Muscle senses & Reflex organization

Csilla Egri, KIN 306, Spring 2012

Ociffer, I’m not drunk, my proprioceptors are askew…

Outline

Proprioception Muscle spindles Golgi tendon organs

Spinal reflexes + clinical importance Stretch reflex Inverse myotatic reflex Flexion reflexes

Withdrawal reflex Crossed extensor reflex

2

Proprioception3

Sense the position of body parts in relation to each other and in space, as well as relative force applied to movements

Balance Vestibular system

Muscle length and force Muscle spindles Golgi tendon organs

B&L Figure 9-1

Muscle Spindles - intro4

Non-force generating intrafusal muscle fibers within a fluid filled capsule (spindle) Lie in parallel with extrafusal muscle

fibers Stretch or shorten along with extrafusal

fibers Innervated by both motor (efferent)

and sensory (afferent) axons Efferent innervation contracts intrafusal

fiber to match length of extrafusal fiber Afferent innervation sends info on relative

amount of muscle stretch Sense change in muscle length

Kandel Figure 36-3

Muscle spindles - structure5

Three types of intrafusal fibers Central regions are non-contractile

Mechanoreceptive sensory innervation Primary Ia afferents

all 3 fibers Secondary II afferents

static nuclear bag and nuclear chain fibers Motor innervation

Dynamic γ efferent Dynamic nuclear bag

Static γ efferent Combination of chain and static nuclear

bag

Kandel Figure 36-3

Muscle spindles – afferent function

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Each afferent has a tonic, baseline level of firing, and responds to relative stretch Ia

Static and dynamic firing responses Firing proportional to amount of and rate of muscle stretch

II Only static firing response Firing proportional to amount of stretch

Firing of which afferent is being assessed during a tendon tap?

B&L Figure 9-2

Muscle spindles – efferent function

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γ motor neurons maintain sensitivity of spindle over a range of muscle lengths α-γ coactivation

Descending input can change relative dynamic vs. static γ activation to modulate spindle sensitivity

http://www.ncbi.nlm.nih.gov/books/NBK11119/bin/ch16f10.jpg

Activation of only dynamic γ motor neurons

increases responsiveness of _____

afferents

Golgi Tendon Organs (GTOs)8

collagen fibers located within a capsule near tendon, in series with extrafusal muscle

innervated by mechanoreceptive Ib afferent fibers intertwined between collagen Activated by muscle

contraction or stretch Sense changes in tendon

tension/force

Kandel Figure 36-6

Reflex organization9

A reflex is a predictable, involuntary and stereotyped response to an eliciting stimulus Can be modulated by stimulus intensity and descending CNS input Testing reflexes is an important clinical tool in assessing neurological

and spinal function

Myotatic or stretch reflex10

Monosynaptic reflex mediated by muscle spindles Contraction in response to lengthening

Reflex arc:1. Muscle stretches2. Ia afferent of muscle spindle increase

firing3. Synapse on α motor neuron and

inhibitory interneuron in spinal cord4. α motor neuron of homonymous muscle

excited, and of antagonist muscle inhibited

5. Homonymous muscle contracts to oppose lengthening, antagonist muscle relaxes

B&L Figure 9-6

Myotatic or stretch reflex11

Stretch reflex has two phases:

Phasic (Ia) phase dynamic change in muscle length (ex. tendon tap) triggers a

transient phasic contraction Physiological importance: reflex contraction prevents

overstretch of extrafusal muscle fiber beyond physiological limits

Clinical importance: tendon tap used to determine integrity of spinal cord at different segmental levels

Myotatic or stretch reflex12

Tonic phase Static stretching of muscles produces a weaker, longer lasting,

tonic contraction Type II afferents also involved

Physiological importance: maintains muscle tone/posture via negative feedback Ex. Soldier standing at attention legs begin to fatigue and flex

quadriceps slowly begin lengthening tonic stretch reflex maintains tone and prevents collapse (to an extent)

Clinical importance: assessing presence of hypertonia Ex. Patients with cerebral palsy have very rigid, tight muscles

resistant to stretch overactive tonic stretch reflex due to upper motor neuron lesion

Motor neuron lesions13

Upper motor neuron lesion of the neural pathway inside the CNS (not including the ventral horn of the spinal cord or motor nuclei of the cranial nerves) stroke, traumatic brain injury or cerebral palsy

Lower motor neuron lesion affects nerve fibers within the ventral horn of the spinal cord travelling to the relevant muscle(s) Nerve trauma, polio

