Spinal_reflexes.pdf

INTEGRATION OF NEUROMUSCULAR ACTIVITY Roger Cameron, Ph.D. Department of Physiology [email protected] 444-7728 How does one automatically initiate a complex movement involving several different muscles? 2 Background • Motor neurons synapse onto muscles at the neuromuscular junction •...

INTEGRATION OF NEUROMUSCULAR ACTIVITY Roger Cameron, Ph.D. Department of Physiology [email protected] 444-7728 How does one automatically initiate a complex movement involving several different muscles? 2 Background • Motor neurons synapse onto muscles at the neuromuscular junction • Neuron action potential (AP)  endplate potential (EPP)  AP in muscle fiber • AP in muscle fiber  muscle contraction – Process involving excitation-contraction (EC) coupling Action potential Mechanical event • Limb movement occurs about joint – Abduction: away from body – Adduction: toward body 3 Motor unit definition • Motor neuron + muscle fiber(s) it innervates – No divergence: neuron + single fiber (e.g., hand and fingers) – Divergence: neuron + multiple fiber (e.g., large muscles in legs) 4 How are muscle fibers organized in a muscle? • Individual fibers contract allor-nothing • Passively relax!!! • Whole-muscle length determined by: – Fiber orientation – Number of contracting motor units 5 Muscles always organized in pairs: terminology • Flexor: moves appendage toward body (typically adduct) Each is the antagonist of the other • Extensor: moves appendage away from body (typically abduct) • Synergists: two (or more) neighboring muscles that act to produce similar movement Example: flexor extensor Biceps is the antagonist of the triceps (and vice versa) 6 Sidedness terminology • Contralateral: opposite side of body (left to right, or right to left) • Ipsilateral: same side of the body (either left or right) Examples: “The contralateral flexor of the left biceps muscle is the right biceps muscle.” “The ipsilateral antagonist of the left biceps muscle is the left triceps muscle.” Etc… 7 Organization of the spinal cord dorsal side (back) Sensory (afferent) enters dorsal side; cell bodies in dorsal root ganglion. Motor (efferent) exits ventral side; cell bodies in ventral horn. left side   right side ventral horn ventral side (front) 8 Withdrawal reflex of hand Decussate = “to cross over to contralateral side” 9 What about same reflex in the legs? • Potential problem: if withdraw one leg, must support body weight with opposite (contralateral) leg • Mediated via the crossed extensor reflex (misnomer since it involves both extensor and flexor muscles) 10 Error! wrong anatomy/wiring 11 Control of muscle length • Involves sensory elements within muscles called muscle spindles • Arranged in parallel with force-producing fibers termed extrafusal fibers • When muscle shortens, so do the spindles • Note difference between “parallel” and “series” arrangements: Parallel (length) Series (force) muscle length bone bone 12 Spindle anatomy 13 Spindle fibers come in two flavors Intrafusal muscle fibers: • at both ends of fibers • alter fiber length • innervated by  motor neurons • do not contribute to muscle force or length (too few/small) Type Ia afferents: • rapidly adapting • phasic receptors • Sense velocity (dx/dt) Type II afferents: • slowly adapting • tonic receptors • sense length 14 What happens when spindle stretched? • Anything that stretches central region of spindle fiber will… –  AP of type Ia afferents (sense velocity of lengthening) –  AP frequency of type II afferents (sense  length) • Fiber doesn’t work if spindle has slack •  motor neuron used to adjust fiber length to muscle length: 15 Myotatic (or stretch) reflex • Fastest reflex in body • Only monosynaptic (no interneuron) in body • Type Ia (or II) synapse directly onto -motor neurons innervating extrafusal fibers in same muscle • Reflex found in all skeletal muscles 16 Type Ia (and II) affect other muscles via interneurons 17 Crossed extensor reflex works too! • Stretching a left-side flexor causes 1. Contraction of left flexor (monosynaptic myotatic reflex) 2. Relaxation of left extensor (polysynaptic) 3. Relaxation of right flexor (polysynaptic) 4. Contraction of right extensor (polysynaptic) • Net effect: opposite movement on contralateral side • Reflex interconnections (e.g., interneurons) impinge on motor neurons, NEVER on -motor neurons – -motor neuron activity set by descending pathways from brain 18 Descending motor neurons initiate movement Upper motor neurons: Cell bodies in cortex Decussate Lower motor neurons: Cell bodies in cord -motor neurons -motor neurons All fibers cross over: ~75% in brain ~25% locally in cord 19 Types of movement Reflex Voluntary Rhythmic Stimulus Sensory receptors (e.g., pain, spindles) Consciously at will Consciously at will Example Withdrawl reflex, myotatic reflex, etc. Playing piano Walking, running Complexity Least complex; integrated at spinal cord Complex; integrated by brain ( motor neurons) Intermediate; initiated by brain ( motor neurons); integrated at spinal cord Comments Sensory receptor activation of built in spinal-cord circuits; rapid Learned movements that improve with practice Brain activation of built in spinal-cord circuits 20 Muscle force measurement: Golgi tendon organs arranged in series with extrafusal fibers Spindle (Type Ib) 21 Golgi tendon organs • Within tendon in series with muscle – “feels” same force as that of muscle •  muscle force   AP frequency of type Ib afferent • Spinal wiring opposite to that of spindles –  muscle force   frequency in muscle -motor neuron – All spinal connections polysynaptic (at least 1 interneuron) 22 -motor neurons exhibit recurrent inhibition • via inhibitory Renshaw cells within ventral root • Thought to be “safety valve” limiting motor-neuron firing rate • Can be overridden by descending fibers from brain - - 23