April 22 2015 The Neuron & Nerve Impulses. Two Types Neurons ◦ Nerve impulse conduction ◦ AP (action potential) Neuroglia ◦ Support & nourish neurons.

The Nervous SystemApril 22 2015

The Neuron & Nerve Impulses

Two Types Neurons

◦ Nerve impulse conduction◦ AP (action potential)

Neuroglia◦ Support & nourish neurons

Nervous Tissue

Three Main Parts1. Cell Body2. Dendrites3. Axon

The Neuron

Nucleus Organelles

such as mitochondria, lysozomes

Cell Body

Many branched extensions off of cell body

RECEIVE SIGNALS/INPUTS

Dendrites

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

Axon

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

The Synapse

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

Neuroglia

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

Action Potentials

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

Ion Channels

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 Phases1. Depolarization – the negative membrane potential becomes less negative, passes 0, and become positive2. Repolarization – the membrane/neuron is returned to its resting potential

Generation of Action Potentials

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

Ion Channels During A.P.

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

“All-or-none principal”

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

Conduction of Nerve Impulses

Depolarizationcauses a series of events that resultsin 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

The Synapse