Neural Control of Human Movement Study Guide

This set of flashcards contains key terms and definitions related to the neural control of human movement, focusing on reflexes, control mechanisms, and muscle activation patterns.

Reflex

A muscle contraction induced by an external stimulus that cannot be changed by pure thinking.

• short, simple, local, low, always

• Voluntary action: long complex, global, high

Monosynaptic Reflex

Reflex involving one synapse, the longest

example:

• Phasic Reflexes

• H-reflex

• T-reflex

Oligosynaptic Reflex

Reflex involving few synapses (2 or 3).

ex:

• Reciprocal inhibition

• Ia muscle spindle

• Ib golgi tendon organ

Polysynaptic Reflex

Reflex involving many synapses (>3), typically middle

examples:

• flexor reflex

• tonic stretch reflexes.

• crossed extensor reflex

• tonic vibration

longer latency, slow, steady-state character (great delay)

Tonic Reflex

A long-lasting, steady muscle contraction response.

Phasic Reflex

A brief, transient muscle contraction response.

Reflex Arc Components

1. Sensory receptors,

2. Sensory/afferent neuron,

3. Central processing unit,

4. Motor/efferent neuron,

5. Muscle/effector.

Ia Spindle Afferents

The neurons involved in monosynaptic reflexes, particularly in knee-jerk responses.

H-reflex

A reflex generated by electrically stimulating afferent fibers in a peripheral nerve and recording the reflexive response in the same muscle

T-reflex

A reflex generated by a tendon tap that mechanically stretches the muscle.

Reciprocal Inhibition

A reflex mechanism in which one muscle relaxes when its antagonist is activated.

Tonic Stretch Reflex

A major mechanism that defines the viscoelastic properties of muscles, joints, and limbs.

Tonic Vibration Reflex

A polysynaptic reflex characterized by slow, reflexive increases in muscle force due to high frequency vibration.

Crossed Extensor Reflex

A polysynaptic reflex that works in coordination with the flexor reflex to maintain balance.

Pre-programmed Reaction

Semi-automatic reactions to stimuli that are faster than voluntary movements but slower than typical reflexes.

Feedforward Control

Open loop control that generates commands without regard to sensory feedback.

advantages: speed (responding to a visual stimulus takes about 200 ms)

Disadvantages: cannot correct for errors and error can compound

ex: giving advice on future task, using project rubrics for preparation. present muscle movement

Feedback Control

Closed loop control that adjusts command signals based on outcome.

advantages: more accurate, can adjust to the environment

disadvantages: slower due to time delay

ex: negative feedback and positive feedback

Merton's Servo Hypothesis

A theory suggesting that the central nervous system uses indirect commands to control muscle activity.

Central Commands Theory

The theory proposing direct specification of muscle activation levels by the brain.

Equilibrium Point Hypothesis

The hypothesis that muscle reflexes specify a relation between muscle force and length, determining the equilibrium point.

Triphasic EMG Pattern

A pattern of muscle activation involving an agonist burst, antagonist burst, and a final agonist burst.

Cocontraction

Coordination between agonist and antagonist muscle activity to stabilize movement.

What are the primary reasons to study reflexes

• can assist in the diagnosis of certain conditions

• help localize injury or disease in the CNS

• absent/weak (hypoactive) reflexes often indicate a disorder in one or more of the components of the reflex arc

• lesion can cause both hyperactive and hypoactive reflexes

what causes the delays in the reflex arc

synaptic transmission it occurs as the nerve impulse passes from a sensory neuron to a motor neuron within the spinal cord

delays are caused by afferent conduction, central processing delay (determined by the number of synapses) and efferent conduction

What neurons are involved in a monosynaptic reflex

Ia spindle afferents and alpha motoneurons

why are both afferent and efferent neurons stimulated when a person elicits the H-reflex

the electrical stimulus is applied to a mixed peripheral nerve containing bother afferent and effferent fibers

What is the typical response of the H-reflex (amplified) as the intensity of the stimulus increases? Why do we see this response?

increases with intensity but decrease at high levels due to action potential collision (where antidromic signals in the motor neurons cancel out the orthodromic reflexie signals)

What is the response of the M-response (amplititude) as the intensity of the stimulus increases? Why do we see this response?

