The motor unit and spinal reflexes - PowerPoint Presentation

Slide1

The motor unit and spinal reflexes

Michael Beierlein, PhD

Department of Neurobiology and Anatomy, MSE R442

McGovern Medical School

Houston, TX

Email: michael.beierlein@uth.tmc.edu

Slide2

Cognition

Action

Environment

Senses

Motor control

Slide3

Components of Proper Motor Control Volition

transformation of abstract goals into activation of specific set of muscles Coordination of signals to many muscle groups

Proprioception

constant updates about muscle length, force, joint position

Postural adjustments

Sensory feedback

Compensation for body and muscles Unconscious processingWalking, postural adjustments, etc.

Adaptability acquisition of motor skills

Slide4

Hierarchical Organization and Functional Segregationof Central Motor Structures

Slide5

Major components involved in motor control

Nolte

Slide6

From J. Nolte (2002)

The Human Brain, 5

th

Edition

Alpha

(lower) motor neurons – the “final common pathway”

Alpha motor neuron

Slide7

Motor Unit:

a single motor neuron and all of the muscle fibers that it

innervates. Each muscle fiber only receives input from

one

motor neuron

Innervation ratio:

number of muscle fibers which are part of a motor unit (~10

to more than 1000)

Slide8

Motor n

euron pool and motor units

Motor

neuron pool (motor

n

ucleus):

The

group of motor neurons (MNs) that innervate a single muscle. Muscles used for finer movements have a larger MN pool

Motor unit:

An individual motor neuron and all the muscle fibers that it

innervates

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Slide10

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Control Of Muscle Force

Rate code:

F

orce generated by a given muscle depends on

firing rate

of alpha motor neuron

Slide12

Size principle: Small motor neurons are recruited first, then larger neurons

Control Of Muscle Force

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Size principle: Small motor neurons are recruited first, then larger neurons

Control Of Muscle Force

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Size principle: Small motor neurons are recruited first, then larger neurons

Control Of Muscle Force

Slide15

From G. E. Loeb & C.

Ghez (2000), in Principles of Neural Science, 4th

Edition

(

Kandel

, Schwartz, &

Jessel

, Eds.)Size

principle: Small motor neurons are recruited first, then larger neurons

smaller neurons have larger input resistance (same synaptic current evokes larger excitatory postsynaptic responses)

Fewer synaptic inputs are needed to generate action potential in smaller neuron

Control Of Muscle Force

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Size principle: 1. Small motor neurons are recruited first, then larger neurons

Control Of Muscle Force

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Control Of Muscle Force

Size principle:

2

. Motor neurons of different size contact muscle fibers with distinct properties

-

small

motor neurons innervate

slow-twitch, fatigue-resistant fibers

- intermediate-sized

motor neurons innervate fast-twitch, fatigue-resistant fibers

- large motor neurons innervate fast-twitch, fatigable muscle fibers.

Slow twitch fibers (type I):

rely on aerobic metabolism, contract more slowly and generateless force, resistant to

fatigueFast twitch fibers (type II): rely on anaerobic metabolism, contract rapidly and generatelarge force, fatigue slowly (type

IIa) or quickly (type IIb)

slow-twitch

fast-twitch

fatigue-resistant

fast-twitch

fatigable

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Control Of Muscle

Force via motor unit with distinct size and properties

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Control Of Muscle Force

Size principle:

3

. Larger motor neurons are part of larger motor units

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Summary - size principle: o

rderly recruitment of motor neurons (according to their size) with increases in afferent activity

slow-twitch, fatigue-resistant muscle fibers are recruited first, then fast-twitch

,

fatigue-resistant fibers, then fast-twitch fatigable fibers

Average size of motor

unit

increases with increases in muscle force production

Physiological Functions:Minimizes amount of muscle fatigue

Precise control of muscle force at all levels of muscle output

Control Of Muscle Force

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Proprioception

Specialized receptors provide

information about muscle:

