F2144 -1- Communication and Homeostasis Flashcards
Outline the process of Positive Feedback.
DOESNT KEEP BODY ENVIRONMENT CONSTANT SO IS NOT A PART OF HOMEOSTASIS
Positive feedback mechanisms amplify a change from the normal level.
1) RECEPTORS detect a change in a level of something and communicate this to EFFECTORS via the nervous or hormonal system.
2) EFFECTORS respond by further increasing the change in level away from normal.
Outline the process of Negative Feedback
1) RECEPTORS in a homeostatic system detect that a level is too high or too low. This information is communicated via the nervous or hormonal system to EFFECTORS.
2) EFFECTORS respond to counteract the change - bringing the level back to normal.
The mechanism that restores the level to normal is called negative feedback.
Pancreatic alpha cells secrete…
Glucagon
Endocrine bit of pancreas
The islets of Langerhans - containing alpha and beta cells
Every other part of the pancreas is exocrine tissue, concerned with secreting digestive enzymes into the gut.
What is the magnitude of action potential?
+40mV
What is summation?
If a low-level stimulus is persistent it will generate several successive action potentials in the presynaptic neurone. In SUMMATION the several low-level signals are amplified as the release of many vesicles of acetylcholine over a short period of time will enable the post synaptic generator potentials to combine together to produce an action potential. Summation can also occur when several presynaptic neurones each release small numbers of vesicles into one synapse.
What is Saltatory Conduction?
Outline how it occurs.
“Jumping conduction”; the way in which the action potential appears to jump from one node of Ranvier to the next.
Sodium and Potassium ions can’t diffuse through fatty myelin sheath. The gaps in the myelin sheath are gaps between adjacent Schwann cells - the gaps are known as nodes of Ranvier. Sodium ions can only diffuse into myelinated neurones at these nodes, so the action potential appears to jump quickly from one node to the next.
Advantages of Saltatory Conduction?
The fact that ions can only diffuse in at the nodes of Ranvier between the Schwann cells speeds up the transmission of the action potential. Myelinated neurones conduct action potentials more quickly than non-myelinated neurones. A myelinated neurone can conduct an action potential at up to 120m/s.
What is acetylcholinesterase?
Acetylcholinesterase is an enzyme in the synaptic cleft. It breaks down the transmitter substance acetylcholine.
Sequence of events of transmission of a signal across the synapse at a nerve junction…
1) Action potential arrives at synaptic
knob.
2) Voltage-gated Calcium ion channels
open.
3) Calcium ions diffuse into synaptic
knob
4) Calcium ions cause synaptic
vesicles to move to and fuse with
the presynaptic membrane
5) Acetylcholine is released by
exocytosis
6) Acetylcholine molecules diffuse
across the synaptic cleft
7) Acetylcholine molecules bind to
receptor sites on the sodium ion
channels in the postsynaptic
membrane
8) Sodium ion channels open
9) Sodium ions diffuse across the
postsynaptic membrane into the
postsynaptic neurone.
10) A generator potential or excitatory
postsynaptic potential (EPSP) is
created.
11) If sufficient generator potentials
combine then the potential across
the postsynaptic membrane
reaches the threshold potential
12) A new action potential is created in
the postsynaptic neurone.
Action of hormones - what are first/second messengers?
The FIRST MESSENGER is the hormone that transmits the signal around the body, targeting a specific receptor site on target cells.
The SECOND MESSENGER is cyclicAMP, which transmits a signal inside the cell once a hormone has bound to the specific receptor site on the cell surface membrane.
Summarise the action of adrenaline
Adrenaline is an amino acid derivative and a hormone, unable to enter a target cell itself. Target cells instead have receptor sites on their surface that are complimentary in shape to the shape of the adrenaline molecule. The receptor is associated with an enzyme on the inner surface of the cell surface membrane. The enzyme is called ADENYL CYCLASE. When adrenaline (the first messenger) in the blood binds to it’s specific receptor on the surface of a target cell it forms an adrenaline-receptor complex which changes the shape of the adenyl cyclase on the other side of the membrane, ACTIVATING the cyclase enzyme. Adenyl cyclase now begins to convert ATP to cyclic AMP (the second messenger) inside the cell. The cAMP can then cause an effect inside the cell by activating enzyme action (activating other enzymes to cause an effect).
