E. Propagation of A Nerve Impulse - nerve impulses,(many depolarizations & repolarizations) “self-propagate”;travel along the axonal membrane from cell body to the end-buttons of the neuron.A Domino Effect…. (with “magic dominos”) - dominos topple-over and then standing back up again in the same order in which they fell.

Two Types of Electrical Signals in Neurons:1) Graded potential – short-distance communication; “localized”within the cell body and dendrites. Na+& Cl- ion channels here.2) Action potentials (nerve impulse) – long and short-distancecommunication; within the axon and synaptic end bulbs.Na+ & K+ ion channels here.

Depolarization of Graded Potential -60 mV

Depolarization of Action Potential +30 mV

(Axon hillock)

Propagation of a Nerve Impulse

aka. “Trigger Zone”

F. All or None Principle: if stimulated (reaching thresholdstimulus of -55mV), the neuron will generate an actionpotential with a constant/maximum strength (+ 30 mV) andduration unless fatigue or toxic material. *like igniting a trailof gun powder; “a flash point when igniting gasoline”

The venom of most scorpions, snakes and many insecticides act by causing Na+ channels to remain open and by blocking the K+ channels - paralysis. What effects would such venoms have on the duration(time/length) of an action potential? Would its amplitude (mV) be changed significantly?

Dendrotoxins -can profoundly affect neuronal excitability

Sharks have electroreceptors in their skin that can detect nanovolts (nv) emitted from other organisms. This is one way they distinguish from certain prey ….. Way cool!

Question?If all action potentials are of the same amplitude/ strength(+30 mV) and duration/time, then how can your sensory &motor systems respond in varying manner? i.e. why does alight touch feel different from firmer pressure; and how canyou control muscle action to be light & delicate or strong &vigorous?

1. Frequency of Nerve Impulse: # of nerve impulses produced/sec.

3 Explanations below:

2. Stimulus Duration: how long the stimulus is applied.

3. Receptive Fields: how many receptors per area of the body.

Action Potentials from the afferent neuron leading from a pressure receptor (in a finger) as the receptor is

subjected to different magnitudes of pressure

Recruitment of sensory receptor firing:

A“calling-in” of additional sensory afferent neurons

CNS Sensory afferent “A”

Sensory afferent “B”

2. Stimulus Duration:• receptors differ in the way they respond to a constantly

maintained stimulus.

Question?Why is it that when you sit on a chair you feel the pressureinitially, but after a few seconds the intensity of pressure isless?

* 2 types of receptor responses to stimulus:A. Tonic receptors “slowly adapting receptors”– e.g. painreceptors.

B. Phasic receptors “rapidly adapting receptors”– e.g. smellingfish, sitting on a stool, acclimating to the Jacuzzi temp.

Phasic receptors Tonic receptors Receptor Firing Pattern

Receptor Stimulation

Relationship between stimulus strength, and the action potential frequency

G. Speed(Velocity) of Impulse Conduction: nerve impulsespeed is related to diameter of axon, temperature & presenceof a myelin sheath around the axon.a) ↑ diameter ; ↑ speed

b) ↑ temp. ; ↑ speed

c) presence of myelin sheath ; ↑ speedQUESTION: Why is ice applied to an injured area to reduce the pain?

From 27 mi/hr. up to speeds of 280 mi/hr.

*The largest nerve fibers are those associated in relaying nerve impulses from external sensory receptors to CNS. Smaller nerve fibers are associated w/impulses from organs (sensing a full bladder for example) to CNS. Question: What might be the physiological advantage of this association mentioned above?

VII. Signal Transmission at Synapses: A chemical signal (NT’s)* remember neuromuscular Jncts. from muscle physiology?* synaptic junction - a tiny space between communicatingneurons or between a neuron and an effector (a muscle orgland).• Presynaptic neurons (info. sender)• Postsynaptic (info. receiver); an effector or neuron.

Different types of synapses between neurons: 1 Neuron may have up to 1000’s of synapses! WOW

Synaptic end-bulbs or

“end-buttons”

(Axon hillock)

(with receptors for chemical signals!) Dendrites

Which end of the neuron is the receiving end? The sending end?

Can you think of a way one neuron might communicate with a neighboring neuron, gland or muscle?

Neurons relay their information using both electrical signals and chemical messages in a process called Neurotransmission

Synaptic Junction “a chemical synapse”

The synaptic junction

Review: Steps in Neurotransmission

Acetylcholinistrase (AChE) plays a role in relaxation!

AChE breaks down the Acetylcholine during relaxation

Enzymes degrade NT’s