Nervous System

Reflexes and Action Potential

How do cells detect and respond to

changes in their internal and external

environment to successfully survive

and maintain homeostasis?

1) Detection of signals from the outside environment or detection of deviation (change) from homeostasis from the internal environment.

2) Integration of multiple signals from outside and inside to produce appropriate response.

BRAIN

3) Response to counteract stimulus being detected.

Draw your own picture to represent RESPONSE

1) Detection of signals from outside environment or detection of deviation (change) from homeostasis from internal environment.

2) Integration of multiple signals from outside and inside to produce appropriate response.

3) Response to counteract stimulus being detected

System builds a Reflex Arc

Sensor Receptor (Detection)

Sensory Input

IntegrationEffector (Response)

Motor Output

Peripheral NervousSystem (PNS)

Central Nervous System(CNS)

Spinal Reflexes

• Mono synaptic reflexes do not reach higher brain levels before responding.

– No thought involved– 1 synapse (sensory to motor – no interneuron)– Example: sneezing

• Polysynaptic reflexes reach the brain during response.

– More than 1 synapse (sensory to interneuron to motor)

– Example- finger on hot stove

Reflex test

• Try the knee-jerk reflex yourself. – Do you have any voluntary

control?

• Can you find any other places on your body that have a similar reflex?– What do the knee-jerk and

other reflex spots have in common?

Action Potential – Resting I

• Resting Potential (-70mV)– Result of unequal distribution of ions– Ion channels– Na+ & K+ (mostly)

• 10X more Na+ outside than inside• Na+/K+ Pump

“At its most basic, an action potential is a very rapid swing in the polarity of the membrane potential from negative to positive and back, the entire cycle lasting a few milliseconds” (http://en.wikipedia.org/wiki/Action_potential )

Animation

Action Potential!!!!!!!!!!

• Stimulus causes Na+ channels to open– Flow into cell causes

depolarization– Adjacent region

subsequently depolarized– Voltage climbs to +30 or

more

• Repolarization– Diffusion of K+ out K+

channels – closing of Na + channels

• All this, 1/1000 sec!

Action Potential – Resting II

• Concentration Gradient• 3 Na+ vs. 2 K+• Plasma Membrane

more permeable to K+– Leak out

Animation

All or none!

• Threshold Stimulus = -55 mV

• Subthreshold – those stimuli that are weaker than threshold (No AP)– Summation

Action Potential III

• How does the myelin sheath improve conductance speed?

• Answer – Blocks the continuous flow of ions through the membrane

• Saltatory Conduction – impulse jumps! between Nodes of Ranvier

• http://bcs.whfreeman.com/thelifewire/content/chp44/4402002.html

Synaptic Gap

• Junction between neurons

• from presynaptic neuron to postsynaptic neuron

• Synaptic vesicles at bulb (end of axon) contain neurotransmitters

Synapse Movie

• The point of contact at which impulses are passed from one cell to another are known as THE SYNAPTIC CLEFT OR SYNAPSE.

• Neurons that transmit impulses to other neurons DO NOT actually touch one another.  The Small Gap or Space between the axon of one neuron and the dendrites of the next neuron is called the Synapse. 

• One importance of the presence of Synapses is that they ensures one-way transmission of impulses in a living person. 

• The Axon Terminals at a Synapse contain tiny vesicles, or sacs. These are known as NEUROTRANSMITTERS.

NEUROTRANSMITTER is a chemical substance that is used by one neuron to signal another.  The impulse is changed from an Electrical Impulse to a Chemical Impulse (Electrochemical Impulses).