Spinal Control of Movement Midterm 2 Results 15 15 12 9 Count Frequency 12 9 6 6 3 3 Mean = 27.90 Std. Dev. = 5.05985 N = 50 0 15.00 0 20.00 25.00 30.00 mid2score 35.00 40.00 A B C mid2grade D F Types of Muscles Smooth – digestive tract, arteries Striated: Cardiac – accelerates or slows heart rate Skeletal – moves bones around joints, moves eyes, facial expression, respiration, speech Skeletal muscles are the somatic motor system and are under voluntary control. Types of Movement Extension – takes limb away from body (opens penknife) Flexion – brings limb toward body (closes penknife) Muscles cannot push so any movement requires coordination Synergists – muscles that work together Antagonists – muscles that pull in opposite directions Muscles are also named by location: Axial (trunk), proximal (shoulder, elbow, knee), distal (toes) Motor Units Each muscle fiber is innervated by an alpha motor neuron. Bundles of fibers form large and small motor units. Small motor units act first and control fine motor movement. Finer control is possible under lighter loads. Fast contracting, fast fatiguing white fibers form “fast” motor units (slow ones are red). Alpha neuron firing rate makes a fiber/motor unit fast or slow. Input to Alpha Motor Neurons Dorsal root ganglion input to muscle spindle. Upper motor neurons in the motor cortex and brain stem. Provides feedback about muscle length. Voluntary control of muscles. Interneurons Muscle Fiber Structure Muscle fibers are enclosed by an excitable cell membrane called sarcolemma. Within the muscle fiber are cylindrical structures called myofibrils which contract in response to the sarcolemma’s action potential. The action potential releases Ca++ from the sarcoplasmic reticulum which starts the chemical reaction resulting in contraction. How Muscles Contract Myofibrils are divided into segments by disks called z lines. The myofibril in between two disks is called a sarcomere. Each sarcomere includes both thick and thin filaments. During contraction, thin filaments slide along the thick filaments bringing the z lines closer together. This is reversed during relaxation. Chemical Reaction Calcium binds with troponin (in the thin filament). This exposes myosin binding sites on the thin filament’s actin. Myosin heads in the thick filament bind with actin in the thin filament, causing the myosin heads to rotate. This results in the sliding. This continues as long as calcium is present. An ATP-driven pump returns calcium to the sarcoplasmic reticulum and muscles relax. Reflexes Reciprocal inhibition – cannot flex and extend the same muscle Myotatic (knee-jerk) Opposes gravity Uses spindle sensory feedback Reverse myotatic (knife-clasp) Relaxes overloaded muscle Responds to golgi tendon organ feedback More Reflexes Flexor reflex – response to pain Crossed-extensor reflex – compensates for flexor reflex One side extends as the other flexes The circuit for coordinated control of walking resides in the spinal cord. Circuits called “central pattern generators” give rise to rhythmic motor activity.