A baseball pitcher planning a pitch…

The mental body image seems to be generated by somatosensory, proprioceptive, and visial inputs to the posterior parietal cortex (area 5, area 7)

The motor control hierarchy have three levels

• The highest level---strategy• Represented by the association areas of

neocortex and basal ganglia of the forebrain• Is concerned with strategy:• The goal of the movement and the

movement strategy that best achieves the goal.

The motor control hierarchy have three levels

• The middle level---tactics• Represented by the motor cortex and

cerebellum• Is concerned with tactics• The sequences of muscle contractions• Arranged in space and time• Required to smoothly and accurately

achieve the strategic goal.

The motor control hierarchy have three levels

• The lowest level—execution• Represented by the brain stem and spinal

cord• Is concerned with execution• Action of the motor neuron and interneuron

pools that generate the goal-directed movement and make any necessary adjustments of posture.

organization of motor subsystems

Part 1

Spinal Control of Movement

Introduction

• The rhythmic movements could be elicited in the hind legs of cats and dogs long after their spinal cords had been severed from the rest of the central nervous system

---Charles Sherington Graham Brown(English)The importance of circuitry within the spinal cord

for the coordinated control of movements

Spinal motor neuronsAlpha motor neuronsInnervate skeletal muscle (extrafusal muscle)Function: directly command muscle contractGamma motor neuronsInnervate muscle spindle (intrafusal muscle)Function: regulating the muscle spindleInterneuronsAllows coordinated motor programs to be generatedFinal common pathwayDirectly command muscle contractCompare: Upper motor neurons

Lower motor neuro

ns

α motor neuron γ motor neuron

Size 30-150μ 10-30μ

Excitability Low High, spontaneous

Controlled High center Peripheral

High center

Effector Largeα ：fast muscle Smallα ：slow muscle

Intrafusal muscle fiber

Effect Muscle contraction Muscle spindle

Spinal interneurons• Most of the input to the alpha motor neurons

comes from interneurons of the spinal cord.• Interneurons receive synaptic input from primary

sensory axons, descending axons from the brain, and collaterals of lower motor neuron axons.

• The interneurons are themselves networked together in a way that allows coordinated motor programs to be generated in response to their many inputs.

The Lower Motor Neurons• Alpha motor neurons are directly responsible for

the generation of force by muscle• Motor unit: one alpha motor neuron and all of the

muscle fibers it innervates collectively make up motor unit, the elementary component of motor control

• Motor neuron pool: the collection of alpha motor neurons that innervates a single muslce.

Anterior motor neurons of the spinal cord

Anterior horns of the cord gray matter

α ， β ， γ

I. Final Common Pathway and Motor Unit

The ventral horn of the spinal cord contains motor neurons that innervate skeletal muscle fibers

Muscle innervation by lower motor neurons

Ventral roots+Dorsal root=Spinal nerve

30 mixed spinal nerves

Cervical 1-8

Thoracic 1-12

Lumbar 1-5

Sacral 1-5

Inputs to alpha motor neurons

This input may be excitatory or inhibitory This input may be excitatory or inhibitory and is part of the circuitry that generates and is part of the circuitry that generates

the spinal motor programsthe spinal motor programsThis input provides feedback about This input provides feedback about

muscle lengthmuscle length

This input is important for the initiation This input is important for the initiation and control of voluntary movementand control of voluntary movement

Alpha-motoneuron – final common pathway

organization of motor systems

Motor Cortex

Brain Stem

Spinal Cord

Skeletal muscle

-motor neuron

Final common pathway

motor neuron: final common pathway

Spinal control of Motor Function Motor neurons in the spinal cord –α motor neurons Causes contraction of the innervated muscle. Final common pathway. – γ motor neurons innervates intrafusal muscle to adjust the sensitivity of muscle spindle to stretch.

•Every striated muscle has encapsulated muscle fibers scattered throughout the muscle called muscle spindles.

•Extrafusal and intrafusal muscle fibers

The extrafusal muscle fibers are innervated by Alpha motor neuron

The intrafusal muscle fibers are innervated by Gamma motor neurons

regulate sensitivity of muscle spindle to stretch stimulation

skeletal muscle contraction

Motor unit A motor unit consists of a single α-motor neuron and the group of muscle fibers which it innervates. The elementary component of motor

control.

