Synapses #
is a physical and functional gap which occur between one neuron and the other and muscle or neuron and gland which serves the purpose of electrical or chemical transmission of impulses. It contains the end of presynaptic neuron + synaptic cleft + end of postsynaptic neuron.
Properties of the synapse
The synapse has the following functions:
- Transmit information form neurone to neurone
- Pass impulses in one direction
- Act as junctions
- Prevents overstimulation
- Filter out low level stimuli
Mechanism of synaptic transmission #
- action potential arrives at presynaptic neurone
- stimulates opening of voltage-gated channels for Ca2+
- Ca2+ diffuse into cytoplasm of presynaptic membrane
- Ca2+ cause vesicles containing acetylcholine (ACh – a type of neurotransmitter) move towards the presynaptic membrane
- Vesicle fuses with the membrane, Ach is released and diffuses across the synaptic cleft
- Ach temporarily binds to receptor proteins on the postsynaptic membrane; causes chemically-gated ion channels for Na+ to open
- Na+ diffuse through postsynaptic membrane à depolarizes membrane à generates action potential
- Recycling of Ach, catalysed by acetylcholinsterase (stops continuous production of action potentials)
- acetylecholine → acetate + choline
- choline moves back into the presynaptic neurone.
- choline + acetyl coenzymeA → acetylcholine
- ACh transported back to presynaptic vesicles
Explain the mechanism of transmission of a nerve impulse across a cholinegic synapse #
Logical sequence of events is very important when explaining a mechanism
- arrival of an impulse
- Depolarization of presynaptic membrane
- opening if calcium channels
- increase in mebrane permeability to Ca2+
- rushing of Ca2+ into presynaptic knob
- synaptic vesicle fuse with presynaptic cleft(by exocytosis)
- acetylecholine released into synaptic cleft
- attaches to a receptor on the post synaptic membrane
- entry of Na+
- depolarisation of membrane
Functions of synapses #
- ensures one-way transmission: neurotransmitters are only released on one side of synapse; receptors on the other side of synapse
- allow connection of nerve pathways
- integration of impulses
- involved in memory making and learning.
- Decreases the overload of information in the brain
Factors affecting the speed of transmission #
The following factors affect the speed of transmission
- Temperature – Increases in temperature increase kinetic energy and therefore speeds up the transport of ions; this speeds up nerve transmission.
- Diameter of the axon – The greater the diameter of the axon the lower the resistance to the movement of ions. Giant squids have axons up to 1mm in diameter allowing them to react quickly in low temperatures.
- Myelination – Schwann cells wrap around the axon and secrete a fatty myelin sheath which is an electrical insulator. Only the Nodes of Ranvier, which are gaps in the myelin sheath exposing the cell membrane, can become depolarized (only these regions have voltage gated Na+ channels). The action potential jumps from node to node and speeds up the rate of transmission; this is called saltatory transmission. The greater the distance between the nodes the greater the rate of transmission. Myelination is only found in vertebrates.
Explain the roles of synapses in the nervous system. #
- ensure one-way transmission;
- receptor (proteins) only in postsynaptic, membrane / neurone;
- vesicles only in presynaptic neurone;
- ref. adaptation;
- increased range of actions;
- due to interconnection of many nerve pathways;
- ref. inhibitory synapses;
- involved in memory / learning;
- due to new synapses being formed;
- e.g. summation / discrimination
The effect of chemicals on the synapse #
Many drugs act at the synapse and can either amplify or inhibit synaptic transmission. Psychoactive drugs act on the CNS by affecting neurotransmitters or their receptors.
| Amplification | Inhibition |
| Accelerating neurotransmitter production in the synaptic knob (cocaine). | Inhibiting neurotransmitter production in the synaptic knob. |
| Opening calcium channels in the pre-synaptic membrane | Closing calcium channels in the pre-synaptic membrane |
| Accelerating the release of neurotransmitter from the synaptic knob by exocytosis. | Inhibiting the release of neurotransmitter from the synaptic knob by exocytosis |
| Amplification | Inhibition |
| Making the post-synaptic receptors more sensitive to the neurotransmitter. | Making the post-synaptic receptors less sensitive to the neurotransmitter. |
| Opening the sodium channels on the post synaptic membrane | Closing sodium channels on the post-synaptic membrane. |
| Inhibiting cholinesterase activity in the synaptic cleft. | Inhibiting cholinesterase activity in the synaptic cleft. |
| Mimicking the neurotransmitter substance (cannabis). | Masking the effect of the neurotransmitter substance |
Nervous Control questions and answers
