Overview Snapshot A 67-year-old male is seen by his physician for a health-maintenance examination. He is currently doing well, but is distressed about poor muscle movement of his right arm. Approximately 6 months ago the patient was hospitalized for the management of an acute stroke. On physical examination, his right arm is hypertonic. When attempting to passively flex the arm, the physician noted resistance. Eventually, the joint rapidly closed. (Clasp-knife reflex - an exaggerated golgi-tendon reflex) Introduction Skeletal muscle contraction and relaxation plays an important role in maintaining posture and movement Motor unit = single motor neuron + muscle fibers motor neuron can innervate few to many muscle fibers Two types of motor neurons: α-motoneurons innervates extrafusal muscle fibers (fibers that cause contraction) γ-motoneurons innervates intrafusal muscle fibers, a component of the muscle spindle adjusts muscle spindle sensitivity α-motoneurons and γ-motoneurons co-activation ensures the muscle spindle remains sensitive to changes in muscle length (eg, contraction) Muscle Spindles / Golgi Tendon Organs Function Physiology Muscle spindle Returns muscle to its resting length when it is either stretched or contracted muscle spindle reflex muscle stretched extrafusal muscle lengthened intrafusal muscle lengthens it is in parallel with extrafusal muscle activation of sensory fibers co-activation of α-motoneurons and γ-motoneurons contraction of extrafusal fibers, and maintains sensitivity of muscle spindle Are in parallel with extrafusal skeletal muscle Innervated by sensory and motor neurons sensory: group Ia and II afferent neurons motor:γ-motoneurons Golgi tendon Senses contractile force golgi tendon reflex extrafusal muscle contraction activation of golgi tendon organ activation of group Ib afferent (sensory) nervesactivation of inhibitor interneurons inhibits α-motoneurons muscle relaxes synergistic muscle relax, antagonistic muscles contract Are in series with the extrafusal skeletal muscle mostly located between the extrafusal muscle and tendon Innervated by group Ib afferent neurons