Neural Control of Human Movement

Unit 1

Sherrington

synapse, reflexes, active inhibition, basis of locomotion

Graham Brown

central pattern generators

Wachholder and Altenburger

electromyography

Bernstein

father of motor control, evolution based approach, multilevel scheme for construction of movements, physiology of activity

Reductionism

Reducing the elements of a system to a basic level of functioning elements, but may miss the big picture

Complex System Approach

knowledge of the elements is essential to understanding the system but greater emphasis is placed on investigating and understanding the system as a whole, but may miss minor details

The complex system approach allows for more _____ description of behavior and the interpretation

succinet

cellular structures exist with

a partial permeability

Three major types of substances can cross membranes

solvents, electrolytes, nonelectrolytes

Convection

movement of a solute based on a pressure gradient. The pressure differential will dictate the rate of flow. The flow always goes from an area of high pressure to an area of low pressure

Diffusion

Movement of the concentration of particles within a solution. Moves from an area of high concentration to an area of low concentration

Movement of Charged Particles

An electric field creates a difference of potentials that induces a flow of charged particles. The current is proportional to the difference of potentials. The inverse of the coefficient of proportionality is termed resistance

Ions move under the influence of what 2 forces

concentration gradient and the difference of potentials

Electric Current

Movement of charged particles (ions) under the action of a difference of potentials

In a state of equilibrium, the movements of all particles are

equilibrium (counterbalanced)

Membrane Channels

Most substances move across the membrane at special locations. These sites usually will allow certain substances across and not others. This acts as a gating mechanism.

3 important properties oof ion channels

• conduct ions

• recognize specific ions

• open and close in response to specific electrical, mechanical, or chemical signals

Ion channels allow for

rapid changes in membrane potential

when does motor development stop?

death

Gaining an understanding of how all the parts or elements work together would be an example of what type of approach?

Complex System Approach

Gaining an understanding of the individual details of the parts of elements would be an example of what type of approach?

Reductionism

What is the difference between convection and diffusion

convection is pressure gradient and diffusion is concentration gradient

Resting membrane potential

difference in electrical potential across the membrane (voltage), descibes the inside of the cell

What is the typical resting membrane potential for a neuron

-70mv

What is an action potential?

regenerative electrical signal in which the amplitude does not attenuate as it moves down the axon

Importance of an Action Potential

allows for the communication of information at a much more rapid pace than convection or diffusion

Speed of Communication

far greater than communication through diffusion or convection, this communication is based upon the potential of the membrane

Active Ion Pump

Maintaining the ion concentration gradients across the membrane requires energy, which is provided by a chemical process that transforms ATP into ADP.

What happens to the membrane potential during depolarization and hyperpolarization? How does this relate to the movement of sodium & potassium ions?

During hyperpolarization the inside of the cell becomes more negative & potassium leaves the cell. Depolarization the cell becomes more positive & sodium comes into the cell

Three important ions

Na, K, Cl (not as important)

Inside of cell

(-) K+ ions

Outside of cell

(+) Na+ ions

What types of stimuli cause action potentials?

light, sound, touch, smell, taste, temp

Threshold

potential when the membrane generates an action potential (-55mv)

All-or-None Law

Either the membrane does not generate an action potential or it generates an AP with a standard shape and magnitude

Hyperpolarizing

Potassium out, more negative

Depolarizing

Sodium in, more positive

does a painful stimulus, such as a sharp or hot object, result in the generation of larger, stronger action potentials?

no, not all stimuli are the same shape or size

Sodium Potassium Pump

an active mechanism maintaining resting potential

Relative Refractory Period

The period following the generation of an AP in which it is possible to generate another AP. The second AP may not be generated with the same intensity of stimulus. An increase in intensity is necessary to generate a new action potential. Hyperpolarized state.

Absolute Refractory Period

The period following the generation of an AP in which it is NOT possible to generate a new ap

Sodium Channel Inactivation

+30mV - Sodium channels inactivation - absolute refractory

• St1 - Action Potential

• St2 - Absolute Refractory

• St3 - Relative Refractory

Features of Action Potential

• caused by membrane depolarization

• has a sharp peak of positive voltage (+30mV)

• followed by an afterpotential (hyperpolarization)

• the duration is about 1 ms

Refractory Period

• caused by inactivation of sodium channels

• absolute (1-2 ms) relative (~10 ms)

• limits frequency of AP generation by a neuron

Positive Feedback

leads to a rapid amplification of the effect, depolarization

Negative Feedback

quickly restores the original state, hyperpolarization

Voltage Clamp Experiments

a constant depolarization is applied to the membrane. Na conductance turns on and off, K conductance changes slowly and stays at a new level. Higher stimuli lead to higher values of Na conductances achieved in shorter times.

Action Potentials Do Not Backfire

the phenomenon of inactivation of sodium channels does not allow an action potential to “backfire”. If an AP comes to point 2 from point 1, it cannot go back; it can travel only forward to point 3.

2 factors that affect Action Potential

axon diameter, myelination

Larger diameter

greater speed, faster

Smaller diameter

slower, less speed

Myelinated neural fibers

covered by myelin, which increases the effective distance of local currents. breaks in the myelin sheath are sites of action potential. few ion channels under the myelin, large number of ion channels in the ranvier nodes.

Conduction of the AP

an AP produces local currents spreading through surrounding tissues. local currents depolarize adjacent segments of the membrane. a new AP is generated. inactivation of the ion channels does not allow AP to backfire

Orthodromic conduction

from the soma to terminal branches, cell body down the axon, motor neurons

Antidromic conduction

from the end of the axon to the soma, up the axon, sensory neurons

Soma

body of the cell; the site of input signals

Dendrites

short branches originating from the soma; sites of inputs

Axon

a long branch; transmits output signals

Axon hillock

the site where the axon exits the soma; typically, the site of generation of AP

Terminal branches

a “brush” at the end of the axon

Myelin

a fatlike substance covering the axon; it increases the speed of conduction of AP

Ranvier nodes

breaks in the myelin sheath; places where AP are generated

Nerve (peripheral) or neural tract (central)

many axons running together

There are virtually __ ion channels under the myelin sheath

No

There are ___ ion channels in the Ranvier nodes

many

What are persistent inward currents (PICs)

Depolarizing inward currents through ion channels, typically Ca++, that do not inactivate with prolonged depolarization

What effect do PICs have on the threshold for AP generation

they cause an effective drop in threshold

What effect do PICs have on AP?

they increase the number of AP generated and can lead to continuous AP generation without other excitatory inputs.

Where are PICs strong during everyday tasks?

in human a-motor neurons

What role do PICs play in motor units?

they help define the patterns of recruitment and derecruitment of motor units

What are PICs sensitive to?

Postsynaptic inhibition, meaning their activity can be controlled by it