Peripheral Nervous System & Reflex Activity Part D: Motor Control & Reflexes Prepared by Janice Meeking & W. Rose. Figures from Marieb & Hoehn 8 th, 9 th eds. Portions copyright Pearson Education

Copyright © 2010 Pearson Education, Inc. Figure 13.13a Feedback Reflex activity Motor output Sensory input (a) Levels of motor control and their interactions Precommand Level (highest) Cerebellum and basal nuclei Programs and instructions (modified by feedback) Projection Level (middle) Motor cortex (pyramidal system) and brain stem nuclei (vestibular, red, reticular formation, etc.) Convey instructions to spinal cord motor neurons and send a copy of that information to higher levels Segmental Level (lowest) Spinal cord Contains central pattern generators (CPGs) Internal feedback Levels of Motor Control Segmental level Projection level Precommand level

Copyright © 2010 Pearson Education, Inc. Figure 13.13b (b) Structures involved Precommand level Cerebellum Basal nuclei Projection level Primary motor cortex Brain stem nuclei Segmental level Spinal cord

Reflexes Inborn (intrinsic) reflex: rapid, involuntary, predictable motor response to a stimulus Learned (acquired) reflex: requires practice and/or repetition Driving Sports

Copyright © 2010 Pearson Education, Inc. Figure Receptor Sensory neuron Integration center Motor neuron Effector Spinal cord (in cross section) Interneuron Stimulus Skin Components of a reflex arc (neural path)

Spinal Reflexes Mediated by spinal cord Regulated by the brain Work (but abnormally) even in spinal cord injury patients Effectors are skeletal muscle Examples: Stretch, Golgi tendon, flexor, crossed extensor, cutaneous Spinal reflex testing an important part of a clinical neurological exam

Stretch and Golgi Tendon Reflexes Help coordinate muscle activity Require proprioceptive input Muscle spindles provide muscle length information Golgi tendon organs provide muscle and tendon force information

Stretch Reflex Maintains muscle tone in large postural muscles Muscle lengthening causes contraction of stretched muscle, relaxation of antagonist Stretch activates muscle spindle IIa sensory neurons make excitatory synapses onto  motor neurons in spinal cord  motor neurons cause stretched muscle to contract Stretch reflex is monosynaptic and ipsilateral

Sensors for the Stretch Reflex: Muscle Spindles 3–10 short modified (intrafusal) muscle fibers in a connective tissue capsule Noncontractile in central region (no myofilaments) Wrapped with two types of afferent endings: primary sensory endings of type Ia fibers and secondary sensory endings of type II fibers Contractile end regions innervated by gamma (  ) efferent fibers that maintain spindle sensitivity Note: extrafusal fibers (regular muscle fibers) are innervated by alpha (  ) efferent fibers

Copyright © 2010 Pearson Education, Inc. Figure Secondary sensory endings (type II fiber) Efferent (motor) fiber to muscle spindle Primary sensory endings (type Ia fiber) Connective tissue capsule Muscle spindle Tendon Sensory fiber Golgi tendon organ  Efferent (motor) fiber to extrafusal muscle fibers Extrafusal muscle fiber Intrafusal muscle fibers

Sensors for the Stretch Reflex: Muscle Spindles Excited by stretch, which could be caused by: 1.External stretch of muscle and muscle spindle 2.Internal stretch of muscle spindle due to activation of  motor neurons, stimulating ends to contract, thereby stretching spindle Stretch causes an increased rate of impulses in Ia fibers  –  coactivation maintains tension and sensitivity of spindle during muscle contraction

Copyright © 2010 Pearson Education, Inc. Figure 13.16a, b (a) Unstretched muscle. Action potentials (APs) are generated at a constant rate in the associated sensory (la) fiber. Muscle spindle Intrafusal muscle fiber Primary sensory (la) nerve fiber Extrafusal muscle fiber Time (b) Stretched muscle. Stretching activates the muscle spindle, increasing the rate of APs. Time (d) - Coactivation. Both extrafusal and intrafusal muscle fibers contract. Muscle spindle tension is main- tained and it can still signal changes in length. Time (c) Only motor neurons activated. Only the extrafusal muscle fibers contract. The muscle spindle becomes slack and no APs are fired. It is unable to signal further length changes.  Time

Copyright © 2010 Pearson Education, Inc. Figure (2 of 2) Muscle spindle Quadriceps (extensors) Hamstrings (flexors) Patella Patellar ligament Spinal cord (L 2 –L 4 ) Tapping the patellar ligament excites muscle spindles in the quadriceps. The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. Excitatory synapse Inhibitory synapse +–+– 1 2 3a 3b 1 2 3a 3b Stretch Reflex Example: Patellar (knee-jerk) reflex

Golgi Tendon Reflex Only kicks in when force is large. May act to prevent muscle tearing due to excessive force. Some evidence for a role in normal muscle coordination too. When tendon stretches, this reflex causes muscle to relax & antagonist to contract Opposite to stretch reflex response to lengthening Polysynaptic

Copyright © 2010 Pearson Education, Inc. Figure Excitatory synapse – Inhibitory synapse Quadriceps strongly contracts. Golgi tendon organs are activated. Afferent fibers synapse with interneurons in the spinal cord. Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. Efferent impulses to antagonist muscle cause it to contract. Interneurons Spinal cord Quadriceps (extensors) Golgi tendon organ Hamstrings (flexors) 1 2 3a 3b

Flexor and Crossed-Extensor Reflexes Flexor (withdrawal) reflex: ipsilateral, polysynaptic Painful stimulus causes automatic withdrawal of the threatened body part Crossed extensor reflex: contralateral; polysynaptic Occurs with flexor reflex in weight-bearing limbs to maintain balance Contralateral extension while ipsi side flexes

Copyright © 2010 Pearson Education, Inc. Figure Afferent fiber Efferent fibers Extensor inhibited Flexor stimulated Site of stimulus: a noxious stimulus causes a flexor reflex on the same side, withdrawing that limb. Site of reciprocal activation: At the same time, the extensor muscles on the opposite side are activated. Arm movements Interneurons Efferent fibers Flexor inhibited Extensor stimulated + Excitatory synapse – Inhibitory synapse

Superficial (cutaneous) reflexes Elicited by gentle cutaneous stimulation Depend on upper motor pathways and cord-level reflex arcs Plantar Abdominal

Plantar reflex Stimulus: stroke lateral aspect of sole of foot Normal response: downward flexion of toes Tests for function of corticospinal tracts Babinski’s sign: abnormal response – Hallux dorsiflexes, smaller toes fan laterally – Normally in infants <1 y.o. due to incomplete myelination – In adults, indicates corticospinal or motor cortex damage

Reflex Testing Normal Babinski: Normal Babinski (infant): &pr=goog-sl &pr=goog-sl Positive Babinski (adult): Movies from the Neurologic Exam and PediNeurologic Exam websites by Paul D. Larsen, M.D., University of Nebraska Medical Center and Suzanne S. Stensaas, Ph.D., University of Utah School of Medicine. Additional materials for Neurologic Exam are drawn from resources provided by Alejandro Stern, Stern Foundation, Buenos Aires, Argentina; Kathleen Digre, M.D., University of Utah; and Daniel Jacobson, M.D., Marshfield Clinic, Wisconsin.

Abdominal reflexes Cause contraction of abdominal muscles and movement of the umbilicus in response to stroking of the skin Vary in intensity from one person to another Absent when corticospinal tract lesions are present