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The nervous system

The nervous system is the network of nerve cells and fibres that transmits impulses around
the body. The nervous system carries messages around the body using neurones.

Neurones are single nerve cells which are highly specialised and adapted for the rapid
transmission of electrical impulses (action potentials)…

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Reflex arcs

The simple nerve pathways are known as reflex arcs and are responsible for our reflexes
­ rapid, involuntary responses to stimuli.

The pathway does not go to the brain; it just arcs through the spinal cord. E.g. the
knee-jerk reflex

Involves just 2 neurones: the impulses enter the…

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The pupil reflex

The iris controls the size of the pupil. It contains a pair of antagonistic muscles: redial and
circular muscles. These are both controlled by the autonomic nervous system. The radial
muscles are controlled by a sympathetic reflex. The circular muscles are controlled by a
parasympathetic reflex. One…

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Neurones are electrically excitable cells, which means that the potential difference across
their cell surface membrane changes when they are conducting an impulse.
If an electrical current above the threshold level is applied to the membrane, it causes a
massive change in potential difference. The potential difference across the membrane…

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The voltage-dependent Na+ channels close and Na+ permeability of the membrane
returned to its usual low levels.

Voltage-dependent K+ channels open due to the depolarisation of the membrane.

As a result, potassium ions move out of the axon, down the electrochemical gradient.

As potassium ions flow out of the…

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How do impulses pass along an axon???

When a neuron is stimulated, the action potential generated does not actually travel
along the axon, but triggers a sequence of action potentials along the length of the axon.

As part of the membrane becomes depolarised at the site of an action potential,…

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1. At resting potential there is a positive charge on the outside of the membrane and
negative charge on the inside, with higher sodium ion concentration outside and higher
potassium ion concentration inside

2. When stimulated, voltage-dependent sodium ion
channels open and sodium ions flow into the axon,
depolarising the…

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Ensure impulses travel only in one direction because receptors are only on the
postsynaptic membrane

Allows neurones to connect with many other neurones ­ increases range of possible
responses to a particular stimulus

Control nerve pathways an give flexibility of response

Integrate information from different neurones to give a coordinated…

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2) Inhibitory synapses (hyperpolarise) make it less likely that an action potential will
result in the postsynaptic cell.

The neurotransmitters open channels for Cl- and K+ ions in the postsynaptic membrane,
and these ions move through the channels down their diffusion gradients

Cl- ions move into the cell carrying a…

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The nervous system is not the only means by which the activities of the body can be
coordinated. Hormones, which are secreted into the bloodstream by endocrine glands, act as
a means of chemical communication with target cells. Contrast between nervous and
hormonal controls in animals.


A growth…


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