Communication and Homeostasis

Ectotherms

An organism that relies on external sources of heat to regulate its body temperature.

• It uses less food in respiration
• Can last long periods of time without food
• More energy is used for growth 

Disadvantages: must warm up in the morning which can lead to predation.

- Can't do much in winter.

Endotherms

An organism that uses internal sources of heat to maintain temperature. Many body reactions are exorgonic. 

• They maintain a fairly constant body temperature
• They can inhabit colder places

Negative Feedback ensures an optimum steady state of maintenance.

Homeostasis: The maintenance of the internal environment in a constant state despite external change.

OPTIMUM

!

Change away from optimum

!

Receptors detect change

!

Communication system informs effector

!

Effector reacts to reverse change

If there is a rise in the core temperature:

1. Thermoregulatory receptors detect the change.

2. Signals are carried to the skin, liver and muscles.

3. Less heat is generated

If there is a fall in the core temperature:

1. Thermoregulatory receptors detect the change.

2. Signals are carried to skin, liver and muscles.

3. More heat is generated.

Motor Neurones

• Transmit nerve impulses from central nervous system to an effector such as a muscle.

Sensory Neurones

• Transmit nerve impulses from receptor to central nervous system.

The myelinated sheath is made up of Schwann cells with nodes of ranvier in between.

Saltatory Conduction

1. Resting potential (POLARISED): pd -70mV. Inside of axon is negatively charged compared to the outside. Channels are closed. 

2. Depolarisation: Voltage gated Na+ open. K+ channels stay closed. Inside of axon becomes more positive. pd=+35mV.

3. Repolarisation: Na+ channels close, K+ open, inside of axon becomes negative.

4. K+ channels remain open, it becomes TOO NEGATIVE.

All or nothing response: For a response to be given, the stimulus must be above the threshold potential.

The size of an action potential is always the same, however the frequency differs.

Resting Potential: sodium/potassium ion pumps use ATP to pump 3 Na+ out for every 2K+ ions in, also leak out due to permeability.  

CELL CYTOPLASM CONTAINS LARGE ANIONS

Local Currents

Local currents involve the movement of ions along the neurone: Occur in the cytoplasm of the neurone. These local currents cause sodium ion channels further along to open.

1. Action potential occurs, Na+ channels open.

2. Na+ diffuses across membrane into neurone.

3. This causes sodium to diffuse sideways. The change in pd causes Na+ channels to open further along to neurone.

Cholinergic synapses- those who use acetylcholine as their transmitter substance.

Synaptic Knob:

• Contains mitochondria, smooth ER
• Vesicles of acetylcholine
• Voltage gated calcium ion channels

1. Action potential arrives at postsynaptic knob.

2. Voltage gated calcium ion channels open, calcium diffuses in.

3. Vesicles move and fuse with presynaptic membrane, acetylcholine released, bind to receptor sites on sodium channels.

4. Sodium ions diffuse into the synaptic neurone.

5. Generator potential formed.

Acetylcholinesterase- hydrolyses acetylcholine into ethanoic acid and choline.

Summation- If a low level stimulus is persistent the stimuli join together.