Ch9. Motor System

Normal control of movement Skeletal muscle Lower motor neurons Spinal region connections Descending tracts Control circuits Motor planning areas

Motor neurons: Descending tracts: Control circuits: nerve cells that control the activity of skeletal muscles (low motor neuron directly innervate skeletal muscle fibers) Descending tracts: from the brain to lower motor neurons in the spinal cord or brain stem postural/gross movement tracts, controlling automatic skeletal muscle activity, fine movement tracts, controlling skilled, voluntary movements Control circuits: basal ganglia, cerebellum (adjust activity in the descending tracts, resulting in excitation or inhibition of the lower motor neurons, muscle contraction)

Ant. Part of frontal lobe Association cortex Cerebellum and BG; regulate the activity in descending motor tract Motor cortex Brain stem

Upper motor neuron Lower motor neuron Cortex 나 brain stem으로부터 시작되어 descending tract 을 통해 axon이 lower motor neuron 이나 spinal interneuron으로 전달됨 Lower motor neuron Cell bodies in the spinal cord or brain stem and synapse with skeletal muscle fiber Spinal interneuron 과 brain stem으로부터 들어오는 여러 input 을 받아 lower motor neuron activity level 이 조절된다.

Sensory function in regulating movement: 수행하고자 하는 동작이 지금 처한 환경에 가장 적합하게 조절되게 하기 위해 필수적임. 여러 sensory mechanism 을 통해 들어온 정보들은 feed forward 나 feedback mechanism 을 통해 운동 조절에 사용됨. 예>얼음판을 걸을 때나 울퉁 불퉁한 길을 걸을 때 상대방의 공격으로부터 방어하거나 피하기 위한 어떤 동작을 할 때 등

1. Muscle fiber structure 2. Lower motor neuron 3. spinal region 4. Descending motor tract

1. Muscle structure and function

Length tension relationship

Adaptation of Number of Sarcomeres to Muscle Length (Structural adaptation) -Prolonged immobilization in a shortened length: decreased number of sarcomeres muscle can generate optimal force at the new resting length. -Also decreased amount of titin (elastic protein in muscle) will limit the extensibility of the muscle

2. Lower motor neuron Alpha motor neuron Gamma motor neuron Extrafusal muscle fiber(Skeletal muscle) -Large myelinated axons Gamma motor neuron Intrafusal muscle fiber(Muscle spindle) -medium-sized myelinated axons

Motor unit A single alpha motor neuron and the muscle fibers(cells) the alpha motor neuron innervates

Hanneman’s size principles: The activity of a motor unit depends on the convergence of information from peripheral sensors, spinal connections, and descending tracts onto the cell body and dendrites of the alpha motor neuron Hanneman’s size principles: the order of recruitment from smaller to larger alpha motor neuron. Fast twitch motor neuron Fast twitch muscle fiber: phasic contraction, fast and powerful movement Slow twitch motor neuron Slow twitch muscle fiber: postural and slowly contracting muscles.

Alpha-Gamma coactivation (simultaneously) Fine motor control muscle as small number of muscle fibers per motor neuron to allow precise control, and postural muscles has large number of muscle fibers per motor neuron Hand intrinsic muscle 10:1 Gastrocnemius 1000:1 Alpha-Gamma coactivation (simultaneously) Important to maintain spindle sensitivity Most of input to the alpha motor neuron has collaterals to gamma.

3. Spinal Region In the spinal region: Reflex: Somatosensory information is integrated with descending motor commands to generate motor output Reflex: involuntary responses to external stimuli Spinal region coordination of voluntary movement Central pattern generator (CPG) Motor neuron pools in the spinal cord

Motor Neuron pools in the spinal cord Located in the ventral horn The actions of these pools correlate with their anatomical position

Spinal motor neuron pool Single muscle 로 공급되는 motor neuron 들의 cell body 들의 모임. Anterior horn 에 정렬 되어있는데, 하나의 근육의 여러 spinal level 에 걸쳐 모여 있기도 하다. Medially located pool: innervate axial and proximal muscles Laterally located pool: distal muscle anteriorly located pool: extensors Posterior located pool: flexors

Spinal Region Coordination Reciprocal inhibition

Muscle Synergies Type 2 afferents contribute to synergies by delivering information to spinal cord neurons from tonic receptors in muscle spindles, certain joint receptors, and cutaneous and subcutaneous touch and pressure receptors Interneurons excites by type 2 afferents project to motor neurons of muscles acting at other joints, providing a spinal region basis for muscle synergies.

