Nerves, homeostasis and communication notes

Notes on chapters one and two of the ocr A2 biology course. Includes the different types of neurones, synapses, myelination, positive and negative feedback and endotherms and ectotherms. Enjoy!

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  • Created on: 28-02-11 16:46
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Communication and Homeostasis
Homeostasis ­The ability to control internal conditions and maintain a stable internal environment.
Homeostasis controls:
Temperature ­ low temperatures will slow metabolic reactions, whereas higher
temperatures may denature proteins, harming enzymes and membranes
Water volume ­ Can cause crenated or lysed cells if the amount of water in the body is not
the same. This can affect metabolic reactions.
Glucose concentration ­ Low levels slows respiration, and higher levels can affect water
potential, and cause water to move out of cells via osmosis.
Homeostasis works through the mechanism of negative feedback. Ref. diagram p3 in textbook
Both the nervous system and the hormonal system are involved in negative feedback. The nervous
system causes fast, sort-term changes and the hormones act slower, but cause more long-term
Positive Feedback ­ Very few examples of this. If you breath in air with a high CO2 conc, this is
sensed and causes breathing rate to increase. So you breath faster, taking in more CO2 and
developing respiratory acidosis.
Endotherms are animals that are able to generate heat within their bodies; this includes mammals.
The hypothalamus continually monitors the core body temperature, by measuring the temperature
of blood flowing through it. This should be around 37°C. The hypothalamus also receives information
from peripheral receptors, which monitor changes in skin temperature and thus can give an advance
warning of a potential fluctuation in core temperature.
Falling Core temperature Increasing Core temperature
Nerve impulses from hypothalamus Vasodilation ­ Smooth muscle in walls of
cause repeatedly contracting muscles, arterioles near the skin surface relax,
causing shivering meaning increased blood flow to the
Vasoconstriction ­ smooth muscle in skin. More heat can be lost through
walls of arterioles near skin contracts, conduction through the skin
lessening blood flow to the skin so less Sweat glands secrete more sweat ­
heat will be lost via conduction. water has a high latent heat of
Sweat glands reduce sweat output vaporisation, so energy in the form of
Erector muscles contract, causing hairs heat is used up
to stand on end. A layer of air is trapped Behavioural differences ­ sitting in the
in these hairs and insulates the body shade, wear fewer clothes and eat ice
Behavioural differences ­ wear more creams
clothes, drink hot drinks, jumping up and

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Ectotherms ­ Animals that obtain most of their heat from outside their bodies. Eg. Reptiles. They
cannot generate large amounts of heat within their bodies. Endotherms tend to be inactive in cold
environments, as their cells are too cold and metabolic reactions therefore take place slowly.
Motor Neurone:
Sensory neurone:
Relay (intermediate) neurone:
The impulse travels from the receptor, along the sensory neurone into the Central Nervous System. It
then crosses a synapse and passes into a relay (or intermediate) neurone.…read more

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Schwann cells wrap themselves around some neurones, called a myelin sheath. Every 1-3mm there is
a Node of Ranvier. About one in three motor and sensory neurones are myelinated. Myelin Sheath
means faster conduction of nerve impulses.
Axons of neurones are always found in bundles. These are protected by a covering called
Perineurium. You get mixed nerves, sensory nerves and motor nerves.
How an action potential travels across a neurone:
1.…read more

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A synapse is a junction between neurones, or between a neurone and an effector cell.
1. An action potential arrives at the synaptic knob of the presynaptic neurone. It stimulates
voltage-gated Calcium ion channels to open.
2. Calcium ions diffuse into the synaptic knob.
3. The influx of Calcium ions causes synaptic vesicles containing a neurotransmitter to move to
the presynaptic membrane.
4. The vesicles fuse with the membrane, releasing the neurotransmitter substance into the
synaptic cleft. (EXOCYTOSIS)
5.…read more

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