Motor System Spinal Reflexes

Georgia Bishop, Ph.D.Professor and Vice ChairDepartment of Neurosciencebishop.9@osu.edu

OBJECTIVES

2

1. Define the terms proprioception and proprioceptor.

2. Define the terms motor unit and recruitment.

3. Explain how motor units function to increase muscle tension is increased

4. Describe muscle spindles and Golgi tendon organs.

5. Differentiate between alpha and gamma motor neurons.

6. Describe the term Gamma Bias and explain its functional role.

7. Differentiate the role of the muscle spindle and the Golgi tendon organ in proprioception.

8. Describe the neural correlates of spinal reflexes including the stretch reflex and flexor

9. withdrawal with crossed extension reflexes.

10.Describe the clinical significance of hyperactive or absent reflexes.

Describe the peripheral receptors and pathways that regulate spinal reflexes.

At the end of the module you will learn to:

REFLEX CIRCUITS

Reflex circuits in the spinal cord produce automated responses adaptive for typical situations. When a specific kind of sensory input consistently elicits a particular response, we call this a reflex.

Spinal or reflex circuits govern many muscle recruitment patterns within and between limbs, including standing and walking.

Reflex circuits require, at a minimum, 2 components:

A sensory input

A motor output

In the circuit, the sensory neuron synapses on the motor neuron which then elicits a muscle contraction.

Many reflex circuits also contain interneurons that may either excite or inhibit populations of motor neurons.

Vertebral Canal Intervertebral

ForamenPeriphery

Dorsal

Sensory

Ventral

Motor

Ventral

Horn

Motor

Neuron

Spinal

Nerve

Ventral

Root

Efferent

Axon of Motor Neuron

Dorsal Root

Ganglion

Peripheral

Process

Dorsal

RootDRG

CellDorsal

Horn

Spinal Nerve

40 m

Innervate extrafusal muscle fibers: Regular contractile portion of muscle

ALPHA MOTOR NEURONS

Skeletal muscle is made up of bundles of cylindrical muscle fibers.

Each muscle fiber is a single, multinucleated cell

MUSCLE FIBER

SKELETAL MUSCLE

Defined as a single alpha motor neuron and ALL THE MUSCLE FIBERS it innervates

Each muscle fiber receives input from a single motorneuron which synapses at the single motor end plate. However, a motor axon may innervate more than one muscle fiber).

MOTOR UNIT

The size of motor units varies from small (10 – 100 fibers/motor neuron) to large (100 – several thousand fibers/motor neuron).

Small motor units provide more precise control of motor activity. These would be found in muscles that control individual digits or muscles that control movements of the eye.

MOTOR UNIT

Motor End-Plate – Site where axons make synaptic contact with muscle fiber

MOTOR UNIT

1 TWITCH (100 msec)

MUSCLE TENSION

The force of contraction of individual muscle fibers is determined by the firing frequency of the motor neuron

Total force of contraction of a muscle is determined by number of alpha motor neurons that are active.

MUSCLE CONTRACTION

Tetany = a sustained muscular contraction caused by a series of stimuli repeated so rapidly that the individual muscular responses are fused. Maximal force a muscle can generate. Temporal summation.

TETANIC CONTRACTION

MUSCLE CONTRACTION

Clostridium tetani is a gram-positive rod-shaped bacterium that is found worldwide in soil; it is usually in its dormant form, spores, and becomes the rod-shaped bacterium when it multiplies.

1. Binds to peripheral nerve terminals and transported within the axon to CNS.2. Binds to proteins at presynaptic inhibitory motor nerve endings and taken up into the neurons.3. Effect is to block release of inhibitory neurotransmitters (GABA, glycine).4. Results in uncontrolled firing of motor neurons resulting in muscular spasms.5. Acts by selective cleavage of a protein required for neurotransmitter release, synaptobrevin II

TETANUS

PROPRIOCEPTION AND PROPRIOCEPTORS

PROPRIOCEPTION include awareness of the body’s position in space, sensation of forces acting on the body, and sensation of body movements underway

Some define proprioception as position awareness and kinesthesia as awareness of movement. For this module, proprioception is used inclusively.

A PROPRIOCEPTOR is a sensory receptor that is principally used for proprioception

SENSORY INPUTS INFLUENCE RECRUITMENT

Two main kinds of proprioceptors influence recruitment levels in the motor pools:Muscle spindles - muscle length, and velocity of muscle length changesGolgi Tendon Organs (GTOs) - muscle tension (force) information

Cutaneous receptors, receptors in joints, and pain receptors also influence recruitment

Muscle Spindle

MUSCLE SPINDLES Muscle spindles detect muscle length, position, velocity, and acceleration

Extrafusal fibers are the muscle fibers we have been talking about. They make up the muscles we see.

The primary sensory afferent from the muscle spindle is called the Group Ia afferent (Ia)

Muscle spindles are miniature, long, thin stretch receptors that are present in all striated muscle. They are made up of specialized intrafusal muscle fibers surrounded by a capsule of connective tissue. They are scattered throughout the muscle and are aligned in parallel with the extrafusal fibers.

EF

SPINDLE

16

MUSCLE SPINDLES HAVE 3 MAIN COMPONENTS

Intrafusal muscle fibers provide regulation of muscle spindle stiffness, for variable sensitivity to stretch

Gamma motor axons synapse on intrafusal fibers to regulate their tension (stiffness)

Primary sensory axons (Ia and II) terminate on central region to sense stretch

The type II afferents only detect length of a muscle whereas type Ia detect length and rate of change in length.