Upper motor neuron lesion

Lower motor neuron lesion

Reflexes Increased, may have pathological reflex signs (Babinski sign)

Decreased,

Muscle tone

Increased, contralateral Decreased, ipsilateral

Weakness Yes, contralateral Yes, ipsilateral

Inverse myotatic or Ib reflex14

Disynaptic reflex mediated by GTOs Relaxation in response to increased

tension Reflex arc:

1. Muscle contracts2. Ib afferent of GTO increase firing3. Synapse on one inhibitory and one

excitatory interneuron4. α motor neuron of homonymous

muscle inhibited, and of antagonist muscle excited

5. Homonymous muscle relaxes to oppose increased force in tendon, antagonist muscle contracts

B&L Figure 9-7

Ib

Inverse myotatic or Ib reflex15

Physiological importance: reflex relaxation thought to prevent excessive force from damaging

muscle tissue. Acts synonymously with the myotatic stretch reflex to maintain posture

and balance

Clinical importance: Clasp knife reflex: seen in patients with upper motor neuron lesions muscle has increased tone and resistance to stretch if sufficient force is applied, limb resistance suddenly decreases thought to be mediated by high threshold firing of GTO afferents (but other receptors may be involved as well)

Flexion withdrawalreflex

16

Polysynaptic reflex mediated by FRAs (flexion reflex afferents: nociceptors, mechanoreceptors etc.) flexion in response to painful stimuli

FRAs synapse on inhibitory and excitatory interneurons which excite ipsilateral flexor motorneurons & inhibit extensor motorneurons

Physiological importance: Rapid flexion away from painful

stimuli Clinical importance: upper motor

neuron lesion impairs flexion reflex pathalogical Babinski sign

B&L Figure 9-8

Upper motor neuron lesion: Babinski sign

17

(Type of flexion reflex) (pathological reflex)

Crossed extension reflex18

occurs in lower limbs as part of reflex arc for flexion reflex

FRAs synapse on interneurons which elicit contralateral limb extension to help maintain balance

Similar neuronal circuits involved in central pattern generators governing locomotion (next lecture)

B&L Figure 9-8

Summary of reflexes19

REFLEX

STIMULUS

(CLINICAL TEST)

RESPONSE

SENSORY RECEPTO

R

SYNAPSES

EFFECT ON

MUSCLEOTHER

EFFECTSFUNCTIO

N

Stretch (Myotatic) Reflex

Rapid Stretch of muscle (test: tap on muscle tendon)

Stretched muscle contracts rapidly (ex. knee jerk)

Muscle Spindle Primary (Ia) and Secondary (II) sensory neurons (tonic phase)

Ia: Mono-synapticII: (tonic phase) monosynaptic and polysnaptic

Excite Homonymous (same muscle)

Also Excite synergist muscles; Inhibit antagonist muscles (Reciprocal Inhibition)

Aid in maintaining posture, counter sudden stretch

Inverse Myotatic Reflex

Large force on tendon (pull on muscle when resisted)

Muscle tension decreases

Golgi Tendon Organ (Ib)

Disynaptic (via interneuron)

Inhibit Homonymous (same muscle)

Also Inhibit synergist muscles; Excite antagonist muscles

Protective, prevent damage to tendon

Flexor Reflex

Sharp, painful stimulus (as in stepping on nail)

Limb is rapidly withdrawn from stimulus

Cutaneous (skin) and pain receptors

Poly-synaptic (via interneuron)

Excite Flexor muscle

Also Inhibit extensor muscle of same limb; Excite extensor muscles and Inhibit flexors of opposite limb (Crossed Extensor Reflex)

Protective, withdraw from painful stimulus; Cross extension aids in maintaining posture when leg is lifted

http://musom.marshall.edu/anatomy/grosshom/spinalreflexes.html

Objectives

After this lecture you should be able to: Compare and contrast the structure and function of

muscle spindles with golgi tendon organs Describe the importance of αγ coactivation Describe the reflex pathway for the myotatic, inverse

myotatic, and flexion reflexes Give an example of a physiological and clinical importance for

each reflex Distinguish between upper and lower motor neuron lesions

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21

1. Type __________ spindle afferents are responsive to the rate of change of muscle length.

2. A Babisnki sign is often associated with _____________ motor neuron lesions.

3. Relaxation of the quadriceps muscle increases/decreases firing of Ia afferents and increases/decreases firing of Ib afferents.

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