increases steadily until it plateaus

what are the effects of high frequency stimulation on the H-reflex

leads to progressively smaller H-reflexes while M-response remain unchanged because of the refractory period for the synapses is longer than for the axon

How is the T-reflex similar and different from the H-reflex

• both are monosynaptic

• h-reflex is triggered by electrical stimulation

• t-reflex is triggered by a mechanical stretch

How does contraction of the agonist muscle group and contraction of the antagonist muscle group affect the amplitude of the H-reflex and T-reflex?

agonist contraction increases the reflex amplitude due to excitation of the mono neuronal pool

antagonist contraction decrease it through reciprocal inhibition form Ia interneurons

What is the purpose of oligosynaptic and polysynaptic

their functional importance is considered limited as they induce reflexive contraction that are poorly controlled voluntarily

What is the difference between phasic and tonic reflex

• phasic- emerge in response to a change in the level of a stimulus specific to the receptors.

typical a burst, of brief depression of muscle activity that leads to a twitch or series of twitchy movements (all monosynaptic reflexes)

• tonic reflexes: emerge in response to the level of a stimulus

Typically leads to sustained muscle contractions and relatively smooth movement (polysynaptic)

Flexor reflex

is a polysynaptic reflex that leads to the activation of flexor muscle within the same (ipsilateral) limb

what triggers the reflex and what neurons are involved (poly. and oligio)

triggered by flexor reflex afferent (secondary spindle endings, cutaneous receptors)

what is the general scheme/purpose of the flexor reflex

it activates flexor muscles in the same limb to move it away from a stimulus

what triggers the tonic stretch reflex and what neurons are involved in tonic stretch

triggered by slow muscle stretch

what is the general scheme/purpose of the tonic stretch reflex?

• is to define the viscoelastic properties (stiffness and damping) fo joints and limbs

• it serves as a mechanism for the brain to control movement by simply adjusting the reflex threshold. ensuring that muscle activity and joint angles result from a balanced interaction between central commands and external loads

what happens at the threshold of the tonic stretch reflex

alpha motor neuron recruitment begins

what triggers the reflex and what neuron are involved for tonic vibration

triggered by high frequency vibration leading to a slow increase in muscle force

what is the general purpose of the tonic vibration reflex

a key feature is that is suppressed monosynaptic reflexes (like H-reflex) in the same muscle through presynaptic inhibition

functionally the TVR can induce muscle contraction that mirror the patterns used during locomotion

what triggers the reflex and what neurons are involved for crossed extensor reflex

triggered by FRA stimulation in one limb in activates extensor muscles in the contralateral (opposite limb)

what is the general scheme/purpose of the crossed extensor reflex

• the primary purpose of this reflex is to maintain postural equilibrium

• by extending the opposite limb, the body provides a stable based of support to compensate for the withdrawal of the stimulated limb

what does the crossed extensor reflex work with

it works in conjunction with the flexor to maintain balance

how is pre-programmed reactions similar to reflex

they are triggered by external stimuli and are involuntary

how is pre-programmed reactions different to reflex

they have longer latency involve the motor cortex (transcortical) and can be modified by instruction

How do the following factors influence a pre-programmed reaction

• Instructions: These reactions demonstrate a strong dependence on the instruction given a subject’s intent. If told to resist a perturbation, the reaction (M2 and M3peaks ) become larger if told to let go it becomes smaller

• Previous experience: The central nervous system uses past events to preprogram fast compensatory reactions for conceivable perturbation allowing for more effective stabilization

• Context: responses are context-dependent meaning they adapt to the specific task or mechanical environment

examples of pre-programmed reactions

the waiter’s response being aware, grasp adjustments, and the corrective stumbling reaction

what is the corrective stumbling reaction

in the swing phase of gait, it triggers a flexion reaction to step over an obstacle in the stance phase it triggers an extension reaction to shorten the phase for that limb

for human movement what part of the body is the controller and what control variables can the controller use to format output commands

the brain/CNS is the controller. Control variables are used by controller to formulate commands signals

how does positive feedback and negative feedback relate to control

negative feedback reduces deviation, positive feedback amplifies them

what is the difference between gain and delay

gain is the ration of output change in input change

delay is measure on time

how does a servo work

a circuit combing feedforward and feedback to keep a variable constrict (like a thermostat)

What are some basic features of a Servo? What type of control does it use?