TYPES OF MOTOR NEURONSAlpha motor neurons (medium – large neurons)

Project to extrafusal (skeletal muscle) fibers

Responsible for generating the muscle forces used to control movement

Gamma motor neurons (very small neurons) Only present to control muscle spindles by synapsing on intrafusal muscle fibers Cannot produce any appreciable muscle force

ALPHA-GAMMA CO-ACTIVATION (GAMMA BIAS)

The CNS independently regulates gamma motor neurons

19

GOLGI TENDON ORGANS

Receptors found at the junction of the muscle fibers with the collagenous tendon. Unlike the muscle spindles that are arranged in parallel to the muscle fibers, the receptors in the GTO intertwine with the collagen fibers of the tendon.

“Ib afferents” coming from the GTOs convey force data to the spinal cord. Detect muscle tension.

CIRCUITRY OF GOLGI TENDON ORGAN

The circuit of the Ib afferent is:Activated Ib afferent synapses on inhibitory interneuron in the spinal cord.

This inhibitory interneuron inhibits the motor neuron that projects to the contracting muscle to decrease tension and prevent damage to the tendon.

Ib excitatoryinterneuron

21

STRETCH REFLEX WITH RECIPROCAL INHIBITION

As described previously, when a muscle is quickly stretched, Ia afferent synapses excite that muscle’s alpha motoneurons

The is the stretch reflex

Muscle length homeostasis

The Ia afferent also synapses on the Ia inhibitory interneuron, which inhibits antagonist moto-neurons, relaxing that muscle. Provides a complementary functional effect.

STRETCH REFLEXMuscle spindles respond to weight of mug and activate the biceps to keep it upright. At same time, inhibitory interneuron blocks activity in the triceps.

Increasing weight causes passive stretch of the muscle and increased activation of Ia afferent which activates more motor neurons to produce stronger contraction. May recruit more motor units.

Increased recruitment of more motor units brings arm back to level position and overcomes increased resistance.

23

FLEXOR WITHDRAWAL WITH CROSSED EXTENSIONResults from activation of a nociceptor.Primary afferents branch into collaterals that activate 2nd order sensory neurons in the dorsal horn

These then project to another set of interneurons in the spinal cord which have multiple effects. They project to:

1. Inhibitory interneurons that inhibit alpha motor neurons that project to the quadriceps (stance muscle) and to excitatory interneurons that activate flexor muscles (e.g., hamstrings).

2. Excitatory interneurons on the contralateral side of the spinal cord that activate the contralateral quadriceps and inhibitory interneurons that inhibit the contralateral flexor muscles.

EFFECTS FROM JOINT RECEPTORS

Joint receptors contribute some proprioceptive awareness, especially at end of the joint range and under high force or strain conditions

Joint receptors can promote stability or safety

Rapid reaction to high joint force is increased muscle tone around the joint

Severe, acute pain from trauma causes extreme recruitment to splint the joint

Long term pain and swelling produces inhibition of muscles to protect the joint

CLINICAL APPLICATIONS

Tendon jerk reflexes reveal CNS excitability state.

When cerebral cortex is damaged, the default state of gamma motoneurons is hyperactive, and stretch reflexes are exaggerated.

Smooth, coordinated movement requires good proprioception

Stretching exercises should be performed slowly - less likely to elicit a stretch reflex.

Absence of a reflex indicates there is damage at the level of the spinal cord. This may result from damage to a sensory neuron or the Ia afferent (e.g., diabetes) or damage to the motor neuron or pools of interneurons.

SUMMARY

1.Sensation and control of reflexes is required for normal, coordinated movements

2.Proprioceptors are sensory receptors dedicated to monitoring movements and forces.

3. Muscle spindles (intrafusal muscle) are miniature muscle fibers that are aligned in parallel with the extrafusal muscle and detect length and rate of movement of a muscle

4. Golgi Tendon Organs are proprioceptors that are entwined with the collagen of a tendon and detect tension in the muscle.

5. Alpha motor neuron innervate extrafusal muscle and gamma motor neurons innervate intrafusal muscle fibers. For proper function, both are co-activated.

6. Gamma motor neurons determine the sensitivity of the muscle to small changes in length.

7. Spinal cord reflexes may be elicited by proprioceptors or by cutaneous (nociceptors) receptors.

8. The stretch reflex elicits muscle contraction following stretching of a muscle. In addition to activation of the stretched muscle, the antagonist is inhibited via interneurons in the spinal cord.

9. The flexor withdrawal reflex with crossed extension involves pools of interneurons that induce removal of a limb from a painful stimulus and extension of the contralateral limb to maintain stability.

10. Absent reflexes may indicate spinal cord or motor neuron damage. Abnormal reflexes (e.g., hyperactive) are a sign of damage to higher parts of the nervous system.

Motor System Spinal Reflexes Quiz

Thank you for completing this module

Questions?Bishop.9@osu.edu

Survey

We would appreciate your feedback on this module. Click on the button below to complete a brief survey. Your responses and comments will be shared with the module’s author, the LSI EdTech team, and LSI curriculum leaders. We will use your feedback to improve future versions of the module.

The survey is both optional and anonymous and should take less than 5 minutes to complete.

Survey