15.1.7
Synapses
Synapses
Synapses
A synapse is a junction between two neurones (e.g. a relay neurone and a motor neurone) or between a neurone and an effector (e.g a motor neurone and a muscle cell). The features of synapses are:


Synaptic cleft
Synaptic cleft
- At a synapse, there is a gap between the cells. This gap is called the synaptic cleft.
- When an action potential reaches a synapse, it must be transmitted across the synaptic cleft.


Presynaptic neurone
Presynaptic neurone
- The presynaptic neurone is the neurone before the synapse.
- When an action potential reaches the end of the neurone, it is transmitted across the presynaptic membrane to the postsynaptic membrane, or to an effector cell.


Synaptic knob
Synaptic knob
- The end of the axon of the presynaptic neurone is called the synaptic knob.
- The synaptic knob is a swelling which contains synaptic vesicles.
- The synaptic knob is the location where the nerve impulse is transmitted across the synpatic cleft.
- There are also lots of mitochondria in the synaptic knob. This is because lots of energy is needed to synthesise neurotransmitters.


Synaptic vesicles
Synaptic vesicles
- Synaptic vesicles are vesicles located in the synaptic knob.
- The vesicles contain neurotransmitters. When the vesicles fuse with the presynaptic membrane, neurotransmitters are released into the synaptic cleft.


Neurotransmitters
Neurotransmitters
- Neurotransmitters are the chemicals that allow an action potential to be transferred across a synapse.
- When neurotransmitters are released from the synaptic vesicles into the synaptic cleft, they bind to specific receptors on the postsynaptic membrane.


Postsynaptic membrane
Postsynaptic membrane
- The postsynaptic membrane is the membrane of the postsynaptic neurone or effector cells.
- Receptors on the postsynaptic membrane have a complementary shape to the neurotransmitters released from the synaptic knob.
- When neurotransmitters bind to their receptors, the action potential continues.
- There are only receptors on the postsynaptic membrane. This ensures the nerve impulse only moves in one direction.
Neurotransmitters
Neurotransmitters
Neurotransmitters are the chemicals that allow an action potential to be transmitted across a synapse. They can be excitatory or inhibitory.


Excitatory
Excitatory
- Excitatory neurotransmitters generate an action potential in the postsynaptic cell.
- When the neurotransmitters bind to the receptors on the postsynaptic membrane, the membrane is depolarised.
- E.g. When acetylcholine binds to receptors on the postsynaptic membrane in the central nervous system (CNS) an action potential is established.


Inhibitory
Inhibitory
- Inhibitory neurotransmitters prevent an action potential from being generated in the postsynaptic cell.
- When the neurotransmitters bind to the receptors on the postsynaptic membrane, the membrane is hyperpolarised.
- E.g. When acetylcholine binds to receptors on the postsynaptic membrane in the heart, potassium ion channels are opened in the membrane. This prevents an action potential from being established.
Summation
Summation
Summation is the process where neurotransmitters from multiple neurones are summed together to produce a response. There are two types of summation: spatial and temporal.


Spatial summation
Spatial summation
- Spatial summation takes place when multiple presynaptic neurones form a junction with a single neurone.
- Each presynaptic neurone releases neurotransmitters. Overall there are many neurotransmitters that bind to the receptors on one postsynaptic membrane.
- Together the neurotransmitters can establish a generator potential that reaches the threshold value and an action potential is generated.


Temporal summation
Temporal summation
- Temporal summation takes place when multiple nerve impulses arrive at the same synaptic knob within a short period of time.
- More neurotransmitter is released into the synaptic cleft, so more neurotransmitter is available to bind to receptors on the postsynaptic membrane.
- Together the neurotransmitters can establish a generator potential that reaches the threshold value and an action potential is generated.
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
Jump to other topics
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
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