Chapter 13 Spinal Control of Movement

Introduction Motor Programs Motor system: Muscles and neurons that control muscles Role: Generation of coordinated movements Parts of motor control Spinal cord coordinated muscle contraction Brain motor programs in spinal cord

The Somatic Motor System Types of Muscles Smooth: digestive tract, arteries, related structures Striated: Cardiac (heart) and skeletal (bulk of body muscle mass)

Lower motor neuron: in ventral horn of spinal cord Lower Motor Neurons Lower motor neuron: in ventral horn of spinal cord The axons of the motor neurons bundle together to form the ventral roots. Together with the sensory (dorsal root, we form a spinal nerve.

Lower Motor Neurons Somatic Musculature and distribution of lower motor neurons in spinal cord Axial muscles: Trunk movement Proximal muscles: Shoulder, elbow, pelvis, knee movement Distal muscles: Hands, feet, digits (fingers and toes) movement

Lower Motor Neurons Distribution of lower motor neurons in the ventral horn Motor neurons controlling flexors lie dorsal to extensors Motor neurons controlling axial muscles lie medial to those controlling distal muscles

Motor unit: Motor neuron and all the muscle fibers it innervates Lower Motor Neurons Alpha Motor Neurons Motor unit: Motor neuron and all the muscle fibers it innervates Motor neuron pool: All the motor neurons that innervate a single muscle Alpha motor neurons trigger the generation of force by muscles… creating a motor unit. Motor neuron pool is the cluster of alpha motor neurons innervating a single muscle group.

Graded Control of Muscle Contraction by Alpha Motor Neurons Lower Motor Neurons Graded Control of Muscle Contraction by Alpha Motor Neurons Varying firing rate of motor neurons Recruit additional synergistic motor units Graded control related to muscle twitch, summation, tetany.

Tetanus – as painted by Sir Charles Bell in 1809.

Lower Motor Neurons Types of Motor Units Red muscle fibers: Large number of mitochondria and enzymes, slow to contract, can sustain contraction White muscle fibers: Few mitochondria, anaerobic metabolism, contract and fatigue rapidly Fast motor units: Rapidly fatiguing white fibers Slow motor units: Slowly fatiguing red fibers

Neuromuscular Matchmaking Crossed Innervation Experiment: Lower Motor Neurons Neuromuscular Matchmaking Crossed Innervation Experiment: Switch nerve input - switch in muscle phenotype (physical characteristics) John Eccles

Figure 9.7a

Excitation-Contraction Coupling Muscle Contraction Alpha motor neurons release ACh ACh produces large EPSP in muscle fiber EPSP evokes muscle action potential Action potential triggers Ca2+ release Fiber contracts Ca2+ reuptake Fiber relaxes

Excitation-Contraction Coupling The Molecular Basis of Muscle Contraction Z lines: Division of myofibril into segments by disks Thin filaments: Series of bristles Thick filaments: Between and among thin filaments Sliding-filament model: Binding of Ca2+ to troponin causes myosin to bind to action Myosin heads pivot, cause filaments to slide

Figure 9.3a–c

Figure 9.3c–e

Figure 9.3d

Figure 9.4

Figure 9.6

Excitation-Contraction Coupling Sliding-filament Model of Muscle Contraction

Figure 9.11

Figure 9.12

Spinal Control of Motor Units Sensory feedback from muscle spindles - stretch receptor A muscle spindle is a stretch receptor… proprioreceptors detect stretch Proprioreception is the sense of body position.

Spinal Control of Motor Units The Myotatic Reflex Stretch reflex: Muscle pulled tendency to pull back Feedback loop Discharge rate of sensory axons: Related to muscle length Monosynaptic e.g., Knee-jerk reflex Myotactic Reflex - Tonic contraction of the muscles in response to a stretching force, due to stimulation of muscle proprioceptors. Also called deep tendon reflex, stretch reflex.

Spinal Control of Motor Units The Myotatic Reflex 1a sensory neurons are closely related to the myotatic reflex

Spinal Control of Motor Units Two Types of Muscle Fiber Extrafusal fibers: Innervated by alpha motor neurons Intrafusal fibers: Innervated by gamma motor neurons

Spinal Control of Motor Units Gamma Loop Keeps spindle “on air” Changes set point of the myotatic feedback loop Additional control of alpha motor neurons and muscle contraction

Spinal Control of Motor Units Golgi Tendon Organs Additional proprioceptive input - acts like strain gauge - monitors muscle tension

Spinal Control of Motor Units Golgi Tendon Organs Spindles in parallel with fibers; Golgi tendon organs in series with fibers

Spinal Control of Motor Units Proprioception from the joints Proprioceptive axons in joint tissues Respond to angle, direction and velocity of movement in a joint Information from joint receptors: Combined with muscle spindle, Golgi tendon organs, skin receptors Most receptors are rapidly adapting, bring information about a moving joint

Spinal Control of Motor Units Inhibitory Input Reciprocal inhibition: Contraction of one muscle set accompanied by relaxation of antagonist muscle Example: Myotatic reflex

Spinal Control of Motor Units Excitatory Input Crossed-extensor reflex: Activation of extensor muscles and inhibition of flexors on opposite side

Interneurons + + – + + – Efferent fibers Afferent fiber Efferent fibers Extensor inhibited Flexor inhibited Flexor stimulated Flexes Arm movements Extensor stimulated Extends Key: + Excitatory synapse – Inhibitory synapse Right arm (site of stimulus) Left arm (site of reciprocal activation)

Information gleaned about nicotinic ACh receptors utilzed the electric organs of electric eels because of their high concentration of the Ach receptor. The Disease Myasthenia gravis is an autoimmune disease where the body's immune system has damaged receptors on your muscles causing long term weakness and eventual, premature death. Individual showing classic, early signs of Myasthenia gravis