Synapses

Synapse

- point where 2 neurones meet but do not touch
- contains the end of presynaptic neurone + synaptic cleft + end of postsynaptic neurone

Mechanism of synaptic transmission

1. action potential arrives at presynaptic neurone

2. stimulates opening of voltage-gated channels for Ca²⁺

3. Ca²⁺diffuse into cytoplasm of presynaptic membrane

4+5. Ca²⁺cause vesicles containing acetylcholine (ACh – a type of neurotransmitter) move towards the presynaptic membrane

6. Vesicle fuses with the membrane, Ach is released and diffuses across the synaptic cleft

7. Ach temporarily bindsto receptor proteins on the postsynaptic membrane; causes chemically-gated ion channels for Na⁺ to open

8. Na⁺diffuse through postsynaptic membrane àdepolarizes membrane à generates action potential

9. Recycling of Ach, catalysed by acetylcholinsterase (stops continuous production of action potentials)

10. choline moves back into the presynaptic neurone

ACh transported back to presynaptic vesicles

Functions of synapses 

1. ensures one-way transmission: neurotransmitters are only released on one side of synapse; receptors on the other side of synapse

2. integration of impulses: the body of motor neurones are covered with terminations of multiple relay neurones. The motor neurone only transmits impulses and initiate action potentials if the threshold potential is reached. This ensures that the brain is not overloaded with sensory information.

3. allow connection of nerve pathways: axons branch out to form more synapses with multiple neurones à humans have a wider range of behaviours

• in dangerous situations: information from 1 neurone spreads throughout the body to reach many relay neurones and effectors
• decision-making in brain: motor neurones have many dendrites à larger surface area for a lot of synapses à neurone can integrate information from many parts of the body

4. involved in memory making and learning: e.g.: brain receives information about 2 things at the same time à new synapses form

15.1  Control and co-ordination in mammals The nervous system provides fast communication between receptors and effectors. Transmission between neurones takes place at synapses. a)   compare the nervous and endocrine systems as communication systems that  co-ordinate responses to changes in the internal and external environment  b)   describe the structure of a sensory neurone and a motor neurone c)   outline  the roles of sensory receptor cells in detecting stimuli and stimulating the transmission of nerve  impulses in sensory neurones (a suitable example is the chemoreceptor cell found in human taste buds) d)   describe the functions of sensory, relay and motor  neurones in a reflex arc e)   describe and explain the transmission of an action potential in a myelinated neurone and its initiation from a resting potential (the importance of sodium and potassium ions in impulse transmission should  be emphasised) f) explain the importance of the myelin sheath (saltatory conduction) in determining the speed of nerve  impulses and the refractory period in determining their frequency g)   describe the structure of a cholinergic synapse and explain how it functions, including the role of calcium  ions h)   outline  the roles of synapses in the nervous system in allowing transmission in one direction  and in allowing connections between one neurone and many  others (summation, facilitation and inhibitory synapses are not required) i) describe the roles of neuromuscular junctions, transverse system tubules and sarcoplasmic reticulum in stimulating contraction in striated muscle j) describe the ultrastructure of striated muscle with particular reference to sarcomere structure k)   explain the sliding filament  model  of muscular contraction including the roles of troponin,  tropomyosin, calcium  ions and ATP. l) explain the roles of the hormones FSH, LH, oestrogen and progesterone in controlling changes in the ovary and uterus during the human menstrual cycle m)  outline  the biological basis  of contraceptive pills containing oestrogen and/or progesterone