1. Motor System Spinal Reflexes
Georgia Bishop, Ph.D.
Professor and Vice Chair
Department of Neuroscience

2. At the end of the module you will learn to:
OBJECTIVES
At the end of the module you will learn to:
Describe the peripheral receptors and pathways that regulate spinal reflexes.
Define the terms proprioception and proprioceptor.
Define the terms motor unit and recruitment.
Explain how motor units function to increase muscle tension is increased
Describe muscle spindles and Golgi tendon organs.
Differentiate between alpha and gamma motor neurons.
Describe the term Gamma Bias and explain its functional role.
Differentiate the role of the muscle spindle and the Golgi tendon organ in proprioception.
Describe the neural correlates of spinal reflexes including the stretch reflex and flexor
withdrawal with crossed extension reflexes.
Describe the clinical significance of hyperactive or absent reflexes.

3. 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.
In this e-learning module we will begin to focus on how sensory information from joints and muscles is translated into muscle contraction at the level of the spinal cord. In a previous lecture you learned about the organization of the spinal cord. In particular, you now know that the dorsal part of the cord is involved in receiving sensory information from the body wall as well as from receptors located within muscles and joints. You also know that the ventral horn contains alpha motor neurons. The axons of alpha motor neurons leave the spinal cord and directly innervate muscles. The goal here is to demonstrate how input to the dorsal horn influences the output of the alpha motor neuron. This circuitry, confined entirely to the spinal cord, is defined as reflex circuits. These circuits function completely independent of input from other areas of the CNS. It is called reflex circuitry. This circuitry is not only activated when someone taps your knee with a hammer but rather may be the basis for automated responses that require integration of motor patterns at the level of the cord - for example, they mediate patterns that may be involved in walking or jumping. Every reflex circuit*** requires 2 components, a sensory input and a motor output. Most reflex circuits, however, are more complex and actually involve pools of interneurons that may excite or inhibit motor neurons. We'll look at some of these interactions in this presentation.

4. Spinal Nerve Vertebral Canal Intervertebral Foramen Periphery Dorsal
Root
Peripheral
Process
Dorsal Root
Ganglion
DRG
Cell
Dorsal
Horn
Dorsal
Sensory
Spinal
Nerve
Ventral
Motor
Ventral
Horn
Motor
Neuron
Let's review the organization of afferent and efferent projections in the spinal cord. Within the vertebral canal *** of the vertebral canal, we find the spinal cord. It is divided into dorsal and ventral halves. The gray matter in the center of the spinal cord likewise is divided into a dorsal horn and a ventral horn. Connected to the dorsal and ventral side of the spinal cord are two connectors called the dorsal*** and ventral*** root. Functionally, the dorsal part of the spinal cord receives sensory information from the periphery, including input from the skin, muscles and joints. The ventral horn represents the output of the spinal cord and contain motor neurons that innervate muscles in the periphery. As we'll see in a few minutes, sensory information is transmitted to the dorsal horn via the dorsal root where it terminates (synapses) on neurons in the dorsal horn. Motor output is carried by axons of alpha motor neurons located within the the ventral horn via the ventral root. Let's move a bit laterally and look at what we see in the area immediately adjacent to the spinal cord in the area near the intervertebral foramen ***. Related to the dorsal root, there is a swelling called the *** dorsal root ganglion which contains neurons called dorsal root ganglion (DRG) cells ***. DRG neurons have a process that extends to the periphery where it detects various types of sensory information called the *** peripheral process. The central process extends from the DRG neuron and enters the spinal cord via the dorsal root. In the ventral root, we find the axons ***of motor neurons. Within the intervertebral foramen, the dorsal root and the ventral root fuse to form a ***spinal nerve. Remember, the dorsal root is a “one way” street carrying information into the spinal cord and the ventral root is a “one way” street carrying information out of the spinal cord. The spinal nerve, then becomes a “two way” street. Let's move just ***a bit more lateral to the point where we are outside the vertebral column. Here we see information flowing to the dorsal and ventral side of the body.
Efferent
Axon of Motor Neuron
Ventral
Root

5. ALPHA MOTOR NEURONS
Let’s focus first on neurons in the ventral horn. Here is a micrograph of an alpha motor neuron. It is large, measuring about 40 microns. The axons of these neurons leave the spinal cord via the ventral root and innervate what is called extrafusal ***muscle fibers which are the regular contractile portion of a muscle. The black line represents the axon of the alpha motor neuron leaving the cord and innervating the quadriceps muscle in the thigh.
40 m
Innervate extrafusal muscle fibers: Regular contractile portion of muscle