For the strength

For the refined motion

Innervation ratioFibers per motor neuronExtraocular muscle 3:1Gastrocnemius 2000:1

※

•The muscle cells of a motor unit are not grouped, but are interspersed among cells from other motor units

•The coordinated movement needs the activation of several motors

organization of motor systems

Motor Cortex

Brain Stem

Spinal Cord

Skeletal muscle

-motor neuron

Final common pathway

Reflex

Voluntary movements

Nociceptive stimulus

Stretch muscles

Posture maintenance

EffectorReceptor

Skin Muscle Joint

Regulating Functions of the Spinal Cord

Spinal shock

Spinal Reflex

spinal animal: is prepared by transecting the spinal cord at the fifth cervical segment

Spinal shock• Concept: Complete transection of the spinal cord

results in the immediate paralysis and loss of sensation in all body regions innervated by spinal cord segments below the lesion. All reflex in the isolated segments of the spinal cord are at once totally depressed. This is called spinal shock.

※

During spinal shock:complete loss of all reflexes, no tone, paralysis, complete anaesthesia, no peristalsis, bladder and rectal reflexes absent (no defecation and micturition )(Feces is detained in rectum and urine in bladder )no sweating (Sweating reflex is suppressed)arterial blood Pressure decrease(40mmHg)(BP drops down, peripheral blood vessels are dilated)

Symptoms of spinal shock

The reason: Is result of the sudden withdrawal of descending control from suprasegmental centers in the brain.

The recovery of spinal neurons excitability.

The time course of recovery from spinal shock varies considerably with the species studied. It is increased with ascent of the phylogenetic scale.

Spinal shock

Extensor reflex weaken Flexor reflex strengthen High central ? Facilitate

Inhibit

Postural reflex ※

Flexion reflex and crossed extension reflex

• -is a protective reflex and a polysynaptic reflex.

• -results in flexion on the ipsilateral and extension on the contralateral.

• -can be innervated by noxious stimulates.

There is a rapid lifting of the affected foot (ipsilateral withdrawal reflex ), while the contralateral response activates the extensor muscles of the opposite leg (contralateral extensor reflex)support the weight shifted to it.

Flexion reflex and crossed extension reflex

Flexion reflex and crossed extension reflexFlexor reflexFlexor reflex

•A noxious stimulus to the spinal animal A noxious stimulus to the spinal animal induces flexion upward and away from the induces flexion upward and away from the stimulus. stimulus. • Reflex response consists of facilitation of Reflex response consists of facilitation of flexor muscles and inhibition of extensor flexor muscles and inhibition of extensor muscle in the stimulated limb. muscle in the stimulated limb. • That can provide protection from the That can provide protection from the injury.injury.

Crossed extensor reflexCrossed extensor reflex If the stimulus sufficiently strong, If the stimulus sufficiently strong, flexion of ipsilateral limb and extension of flexion of ipsilateral limb and extension of the contra-lateral limb which serves to the contra-lateral limb which serves to support the body weight and maintain the support the body weight and maintain the body balance.body balance.

※

• Stretch reflex includes:• Tendon reflex (Phasic stretch reflex ) ：

• short lasting and relatively intense ( a),Ⅰ strong reflex contraction.(single synaptic reflex).

• Muscle tonus (Tonic stretch reflex ) : • less intense but lasts longer, maintain the body

posture. （ polysynaptic reflex ）

Caused by rapid stretch of the muscle, Transmitted from the Ia sensory ending of the M. S. Causes an instantaneous, strong reflex contraction of the same muscle; Opposing sudden changes in length of the M.S; A monosynaptic pathway:being over within 0.7 ms;

Tendon reflex—knee-jerk reflex;

Achilles tendon reflex

elbow jerk

Caused by a weaker and continues stretch of the muscle, Transmitted from the Ia and II sensory ending of the M. S. Multiple synaptic pathway, continues for a prolonged period. Non-synchronized contraction, for at least many seconds or minutes, maintaining the posture of the body.

