
Their shoots grow towards light and against the force of gravity,
Their roots grow towards moisture and in the direction of the force of gravity.…read more