6 5 NEURONS SYNAPSES The Nervous System Divided

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 -70 mv • Uses ATP to pump 3 Na+ out of cell and 2 K+ 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