Larry M. Frolich, Ph.D. Biology Department, Yavapai College.

Larry M. Frolich, Ph.D.Biology Department, Yavapai College


Nervous Function• What does the nervous system do?• Action Potentials—rapid transmission

of messages• Reflex arc (simple somatic function)

and autonomic function• What can we sense?


Nervous Function• What does the nervous

system do?• Action Potentials—rapid transmission of

messages• Reflex arc (simple somatic function) and

autonomic function• What can we sense?


What does the nervous system do?

• Sensory input—brings information in (last part of this presentation shows what kind of information)

• Processing information (brain and spinal cord—won’t go into details in this class)

• Motor output—initiates response – Muscles—movement– Glands—secretion

This is all we can do!


movement

sensory

Processing—this is the complicated part that 10,000 neurobiologists are working on


Nervous Function• What does the nervous system do?

• Action Potentials—rapid transmission of messages

• Reflex arc (simple somatic function) and autonomic function

• What can we sense?


Action potentials

• Ability to sense environment, process information rapidly and respond requires rapid transmission of messages within body

• Ability to transmit messages rapidly unique to animals

• Accomplished by cell membrane changes called action potentials

• Happens in nervous tissue cells called neurons and in muscle cells

• Focus for now on neurons


Neurons• Main cells of nervous system• Transmit rapid messages

called action potentials• Axon carries message• Dendrites connect to other

cells, usually neurons or muscle cells

• Cell body contains nucleus, maintains cell function

• Single neuron carries message to and from periphery (axons over 1 meter long but barely visible in cross section in microscope)

• Most neurons never die or divide (undergo mitosis) after initial nervous system development (life-time cells!)


Myelin sheath• Single nerve contains

hundreds or thousands of axons

• Each axon is surrounded by a myelin sheath

• Myelin is a fat-based insulating substance that isolates neurons and helps speed of action potential propagation (more later)

• Myelin is contained within cells that wrap around axon called Schwann cells


Types of neurons• Sensory neurons (bundled in

nerves) bring information in from virtually every tissue, structure and organ of body (except brain and spinal cord—thus brain surgery with no anesthesia)

• Motor neurons (bundled in nerves) to every muscle and gland, including blood vessels


How do neurons carry messages?

• ACTION POTENTIALS– Unique to animal nerve and muscle tissue– Ability to rapidly carry an ion diffusion

mediated change in voltage along the cell membrane

– Only neurons and muscle cells can do it– Here’s how (more or less)….


Resting potential• At rest, one-third of total body

energy used in neurons and muscle cells maintaining resting potential

• Sodium-potassium “pump” actively moves + ions out of cell so that inside cell is -70mV relative to outside

• Happens at sub-microscopic molecular level (“pump” is protein structures in cell membrane)

• Happens at micro-time scale (in 1000’s of a second)

• Constantly maintains resting state for all neurons and muscle cells so they are ready to “fire”


Action Potential—cell membrane “depolarizes”• A stimulus causes Na+ and

K+ “gates” to open• Positive ions rush in

changing cell interior voltage to +40 mV relative to outside cell

• Stimulus at one end of cell causes adjacent membrane “gates” to open so that action potential “propagates” or moves along cell membrane. This is how message is rapidly transmitted (at about 100 Miles per hour)

• “Gates” are actually protein structures in cell membrane)


Role of myelin sheath/Schwann cells

• Myelin in Schwann cells wrapped around axon means that action potential can jump to nodes where cell membrane is exposed

• This is called saltatory (“jumping”) propagatino of action potential• It saves energy (so entire membrane doesn’t depolarize) and makes

action potential move faster


• Immediately after depolarization, sodium potassium pump returns cell to resting state (resting potential

• Voltage change from negative resting potential to positive actino potential and back to negative resting potential can all happen within 3/1000 of a second

Speed of depolarization or ion movement


Synapse

• What happens when action potential arrives at end of axon?

• Neurotransmitter is secreted to stimulate connecting neurons or muscle cells


Nervous Function• What does the nervous system do?• Action Potentials—rapid transmission of messages

• Reflex arc (simple somatic function) and autonomic function



Total Nervous Function

• Sensory neurons—action potential brings message to brain or spinal cord with sensory input information from sensory receptors

• Motor neurons—action potential takes message away from brain or spinal cord with motor output command (for movement to muscle cells or secretion to gland cells)

• Within brain and spinal cord, action potentials among neurons process information allowing for decision-making—what is the motor response to the sensory input?


Simplest case—spinal cord reflex

• Sensory neurons action potential bring in pain information from skin• Neurons of spinal cord process information, take decision• Motor neurons carry output to muscles to move limb away from pain

NOTE: Image shows single neuron but pain would stimulate many neurons that are become bundled together in nerve. Motor output would also be carried by many neurons that stimulate many muscle cells causing movement response.


Autonomic function• Spinal cord reflex example is “voluntary” or somatic function• Most visceral organs (digestive system, glands, etc.), blood flow

(heart, blood vessels) have autonomic “involuntary” control• Same principles, same kinds of action potential• Control is sub-conscious• Always dual control or innervation:

– Sympathetic: Speed up (“fight or flight”) response– Parasympathetic: Slow down (“meditative”) response


Nervous Function• What does the nervous system do?• Action Potentials—rapid transmission of

messages• Reflex arc (simple somatic function) and

autonomic function



How does sensation happen?

• Sensory receptors respond to stimulus from body or environment

• These receptor cells trigger action potential in connecting sensory neurons

• Spinal cord and/or brain interpret and analyze information– Where?– What?– How much/how strong?


Sensory fields in brain• For conscious somatic sensory perception, large

fields in brain organize information spatially– Visual cortex forms visual field or complete visual

image– Sensory cortex maps touch sensation from entire skin

surface


What can we sense—sensory all over body

• Cutaneous receptors of skin• Bring touch, pressure, pain, heat cold


• Proprioception—gives body position by sensing muscle tension• Allows touching nose with eyes closed



Special senses—in head

• Taste

• Smell

• Vision

• Hearing

• Equilibrium


Taste


Smell


Vision—eye as optical device


Vision—retina receptors

respond to light


Hearing


Equilibrium


Nervous Function--review• What does the nervous system do?• Action Potentials—rapid transmission

of messages• Reflex arc (simple somatic function)

and autonomic function• What can we sense?