3 - Spinal Cord - Motor Unit and Reflexes 3

Question 1

What is a reflex?

Answer 1

An involuntary, stereotyped motor response to a sensory stimulus; coordinated pattern of muscle contraction and relaxation. Not fixed, adaptable for the task.

Question 2

What are the components involved in a reflex?

Answer 2

Sensory stimuli, spinal interneurons, descending neurons. All can influence and modulate reflex.

Question 3

What is the function/purpose of spinal reflexes?

Answer 3

To automatically keep us upright, maintain our tone, and hold still = posture and balance. Smooth out fine motor movement and adjust force in muscle for a specific task.

Question 4

Why are reflexes useful for the brain and our body?

Answer 4

Provides fast-acting safety reactions to avoid danger/injury. Frees up the brain to do other, more complicated things such.

Question 5

How do some reflexes differ based on their components?

Answer 5

Some involve the spinal cord and peripheral nerves: simple stretch Others are modulated by descending pathways from the cortex and brainstem

Question 6

What results in altered strength of reflexes?

Answer 6

Damage to CNS descending pathways.

Question 7

What is areflexia, hyporeflexia, and hyperreflexia?

Answer 7

Areflexia (negative sign): loss of a reflex Hyporeflexia: reduced reflex strength Hyperreflexia (positive sign): overactive reflex

Question 8

What two things in a reflex are used to sense the status of a muscle?

Answer 8

Receptors in muscle that monitor length and tension of muscle spindles and golgi tendon organs (these are the two types of receptors) Sensory neuron (afferents) that innervate receptors relay the info to the spinal cord (cell bodies in DRG or TG).

Question 9

What is needed in a reflex to cause an effect on the muscle?

Answer 9

Effector neuron: lower motor neuron in spinal cord or brainstem.

Question 10

What is needed to modify information between the components that are sensing and causing effect? What is the net effect of these modifications?

Answer 10

1. Interneurons in sp cd modify reflex locally 2. descending neurons from cortex, brainstem project down and modify spinal reflex Net effect of descending control: excitatory or inhibitory

Question 11

What occurs in a simple reflex?

Answer 11

1. Muscle fiber innervated by the sensory afferent neuron 2. Sensory afferent neuron has a cell body within the DRG and projects its central axon to the motor neuron 3. This synapse elicits some action in the MN to cause an effect back on the same muscle fiber

Question 12

What is the structure of muscle spindles? What are they important for? When do they discharge?

Answer 12

Made of intrafusal fibers, lie parallel to extrafusal fibers, and sense muscle LENGTH. Important for proprioception. Discharge when muscle is stretched, silent when muscle contracts.

Question 13

What are the types of intrafusal fibers in a muscle spindle? What do they each sense?

Answer 13

Dynamic nuclear bag fibers sense CHANGE in length and response adapts. Static nuclear bag fibers and nuclear chain fibers sense STATIC length and response remains steady.

Question 14

What are the two types of muscle spindle sensory afferents? What is their structure?

Answer 14

Ia: sense length and rate of change in length and convey fast, dynamic responses. Code velocity; very sensitive. II: sense static length, slow tonic response. Code duration of stretch. Both spiral around intrafusal fibers.

Question 15

What happens when you stretch or unload (contract) intrafusal fibers?

Answer 15

Stretch: sensory afferents activated and increase their action potential firing rate Unload (contraction): afferent stop firing

Question 16

What do we need dynamic and static intrafusal fibers in the muscle spindle?

Answer 16

Need dynamic fibers to sense when muscle is changing and static fibers to sense when muscle has stabilized at new length. Provides feedback about unexpected changes in muscle length and allows quick corrective measures in muscle contraction.

Question 17

What does the CNS use muscle spindles to do?

Answer 17

sense and correctly change position of limbs and body segments.

Question 18

What is the location of golgi tendon organs? What is their function? What are they innervated by?

Answer 18

Located at the juntion of msucle fibers and tendon and are in series with 15-20 extrafusal fibers. Sense tension; capsule innervated by Ib afferents. Intertwines between collagen fiber mesh and stretch compresses nerve ending.

Question 19

When do golgi tendon organs best discharge? What is their main function?

Answer 19

When the muscle connected to them contractsL extrafusal fibers pull on golgi and golgi is very sensitive to this. Precisely, continuously measure FORCE in a contracting muscle .

Question 20

Why do we need golgi tendon organs?

Answer 20

1. Gives precise control of muscle tension so appropriate force is exerted for task (pick up delicate thing without crushing it) 2. Provide negative feedback to CNS: safety device to protect against too much muscle tension, damage inhibiting motor neurons.

Question 21

Muscle spindles sense ______, while golgi tendon organs sense ______.

Answer 21

Muscle spindles sense LENGTH while golgi tendon organs sense TENSION.