Length

velocity

(change in length)

load

(force

)Receptors

involved in proprioception

muscle spindles

golgi

tendon organs

Slide23

Motor control and proprioceptive input

The case of Ian Waterman (“IW – the man who lost his body”)

at age 19 had never-diagnosed

fever that is

believed to

have set off an auto-immune

reaction

l

ost all somatosensory and proprioceptive input from the neck down, no paralysisinitially unable to make coordinated movements

trained himself to make movements under visual guidance

Requires total concentration to move and even maintain postureCollapses when blindfolded

Clip here

https://www.youtube.com/watch?v=pMEROPOK6v8

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Proprioception

Receptors

muscle spindles

-

in parallel with

muscle

- signal length and velocity

g

olgi

tendon organs

- in series with muscle

- signal tension (force)

Slide25

Muscle spindle

Slide26

Three types of intrafusal muscle fibers in a muscle spindle

Nuclear chain fibers - signal static

length

of

muscle

Static nuclear bag fibers

- signal static length of muscleDynamic

nuclear bag fibers - signal rate of change

of muscle length

A typical muscle spindle is composed of

1 dynamic

nuclear bag fiber, 1 static nuclear bag fiber, and ~5 nuclear chain fibers

Slide27

Innervation of muscle spindles

Sensory

Type

Ia

:

innervate all fibers via

annulospiral

endings

, report both muscle velocity

and

length

Type II: innervate nuclear chain and static nuclear bag fibers via

flower spray endings, report only muscle lengthMotor

Gamma motor neurons: innervate only intrafusal muscle fibers

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Gamma motor neurons – control sensitivity of muscle spindles

innervate intrafusal fibers only

ensure that muscle spindle stays taut and responsive during changes in muscle length

alpha-gamma

coactivation

(coordinated contraction of extra- and

intrafusal fibers

Slide29

Golgi Tendon Organ

- in series with muscle

-

a

ctivated following increases in muscle tension

- innervated by Group

Ib

fibers, no motor neuron innervation

Slide30

Muscle spindles: respond to changes in length and velocity

of muscle Golgi tendon organs: respond to changes in load or force applied

muscle

Slide31

Spinal

cord circuits

Motor control of limbs and

body

Rhythmic Movements (e.g. walking)

Reflexes

Slide32

Myotatic

reflex (stretch reflex)

Activated by:

muscle spindle (in e.g. flexor), in response to stretched muscle

Mediated by:

Ia

afferent fibers

+

alpha motor neuronsActivation of:

alpha motor

neuronsResults in: homonymous muscle contraction (e.g. flexor)

Function: control of posture, e

tc.

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Activated by:

muscle spindle (in e.g. flexor

)

Mediated by:

Ia

afferent fibers

+

Ia

inhib. interneuron + alpha motor neuronInhibition of:

alpha motor neuron of opposing muscleResults in:

inhibition of opposing muscle contraction (e.g. extensor)Function: minimizes co-contraction of opposing muscles

Inhibition

in the myotatic reflex (stretch reflex)

Slide34

Stretch reflex

Hyporeflexia:

problem with the reflex arc itself

Hyperreflexia

: problem with descending control of reflex

Slide35

Flexor reflex

Activated by:

c

utaneous or pain receptors

Mediated by:

g

roup III afferent fibers

+

excitatory interneurons +

alpha motor neuronsExcitation of:

alpha motor neurons for separate flexor muscles in thigh and hip

Results in:

contraction of thigh and hip flexors Function: coordination of muscle activityover multiple joints

Reciprocal inhibition in the flexor reflex:Mediated by:

group III afferents + inh. interneurons +

alpha motor neurons in extensor muscle

Slide36

Crossed extension reflex (part of flexor reflex)

Activated by:

c

utaneous and pain receptors

Mediated by:

g

roup III afferent fibers

+

excitatory interneurons crossing midline

+ alpha motor neurons on contralateral side

Excitation of:alpha motor

neurons in extensor ino

pposite legResults in:contraction of extensor musclesFunction: m

aintain balance and posture duringflexor reflex