Homeostasis
Homeostasis is the maintenance of a constant internal environment (within a range)
despite external changes.
Communication
Cell signalling = how cells communicate with one another using signal
molecules. Messenger molecules bind to receptors with complementary
shapes in order to generate a response from the target cell.
Nervous system deals with short responses.
Hormones deal with long term responses.
Animals and plants increase their chance of survival by being able to respond
to a change in the environment. They also respond to changes in their internal
environment to make sure that conditions are always optimal for metabolism.
Negative feedback = a change in the system results in an action that will
reverse the change, bringing the system back to its normal state.
Positive feedback = a change in the system results in an action that will
increase the change, therefore increasing the deviation from optimum
conditions.
Ectotherms Endotherms
An organism that relies on external An organism that uses internal
sources of heat to regulate its body sources of heat to maintain its body
temperature. temperature.
Eg amphibians, reptiles, fish, insects. Eg mammals, birds.
Their body temperature fluctuates with Body temperature is maintained at
the environment. fairly constant level.
Use reorientation, wallowing, shelter Positives = activity possible in various
and colour changes. conditions, inhabit colder parts of the
Positives = need less food, greater earth.
proportion of food used for growth. Negatives = most energy intake used
Negatives = less active in cooler to generate heat when cold, less
temperatures, may become incapable energy available for growth.
of activity in winter.
The hypothalamus receives information about internal temperature from
thermoreceptors.
Peripheral thermoreceptors in the ski detect external temperature.
Thermoreceptors send impulses along sensory neurones to the
thermoregulatory centre in the hypothalamus which then sends impulses
along the motor neurone to effectors.
Effectors work to restore temperature to normal.
Nervous system
, Electrical impulses are passed along neurones as action potentials.
Sensory neurone = transmit action potential from sensory receptors to relay
neurone.
Relay neurone = have cell bodies within the CNS to connect sensory and
motor neurone.
Motor neurone = transmit action potentials from CNS to effectors.
Neurones have a cell body, short dendrites/dendron (to carry impulses
towards cell body) and a long axon (carry impulses away from cell body).
Dendrites are thin strands of cytoplasm that receive impulses and conduct
them towards the cell body (also used to communicate with other neurones).
Sensory neurones have a single dendron - carries impulses towards cell
body.
The axon (nerve fibre) is a long cytoplasmic extension that carries impulses
away from the cell body.
Adaptations:
, Very long
Plasma membrane with many gated ion channel proteins and
sodium/potassium ion pumps.
Able to main potential across membrane.
Gaps in myelin sheath allow rapid salutatory conduction.
Motor neurones have long axon with cell body inside CNS.
Sensory neurones have long dendron from receptor to cell body in the PNS
and the short axon to the CNS.
Numerous dendrites to allow connections to other neurons.
Pascinian corpuscle:
Mechanoreceptor in the skin.
End of sensory neurone wrapped in layers of connective tissue called
lamellae.
When stimulated, the lamellae are deformed and press the end of the sensory
neurone.
This causes deformation of the stretch-mediated sodium ion channels in the
neurones membrane which triggers a generator potential.
Resting state:
When a receptor is in its resting state there’s a difference in voltage between
the inside and outside of the cell.
The difference in voltage is called the potential difference.
Resting potential = -70mV
Potential difference is generated by ion pumps and ion channels in the
membrane.
Ion pumps constantly move sodium ions out of the cell and potassium ions
into the cell (3 sodium removed for every 2 potassium that enter).
At rest the membrane is impermeable to sodium ions (only removed by ion
pump).
Action potential generation:
1. At rest, the inside of the cell is more negative than the outside which is more
positive because there are less potassium ions inside the cell than there are
sodium ions outside the cell.
2. When a receptor is stimulated, this excites the neurone membrane, causing
sodium ion channels to open and sodium ions diffuse into the membrane.
3. This depolarises the membrane (makes the inside more positive).
4. When the potential difference reaches the threshold value of -50mV, voltage
gated sodium ion channels open causing more sodium ions to flood in (action
potential can only be generated if the stimulus reaches a certain threshold
intensity). Once this threshold is reached the size of the action potential is
, independent to the intensity of the stimulus so a more intense stimulus will not
give a larger action potential (all or nothing).
5. Action potential (+40mV) is generated.
6. Stronger stimulus will not produce a larger action potential but there will be
more frequent action potentials so the intensity of stimulations increases.
Repolarisation:
1. Sodium ion channels close and potassium ion channels open and potassium
ions diffuse out of the cell bringing the potential difference back to more
negative.
2. Potential difference overshoots slightly (hyperpolarisation).
3. The resting potential difference is restored using the sodium potassium ion
pump to get ions back onto the right side of the membrane (refactory period).
Absolute refactory period = another impulse cannot be conducted no matter
how intense the stimulus as the ions are on the wrong sides of the membrane.
Relative refactory period = an action potential can occur if the stimulus is
more intense than the usual threshold.
Local current in unmyelinated neurones:
1. Signal causes sodium ion channels to open allowing sodium ions to diffuse
through the channel.
2. There is a localised increase in the concentration of sodium ions inside the
cell.
3. Sodium ions diffuse along the axon along a concentration gradient.
4. The voltage gated sodium ion channels which was originally closed will open
due to the movement of sodium ions. More sodium ions can enter and create
another action potential.
5. Waves of depolarisation occur along the axon followed by waves of
repolarisation where potassium ions move out of the axon and resting
potential is restored by sodium potassium pumps.
Salutatory conduction:
1. Sodium and potassium ions can’t diffuse through the myelin sheath (fatty
layer).
2. The ionic exchanges that cause an action potential can only occur at the
nodes of Ranvier (gaps in the myelin sheath).
3. The local currents are elongated and sodium ions diffuse along the axon from
one node to another where depolarisation can take place.
4. This means the action potential appears to jump between nodes. This is
called salutatory conduction.
Myelinated neurone Unmyelinated neurone
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