Download Neurotransmission Notes

Neurotransmission Notes
Neuron – nerve cell
Soma – cell body
Axon – sends message
Dendrite –
receives
message
Myelin sheath – insulates and
speeds up transmission
synapse
Synaptic terminal
Transmission of message:
1. Along the neuron – electrical (action potential)
2. between neurons – chemical (synaptic transmission)
Electrical Transmission:
Background: At rest, the inside of the neuron is negatively charged and the outside is positively
charged. This is due to unequal distribution of charged particles (ions). Positive ions exist both in
and out of the axon, but there are more outside. Most of the positive ions outside are Na+ ions,
while the inside is primarily K+ ions.
The unequal distribution of charged particles creates a voltage (known as the resting potential) inside
the axon of -70 mV.
Transmission:
When a dendrite is stimulated:
1. positive ions rush in. If enough rush in (ie, if it reaches threshold), channels at the base of
the axon open, allowing even more positive ions in.
2. This causes the axon to become positively charged or depolarized.
3. The axon is covered with gates that are sensitive to voltage. The movement of positive ions
into the axon triggers these gates to open and let in MORE positive Na+ ions. Thus,
depolarization travels down the axon to the end.
4. The voltage sensitive Na+ gates only stay open a moment and then K+ gates open. These
allow K+ to rush out of the axon, bringing it back to a negative charge (when called this
repolarization or hyperpolarization).
5. Although the charge is back to rest, the Na+ and K+ ions are on opposite sides from where
they began. A Na+/K+ pump returns them to their original position. For a short time,
before the ions are returned, depolarization cannot happen again. Thus, there is a refractory
period that prevents the message from traveling backwards.
6. This entire process is called an action potential.
Graph of an Action Potential
Source: http://artsci-ccwin.concordia.ca/psychology/psyc358/Lectures/actpotent1.htm
All-or-none effect – if the signal hits threshold, the signal is sent at the same strength regardless of
initial stimulus. How does our brain interpret the intensity of the stimulus? The greater the
frequency of action potentials, the greater the stimulus.
Chemical (Synaptic) Transmission:
When an action potential (AP) reaches the synaptic terminal, the neuron releases chemicals call
neurotransmitters (NT’s), into the synapse. They diffuse across and bind to receptors on the
post-synaptic neuron, stimulating the dendrite (and maybe causing an action potential if it hits
threshold).
The sending neuron reabsorbs any excess NT’s left in the synapse. This is called reuptake.
Neurotransmitters can be excitatory (causing EPSP’s or excitatory post-synaptic potential) or
inhibitory (causing IPSPs or inhibitory post-synaptic potential). Excitatory NTs bring the dendrite
closer to threshold, while inhibitory NT’s take it further from threshold.
Agonist – any substance that mimics or has a similar effect to a neurotransmitter
Antagonist – Any substance that inhibits the effect of a neurotransmitter.