Muscles, Proprioception & Spinal Cord Mechanisms

- Rene Descartes (1664) Treatise on Man

• Muscles needed to move the body.  A system of coordinates and nomenclature has been developed to describe these movements.
  
• History
• Descartes - muscle inflated by animal spirits
• Realized the nerves not hollow tubes...
• and realized that muscles did not increase volume during contraction...
• but no adequate theory to replace until
• 1791, L. Galvani
• frog nerve-limb preparation contracted if nerve in contact with two different metals (iron & copper). Also limb contracted when nerve in contact with another freshly cut muscle. Thus, irritable tissue not only was sensitive to electricity, but it also produced it.
• 1848, E. du Bois-Reymond
• measured action potential
• early 1950s
• muscle contraction thought due to contraction of protein in muscle
• 1954, Hugh Huxley & Andrew Huxley proposed sliding filament theory
• Molecular biology of muscle contraction
• muscle contracts when myosin filaments heads attach to actin filaments and change shape which causes movement of the fibers relative to one another. myosin detaches, reverts to original shape and reattaches to actin again.
• other molecules - tropomyosin and troponin - regulate whether the myosin can attach to the actin
• depolarization of muscle fiber causes increase [Ca²⁺ ] in muscle fiber cells; Ca²⁺ binds to troponin; receptor on actin for myosin becomes available; myosin binds to actin and changes shape; changed conformation of myosin moves actin relative to myosin filament; myosin detaches by converting ATP toADP + P_i in a process that requires Ca^2+; upon detaching myosin converts back to original shape
• The length-tension relation
• The force of contraction depends on the length of the muscles
• If fiber is innervated, it produces more tension.
• active tension is inverted-U shaped function due to physical properties of actin-myosin filament interactions
• Force produced by spring-action of muscles
• e.g., Marlon Shirley
• motor units and recruitment
• motor neuron + innervated fibers = motor unit
• Defined by Sherrington
• number of motor fibers / motor unit varies with effector: in hand and eye there are fewer than 100 fibers per motor unit; in lower leg there are 1000 fibers per motor unit
• different kinds of muscle fibers
• same fibers in given motor unit
• fast (white meat) - fast twitch, easily fatigued, generate greatest force, use glycolysis (anaerobic)
• slow (red meat) - slow twitch, not fatigable, longer contraction time but generate 1-10% of force of fast fatigable, more mitochondria, more oxidative metabolism, high [myoglobin]
• motor neurons innervating fast/strong and slow/weak fibers are different and matched in properties to the muscle fibers
• during normal action, motor units are recruited in orderly fashion
• size principle - first motor units activated are the smallest and weakest; subsequent motor units are larger and stronger
• functional advantages of the size principle
• force is more easily regulated / modulated; by recruiting motor units in order of increasing tension production, recruitment can stop when the proper amount of force is generated. amount of force added is proportional to amount of force currently generated
• Production of joint angle
• neither force of muscle contraction nor length of muscle can be determined by neural activation
• because muscles act like springs, motor system controls stiffness and set-point
• but antagonist muscles activation must be considered too; control of opposing muscles allows motor system to effectively produce and maintain desired joint angles
• two mechanisms:
• reciprocal innervation - conserves energy, but to maintain joint angle the central motor system must anticipate loads accurately
• co-contraction - uses more energy, but increased joint stiffness prevents movement by unexpected loads
• muscles contract slowly (10-100 ms) relative to speed of neural input (1-3 ms); to generate rapid, high forces motor system over-activates agonist and then activates antagonist to brake movemen

• Sherrington - defined proprioceptors as sensory receptors for stimuli that "are traceable to actions of the organism itself, and since the stimuli to the receptors are delivered by the organism itself, the deep receptors may be termed proprioceptors..." (1906) Brain 29:467-482
• two essential properties
• the amount of muscle activity mobilized by proprioceptive inputs is small
• the adequate stimuli for proprioceptors arise from the actions of the organism itself
• Behavioral effects of loss of proprioception
• e.g., The Disembodied Lady, Oliver Sacks
• Proprioception receptors
• Joint receptors - sensory endings in joint
• Some recordings from joint afferents report activity only at extreme angles, but other recordings have found variation in activity through intermediate joint angles
• experiments in which input from different proprioceptive sensors is dissociated indicates that joint (& cutaneous) information can be used to sense joint angle -- the experiment involved posturing the hand with all but middle digit extended and middle digit flexed. In this posture, the distal joint cannot be flexed. so no muscle spindle or tendon organ input, but there are clear improvement in sensitivity when muscles engaged though
• artificial joint replacement does not prevent limb sense
• Cutaneous receptors - Movements and postures cause deformations of skin to which mechanoreceptors in skin are sensitive. damage to mechanoreceptors can seriously impair movement and posture maintenance
• muscles contain specialized receptors that sense different features of the state of the muscle
• muscle spindles respond to stretch of specialized intrafuscal muscle fibers
• golgi tendon organs are sensitive to changes in tension
• functional differences between spindles and tendon organs derive from their different anatomical arrangements within muscle
• muscle spindles
• sensitive to muscle stretch
• 3 main components
• group of specialized muscle fibers - 2 main types: nuclear chain fibers & nuclear bag fibers (2 types: dynamic & static)
• sensory axons that terminate on muscle fibers
• primary ending - Ia sensory afferent, wrapped around central region of nuclear bag fiber and nuclear chain fiber
• secondary ending - group II afferent which terminates on peripheral end of muscle spindle
• motor axons that regulate sensitivity of spindle
• the primary and secondary endings of spindle afferents respond differently to phasic changes in length
• during muscle stretch or relaxation there are two phases - early dynamic phase when length is changing and later static phase when length reaches endpoint.
• primary spindle afferents fires burst during dynamic phase and less during static phase
• secondary afferent changes activity gradually, reaching new steady state level during static phase
• primary endings activity proportional to speed of stretch, so they can signal speed of movement
• the central nervous system can control sensitivity of the muscle spindles through the gamma motor neurons
• the fusimotor system maintains spindle sensitivity during muscle contraction
• stimulate motor neurons alone results in pause in firing of spindle afferents because muscle contracted and unloaded spindle
• stimulate and motor neurons simultaneously prevented pause in spindle Ia afferent during contraction
• stimulation of motor cortex activates and motor neurons together - "- coactivation"
• two types of gamma motor neurons alter the responsiveness of spindles
• dynamic motor neurons tune the spindle to signal more about the dynamic phase of stretch
• static motor neurons tune the spindle to signal more about the static phase of stretch
• the relative amounts of static and dynamic activity is preset according to activity
• discharge of muscle spindle afferents produces stretch reflexes
• group Ia afferents make monosynaptic connections to motor neurons
• stretch reflexes regulate muscle tone through negative feedback

• interneurons are the building blocks of spinal reflexes
• convergent and divergent connections are the basis of reflex pathways
• networks of interneurons coordinate the timing of reflex components
• inhibitory interneurons coordinate muscle action around a joint
• group Ia inhibitory interneurons coordinate opposing muscles
• balance between reciprocal inhibition and cocontraction
• Renshaw cells are part of a negative feedback loop to motor neurons
• negative feedback stabilizes motor neuron activity, preventing large transient changes
• cutaneous stimuli elicit complex reflexes that serve protective and postural functions
• certain reflexes adapt to different body postures, e.g., frog wiping reflex