6. SKELETAL MUSCLE
MUSCLE FIBER
Skeletal muscle is made up of bundles of cylindrical muscle fibers.
Each muscle fiber is a single, multinucleated cell
Remember from your bone and muscle block, skeletal muscle is made up of bundles fibers*** Also remember, *** each individual fiber is actually a single cell that contains multiple nuclei

7. MOTOR UNIT
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).
Now let’s put the alpha motor neuron and the muscle together by defining a motor unit. A motor unit consists of a single alpha motor neuron and all the muscle fibers it innervates. Remember a muscles is made up of hundred of individual muscle fibers. A single alpha motor unit innervates a sub-set of these fibers; thus more than one motor neuron innervates a single muscle (e.g., biceps). Another important *** fact is that each motor neuron innervates more than one muscle fiber. This is important as we’ll see on the next slide.

8. MOTOR UNIT
The size of motor units varies from small (10 – 100 fibers/motor neuron) to large (100 – several thousand fibers/motor neuron).
We just defined a motor unit. Let’s extend that concept. Motor units may be large or small. If a motor neuron innervates only muscle fibers in a given muscle, we would call that a small motor unit. If it innervates more that 100 fibers we would call that a large motor unit. What does that mean functionally? **** Small motor units mean only a few fibers contract with activation of the motor neuron. Thus very precise movements could be made. Examples where small motor units are prevalent would be muscles that control eye movements, or the small muscles in your hand that control digital movement. In contrast, large motor units would provide for less precise movement. Examples of this would be muscles in our trunk that sustain posture or in the large muscles of our joints (e.g., biceps or quadriceps). These would mediate activities that require sustained contractions or strength moves.
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.

9. MOTOR UNIT
This is an actual image of the axon of an alpha motor neuron, labeled nerve, as approaches a muscle. The axon divides into several branches that innervate 2 muscle fibers that are in view. Other branches go deeper into the muscle to innervate other muscle fibers not seen.
Motor End-Plate – Site where axons make synaptic contact with muscle fiber

10. MUSCLE CONTRACTION MUSCLE TENSION 1 TWITCH (100 msec)
The force of contraction of individual muscle fibers is determined by the firing frequency of the motor neuron
Let’s cover one more concept related to the motor unit. That is what determines the force of contraction of individual muscle fibers in a motor unit and how do we obtain maximal force of contraction of a muscle. ***If you stimulate a single motor unit, and you get a single action potential, the muscle fibers it innervates *** will contract and the muscle appears to twitch. If you want more than a twitch *** you need to activate more of the motor neurons that innervate that muscle. The more motor units that are activated, the stronger the contraction as more muscle fibers are stimulated to contract.
Total force of contraction of a muscle is determined by number of alpha motor neurons that are active.

11. MUSCLE CONTRACTION TETANIC 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
Now we'll combine both concepts. As noted on the last slide, if nerve stimulation is given at low frequency, the muscle twitches. The number of fibers that twitch is determined by the number of motor units that are activated. What if the frequency of stimulation is increased? *** As the impulses come closer together, the muscle fiber cannot completely relax. If *** the stimulation frequency continues to increase the muscles reaches a stated called tetany*** and produces a sustained contraction. When all muscle fibers are activated and contracting at their maximal force a state of complete tetanus is reached and this is the maximal force the muscle can generate. The activation of multiple motor neurons is referred to as recruitment

12. TETANUS
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.
Just as an aside, the term tetany is also applied to the clinical presentation of a patient suffering from a tetanus infection. You’ll learn more about this in the Host-Defense block, but clostridium tetani are bacteria that live in the soil. The infectious pathway of this bacterium is quite unique. You heard about this in Dr. Askwith’s lecture on synaptic transmission so I’ll just review it here. *** The bacteria enters the body through open cuts where they bind to the endings of nerves in the skin. These actually are sensory nerves. They are transported back to the spinal cord where they are released in the gray matter. They then bind to proteins at the nerve endings of inhibitory interneurons in the spinal cord and block release of GABA by cleaving a protein, synaptobrevin II, that is required for transmitter release. Without inhibitory control, motor neurons begin to fire uncontrollably at a very high frequency and cause muscles throughout the body to contract. This is*** a classic picture of an individual in full tetany where all muscles are contracting at the same time. Get your tetanus shots to prevent this from happening.
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

13. 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
We've been focusing on the motor side of reflex. Now let's spend just a little time on the sensory side. Let's first define some terms. Proprioception basically means being aware of where we are in space. That is not only our bodies, but also our limbs. I can put my arm behind my back where I can't see it, but I know it is there. I can put my hand in a box where I can't see it but I am still aware of where my fingers are and what they are trying to pick up. It also refers to being aware of how our body is moving in space. Another term you may hear related to movement awareness is kinesthesia which literally means movement. For our purposes, we will use the term proprioception to refer both to static awareness of the body as well as awareness of movement. Another term we will define is *** Proprioceptor. This refers to specializations in muscles and joints that transmit the sensory information on where we are in space. That is, a proprioceptor is a sensory receptor that detect muscle contraction or movement.

