• point where nerve and muscle communicate
• product of communication will be muscle twitch rather than an action potential (AP)

• consists of a motor neuron, its axon, and muscle fibers it innervates

• if there are sufficient MEPPs, muscle AP will be generated
• it takes many activated regions to excite muscle fiber
• you need to activate many muscle fibers to actually move muscle and do work
• one will not be enough to make muscle contract

striated (skeletal) muscle

• long ropelike structure made up of strands of muscle fiber
• each muscle fiber is made up of long myofibrils
• each myofibril is composed of either thin or thick myofilaments

• occurs as result of complex chemical reactions
• many myofilaments make up one muscle fiber
• muscle fibers make up a muscle bundle

• neuromotor innervation
• slow and fast twitch muscle fibers

• each muscle fiber is innervated by one motor neuron
• but each motor neuron may innervate a large number of muscle fibers

• use of many motor neurons to activate muscle
• more motor neurons that are activated; greater number of fibers that will contract

• most simple reflex
• classic example is knee jerk reflex
• monosynaptic or two-neuron reflex (one synapse or one sensory neuron and one motor neuron)

• tap patellar tendon of quadriceps muscle
• leads to brief stretch of muscle, which stimulates sensory endings of spindles
• sends volley of impulses to alpha-motor neurons
• activation of alpha-motor neurons causes a quick contraction (muscle jerk) of same muscle

• common element in all stretch reflexes: stretched muscle contracts after very brief delay
• crossed extensor reflex (for your information)

• central region of intrafusal fiber is wrapped by these sensory endings
• fast-conducting
• two types of intrafusal fibers: nuclear bag fibers and nuclear chain fibers

• as spindles stretch, surge of sensory input is directed to alpha-motor neurons, which reflexively contract muscle mass to decrease muscle length progressively
• contraction of entire muscle halts stretch of spindles
• stretching of central portion of intrafusal fibers can induce stretch reflex

• neuronal circuitry for spinal reflex
• circuitry includes following structures: muscle spindles, afferent fibers, alpha motor neurons (efferent fibers), neuromuscular junction, and muscle tissue

• mostly, independent of voluntary control
• however, reflexes are indirectly influenced by descending impulses from motor cortex and brainstem motor centers
• clinical example: in case of an upper motor neuron (UMN) lesion, spinal cord reflexes are released from higher levels of motor control and become hyperactive