Neuron Structure HBS 2 2 1 Neurons Neurons

Neuron Structure HBS 2. 2. 1

Neurons • Neurons are specialized cells that carry information either to the brain or away from the brain to other body parts • There are three types: 1. Motor neuron 2. Sensory neuron 3. Associative or interneuron

Three Neurons

Three Neurons • Motor neuron – Carries impulses from the brain to a target structure to produce some kind of movement • Sensory neuron – Carries information from parts of body up to the brain to be interpreted as sense • Interneuron or associative neuron – Special neurons found in reflex arcs that connect a sensory neuron to a motor neuron

Motor Neuron Major Structures Direction of impulse travel

Neuron Smear

Myelin Sheath and Nodes of Ranvier • Not all neurons have myelin sheaths • Those that do transmit faster • Myelin is fat wrapped around an axon • Produced by Schwann cells • Has gaps called Nodes of Ranvier

Three Classifications of Neurons Multipolar neuron Bipolar neuron Unipolar neuron

Neural Conduction • Nerve signals are electrochemical in nature • They are called action potentials • Must understand cell membrane structure first • Made of phospholipids • Hydrophobic and hydrophilic • Forms two layers

Cell membrane This configuration does not allow water-soluble or charged particles like ions to pass through

Path of Action Potential

-All or None law

ACTION POTENTIAL: REPOLARIZATION PHASE • • • The positive intracellular charge stops Na+ entry Sodium inactivation gates close Slow voltage-sensitive K+ gates open K+ leaves cell Internal cell negative charge is restored RMP is restored with Na+-K+ ATPases (pumps)

Measuring Resting Potential Na+ on outside of cell K+ on inside of cell

Action Potential Depolarization – opening of voltage gated Na+ channels If threshold met, Na gates open membrane depolarizes K+ channels open and K+ begins to leave cells. Na+ close Repolarization – returning to Na+ on outside of cell and K+ on inside Hyperpolarization - Voltage gate K+ channels remain open after potential reaches resting level Stimulus -Na+ gates start to open Hyperpolarized -Is in refractory and can not fire. Na/K pumps working

Speed of Conduction • Typical neurons conduct at 10 to 100 meters per second • Conduction speed: – Varies with diameter of axon: larger = faster – Presence of myelin: myelin = faster

Neurotransmitters • Neurotransmitters are chemicals that allow neurons to communicate • Neurons do not touch • Produced at the axon terminal of one neuron and carry the message across the synaptic gap to the dendrites of another neuron

Synaptic Gap Transmission • Presynaptic cell makes serotonin from Tryptophan and packages it in vesicles • Action potential reaches terminal • Calcium moves into neuron and moves vesicles to cell membrane • Vesicle opens, serotonin passes across gap to receptors on postsynaptic cell • Starts depolarization • If threshold is reached, new action potential is propagated • Remaining serotonin in gap is destroyed by monoamine oxidase (MOA) and catecholomethyl transferase (COMT) • Other neurotransmitters: acetylcholine, norepinephrine, dopamine, gamma amino butyric acid. Nerves produce only 1

Saltatory Conduction • Current passes through the membrane of a myelinated axon only at the Nodes of Ranvier • Na+ channels are concentrated at the nodes • Electrical signal jumps from node to node 30 x faster than if there was continuous conduction.