Homeostasis: The maintainance of a constant internal enviroment despite external changes.
Stimulus: Any change in the enviroment of an organism that causes a response.
Homeostasis is vital for cells to function normally and to prevent them from being damaged. It is particularly important to maintain the right body temperature because it effects enzyme activity, and enzymes control metabolic reactions.
Too high - enzymes change shape, denaturing them.
Too low - enzyme activity is reduced, slowing the rate of reactions.
Optimum temperature - 37 degrees
Negative feeback: A process in which any change brings about the reversal of that change so that it can keep a constant level.
Positive feeback: A process in which any change is amplified.
Receptors dectect when the level is too high or too low, and the information is communicated via the nervous system or the hormonal system to the effectors. The effectors counteract the change, bringing the level back to normal. Negative feeback keeps body temperature within 0.5 degrees above or below 37 degrees.
Some changes trigger a positive feedback mechanism, which amplifies the change. Positive feedback is useful to rapidly activate something, e.g. a blood clot after injury to prevent bleeding. It is not involved in homeostasis as it doesn't keep the internal enviroment constant.
The Nervous System
Motor neurone: Transmits nerve impulses from the CNS to the effectors
Sensory neurone: Transmits nerve impulses from receptors to the CNS (the brain and spinal cord).
Resting potential: The potential difference across the neurone cell membrane while the neurone is at rest.
Action potential: A brief reversal of the resting potenial across the cell surface membrane of a neurone. All action potenials have a value of +40mv.
Nerve impulse chain
---------Stimulus --------- Receptors --------- CNS ---------- Effectors -------- Response --------
sensory neurone motor neurone
Neurones - Motor
Neurones - Sensory
Action Potential - Graph
Action Potential - Explanation
Stimulus - this excites the neurone cell membrane, causing sodium ion channels to open. The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. This makes the inside less negative.
Depolarization - if the potential difference reaches threshold (-55mv), voltage gated sodium ion channels open. More sodium ions diffuse into the neurone.
Repolarisation - at a potential difference of around +30mv the sodium ion channels close and voltage-gated potassium ion channels open. The membrane is more permeable to potassium so potassium ions diffuse out of the neurone down the potassium ion concentration gradient. The begins to bring the membrane back to its resting potential.
Hyperpolarisation - potassium ion channels are slow to close so there's a slight overshoot, where too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential (less than -70mv).
Resting potential - ion channels are reset. The sodium-potassium pump returns the membrane to its resting potential and maintains it until the membrane is excited by another stimulus.
Sodium - Potassium Pump
Sodium - Potassium Pump
These pumps use active transport to move 3 sodium ions (Na+) out of the neurone for every 2 potassium ions (K+) moved in. ATP is required for this process.
These channels allow facilitated diffusion of potassium ions out of the neurone, down their concentration gradient.