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