SNAB Topic 8

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  • Created on: 27-08-14 13:09
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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) around the body.
A Nerve is a more complex structure containing a
bundle of the axons of many neurones surrounded by
protective covering.
Adaptations of neurones ­ long axon and dendrites to increase sensitivity, Schwann cells
are flattened so it can wrap around the axon (insulator)
Stimulus Receptor cells Sensory neurone CNS Motor neurone Effectors

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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 CNS along sensory neurones via the
dorsal route.
A sensory neurone connects to motor neurones within the CNS and passes impulses to
the brain to produce a coordinated response with the effector cells.…read more

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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 reflex dilates and the other constricts the pupil.…read more

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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 is
locally revered, making the inside of the axon positive and the outside negative. This is
known as depolarisation.…read more

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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 cell, the inside of the cell once again becomes more
negative than the outside.…read more

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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, a local
electrical current is created as the charged sodium ions flow between the depolarised
part of the membrane and the adjacent resting region.…read more

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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 membrane. Localised electric currents
are generated. Sodium ions move to the adjacent
polarised (resting0 region causing a change in
electrical charge (potential difference) across this part
of the membrane
3.…read more

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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 response
The main factors affecting the likelihood that the postsynaptic membrane will depolarise are:
The type of synapse
How much neurotransmitter reaches the postsynaptic membrane which depends
partly on the frequency of impulses reaching the…read more

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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 negative charge and K+ ions move out carrying a
positive charge.
This results in a greater potential difference across the membrane as the inside becomes
more negative than usual (-90mV) ­ A.K.A hyperpolarisation.…read more

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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 response to directional light. Shoots are positively phototrophic, growing towards
the source of light. Roots are negatively phototropic, growing away from the source of light.…read more


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