Proprioceptive Body Schema The spinal cord interprets proprioceptive information as a whole, and computes a complete proprioceptive image(schema) of the body in time and space. Schema is essential for adapting movements to the environment, based on the proprioceptive feedback. This nonconscious schema is used to plan and adapt movements.

A group of muscles innervated by a single spinal nerve: myotome

Reflexs Proprioceptive Reflexs Cutaneous Reflexs Muscle spindle Phasic stretch reflex Tonic stretch reflex Golgi tendon organ reflex Cutaneous Reflexs

1) Muscle spindle reflex: quick stretch reflex, phasic stretch reflex, myotatic reflex, muscle stretch reflex, deep tendon reflex, tendon jerk

2) Tonic stretch reflex: secondary spindle ending Maintained stretch 가 muscle의 central spindle 에 지속적으로 가해졌을 때 Primary and secondary nerve ending 을 통해 Ia, II afferent fiber 흥분, interneuron 을 통해 alpha motor neuron으로 전달됨 When slow or sustained stretch is applied to the central spindle, the tonic stretch reflex facilitates contraction in the muscle

3) Golgi tendon organ reflex Tendon tension is registered by GTO, the information conveyed into the spinal cord by Ib afferent  stimulate interneuron that inhibit the alpha motor neurons to the same muscle, autogenic inhibition

Cutaneous reflexes withdrawl reflex

Reflexes can be elicited by stimulation of musculoskeletal or cutaneous receptors Stimulation of the muscle spindle receptors can result in phasic or tonic stretch reflexes Activation of the Golgi tendon organ can inhibit activity of the corresponding muscle Noxious cutaneous information can result in a withdrawl reflex

H-reflex : to quantify whether alpha motor neurons are facilitated or inhibited

Spinal region coordination 1) Reciprocal inhibition

Spinal region coordination 2) Central pattern generator Many parts of the nervous system produce patterns independent of either their sensory input or supraspinal input. Neural circuits that produce self-sustaining patterns of behavior are called central pattern generators. Animal vs. human CPG in the spinal cord probably contribute to normal walking in humans but do not provide adequate control to support walking without descending input

Descending Motor Tracts (upper motor neurons) Upper motor neuron project from supraspinal centers to lower motor neurons(alpha and gamma) and to interneuron in the brain stem and spinal cord Medial activation system : controls lower motor neurons that innervate postural and girdle muscles Lateral activation system : controls lower motor neuron that innervate distally located muscles used for fine movements Nonspecific activating tracts : contributes to background levels of excitation in the cord and facilitates local reflex arcs

Postural and gross movement tracts : Medial activation system Brain stem tracts arise in the tectum(posterior midbrain), medial reticular formation, and vestibular nuclei(brain stem nuclei concerned with equilibrium) Tectospinal : 갑작스러운 시각 또는 청각자극이 들어오는 방향으로 머리를 돌리는 반응에 관여 Medial reticulospinal : postural muscle and limb extensor Medial and lateral vestibulospinal : 자세반사 The tracts from the cortex : medial corticospinal : control of neck, shoulder, trunk muscles

Limb and fine movements tracts: lateral activation system Lateral corticospinal : provide fractionation of distal movements Rubrospinal : innervate upper limb flexor muscle Lateral reticulospinal : facilitate flexor muscle motor neurons and to inhibit extensor motor neuron, but during movement, can be reversed Corticobulbar fibers : project to cranial nerve nuclei in the brain stem and do not reach the spinal cord -> control lower motor neurons innervating the muscles of the face, tongue, pharynx, and larynx

Nonspecific activating tracts Ceruleospinal Raphespinal Not related to specific movements Contribute of changes in motor performance with varying levels of motivation

Control Circuits Basal Ganglia Cerebellum