AQA Psychology for A Level Year 2 - Student Bk

1. Briefly explain the process of synaptic transmission. [4 marks] 2. With reference to neurotransmitters, explain what is meant by both excitation and inhibition . [4 marks] 3. Distinguish between a sensory neuron and a relay neuron. [2 marks] Check It Concepts: The reflex arc Fill in the gaps using the terms provided at the bottom of the box. The knee-jerk reflex is an example of a reflex arc: A stimulus, such as a hammer, hits the knee. This is detected by sense organs in the _________________, which convey a message along a _________________, The message reaches the _________________, where it connects with a _________________, This then transfers the message to a _________________, This then carries the message to an _________________, such as a muscle, which causes the muscle to contract and, hence, causes the knee to move or jerk. Missing words: effector CNS (central nervous system), sensory neuron PNS (peripheral nervous system) motor neuron relay neuron @ookx hs @ookx hs Synaptic transmission Chemical transmission – synapses Neurons communicate with each other within groups known as neural networks . Each neuron is separated from the next by a synapse . The synapse includes the space between them (called the synaptic cleft) as well as the presynaptic terminal and postsynaptic receptor site. Signals within neurons are transmitted electrically; however, signals between neurons are transmitted chemically by synaptic transmission . When the electrical impulse reaches the end of the neuron (the presynaptic terminal ) it triggers the release of neurotransmitter from tiny sacs called synaptic vesicles . Neurotransmitter Neurotransmitters are chemicals that diffuse across the synapse to the next neuron in the chain. Once the neurotransmitter crosses the gap, it is taken up by the postsynaptic receptor sites – in other words, the dendrites of the next neuron. Here, the chemical message is converted back into an electrical impulse and the process of transmission begins again in this other neuron. Several dozen types of neurotransmitter have been identified in the brain (as well as in the spinal cord and some glands ). Each neurotransmitter has its own specific molecular structure that fits perfectly into a post-synaptic receptor site, similar to a lock and a key. Neurotransmitters also have specialist functions. For instance, acetylcholine (ACh) is found at each point where a motor neuron meets a muscle, and upon its release, it will cause muscles to contract. Excitation and inhibition Neurotransmitters have either an excitatory or inhibitory effect on the neighbouring neuron. For instance, the neurotransmitter serotonin causes inhibition in the receiving neuron, resulting in the neuron becoming more negatively charged and less likely to fire. In contrast, adrenaline (an element of the stress response which is both a hormone and a neurotransmitter) causes excitation of the postsynaptic neuron by increasing its positive charge and making it more likely to fire. Key terms Synaptic transmission – The process by which neighbouring neurons communicate with each other by sending chemical messages across the gap (the synaptic cleft) that separates them. Neurotransmitter – Brain chemicals released from synaptic vesicles that relay signals across the synapse from one neuron to another. Neurotransmitters can be broadly divided into those that perform an excitatory function and those that perform an inhibitory function. Excitation – When a neurotransmitter, such as adrenaline, increases the positive charge of the postsynaptic neuron. This increases the likelihood that the neuron will fire and pass on the electrical impulse. Inhibition – When a neurotransmitter, such as serotonin, makes the charge of the postsynaptic neuron more negative. This decreases the likelihood that the neuron will fire and pass on the electrical impulse. postsynaptic receptor sites dendrite axon presynaptic nerve terminal neurotransmitter synaptic vesicle synaptic cleft Concepts: Psychoactive drugs Increased understanding of the mode of action of neurotransmitters in the brain has led to the development of psychoactive drugs to treat mental disorders. For instance, depression has been linked to a lack of serotonin, which is thought to play an important role in stabilising mood. A category of drugs known as SSRIs (selective serotonin reuptake inhibitors) such as Prozac , slow down the reuptake of serotonin after it has crossed the synapse, ensuring it stays active for longer in the brain. Question Use your knowledge of synaptic transmission to explain what is happening at the synapse. @ookx hs @ookx hs Neurons and synaptic transmission 37 •