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1 Communication 2 Nerves 1
Cells communicate by cell signalling one cell Sensory receptors (e.g. rods and cones in the retina)
produces a chemical that is detected by another convert one form of energy (e.g. light) into electrical
cell. energy in a nerve impulse.
Cell signalling can be nervous (i.e. a Neurones carry impulses: sensory (from receptors to
neurotransmitter at a synapse) or hormonal (i.e. a CNS); motor (from CNS to effectors); and relay
hormone carried in blood). (connects sensory and motor).
It coordinates responses to stimuli from internal More Na ions pumped out of nerve membrane than
(e.g. temp and pH) and external (e.g. light, temp, K in, using ATP. Negative ions trapped inside, so
humidity) environments. inside of neurone membrane is negative.
A response to stimuli is a change in behaviour or A negative resting potential (voltage) of 60mV
physiology to cope with the change in environment. compared with outside known as polarised.
3 Nerves 2 4 Nerves 3
Nerve membranes have voltage-gated channels Na diffuses sideways and creates local currents that
for Na and K ions closed during resting potential depolarise the next piece of membrane the
so it stays at 60mV. depolarisation travels along the neurone.
A generator potential or other stimulation can open Mammalian neurones have a myelin sheath
the channels. Na diffuses in and depolarises the (electrical insulation) formed by Schwann cells.
membrane to +40mV inside cell. Gaps in the myelin insulation are called nodes of
Na channels close and more K channels open so
+ + Ranvier which allow impulses to jump from node to
K diffuses out of the cell. The inside becomes node.
negative (60mV) again i.e. repolarised. This jumping is called saltatory conduction and
Depolarisation followed by repolarisation in a makes the speed of impulse much faster (120m/s)
section of nerve membrane is called an action than in unmyelinated neurones (30m/s).
5 Synapses and signals 6 Hormones 1
Depolarisation (nerve impulse) travels at a fixed Hormones are chemicals secreted by endocrine
speed a stronger stimulus gives more frequent glands. They travel to all cells via the blood but only
impulses not faster ones. affect cells in target tissues.
A synapse is a junction between two nerve cells: The adrenal gland produces adrenaline which
presynaptic membrane synaptic cleft (gap) prepares the body for activity e.g. increases heart
postsynaptic membrane. rate, glycogen conversion to glucose, etc.
An impulse causes release of a neurotransmitter Adrenaline is an example of a first messenger and
(acetylcholine) from vesicles, which fuses with the cyclic AMP is an example of a second messenger.
presynaptic membrane. An exocrine gland is one that secretes molecules
Acetylcholine diffuses across the cleft and into a duct, which then carries the molecules to where
depolarises the postsynaptic membrane of the next they are used.
neurone a new impulse starts.
7 Hormones 2 8 Hormones 3
The pancreas secretes enzymes into the gut Type I diabetes (insulin dependent) not enough
(exocrine gland) and hormones into blood insulin produced. Type II (non-insulin-dependent)
(endocrine gland). liver and muscle cells not responding to insulin.
Pancreatic cells detect high blood glucose Bacteria can be genetically modified to produce
hormone insulin secreted. cells detect low blood human insulin. Pancreatic stem cells could produce
glucose hormone glucagon secreted. new cells in diabetics.
Insulin stimulates liver cells to take up glucose and Stress, shock or excitement causes adrenaline
convert it to glycogen (lowers blood glucose). secretion which increases heart rate.
Glucagon does the opposite. Cardiovascular centre in medulla oblongata of
High glucose causes K channels in liver cell
brain sends nerve impulses to Sino Atrial Node
membrane to close and Ca channels to open (pacemaker) in heart to control heart rate.
Ca diffuses in and insulin is secreted.
© Pearson Education Ltd 2009
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