Motor Functions of the Spinal Cord; The Cord Reflexes

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How do reflexes mediate homeostasis?

By negative feedback

What are two classifications of reflexes by development?

1- Acquired learned reflexes
2- Genetically determined innate reflexes

What are two classifications of reflexes by response?

1- Somatic reflexes controls skeletal muscles
2- Visceral reflexes autonomic control actions of smooth and cardiac muscles and glands

What are two classifications of reflexes by complexity?

1- Polysynaptic reflexes
2- Monosynaptic reflexes

What are two classifications of reflexes by processing?

1- Cranial reflexes processing in the brain
2 Spinal reflexes processing in the spinal cord

What is a reflex arc?

A neural pathway taking sensory information to the CNS, where it is "reflected" back by interneurons to motor neurons.

It is an involuntary reflex that is responsible for muscle contraction

What two neurons does the reflex arc involve?

1- Afferent neuron to the spinal cord
2- Motor neuron from the spinal cord to the muscle

Is the brain involved in the reflex arc?

NO!

What is a motor unit?

A motor neuron and the muscle fibers it innervates

What is an alpha motor neuron?

These are a link to the largest myelinated axons that average about 14 micrometers in diameter and conduct action potentials very rapidly.

What is a gamma motor neuron?

Smaller then an alpha and these give rise to smaller axons that average about 5 micrometers in diameter. They conduct action potentials at a lower velocity

There are ___ more interneurons then there are motor neurons and they are highly excitable.

30 times

What is the spontaneous firing rate of an interneuron?

1500 times per second.

Interneurons receive synapse in what two ways?

1- Incoming sensory information
2- Signals descending from higher centers in the brain

What does a renshaw cell receive input from?

Collateral branches of motor neuron axons

What does a renshaw cell provide?

Inhibitory connections with the same or neighboring motor neurons to focus or sharpen its signals (lateral inhibition).

What 2 things does sensory feedback from skeletal muscle depend on?

1- The current length of the muscle (muscle spindle).
2- The current tension in the muscle (golgi tendon organ).

What is a primary ending?

Type Ia myelinated primar sensory fiber.

What is the average diameter and conduction velocity of a primary ending (type Ia fiber)?

17 micrometers

70-120 m/sec

What is a secondary or flower spray-ending and what is the average diameter of these?

These are type II fibers and are an average of 8 micrometers in diameter, lightly myelinated, and conduct at slower velocities then type Ia.

Where are nuclear bag fibers located?

They are clumped in the central region.

What are nuclear chain fibers?

They are nuclei arranged linearly.

How many nuclear bag fibers does a muscle spindle contain?

1 to 3

How many nuclear chain fibers does a muscle spindle usually contain?

3 to 9

What is the static response of the muscle spindle?

This is when the central region of a spindle is slowly stretched, the number of impulses in both the primary and secondary endings increases with proportion to the degree of stretch

What is the dynamic response of the muscle spindle?

This signals the rate of change in length.

What fibers are believed to be responsible for dynamic response?

Nuclear bag fibers that are associated with primary endings.

When Gamma motor neurons are distributing to nuclear bag fibers, the stimulation enhances which response?

The dynamic response

Nuclear chain fibers stimulation enhances which response?

The static response

Type Ia sensory fibers terminate in the cord near their level of entry, true or false?

True

What do type Ia sensory gibers synapse with?

The alpha motor neurons in the ventral horn that innervate extrafusal fibers in the same muscle in which the primary sensory fibers originated.

What are the two components of the stretch reflex?

1- The dynamic phase occurs when the spindle is stretched.

2- The static phase occurs when he muscle has stopped increasing in length and reached a new static length

How does the stretch reflex maintain posture?

It opposes muscle stretch by contracting the lengthening muscle.

What is a clonus?

An alternating contraction of the agonist and antagonistic muscles crossing a joint. It is an abnormal stretch reflex function.

What body part is a clonus most prominent at?

The ankle

How could an examiner demonstrate a clonus?

When rapid and maintained dorsiflexion of the foot elicits sustained jerking movements of the foot at the ankle joint

What is the clinical significance or signs of a clonus?

It is a sign that the spinal cord circuits that mediate the stretch reflex are not being properly influenced by descending projections from the brain.

At what key joint locations can an examiner asses the general state of reflex activity?

The patellar tendon at the knee (tapping with a hammer stretches the spindles in the quadriceps and normally elicits a reflex contraction of that muscle group (stretch reflex), which produces a knee jerk.

Approximately what percent of axons distributing to any given muscle are from gamma motor neurons?

31%

What happens to the alpha and gamma motor neurons when signals are transmitted from the motor cortex or other control centers?

