A. General Overview
Synapse à point at which a neuron
meets its target cell e.g. another neuron, muscle cell.
Generally the presynaptic cell ends next to the dendrites or cell body of the post synaptic cell.
A moderate # of synapses on a cell is 10,000 but some cells are estimated to have 150,000 synapses.
Post synaptic potentials travel to the trigger zone of the neuron - the integrating center of the neuron.
It is the summation of all the synaptic inputs that determines whether the post synaptic cell will be activated or will be inhibited
Thus, if the sum of all the potentials reaching the trigger zone reaches a sufficient level of depolarization to open voltage gated Na channels the post synaptic cell will fire an action potential.
If the sum of all the potentials leads to a net hyperpolarization, the post synaptic cell will be inhibited.
Two types of synapses
(i) Electrical
Current passes directly from the cytoplasm of one cell to another through gap junctions.
(ii) Chemical
Vast majority of synapses are chemical. Synapse made up of 3 parts:
(i) Presynaptic cell
(ii) Synaptic cleft
(iii) Postsynaptic cell
Synaptic transmission involves 4 phases:
(i) synthesis of transmitter
(ii) storage of transmitter in synaptic vesicles
(iii) interaction of transmitter with postsynaptic
membrane
(iv) termination of synaptic transmission.
(i) Action potential coming down the axon depolarizes the membrane of the axon terminal
(ii)
The depolarization opens voltage gated Ca channels in the
membrane Ca enters the cell
(iii) Ca entry signals the synaptic vesicles to release their contents by exocytosis
(iv) Transmitter diffuses across synaptic cleft and binds with receptor à (chemically activated) ion channel complexes on post synaptic cell
(v) Receptor-transmitter complex opens ion channels
in the post synaptic membrane initiating response
(vi) Response is terminated by transmitter uptake back into presynaptic cell
Post synaptic potentials are
(i) excitatory (EPSP) - increases the probability
that the postsynaptic cell will fire an action potential
(ii) inhibitory
(IPSP) Ã decreases the probability that the postsynaptic
cell will fire an action potential
Post synaptic
potentials (unlike action potentials) are graded i.e. the
amplitude of the post synaptic potential is directly proportional
to the strength of the stimulus.
Binding of transmitter to postsynaptic
receptor can produce:
(i)
Opening of channels selective for Na ions in the postsynaptic
membrane à Na enters cell along electrical and
concentration gradients, depolarizing membrane potential.
(ii)
Opening of relatively non-selective channels in membrane that
allow the passage of both Na and K ions. Na ions move into cell,
K ions move out of cell. Driving force for Na is greater
than that for K so more Na enters cell than K leaves. Net
depolarization results.
IPSP
Can be produced by a variety of different
ionic mechanisms
(i)
Opening of K channels - K leaves the cell, cell
becomes less +ive (hyperpolarized)
(ii)
Opening of Cl channels. Cl flows into cell
(iii)
Closing Na channels. K efflux becomes predominant, cell
hyperpolarizes.
G. Neuromuscular Junction
The ionic
mechanism that occurs at the neuromuscular junction (the
junction between motor neurons and muscle cells) is opening of
non selective chemically activated channels in the post synaptic
membrane.
Acetylcholine is
released from the motor terminal and interacts with acetylcholine
receptors on the muscle cell. The resulting EPSP is called an end
plate potential.
Myasthenia gravis
à a debilitating neuromuscular disease associated with
weakness and fatigability of skeletal muscle.
End plate
potential of smaller amplitude than normal.
(i)
Neurotransmitter binds directly to receptor Ã
(chemically gated) channel complex on post synaptic membrane
leading to opening of the channel.
(ii)
Resulting synaptic event is fast; postsynaptic response is short
lasting.
(iii)
The receptors are called ionotropic.
(i) Neurotransmitters bind to receptors liked to G proteins and second messanger systems not directly to the receptor-channel complex.
(ii) Second messangers act from the cytoplasmic side of the cell membrane to open or close ion channels.
(iii) Resulting synaptic event is slow; postsynaptic response is long lasting.
(iv) Receptors are called metabotropic.
A post synaptic cell will have many synaptic contacts from many different presynaptic cells. It is the summation of all these inputs that determines whether the post synaptic cell will fire or not. The trigger zone (initial segment) of the post synaptic axon needs to be depolarized to a sufficient level (about Ã45 mV) to initiate an action potential in the post synaptic cell.
Two inputs A and B when applied separately are insufficient to produce an action potential in the postsynaptic cell.
When activated simultaneously post synaptic potentials A +B summate to produce sufficient depolarization at the initial segment of the postsynaptic cell to initiate an action potential.
Involves the summation of postsynaptic potentials that arise from activation of a single input twice with a short latency in between stimuli.