The Cells of the Nervous System and Neurotransmitters at Synapses U3

Composed of: Dendrites - Receive nerve impulses and carry them towards the cell body. Cell Body - Contains a nucleus and cytoplasm. The cytoplasm contains organelles such as mitochondria to provide energy for impulses and synthesis of neurotransmitters. Axon - Carries nerve impulses away from the cell body.

Axons are surrounded by a fatty layer called the myelin sheath. Insulates the axon. Increases the speed of impulse conduction.

Myelinated. Unmyelinated. Continues from birth to adolescence, therefore responses to stimuli in the first 2 years of life are not as rapid or coordinated as an older child or adult.

Physically support neurons. Produce the myelin sheath.

Allow axons to connect with other dendrites or the post synaptic neuron. It is a gap between neurons. Neurotransmitters diffuse across the synaptic cleft. The neuron before the synaptic cleft is known as the presynaptic neuron. The neuron after the synaptic cleft is known as the postsynaptic neuron.

At the end of the presynaptic neuron, neurotransmitters are stored in vesicles. On arrival of the impulse, the vesicle fuses with the membrane, releasing the neurotransmitter into the synaptic cleft. The neurotransmitter diffuse across the synaptic cleft and bind to receptors on the membrane of the post synaptic neuron. This allows the impulse to continue.

Must be removed from the synaptic cleft to prevent the continuous stimulation of the post synaptic neuron. This can be achieved by: Enzyme degredation (broken down into non active parts) Re-uptake (products are absorbed by the presynaptic neuron and resynthesised int active transmitters)

At the synapse, the receptors on the postsynaptic membrane can determine if the signal is: Excitatory (causes an increase in action e.g. causes muscles to contract). Inhibitory (causes a decrease in action e.g. slows heart rate).

Synapses can filter out weak stimuli when there is an insufficient secretion of neurotransmitters. Therefore, a sufficient number of neurotransmitter molecules must bind to receptors for a threshold to be reached so the impulse can be transmitted.

Where a series of weak stimuli can release enough neurotransmitters to trigger an impulse. Convergent neural pathways can release about neurotransmitters to reach a threshold so the impulse can be transmitted.

Neurotransmitters that stimulate neurons. Function: Reduce sensitivity to pain. Induce feelings of pleasure. Production increases through severe injury, exercise, eating certain foods and stress.

Dopamine is a neurotransmitter. It induces the feeling of pleasure. Reinforces behaviour by activating the reward pathway in the brain.

Each neurotransmitter can positively or negatively influence neurons in a specific portion of the brain, thereby affecting mood, perception and function of the brain (cognition). Therefore action of certain neurotransmitters can lead to certain behaviours or conditions of the brain. E.g., parkinson’s (lack of dopamine), alzheimer’s (lack of acetylcholine), schizophrenia (overactive dopamine) etc.

Agonists. Antagonists. Other drugs act by inhibiting the enzymes that remove neurotransmitters or by inhibiting re-uptake of the neurotransmitters at the synapse causing an enhanced effect.

Agonists bind to and stimulate specific receptors and mimic the action of the neurotransmitter at the synapse. This allows normal cell responses to occur. Therefore, parkinson’s could be treated with a dopamine agonist called L-dopa.

Antagonists bind to specific receptors and block the action of the neurotransmitter at the synapse. This prevents normal cell responses occurring. Therefore, schizophrenia could be treated with a dopamine antagonist.

Act as agonists and antagonists, therefore affect neurotransmission at a synapse. This can lead to changes in: - Mood - Cognition - Perception - Behaviour Many recreational drugs affect neurotransmission in the reward pathway of the brain.

Caused by repeated use of drugs that act as antagonists. Antagonists block specific receptors which causes the increased number and sensitivity of receptors (sensitisation). This sensitisation leads to addiction where the individual craves more of the drug.

Caused by repeated use of drugs that act as agonists. Agonists mimic specific receptors which causes the decreased number and sensitivity of receptors (desensitisation). This desensitisation leads to tolerance where the individual needs to take more to achieve the same effect.