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Peripheral Nervous System & Reflex Activity
Part D: Motor Control & Reflexes
Prepared by Janice Meeking & W. Rose.
Figures from Marieb & Hoehn 8th , 9th eds.
Portions copyright Pearson Education
Copyright © 2010 Pearson Education, Inc. Figure 13.13a
Feedback
Reflex activity Motoroutput
Sensoryinput
(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)
Internalfeedback
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 13.14
Receptor
Sensory neuron
Integration center
Motor neuron
Effector
Spinal cord(in cross section)
Interneuron
Stimulus
Skin
1
2
3
4
5
Components of a reflex arc (neural path)
Spinal ReflexesMediated 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 13.15
Secondary sensoryendings (type II fiber)
Efferent (motor)fiber to muscle spindle
Primary sensoryendings (type Iafiber)
Connectivetissue capsule
Muscle spindle
Tendon
Sensory fiber
Golgi tendonorgan
Efferent (motor)fiber to extrafusalmuscle fibers
Extrafusal musclefiber
Intrafusal musclefibers
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.
Musclespindle
Intrafusalmuscle fiber
Primarysensory (la)nerve fiberExtrafusalmuscle 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 13.17 (2 of 2)
Musclespindle
Quadriceps(extensors)
Hamstrings(flexors)
Patella
Patellarligament
Spinal cord(L2–L4)
Tapping the patellar ligament excitesmuscle spindles in the quadriceps.
The motor neurons (red) sendactivating impulses to the quadricepscausing it to contract, extending theknee.
Afferent impulses (blue) travel to thespinal cord, where synapses occur withmotor neurons and interneurons.
The interneurons (green) makeinhibitory synapses with ventral horn neurons (purple) that prevent theantagonist muscles (hamstrings) fromresisting the contraction of thequadriceps.
Excitatory synapseInhibitory synapse
+
–
1
2
3a
3b
1
2
3a3b 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 13.18
+ 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)
Golgitendon
organHamstrings
(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 13.19
Afferentfiber
Efferentfibers
Extensorinhibited
Flexorstimulated
Site of stimulus: a noxiousstimulus causes a flexorreflex on the same side,withdrawing that limb.
Site of reciprocalactivation: At thesame time, theextensor muscleson the oppositeside are activated.
Armmovements
Interneurons
Efferentfibers
FlexorinhibitedExtensorstimulated
+ 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 TestingNormal Babinski: http://library.med.utah.edu/neurologicexam/html/motor_normal.html#10
Normal Babinski (infant): http://video.google.com/videoplay?docid=-3102473882446365023&pr=goog-sl
Positive Babinski (adult): http://www.youtube.com/watch?v=bWKTrUjxkqs
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