6.5 Neurons & Synapses

The Nervous System Divided into 2 parts CNS: Central Nervous System Consists of brain and spinal cord Composed of neurons PNS: Peripheral Nervous System Consists of all the nerves that branch from the CNS and go to the rest of the body Made up of sensory receptors and nerves Afferent and efferent

Neurons

Neurons transmit electrical impulses Send out rapid electrical impulses called action potentials must be changed into a chemical message in order to cross the synapse (space between 2 neurons) Carry message to a relay neuron inside the CNS Relay neuron turns it into an electrical impulse Then it travels to an effector cell (elicits a response)

Resting potential & Action Potential Difference in electrical charge inside (cytoplasm) and outside (extracellular fluid) Membrane potential Controlled by sodium NA+ and potassium k+ Neurons pump sodium and potassium ions across their membrane to generate a resting potential

Resting potential The charge of a cell at “rest”. The neuron is not sending an action potential. Resting potential is maintained by the sodium-potassium pump. At rest charge inside cells is more negative by -70mv Uses ATP to pump 3Na+ out of cell and 2K+ into cell Some “leaky” channels (always working to maintain resting potential)

Action Potential The changing charge in a cell due to the opening of sodium-gated ion channels (depolarization) followed by the opening of potassium-gated ion channels (repolarization) Occur when a stimulus is detected If cells depolarized to a charge around -50 mv and action potential is propagated (threshold potential)

Propagation of an action potential

Nerve Impulses When action potential is propagated it moves down the axon to the terminal buttons Axon is depolarized and then repolarized Wave of traveling membrane charge changes Myelinated axons speed up the transmission Axon potential generated between nodes (between myelination, across Schwann cells) Jumping of the impulse from one node to another is called saltatory conduction

Graph of an Action Potential

Synaptic Transmission Synapse: space between two neurons or a neuron and a muscle cell In order to carry and impulse across a synapse it needs to be changed from and electrical signal to a chemical signal Chemical form is called a neurotransmitter Stored in the terminal buttons of neurons Arrival of action potential at terminal buttons triggers the opening of the calcium gated channels in the presynaptic membrane Causes the release of neurotransmitters

Neurotransmitters Excitatory: when they open sodium channels Inhibitory: when they open potassium channels Examples Acetylcholine (cholinergic synapses) Cholinergic synapses control muscle contractions Dopamine Serotonin Epinephrine norepinephrine

Useful applications for blocking cholinergic synapses Pesticides: neuroactive insecticides bind to acetylcholine receptors causing the paralysis of insects. Botox: toxin derived from the bacteria that causes botulism, stops muscle contractions by blocking the release of acetylcholine. The relaxed muscles reduce the appearance of wrinkles.

Removal of neurotransmitters Must be removed from the synapse to stop the message being sent Removed by enzymes Removed by terminal buttons by reuptake (endocytosis)

Receptor  Interneuron  Effector = Reflex Does not involve the brain Involuntary (ex. pain) Adaptive behavior for survival