April 22 2015 The Neuron & Nerve Impulses The Nervous System April 22 2015 The Neuron & Nerve Impulses

Nervous Tissue Two Types Neurons Neuroglia Nerve impulse conduction AP (action potential) Neuroglia Support & nourish neurons

The Neuron Three Main Parts 1. Cell Body 2. Dendrites 3. Axon

Cell Body Nucleus Organelles such as mitochondria, lysozomes

Dendrites Many branched extensions off of cell body RECEIVE SIGNALS/INPUTS

Axon Long & thin extension off cell body Conducts nerve impulses toward another neuron, a muscle fiber, or a gland cell (effector cells) Axons end by dividing into many fine processes called axon terminals Nerve impulses travel from cell body to the axon terminals

Axon Myelin Sheath Most axons are surrounded by the myelin sheath Myelin sheath is made up of lipids & proteins Insulates the axon of the neuron Increases the speed of the signals that travel down the axon

The Synapse The site where two neurons meet (or a neuron meets an effector cell) is called a Synapse

Neuroglia These cells are smaller than neurons Much more numerous Do not conduct impulses Six types of neuroglia cells

Action Potentials Cells, like neurons, have a difference in the charge within their plasma membrane versus the charge outside The voltage across the plasma membrane when the neuron is not conducting an AP is the resting membrane potential At rest, the inside of the neuron has a negative charge and the outside of the neuron has a positive charge The flow of ions is the current carried between cells in living tissues Electrical charge Voltage = the difference of electric potential between two points

Ion Channels Potassium (K+) and Sodium (Na+) 2 types of Ion Channels Leakage Channels Gated Channels Leakage channels – small, constant stream of ions Gated channels – open in response to a change in membrane potential

Generation of Action Potentials A stimulus causes a change in the resting membrane potential If the resting membrane potential reaches a critical level (threshold) an action potential begins Two Phases 1. Depolarization – the negative membrane potential becomes less negative, passes 0, and become positive 2. Repolarization – the membrane/neuron is returned to its resting potential Remember the neuron is usually negatively charged compared to outside

Ion Channels During A.P. First opens voltage-gated Na+ channels Na+ rush into the cell Voltage-gated K+ channels open more slowly K+ open as Na+ channels close K+ ions flowing out of the cell allow repolarization Remember the neuron is usually negatively charged compared to outside

“All-or-none principal” As long as a stimulus meets the threshold, an action potential will occur A stronger stimulus does not cause a larger action potential Size of AP is always the same Remember the neuron is usually negatively charged compared to outside

Conduction of Nerve Impulses Nerve impulses travel from the area where the axon arises, down the axon, to the axon terminals At the end of each axon terminal is a synaptic end bulb The synaptic bulb of one neuron meets (but does not touch) a dendrite of another at the SYNAPSE Remember the neuron is usually negatively charged compared to outside

The Synapse Depolarization causes a series of events that results in the release of neurotransmitters from the axon terminal into the synaptic cleft The neurotransmitters bind to receptors on the dendrites Neurotransmitters cause gates to open in the post-synaptic neuron, and the signal continues Remember the neuron is usually negatively charged compared to outside