Nervous system

• A synapse is where the axon endings of one neurone meet an effector or another neurone.
• The gap between the two cells is a synapse.
• The wave of depolarisation cannot cross the gap.
• Two main neurotransmitters are acetylcholine and noradrenaline.
• The structure of the synapse includes a synaptic vesicle containing neurotransmitter, a Calcium ion channel, a presynaptic membrane, a postsynaptic membrane, and postsynaptic receptors.
• Depolarisation of the axon membrane causes Ca 2+ channels to open.
• Ca 2+ diffuse into the synaptic knob.
• Ca 2+ cause vesicles containing neurotransmitter to fuse with the presynaptic membrane.
• Neurotransmitter is released into the synaptic cleft by exocytosis.
• Neurotransmitter diffuses over the synaptic cleft and binds to receptors on the postsynaptic membrane.
• The binding causes Na + channels on the postsynaptic membrane to open and the postsynaptic membrane depolarises.
• An enzyme breaks the neurotransmitter down and the products are taken into the synaptic knob where they are resynthesised and packaged.
• Antagonists affect synapses by preventing postsynaptic depolarisations.
• Agonists cause more postsynaptic depolarisations.
• Antagonists work in a number of ways, such as preventing Ca 2+ channels opening and thus preventing exocytosis, blocking receptors, and hyperpolarising the postsynaptic membrane so it is harder to reach threshold.
• Ways in which agonists may work include inhibiting the enzymes that break down neurotransmitter, causing exocytosis, preventing uptake of neurotransmitter from the synaptic cleft, and binding to receptors and mimicking the neurotransmitter.
• Curare is an example of an organophosphate antagonist that blocks receptors.
• Organophosphate agonists inhibit acetylcholinesterase, an enzyme that breaks down neurotransmitter.
• The nervous system enables animals to respond to changes in the external or internal environment, also known as stimuli.
• The neurons are unmyelinated and send impulses in all directions.
• The nervous system has a nerve net of identical branching neurons that are interconnected.
• A nervous response follows this pathway: Stimulus  Receptor  Sensory neurone  Relay neurone in CNS  Motor neurone  Effector  Response.
• An effector is a muscle or gland.
• The motor and sensory neurones are part of the peripheral nervous system.
• The structure of the spinal cord includes grey matter, which is grey as it contains many cell bodies and nuclei, and white matter, which is white as many axons, wrapped in the lipid myelin, pass through this region.
• The central canal of the spinal cord contains cerebrospinal fluid.
• The spinal cord is surrounded by membranes called meninges.
• Motor neurons consist of a cell body, dendrites, an axon, axon endings, and a node of Ranvier.
• This sends an impulse through a motor neurone to an effector, which responds to protect the body.
• Dendrites transmit impulses into the cell body.
• In simple organisms such as Hydra, receptors respond to a limited number of stimuli, so the number of effectors is small.
• Schwann cells wrap around the axon and form the myelin sheath, an electrical insulator.
• A reflex action is a fast, automatic response to a stimulus that has a protective function.
• Receptors detect the harmful stimulus and send impulses through sensory neurons to a relay neurone in the spinal cord.
• The axon transmits the impulse to the axon endings.
• Axon endings innervate effectors.
• Nodes of Ranvier are gaps in the myelin sheath that form long local circuits with the next node, speeding up transmission.
• The action potential is transmitted along a neurone when the depolarisation of the membrane sets up a local circuit with the next section.
• Transmission is in one direction as during repolarisation, the membrane cannot form an action potential.
• This is called the refractory period.