14. 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 changes Golgi Tendon Organs (GTOs) - muscle tension (force) information
We used the term recruitment earlier to refer to activation of multiple motor units. What determines how many are active at a given time? That is determined by proprioceptors in the periphery. There are two type of proprioceptors that influence recruitment that are called muscle spindles and Golgi tendon organs. Muscle spindles, as their name implies, are receptors that are found within the muscle itself. Their role is to detect changes in muscle length and how quickly the muscle is contracting. The Golgi tendon organ is found in the tendon. It’s role is to detect muscle tension. ***Other receptors, such as those in the skin or in joints also influence recruitment, but we will not focus on those in this presentation. Let’s start with the muscle spindle.
Cutaneous receptors, receptors in joints, and pain receptors also influence recruitment

15. MUSCLE SPINDLES
Muscle Spindle
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.
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
As just defined, muscle spindles are proprioceptors that detect muscle length and position. They also provide information on how fast the fiber is contracting. Where do we find muscle spindles? Let’s define a few more terms. The muscle fibers that make up a muscle, the ones we actually see and feel are called ***extrafusal fibers. Embedded within the muscle itself are several miniature structures called the muscle spindle ***. These are actually stretch receptors that are made up of miniature muscle fiber, called intrafusal fibers, that are surrounded by a fibrous capsule. They are oriented parallel to the extrafusal muscle fiber. We see *** here an actual histological preparation showing the extrafusal muscle fibers and the muscle spindle. One last term, *** the primary sensory axon that carries stretch information from the spindle to the spinal cord is called a Group one A afferent. We’ll talk more about this designation in a future guided learning module. Since the spindle is attached to the extrafusal muscle fibers and they are in parallel, when the muscle is stretched so is the spindle. Stretching of the spindle activates afferent axons that respond to this mechanical distortion. When activated, they generate action potentials that are sent from the periphery to the dorsal horn of the spinal cord. More about this later. Let’s look in a bit more detail at this system.
The primary sensory afferent from the muscle spindle is called the Group Ia afferent (Ia)

16. MUSCLE SPINDLES HAVE 3 MAIN COMPONENTS
Intrafusal muscle fibers provide regulation of muscle spindle stiffness, for variable sensitivity to stretch
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.
Gamma motor axons synapse on intrafusal fibers to regulate their tension (stiffness)
Muscle spindles basically are componsed of 3 elements. This includes the ***intrafusal muscle fibers we just described on the previous slide. The second component are the ***primary sensory axons designated IA and II that terminate in the central region of the spindle. This is an area that contains multiple nuclei as depicted by the red dots. We will focus primarily on the 1A or primary afferents. In this diagram they are the fibers forming a spiral around the nuclear part of the muscle spindle (dark red dots). Type II afferents are secondary and are located a bit further away from the central region. They only detect changes in length of a muscle. The Ia afferents detect changes in length and rate of change in length during a contraction. The third component are ***called Gamma motor axons. The muscle spindle has it’s own motor innervation that is distinct from the innervation of the extrafusal muscle fibers. Let’s look at that on the next slide.

17. TYPES OF MOTOR NEURONS
Alpha 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
Let’s try to put this all together. In this diagram we see the spinal cord. Dorsal is to the top and ventral to the bottom. The dorsal root ganglion neuron (green) sends a process to the muscle spindle. This is the 1A afferent. The other part of the process enters the spinal cord through the dorsal root where it enters the dorsal horn of the spinal cord. In this simple diagram, the 1A afferent goes directly to the ventral horn where it synapses on a large motor neuron called the ***alpha motor neuron which we described earlier. The axon of the alpha motor neuron in turn returns to the extrafusal muscle where it causes muscle fibers within it’s motor unit to contract. This is a basic circuit we will return to. In addition, we see another neuron in the ventral horn*** that is smaller (red cell) **** This is the gamma motor neuron. Notice that it’s axon projects back to the muscle spindle where it innervates the intrafusal muscle fibers. Why do we need a gamma motor neuron?