Muscle tonus

tendon reflex and the tonic stretch reflex

Type Tendon reflex Tonic stretch reflex

stimulation suddenly stretched Weaker, continuous stretch

Afferent fiber Ia sensory ending Ia and sensory Ⅱending

Synapse Monosynaptic reflex Polysynaptic reflex

Significance Determine the lesion areas

maintenance of posture

Stretch reflex Muscle sensors

• Types of muscle fibers• a.  Extrafusal fibers• -make up the bulk of muscle• -are innervated by α-motoneurons.• -provide the force for muscle contraction.

• b.  Intrafusal fibers• -are smaller than extrafusal fibers• -are innervated by γ-

motoneurons.• -are encapsulated in sheaths to form muscle spindles

Stretch reflex Muscle sensors

• Muscle spindles:• -are distributed throughout muscle• -intrafusal fibers is in parallel with extrafusal fibers. • -Nuclear bag fibers and Nuclear chain fibers.

Stretch reflex Muscle sensors

• -When a muscle is stretched → the muscle spindle is also stretched → stimulates group a Ⅰ and afferent fibers.Ⅱ• →α-motoneurons →• Innervates the extrafusal

fibers → the muscle contracts

Stretch reflex  How the muscle spindle works

• γ-motoneurons• -innervate intrafusal muscle fibers.• -adjust the sensitivity of the muscle spindle.• -receive control of high center .

Stretch reflex  How the muscle spindle works

MUSCLE

Muscle spindle

Descending influence

Ⅰa

α

γActivation of the γ-loop

results in increased muscle tone

WHAT IS THE γ-LOOP?

The Gamma loop• Changing the activity of the gamma motor

neurons changes the set point of the myotatic feedback loop

• The Gamma loop• Gamma motor neuron—intrafusal muscle fiber---

a afferent---alpha motor neuron—extrafusal Ⅰmuscle fibers

• Alpha and gamma motor neurons are simultaneously activated by descending commands from the brain.

Tendon organ and inverse stretch reflex

Tendon organ

Change of tension

Ⅰb afferent fiber Tendon organ

Action of Tendon OrganTendon organ embedded in the tendon at the end of the muscle measure the tension of muscles can be activated by isometric contraction of extrafusal fibers. Tendon organ is innervated by Ib afferent fibers, which synapses with inhibitory interneurons and inhibit α motor neurons .

Proprioception from Golgi tendon organs

• Acts like a strain gauge, it monitors muscle tension, or the force of contraction.

• Located at the junction of the muscle and the tendon and are innervated by group bⅠ sensory axons that are slightly smaller than the a axons innervating the muscle Ⅰspindles.

Tendon organ

Mechanism of stretch reflex • A skeletal muscle is stretched→ spindle is stretched

→the impulses are conducted to the spinal cord by Ⅰa/Ⅱfibers to activate the α-motor neurons→which supply the same muscle → the muscle contracts to oppose the stretch (the antagonist muscle relaxes ).

• if stronger stretch applied on muscle, it changes the muscle tone → stretch the tendon organ → in turn to inhibit αmotoneurons by inhibitory interneurons → to inhibits stretch reflex to prevent the stretched muscles from injury.

Summary

• Spinal shock • Stretch reflex

Question

• Describe the neuronal circuit and function of the skeletal muscle stretch reflexes in detail, and difference in the different types of the reflexes.

• Sudden stretch of a muscle excites the muscle spindle, and Ia proprioceptor nerve fiber sends signals to the spinal cord, synapses directly with anterior motor neurons that send nerve fiber back to the extrafusal muscle fibers of the same muscle, causing reflex contraction of the muscle.

(1) The basic circuit:

(2) tendon reflex and the tonic stretch reflex

Type Tendon reflex Tonic stretch reflex

stimulation suddenly stretched Weaker, continuous stretch

Afferent fiber Ia sensory ending Ia and sensory Ⅱ

ending

Synapse Monosynaptic reflex Polysynaptic reflex

Significance Determine the lesion areas maintenance of posture

(3)Functions:

• Tendon reflex causes an instantaneous, strong reflex contraction of the same muscle; Muscle tonus is good for maintaining the posture of the body.