Question 22

What are the two types of muscle spindle efferents? What is the function of each?

Answer 22

Alpha motor neurons: innervate extrafusal fibers Gamma motor neurons: innervate spindles by contracting to shorten intrafusal fibers when they become slack so that sensory afferents keep firing.

Question 23

What is the role of gamma motor neurons?

Answer 23

Ia aferrents fire when muscle is stretch but stop during contraction. Gamma motor neurons shorten spindle by causing contraction of intrafusal fibers so spindle remains taut and Ia afferents keep firing. Gamma MNs automatically maintain spindle sensitivity at all muscle lengths.

Question 24

When are alpha and gamma MNs co-activated? What is the purpose of this?

Answer 24

During voluntary movements. Alpha MNs contract muscle and gamma MNs keep spindles sensitive as muscle shortens to prevent sensory afferent from falling silent. Gamma provides + feedback to reinforce activation of alpha so contraction continues.

Question 25

What is the monosynaptic excitatory reflex? What is the speed of this?

Answer 25

“Deep tendon reflex” Strike bicep tendon with hammer and bicep should contract. Speed very fast: <20 ms

Question 26

How does a stretch reflex work when there is a stimulus of the biceps muscle being stretched? What neurons are excited in this process?

Answer 26

Spindles lengthened and Ia afferents are excited; they project to sp cd to excite alpha MNs that project to biceps (homonymous) and contracts. Also excites and contracts brachialis (synergistic). Monosynaptic. Ia afferents also excite inhibitory interneuron which inhibits alpha MNs projecting to the triceps. Disynaptic. Called reciprocal innervation.

Question 27

What does it mean if there is a decreased of absent reflex (hyporeflexia; areflexia) with decreased muscle tone?

Answer 27

Indicates problem with some component of reflex arc. Indicates disease of muscles, NMJ; sensory neurons, lower motor neurons.

Question 28

What does it means if there’s an increased reflex (hyperreflexia or spasticity) with increased muscle tone?

Answer 28

Indicates an upper motor neuron lesion. Indicates loss of descending inhibitory control over lower MNs.

Question 29

Why do we need the monosynaptic reflex?

Answer 29

Maintains appropriate amount of tension for proper motor control. Maintains tone for posture, balance, holding still, smooths movements, increases efficiency for walking and running.

Question 30

What is the role of the flexor reflex? What is an example?

Answer 30

Protection: to withdraw from painful stimulus. Polysynaptic: multiple synapses, interneurons involved. When you step on something sharp you automatically withdraw the injured foot by contracting flexors in leg and raise knee AND sijmultaneously get increased support of opposite leg.

Question 31

What are the neurons involved when you step on broken glass? What is the speed of this?

Answer 31

alphagamma nociceptors activated interneurons in the sp cd on the same side to excite ipso flexor and inhibit ipsi extensor –> withdraw leg from glass Activates interneurons that cross sp cd to excite contralateral extensor and inhibit contralateral flexor (polysynaptic) –> extends opposite leg for support. The stronger the stimulus, the faster the reflex.

Question 32

What is the golgi tendon reflex?

Answer 32

Ib afferents and Ib inhibitory interneurons activated. Other cutaneous and joint receptors and descending pathways also converge on the same Ib inhibitory interneuron.

Question 33

During the golgi tendon reflex, what is the result of activation of the Ib inhibitory interneuron?

Answer 33

It inhibits alpha motor neurons that innervate the original muscle to inhibit the muscle from contracting.

Question 34

What reflexes occur when you pick up delicate things such as touching a bubble?

Answer 34

Increase in muscle tension activates golgi tendon reflex. Inhibits motor neurons, grasp steps; don’t pop bubble. Also prevents too much force exertion, muscle injury.

Question 35

What are the two mechanisms that modulate spinal reflexes?

Answer 35

1. Inhibitory interneurons in sp cd help coordinate reflex and coordinate reciprocal innervation: excite synergistic muscle and inhibit antagonists. 2. Descending control: descending neurons (upper MNs frmo cortex, brainstem) sunapse on neurons in the sp cd to modulate reflexes.

Question 36

What is the purpose of inhibitory interneurons in the sp. cd that coordinate reflex actions?

Answer 36

They simplify voluntary control so higher commands don’t have to send separate commons to opposing muscles.

Question 37

What are the spinal cord sites for descending modulation? Is the force of a reflex always constant when the stimulus is constant?

Answer 37

Alpha motor neuron Interneurons Presynaptic terminals of sensory afferent fibers Due to many descending influences, force of a reflex varies even when stimulus is constant.

Question 38

Reflexes can be modified and depend on what?

Answer 38

Stimulus intensity, context, posture, task requirements, prior training, and cognitive influences.