• Feedback loop: the servo itself is a feedback loop designed to keep a specific variable constant at a value specified by controller despite changes in external conditions

• Error correction: it functions by comparing the intended output to the actual output and making corrections

• Sensitivity to delay: a key feature is its relationship with time longer time delays allow larger errors to accumulate before the servo caninitiate a corrective action

what are the main neruons involved in merton’s servo hypothesis

o Gamma Motoneuron- these neurons receive descending central

commands from the brain which set the desired muscle length by setting

the activity level of the y- system

o Muscle spindles (afferents)- these serve as the sensory element that

detects changes in muscle length and sensitivity based on the y-

motoneuron activity

o Alpha motoneurons- these neurons receive the reflexive signal from the

muscle spindle afferents via the tonic stretch reflex (TSR) and innervate

the muscle to produce a contraction

what is the basic property of this Merton’s Servo hypothesis

indirect command- the central nervous system does not directly activate alpha motoneurons to initiate a movement

Gamma-driven sensitivity: instead descending commands specify the activity level of gamma motoneurons which change the sensitivity of muscle spindles to muscle length

Two main problems with this hypothesis

• no direct alpha motorneuron activation

• TSR as a perfect servo

basic scheme of central commands theory

The theory proposes that central commands from the brain directly specify the activity levels of alpha motoneuron pools (the brain is responsible for directly determining the level of muscle activation

main neurons in central commands theory

the primary neurons involved are the alpha motoneurons which receive direct signals from the central nervous system to initiate muscle contraction

what is the basic premises of central commands theory

• Direct control: central commands directly specify the level of muscle activation rather than using indirect loops

• Minor reflex role: reflexes are considered to play only a minor role in voluntary movement, contributing mostly to unexpected changes

• Parameter specification: the theory suggests that central commands use reflexes to change muscle activity level by specifying the parameters of these reflexes

What is the main problem with command central theory

When a subject holds a steady position against a load that is suddenly removed the EMG of the active muscle shoes a period of complete silence (unloading) reflex even if the subject is voluntarily trying to keep the muscle active. The silence occurs because moving the load causes muscle spindle activity to drop which turns off the reflexive support for the motoneuron pool

what is the basic scheme of the equilibrium point hypothesis

muscle reflexes specify a relation between muscle force and muscle length, referred to as an invariant characteristics (IC)

the central nervous system does not directly specify muscle force or length but instead specifies the location of this IC

a central command is viewed as a balanced combination of descending signals to various spinal neurons

what is the main neuron involved in equilibrium point hypothesis

alpha motorneurons

gamma motor neurons

interneuron

they work together within the tonic stretch reflex mechanism

how does equilibrium point hypothesis work

• Specifying Threshold (): The central command specifies the location of the IC by shifting the threshold of the tonic stretch reflex ().

• Reaching Equilibrium: The system (comprised of the muscle and the load) is in equilibrium when the muscle force equals the external force (the load). This specific state is called the equilibrium point (EP).

• Adjusting to Loads: If the external load changes, the muscle force and length will both change to find a new equilibrium point.

• Joint Contribution: Changes in muscle activation (EMG), force, and length emerge from equal contributions of central commands and peripheral signals from the load via the TSR. This means the brain provides the initial shift, but the final outcome is determined by the interaction with the environment

what is the main benefits of this hypothesis EMG

correctly accounts of for the independence of muscle length, force and the EMG activity when external loads change

Why study single joint movement

to understand motor variability progress from simple to complex control and for clinical relevance

what is the difference between task parameters and performance variables

• task parameters are what the subject is instructed to do

• Performance variables are what the subject actually does

What are the three parts of the triphasic EMG pattern (using the agonist and antagonist muscle groups)?

• First agonist burst: the initial burst of activity in the agonist muscle that starts the movement

• antagonist Burst- a burst in the antagonist muscle that acts as a “brake” to slow down and control the movement

• second agonist burst- a final burst in the agonist muscle that finishes the movement or provides corrections

What causes each of the three parts of this pattern for the triphasic EMG pattern

• First agonist burst: caused by the voluntary central command to imitate the joint movement

• Antagonist burst: the reflexive in nature. The quick initial burst of the agonist muscle stretches the antagonist muscle, activating its muscle spindles which in turn activate the alpha motoneurons of the antagonist

• Second agonist burst: general serves to finish the movement and bring the joint to its final target position

Using an actual joint action and muscle groups, apply the triphasic EMG pattern to a movement (elbow flexion, knee extension, etc)

• In an elbow extension movement, the triceps is the agonist and the biceps are the antagonist

• The pattern begins with a triceps burst (starts the extension), followed by a biceps burst (brakes the extension) and ends with a second triceps burst (complete the extension)

How does increasing the velocity of the movement affect the triphasic EMG pattern?