18. ALPHA-GAMMA CO-ACTIVATION (GAMMA BIAS) 
Muscle spindles are under what is called gamma bias. To function properly there has to be co-activation of both alpha and gamma motor neurons. If the muscle spindle was not contractile, muscles would stop contracting after an initial stretch. We’ll work through this process with some diagrams. In the top picture, we have the spinal cord and the major players, namely the alpha and gamma motor neurons and the sensory neuron in the dorsal root ganglion. In the periphery we have the muscle and its muscle spindle. Let’s stretch the muscle*** Here is muscle where the extrafusal fibers have been stretched. Please note muscles are always under some tension. It doesn’t require someone pulling on your arm or hitting your knee cap. Just the act of using them exerts tension or stretch. As the extrafusal fibers are stretched, so too are the intrafusal fibers. When they stretch action potentials are generated in the 1A afferent. As we saw ***on the previous slide this activation of 1A afferents ultimately results in excitation of an alpha motor neuron. The axon of the *** alpha motor neuron project back to the muscle and all muscle fibers innervated by this neuron contract. But now we have a problem, **** when the muscle contracts, there is deceased tension on the muscle spindle. Think of having arubber band between 2 points. When the points are farther away, the rubber band stretches. When they move closer together the rubber band relaxes. This issues here, is that when this happens, activity in the 1A afferent is decreased or eliminated. That means there is no excitatory drive on the alpha motor neuron and the muscle relaxes. We don’t want our muscle to contract once and then relax, we want some level of excitation to be maintained. That is the role of the gamma motor neuron. The gamma motor neuron projects to the intrafusal muscle *** fibers causing them to contract, even though the extrafusal fiber is in a contracted state. Thus there is always some level of activity in the 1A afferent and thus the alpha motor neuron continues to fire to sustain a contraction. Last point, what activates gamma motor neurons? This will be discussed in a future presentation. Spoiler alert *** – they are controlled by centers in the brainstem that respond to movement such as the vestibular system and others.
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.
One last receptor to describe before finally getting to reflexes. There are receptors located at the junction of the muscle fibers and tendon made up of collagen. These receptors intertwine with the collagen fibers. If these muscle contracts, these collagen fibers are stretched essentially trapping the receptors in the web. This pressure activates them and they send an action potential to the nervous system via a system of axons referred to as Ib afferents. ***Their role is to sense how much tension is being generated in the muscle. If a muscle is strongly contracting, there is high tension on the tendon and the Ib afferents are excited. We’ll look at the effect of this on the next slide. Basically the role of the GTO is to monitor tension in muscles to prevent them from “over-contracting”.

20. 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 excitatory
interneuron
The role of the GTO is to decrease tension on the tendon by inhibiting motor neurons that project to a contracting muscle. As the muscle contracts, tension is put on the non-contractile tendon. This causes the collagen fibers to stretch and put pressure on the Ib endings that are intertwined between them causing them to generate an action potential. This action potential is conveyed into the spinal cord via the Ib afferent where it synapses on an inhibitory interneuron (Ib-IN). *** The inhibitory interneuron projects to and suppresses motor neurons that project back to the same muscle to decrease tension and prevent damage to the tendon. Also note, the Ib afferent may also synapse on a Ib excitatory interneuron (Ib-EN)*** that projects to the antagonist muscle again to counteract the force generated by the contracting muscle.

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.
We will now describe some basic reflexes that are elicited at the level of the spinal cord using the circuitry we just learned. Let’s start with the simplest reflex, the Strech reflex with reciprocal inhibition. Basically, if a muscle spindle perceives that the muscle is stretched, it sends a signal to the spinal cord that directly activates an alpha motor neuron that projects back to the stretched muscle to make it contract. *** It also synapses on an inhibitory interneuron that suppresses activity in an antagonist muscle.

22. STRETCH REFLEX
Muscle 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.
How do we activate the stretch reflex in real life. One way would be to have someone tap a tendon with a hammer. This actually produces stretch in the muscle which demonstrates how sensitive muscle spindles are. That doesn’t happen in normal life. Here is a real world situation. The individual is holding mug which puts stretch on their biceps inducing activity in the Ia afferent. We will assume there is co-activation of gamma motor neurons to retain a level of muscle tension. ***If a beverage is added to the mug, the weight increases placing more stretch on the muscle, and a subsequent stronger activation of the Ia afferents. Within the spinal cord, this increased activity may recruit more motor units to increase the force of contraction (Go back to the beginning of this presentation if necessary to review recruitment and motor units). *** This increase in the number of active alpha motor neurons enhances contraction of the muscle and the limb is brought back to a neutral position. Also note, the Ia afferent also synapses on an inhibitory interneuron that suppresses activity in the antagonist muscle, the triceps.
Increased recruitment of more motor units brings arm back to level position and overcomes increased resistance.

