Question

1)Compare and contrast synaptic potentials and action potentials. How each of these encode stimulus strength.

2)Describe in detail how one messenger can trigger the release of another second messenger.

3)On a molecular level, what absolutely refractory period? Can another action potentials be fired during this period? What principle of AP firing is due to the absolute refractory period?

Answer 1

1)

PROPERTY	SYNAPTIC POTENTIAL	ACTION POTENTIAL
LOCATION	dendrites,cell body,sensory receptors	axon
IONS	usually Na+,CL- or K+	Na+ and k+
STRENGTH	proportion to strength of stimulus, dissipates with distance	all or none
CHANNEL	ligand gated, mechanically gated	voltage gated
SUMMATION	temporal and spatial	none
DIRECTION	depolarising or hyperpolarising	only depolarising

2) The first messengers usually are extracellular factors often hormones or nuerotransmitters which donot cross the membrane phospholipid bilayer but attach to it externally to initiate set of responses inside the cell and activate the secondary messengers which then bring about the desired changes or reactions according to their function. examples of secondary messengers are, cAMP, cGMP, IP3 etc.

Explaining with the following example:

there are several different secondary messenger systems (cAMP system, phosphoinositol system, and arachidonic acid system), but they all are quite similar in overall mechanism, although the substances involved and overall effects can vary.

In most cases, a ligand (first messenger) binds to a membrane-spanning receptor protein molecule. The binding of a ligand to the receptor causes a conformation change in the receptor. This conformation change can affect the activity of the receptor and result in the production of active second messages.

In the case of G protein-coupled receptors, the conformation change exposes a binding site for a G-protein. The G-protein (named for the GDP and GTP molecules that bind to it) is bound to the inner membrane of the cell and consists of three subunits: alpha, beta and gamma. The G-protein is known as the "transducer."

When the G-protein binds with the receptor, it becomes able to exchange a GDP (guanosine diphosphate) molecule on its alpha subunit for a GTP (guanosine triphosphate) molecule. Once this exchange takes place, the alpha subunit of the G-protein transducer breaks free from the beta and gamma subunits, all parts remaining membrane-bound. The alpha subunit, now free to move along the inner membrane, eventually contacts another membrane-bound protein - the "primary effector."

The primary effector then has an action, which creates a signal that can diffuse within the cell. This signal is called the "second (or secondary) messenger." The secondary messenger may then activate a "secondary effector" whose effects depend on the particular secondary messenger system.

3) the time period when nerve fiber cannot be stimulated no matter how high the stimulus is called absolute action potential. It follows immediately after an action potential is generated. molecular cause is due to INACTIVATION OF THE Na+ CHANNEL.

NO. No other potentials can be fired during this period