• Agonist Burst: There is an increase in the rate of EMG rise, the peak amplitude, and the total area of the burst.

• Antagonist Delay: The delay decreases, meaning the antagonist "brake" kicks in sooner.

• Antagonist Amplitude: There is an increase in the amplitude and area of the antagonist burst.

• Cocontraction: The level of final cocontraction between the muscles increases

What are the two methods to analyze and interpret EMG activity?

• Viewing EMGs as reliable indices of central control signals, reflecting the processes of voluntary motor control.

• Viewing EMGs as being generated by equal contributions from both central control signals and peripheral reflex loops (similar to the Equilibrium Point Hypothesis

What is the term for diminished reflex? What is the term for an exaggerated reflex

hypo-reflex / Exaggerated

what happens to muscle spindle activity when a muscle lengthens

spindles activity increases in the lengthen muscle

what happens to the activity of the Ia afferent when a muscle lengthens

Ia afferent (primary ending) increase their activity (sends more APs towards the spinal cord)

what happens to muscle force due to the activity of the alpha motoneurons

muscle force will increase

what happens at the threshold of the tonic stretch reflex? What happens if themuscle continues to lengthen?

this is the point where alpha motoneuron activity causes reflexive force development in muscle

what happens to the activity of the alpha motor neurons due to the activity of the Ia afferents

alpha motorneuron activity will increase (sends more APs to the lengthened muscle, gamma, and alpha gamma only sends excitatory)

how does the crossed extensor reflex work with the flexor reflex

muscle force will continue to increase the muscle length

what is a pre-programmed reaction/long loop reflex

brain involvement

put the following in order from the fastest to the slowest

reflex

preprogrammed reaction

voluntary correction

what type of stimulus can trigger a pre-programmed reaction

sensory

• visual

• auditory

• touch

what is the primary advantage and disadvantage of feedforward control? What is an example of feedforward control

• advantage: doesn’t have to use sensory information faster

• disadvantage: cant correct it

what is the primary advantage and disadvantage of feedback control? What is an example of feedback control

advantage: accuracy

disadvantage: slower;

ex: desk in the way and you adjust to go around it

Imagine that you are eating your favorite meal. As you eat your favorite food, what type of control would be used? What role would the comparator play as you move the food to your mouth and your cup/glass to your mouth and back to the table?

feedforward to start a movement

feedback: guide/finish movement

comparator: decide if the piece was big enough or small enough

what was the primary problem with the central command viewpoint? Which reflex/reaction provides evidence against this viewpoint?

unloading reflex reaction

which of the following reflexes would have the shortest central processing delay (which reflex is the fastest/occurs in the least amount of time )

H-reflex

what happens at the threshold of the tonic stretch reflex

reflexive (Autogenic) activation of alpha motor neurons

Which of the following is NOT a component of the standard reflex arc?

high brain centers

what causes the H-reflex

direct electrical stimulation of the sensory neurons

what causes M-response

direct electrical stimulation of the motor neurons

which of the following is an example of a phasic reflex

t-reflex

muscle spindles are involved in which of the following reflexes

mono synaptics

oligosynaptic

polysynaptic

You are walking and step on a sharp/painful object. Which of the following reflexes would you expect would be elicited

flexor reflex

crossed extensor reflex

which of the following order of response is correct after a stimulus/pertubation is applied to a person

reflex

preprogramed reaction

voluntary correction

what is the typical latency of the H-reflex and T-reflex

35ms

what is the latency of a pre-programmed reaction

50-100 ms

which reflex supressed/reduces the H-reflex through the process of presynaptic inhibition

tonic vibration reflex

What conclusion can be drawn from the unloading reflex/reaction and the loading reaction (the experiment when the load is increased by 4x its current magnitude)

shows voluntary & reflexive commands