23. FLEXOR WITHDRAWAL WITH CROSSED EXTENSION
Results 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:
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).
One other reflex we’ll look at is called the Flexor Withdrawal with Crossed Extension Reflex. This is a much more complicated response involving several pools of interneurons. This occurs when there is a painful or noxious stimulus that may be detected by a cutaneous receptor. In this case, the individual steps on a tack, activating sensory fibers that are sensitive to painful input. The signal is again sent to the spinal cord where the processes terminate on neurons in the dorsal horn of the spinal cord. *** These dorsal horn neurons*** in turn project to another pool of interneurons that have multiple effects. First *** in order to remove the limb from the painful stimulus inhibitory interneurons projects to the quadriceps muscle (the stance muscle) to begin the process of withdrawal of the limb. At the same time, the dorsal horn neurons project to excitatory interneurons that stimulate flexor muscles in the limb to initiate withdrawal. This is the withdrawal part of the reflex. However, if we lift one foot off the ground, the other one better be firmly planted and extended to maintain us in an upright position. This is the crossed extensor part of the reflex. The dorsal horn neurons activated by the painful input also activate interneurons that ***excite the contralateral quadriceps and interneurons that inhibit the contralateral hamstrings. The net effect is flexion or withdrawal of the limb from the painful stimulus and concomitant extension on the contralateral side to prevent us from falling down.
Excitatory interneurons on the contralateral side of the spinal cord that activate the contralateral quadriceps and inhibitory interneurons that inhibit the contralateral flexor muscles.

24. 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
We won’t go into these in any detail but be aware that there are receptors in joints as well that provide some proprioceptive information. If there is a force on a joint, it can produce a response where there is increased muscle tone to protect the joint. Severe pain may cause muscles on both side of the joint to contract to prevent it from moving. This is referred to as splinting. However, if there is prolonged pain or swelling, the effect is reversed and muscle tone actually decreases, likely through interference with receptors in the joint due to the prolonged inflammation.

25. 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.
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.
Stretching exercises should be performed slowly - less likely to elicit a stretch reflex.
How can these reflexes be used clinically. You will be testing tendon reflexes in your practice. The response to a tap on the quadriceps tendon or the biceps tendon in the arm provide some information on the excitability state of the CNS. Remember, earlier we said that gamma motor neurons are controlled by axons that arise from neurons in the brain such as the cerebral cortex or brainstem. The descending inputs, in general, suppress the activity of the gamma motor neurons. If not controlled, their default state is to be hyperactive. If that occurs, the sensitivity of the muscle to slight perturbations is to produce a very strong contraction. This exaggerated response to a mild stimulus is suggestive of damage at higher levels of the CNS. On the other hand,*** if reflexes are absent, it suggests damage at the level of spinal cord or the afferent axon. Remember, a reflex requires both afferent and efferent axons. Damage to a Ia afferent, as in diabetes, removes the input side of the reflex. Damage to the spinal cord may kill motor neurons and interneurons resulting in loss of the output side of the reflex. ***Another practical application is that if you are doing stretching exercises, be sure to do them slowly so you don’t activate a stretch reflex which would cause the muscle to contract; this would be counter-productive. Also, remember *** smooth coordinated movement requires good proprioception. You’ll learn more about this is future lecture on the cerebellum which also plays a major role in modulating gamma and alpha motor neuron activity based on proprioceptive information it receives.
Smooth, coordinated movement requires good proprioception

26. SUMMARY
Sensation and control of reflexes is required for normal, coordinated movements
Proprioceptors are sensory receptors dedicated to monitoring movements and forces.
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
Golgi Tendon Organs are proprioceptors that are entwined with the collagen of a tendon and detect tension in the muscle.
Alpha motor neuron innervate extrafusal muscle and gamma motor neurons innervate intrafusal muscle fibers. For proper function, both are co-activated.
Gamma motor neurons determine the sensitivity of the muscle to small changes in length.
Spinal cord reflexes may be elicited by proprioceptors or by cutaneous (nociceptors) receptors.
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.
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.
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.
This is a summary of the material covered in this guided learning module.

27. Motor System Spinal Reflexes Quiz

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