They become co-activated

What 2 things does the stimulation of gamma motor neurons during contraction of a muscle maintain?

1- The sensitivity of the muscle spindle
2- Prevents it from going slack and stopping its output

What is the gamma motor system most strongly influenced by?

Descending projections from facilitatory regions of the brain stem reticular formation.

What 3 CNS elements influence the brain stem reticular formation?

1- Cerebellum
2- Basal ganglia
3- Cerebral Cortex

What do the gamma motor neurons do during movement that requires substantial force generation?

They act on muscle spindles located in muscles on both sides of a joint and are activated in tandem by the brain stem reticular formation.

When gamma motor neurons act on muscle spindles during movement that requires substantial force it enhances what?

Muscle tone on both sides of the joint, providing stabilizing effects on movement at that joint.

Why do intrafusal fibers contract during movement that requires substantial force?

To keep stretch receptors taught and sensitive.

What do tendon reflexes react to and inhibit?

They react to excessive tension and inhibit the muscle

What types of fibers are tendon organ signals conducted by?

Large myelinated type Ib that conduct nearly as rapidly as the type Ia fibers from muscle spindles.

What do group Ib fibers form branches with upon entering the spinal cord?

On the pool of interneruons and others entering a long ascending pathway.

What do local inhibitory interneurons link the tendon organ input with?

Alpha motor neurons that innervate those muscles with which the tendon organ is associated.

What is the difference between muscle spindle input and tendon organ input?

Muscle spindle input excites its related motor neurons while tendon organ input produces inhibition of the motor neurons to which it is connected.

What is a golgi tendon organ?

Its an encapsulated receptor through which a small bundle of muscle tendon fibers pass just before their bony insertion.

What does the negative feedback of golgi tendon reflex prevent?

Injury to the muscle when it exceeds its upper limit of tension

What does the tendon organ reflex serve in terms of redistribution?

It redistributes the muscle load more evenly over a larger number of muscle fibers.

Where do the tendon organs ascending projections input to in the CNS?

The cerebellum and motor areas of the cerebral cortex that will be used by these centers in the control of movement

What do withdrawal reflexes do?

Flex the limb and relax extensors via reciprocal innervation.

What elicits the withdrawal reflex?

Pain receptors

What muscle spindles get activated in the withdrawal reflex?

The ones that require the removal of body parts from the painful stimulus

What is typically the category of muscles that get excited during the withdrawal reflex?

Flexor muscles of the limbs

Where do the sensory fibers that carry the signals for reciprocal inhibition of the withdrawal reflex usually terminate?

The pool of interneurons in the spinal cord

What do the interneurons of reciprocal inhibition of the withdrawal reflex usually provide?

Exitatory input to the appropriate ventral horn motor neurons.

How long does the crossed extensor reflex take to activate?

About 0.2 to 0.5 seconds after the stimulus

What is the method of reflex of the crossed extensor reflex?

A flexor reflex in one limb while the opposite limb begins to extend.

What does the crossed extensor reflex often occur in conjunction with?

The withdrawal/flexor reflex

What is the cords righting reflex?

This is when a cervical cord transection occurs in an animal. The animal is placed on its side and it tries to raise itself in standing position, and the maneuver is rarely successful.

What does the cords righting reflex indicate?

That circuits intrinsic to the spinal cord are capable of generating movements in a single extremity, pair of extremities, or all four extremities.

What connections does the righting reflex circuitry involve?

Flexor and extensor motor neurons in a single cord segment, across the midline, and rostrally and caudally through gut propriospinal system.

What can help facilitate or depress the synapses in reflex arcs?

Descending synaptic input from the brain.

What is the cause of spinal shock?

When the spinal cord is transected.

What happens to the cord functions below the transected cord in spinal shock?

All functions become substantially depressed.

How long can conditions of spinal shock persist? What happens to the excitability of spinal neurons in this time?

Hours, days or weeks.

The excitability of spinal neurons dramatically reduce the loss of all descending projections.

How do the affected neurons of spinal shock gradually regain their excitability?

They reorganize and adapt to the new levels of synaptic input.

What happens when the affected neurons of spinal shock become hyper excitable?

They produce motor abnormalities such as spasm, tremor, cramping and they may be difficult to overcome.

What are 3 common symptoms of spinal shock?

1- Arterial blood pressure may fall significantly (rapid to recover if there is no structural damage to the sympathetic nervous system).

2- All skeletal muscle reflexes become nonfunctional.

3- Sacral autonomic reflexes that regulate bladder and bowel movement may be suppressed for several weeks.

What can happen to skeletal muscle reflexes if the transection is incomplete?

Some descending pathways that remain intact can cause some reflexes to become hyperactive.

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