6.5.6

Types of Synapse

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Neuromuscular Junction

A neuromuscular junction is a synapse between a motor neurone and a muscle cell. An action potential is transmitted across the synapse using the neurotransmitter acetylcholine. The steps involved are:

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Arrival at the synaptic knob

  • An action potential arrives at the synaptic knob at the end of the motor neurone.
  • The action potential depolarises the membrane of the synaptic knob. This causes voltage-gated calcium (Ca2+) ion channels to open.
  • Ca2+ ions diffuse into the synaptic knob.
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Release of acetylcholine (ACh)

  • The Ca2+ ion concentration inside the synaptic knob begins to increase.
  • This causes the synaptic vesicles to move and fuse with the presynaptic membrane.
  • Acetylcholine, the neurotransmitter inside the vesicles, is released into the synaptic cleft.
    • This process is called exocytosis.
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Binding to receptors

  • Acetylcholine binds to specific receptors on the postsynaptic membrane called nicotinic cholinergic receptors.
  • Binding of the neurotransmitter opens sodium ion channels in the postsynaptic muscle cell.
  • As Na+ ions diffuse into the cell, the membrane becomes depolarised.
  • If the potential difference reaches the threshold value, an action potential is generated and flows along the motor cell.
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Removal of acetylcholine

  • An enzyme called acetylcholinesterase (AChE) breaks down acetylcholine in the synaptic cleft.
  • The products of the break down are reabsorbed by the presynaptic neurone and reused to synthesise more acetylcholine.
  • It is important that the acetylcholine is removed from the receptors. This stops action potentials from being continuously generated in the postsynaptic cell.

Cholinergic Synapses

Synapses that use acetylcholine as a neurotransmitter are called cholinergic synapses. Although acetylcholine is also used at a neuromuscular junction, there are some key differences:

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Type of postsynaptic cell

  • Cholinergic synapses are between two neurones.
  • Neuromuscular junctions are between a motor neurone and a muscle cell.
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Number of receptors

  • There are less receptors in the postsynaptic membrane at a cholinergic synapse than at a neuromuscular junction.
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Type of response

  • A cholinergic synapse can trigger an inhibitory or excitatory response in the postsynaptic membrane.
  • An action potential at a neuromuscular junction always triggers an excitatory response in the muscle cell.
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Result of depolarisation

  • In a cholinergic synapse, depolarisation of the postsynaptic membrane results in an action potential.
  • At a neuromuscular junction, depolarisation of the postsynaptic membrane results in muscle contraction.
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Acetylcholinesterase

  • Acetylcholinesterase is the enzyme that breaks down acetylcholine after it has bound to the receptors on the postsynaptic membrane.
    • In cholinergic synapses, the enzyme is located in the synaptic cleft.
    • At a neuromuscular junction, the enzyme is stored in clefts in the postsynaptic membrane.

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1Cell Biology

1.1Introduction to Cells

1.1.1Introduction to Cells

1.1.2Cell Size & Surface Area

1.1.3Surface Area to Volume Ratio

1.1.4Eukaryotic vs Prokaryotic Cells

1.1.5Stem Cells

1.1.6Stem Cells in Disease

1.2Ultrastructure of Cells

1.2.1Eukaryotic Cells & Organelles

1.2.2Golgi, Lysosomes & Ribosomes

1.2.3Cell Walls & Vacuole

1.2.4Prokaryotes

1.2.5Binary Fission

1.2.6Subcellular Components & Organelles

1.3Membrane Structure

1.3.1Cell Membrane Structure & Permeability

1.4Membrane Transport

1.4.1Passive Movement Across Membranes

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1.4.3Endocytosis & Exocystosis

1.5The Origin of Cells

1.5.1Origins - Endosymbiosis

1.6Cell Division

1.6.1The Cell Cycle

1.6.2Mitosis

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1.6.5IB Multiple Choice - The Cell Cycle

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2Molecular Biology

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2.2.4Cellulose

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2.3Proteins

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2.4Enzymes

2.4.1Enzymes

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2.5.1DNA & RNA

2.6DNA Replication, Transcription & Translation

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3Genetics

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4.1.7Eutrophication

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6Human Physiology

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6.1.2Digestion in Mammals

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6.5Neurons & Synapses

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6.5.3Speed of Transmission

6.5.4End of Topic Test - Neurones & Action Potentials

6.5.5Synapses

6.5.6Types of Synapse

6.5.7Medical Application

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10AHL: Genetics & Evolution

10.1Meiosis

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10.2.1Linked Genes

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10.2.9Grade 4-5 (Scientific Practices) - Inheritance

10.3Gene Pools & Speciation

10.3.1Populations

10.3.2Mutations, Genetic Drift, & Gene Flow

10.3.3Speciation

10.3.4Rate of Speciation

10.3.5Allopatric & Sympatric Speciation

11AHL: Animal Physiology

11.1Antibody Production & Vaccination

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11.1.2The Immune Response

11.1.3Antibodies

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11.1.5Vaccines

11.2Movement

11.2.1Skeletal Muscle

11.2.2Sliding Filament Theory

11.2.3Contraction

11.3The Kidney & Osmoregulation

11.3.1Kidneys & Osmoregulation

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Synapses

Medical Application