Neuronal Signals

Main Point #1 Neurons sense stimuli, interpret that stimuli, and react

3 Functions Sensory input Receiving signals Integration Interpretation and thinking Motor output Reactions

Main Point #2 The nervous system consists of neurons and supporting cells, which greatly outnumber the number of neurons

Cells of the Nervous System Neurons Carry signal throughout the body Supporting cells (glia) Provide structure Protect/insulat e neurons

Neuron Structure Large cell body Dendrites – receive signals Axons – carry signals to other neurons

Dendrites and Axons

Speeds up transmission of signals Myelin Sheath Insulation layer Speeds up transmission of signals

Synapse The end of an axon is called the synaptic terminal Connects to a target cell

Nerve vs. Neuron A nerve is a bundle of neurons with similar functions Neurons only convey signals in one direction, but a nerve can be multidirectional as it has many neurons

Main Point #3 Neural signals are transmitted as electrical signals in the form of action potentials, rapid changes in the voltage of a neuron

Neural Signals Based on ions moving in and out of neurons Changes the voltage across the membrane

Sodium Potassium Pump Pumps 3 Na+ molecules out for every 2 K+ molecules that get in Makes inside more negative relative to outside

Na+ / K+ Na+ is much more concentrated outside cell, wants to diffuse in K+ is much more concentrated in the cell, wants to move out

The Resting Voltage The inside is negatively charged – relative to the outside which is by definition 0 Resting voltage is about -70mV (5% of the voltage of a flashlight)

Voltage gated Ion channels Open or close depending on voltage Let ions in or out

Hyperpolarization/Depolarization Opening of the K+ channel lets K+ flow out, making cell more negative Opening Na+ channel lets Na+ flow in, making cell become positive

Action Potential When a cell reaches a depolarization threshold, voltage- gated Na channels open Causes the neuron to quickly become very positive

How an Action Potential Works 2. Na+ channels close, K+ channels open, cell returns to negative 1. Na+ channels open, cell becomes positive 1. Na+ channels open, cell becomes positive 3. K+ channels stay open, cell becomes even more negative REFRACTORY PERIOD 4. Cell returns to normal state

The Refractory Period Cell becomes hyperpolarized Won't react to another stimulus Limits how fast neurons can fire Strong stimuli cause nerves to fire immediately after this period

Main Point #5 Action potentials travel down a neuron and are sped up by myelination of the axon

Travel of Action Potential Na+ flowing into one part of the axon makes the portions next to it positive This causes a new action potential Keeps getting passed down the neuron

Speeding up Transmission Bigger axons convey messages quicker Myelinated axons have evolved in vertebrates

How Myelination Works Ions can only diffuse at gaps in the myelin So the action potential jumps from node to node Signals can move 150m/s!

White Matter vs. Gray Matter Myelinated axons appear white and are bundled together Cell bodies, dendrites and un-myelinated axons appear gray

Main Point #6 Signals are transmitted from a neuron to another cell at synapses Can be stimulatory or inhibitory

Synapses Small gap between neuron and another cell Can be another neuron, a muscle cell or a gland

Role of Calcium At the end of the axon terminal depolarization causes opening of Ca++ channels Inflow of calcium causes neurotransmitters stores in vesicles to be released

Chemical Synapses Axon terminal releases a neurotransmitter into the synaptic cleft The neurotransmitter binds to a receptor on the receiving cell, opening ion channels

Postsynaptic Effects Some channels cause depolarization or the creation of an action potential Some cause hyperpolarization, inhibiting action potentials

Summation Usually one neuron doesn't cause a strong enough effect to cause an action potential But a neuron receives signals from several neurons

Response Eventually the signal induces a response I.e. flexing of a muscle, opening/closing of a channel, release of a hormone etc.

=sodium (Na+) V -50 -70 =potassium (K+) Time

Ca Ca Ca