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

Spinal Control of Movement 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 neurons Alpha motor neurons Innervate skeletal muscle (extrafusal muscle) Function: directly command muscle contract Gamma motor neurons Innervate muscle spindle (intrafusal muscle) Function: regulating the muscle spindle Interneurons Allows coordinated motor programs to be generated Final common pathway Directly command muscle contract Compare: Upper motor neurons Lower motor neurons

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.

α，β，γ I. Final Common Pathway and Motor Unit Anterior motor neurons of the spinal cord Anterior horns of the cord gray matter α，β，γ

Muscle innervation by lower motor neurons The ventral horn of the spinal cord contains motor neurons that innervate skeletal muscle fibers 30 mixed spinal nerves Cervical 1-8 Thoracic 1-12 Lumbar 1-5 Sacral 1-5 Ventral roots+Dorsal root=Spinal nerve

Inputs to alpha motor neurons This input may be excitatory or inhibitory and is part of the circuitry that generates the spinal motor programs This input provides feedback about muscle length This input is important for the initiation and control of voluntary movement

Alpha-motoneuron – final common pathway

organization of motor systems Motor Cortex Brain Stem Spinal Cord -motor neuron Final common pathway Skeletal muscle

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

regulate sensitivity of muscle spindle to stretch stimulation The extrafusal muscle fibers are innervated by Alpha motor neuron The intrafusal muscle fibers are innervated by Gamma motor neurons skeletal muscle contraction regulate sensitivity of muscle spindle to stretch stimulation

※ 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 refined motion Innervation ratio Fibers per motor neuron Extraocular muscle 3:1 Gastrocnemius 2000:1 For the strength

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 Voluntary movements Nociceptive stimulus Stretch muscles Posture maintenance Motor Cortex Reflex Brain Stem Spinal Cord Final common pathway -motor neuron Effector Skeletal muscle Receptor 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.

Symptoms of 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)

Spinal shock Facilitate Inhibit 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. Extensor reflex weaken Flexor reflex strengthen Facilitate Inhibit High central ?

※ 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.

Flexion reflex and crossed extension reflex 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 Flexor reflex A noxious stimulus to the spinal animal induces flexion upward and away from the stimulus. Reflex response consists of facilitation of flexor muscles and inhibition of extensor muscle in the stimulated limb. That can provide protection from the injury. Crossed extensor reflex If the stimulus sufficiently strong, flexion of ipsilateral limb and extension of the contra-lateral limb which serves to support the body weight and maintain the body balance.

※

Stretch reflex includes: Tendon reflex (Phasic stretch reflex ) ： short lasting and relatively intense (Ⅰa), strong reflex contraction.(single synaptic reflex). Muscle tonus (Tonic stretch reflex ) : less intense but lasts longer, maintain the body posture.（polysynaptic reflex）

Tendon reflex—knee-jerk 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;

elbow jerk Achilles tendon reflex

Muscle tonus  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.

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 Ⅱ sensory ending 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.

Stretch reflex Muscle sensors 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 How the muscle spindle works -When a muscle is stretched → the muscle spindle is also stretched → stimulates group Ⅰa and Ⅱ afferent fibers. →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 .

Activation of the γ-loop WHAT IS THE γ-LOOP? Descending influence Muscle spindle Ⅰa γ Activation of the γ-loop results in increased muscle tone MUSCLE α

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---Ⅰa afferent---alpha motor neuron—extrafusal muscle fibers Alpha and gamma motor neurons are simultaneously activated by descending commands from the brain.

Tendon organ and inverse stretch reflex Change of tension Ⅰb afferent fiber Tendon organ

Action of Tendon Organ Tendon 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 Ⅰb sensory axons that are slightly smaller than the Ⅰa axons innervating the muscle spindles.

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.

(1) The basic circuit: 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.

(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 Ⅱ sensory 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.