Download Nerve Impulse Transmission

Synaptic Transmission
Impulses Along a Neuron
• Dendrites receive the nerve impulse and
carry it toward the cell body, which
contains the nucleus.
• The axon carries the impulse from the cell
body toward the synaptic knobs where it
will be transferred to other neurons.
Transmission at the Synapse
• There is a tiny gap between the synaptic
knobs of one neuron and the dendrites of
the next one.
• This gap is called the synapse or synaptic
cleft.
• The nerve impulse needs to cross this gap
and it does so by the release of special
chemicals called neurotransmitters.
The Synapse
Neurotransmitters
1. Are released from the presynaptic knobs by
exocytosis (active)
2. Diffuse across the synapse (passive)
3. React with special receptors in the dendrites
of the postsynaptic neuron
4. This causes sodium gates to open, an action
potential starts and the nerve impulse
continues
5. Once the neurotransmitters have passed
along the impulse, the “extra” neurotransmitters
still remaining in the synapse are either:
a) broken down by enzymes
or
b) transported back into vesicles in the synaptic
knob by endocytosis.
Synapse animation
Structure of the Synapse
Diagram
1.
2.
3.
4.
axon
mitochondria
synaptic knob
vesicles containing
neurotransmitters
5. synapse
6. neurotransmitter
7. receptors
Diagram of Synapse
1.________________
2.________________
3.________________
4.________________
5.________________
6.________________
7.________________
Neurotransmitters, cont’d
•
•
There are excitatory neurotransmitters
which start impulses in neighboring
neurons
Ex: acetylcholine (Ach) – opens sodium
ion channels on postsynaptic neurons,
broken down by cholinesterase
Neurotransmitters, cont’d
•
Inhibitory neurotransmitters which
prevent impulses (ex. serotonin,
dopamine). They make the postsynaptic
membrane more permeable to K+ so that
the membrane becomes even more
negative in its resting state (called
hyperpolarized)
Neuron Impulse and Synaptic
Transmission
Neurotoxins
Neurotoxins
• Many animals (snakes, spiders,
pufferfish, scorpion) use
neurotoxins as a form of defence.
• Most act to block or open Na+ or
K+ channels or block the release of
acetylcholine.