70 - Spinal Mechanisms of Motor Control Flashcards
Muscle spindles
Muscle spindles are sensory receptors within the belly of a muscle that primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, by activating motoneurons via the stretch reflex to resist muscle stretch.
Location of muscle spindles
Parallel to the extrafusal muscle fibers
Extrafusal muscle fibers
Extrafusal muscle fibers are the skeletal standard muscle fibers that are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement.
Intrafusal muscle fibers
Intrafusal muscle fibers are skeletal muscle fibers that serve as specialized sensory organs (proprioceptors) that detect the amount and rate of change in length of a muscle.[1] They constitute the muscle spindle and are innervated by two axons, one sensory and one motor. Intrafusal muscle fibers are walled off from the rest of the muscle by a collagen sheath. This sheath has a spindle or “fusiform” shape, hence the name “intrafusal.”
Three types of intrafusal muscle fibers
- Dynamic nuclear bag
- Static nuclear bag
- Nuclear chain fibers
Sensory innervation of muscle spindles
Primary ending and secondary endings are present
Primary ending = Group Ia fibers
Secondary ending = Group II fibers
Primary ending - Group Ia fiber
- One per muscle spindle
- Found on both nuclear bag & chain fibers
- Shows both dynamic & static response
• Static = firing pattern signals amount of stretch or muscle length
• Dynamic = firing pattern signals rate at which muscle is being stretched or rate of changes in muscle length
Secondary ending - Group II fiber
- Found on nuclear chain fibers ONLY (also on static nuclear bag)
- Shows static response ONLY ***
- Firing rate gives info about muscle length but does not emphasize changes in muscle length
Motor innervation of muscle spindles
a. Gamma efferent fibers – provide fusimotor drive to muscle spindles
b. Skeletofusimotor fibers - some motor neurons innervate both extrafusal and intrafusal fibers; sometimes called beta innervation.
Fusiform organ and fusimotor innervation
The muscle spindle is sometimes called the fusiform organ and the gamma motor axons that innervate it can be called fusimotor innervation
Golgi tendon organ (GTO)
GTO senses changes in muscle tension. It is a proprioceptive sensory receptor organ that is at the origins and insertion[1] of skeletal muscle fibers into the tendons of skeletal muscle. It provides the sensory component of the Golgi tendon reflex.
Golgi tendon organ innervation
GTO is innervated by a single group Ib fiber
Where is the GTO located in the muscle fiber?
Located in series with muscle
Alpha motor neurons
- Large lower motor neurons of the brainstem and spinal cord. - They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction.
- Alpha motor neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles
Alpha motor neuron characteristics
- Innervate extrafusal motor neurons
- Large neurons
- Heavily myelinated
- Rapid transmission
- Responsible for initiating contraction
Gamma motor neuron characteristics
- Innervate intrafusal motor neurons
- Slender neurons
- Lightly myelinated
- Less rapid than alpha
- Control skeletal muscle by adjusting tension
What is the function of beta motor neurons (skeletofusimotor neurons)?
Beta motor neurons innervate intrafusal fibers of muscle spindles with collaterals to extrafusal fibers.
Be able to explain the function of coactivation of alpha and gamma motorneurons.
a. When extrafusal muscle fibers contract, the muscle shortens and muscle spindles would become unloaded (not able to sense a small lengthening of the muscle).
b. To prevent unloading, gamma motor neurons are activated and they make intrafusal fibers contract. Now a small lengthening of the muscle can be sensed immediately.
c. In intact animals, Ia fibers normally facilitate alpha motor neurons during muscle contraction. This facilitation would be lost if muscle spindles became unloaded during muscle contraction.
Static response of muscle spindles
Firing pattern signals amount of stretch or muscle length
What type of fibers carry information regarding the static response of muscle spindles?
Both type Ia and II
Dynamic response of muscle spindles
Firing pattern signals rate at which the muscle is being stretched or the rate of change in muscle length
What type of fibers carry information regarding the dynamic response of muscle spindles?
Type Ia ONLY
Be able to describe the role of the gamma motorneuron in controlling spindle response.
The firing rate of muscle spindle afferents depends on both muscle length and the level of gamma activation of the intrafusal muscle fibers.
When the nervous system
interprets the signals from muscle spindle it must take into account the amount of fusimotor drive.
Be able to describe how muscle spindles signal changes in muscle length
Muscle spindle functions:
i. Sensory input for stretch reflexes
ii. Sensory input to cerebellum
iii. Muscle spindle output relayed to cerebral cortex for conscious proprioception
iv. RESULT = Change in muscle length (via gamma motor neuron activation)
Be able to draw the neuronal circuit for the stretch reflex (or myotatic reflex)
See photo on phone
What’s another name for a tendon stretch reflex?
Myotatic reflex
What is a phasic stretch (myotatic) reflex?
The type of reflex initiated by tapping the tendon with a reflex hammer
What type of fibers are involved
ONLY Ia fibers
Describe the role of Ia fibers in the phasic stretch reflex
Ia fibers synapse directly on alpha motor neurons of homonymous and synergistic muscles
Describe the clinical relevance of a phasic stretch reflex
- Sometimes called DTR, Tendonr Jerk Reflex or Muscle Jerk Reflex
- MSR = Muscle Stretch Reflex
- Clinician must decide if the reflex is:
- Hypoactive – may be reduced or even absent
- Normal - Hyperactive – elevated response
- Hypoactive – may be reduced or even absent
- Can demonstrate evidence of LMN vs UMN problems
- Jendrassik maneuver sometimes used to make stretch reflexes of leg more visible
What is a phasic stretch (myotatic) reflex?
The type of reflex that can be initiated by passively bending a joint in some individuals
What type of fibers are used by the tonic stretch reflex?
BOTH Ia fibers and type II fibers
How are type Ia fibers used in the tonic stretch reflex?
Synapse directly on alpha motor neuron of homonymous & synergist muscles
How are type II fibers used in the tonic stretch reflex?
Synapse directly on alpha motor neurons of homonymous muscle
Describe the clinical relevance of the tonic stretch reflex
NOT noticeable in neurologically normal patients
- In patients w/ spasticity, the strength of the reflex in spastic muscles depends on the speed of muscle stretch.
- The faster the stretch, the stronger the reflex
- Ex. The tonic reflex is exaggerated in Parkinsonism
Reciprocal innervation and inhibition
Reciprocal innervation describes skeletal muscles as existing in antagonistic pairs, with contraction of one muscle producing forces opposite to those generated by contraction of the other. For example, in the human arm, the triceps acts to extend the lower arm outward while the biceps acts to flex the lower arm inward. To reach optimum efficiency, contraction of opposing muscles must be inhibited while muscles with the desired action are excited. This reciprocal innervation occurs so that the contraction of a muscle results in the simultaneous relaxation of its corresponding antagonist.
Be able to describe reciprocal innervations in the spinal cord. Be able to relate this concept to the stretch reflex.
Group Ia fibers excite homonymous muscle (agonist muscle) & its synergists (i.e. flexors)
Group Ia fiber collaterals end on Ia inhibitory interneurons
- These neurons inhibit the alpha motor neurons on the antagonist muscles (i.e. extensors)
Which stretch reflex is used to determine muscle tone?
Tonic stretch reflex
Describe the process of testing the tonic stretch reflex
Clinically, this reflex is used to measure muscle tone, the resistance of a relaxed muscle to passive stretch. The patient is told to relax a limb. The examiner then passively flexes or extends a joint. The muscle can be stretched at various speeds. Fast stretch is necessary to demonstrate some pathological conditions (Spastic limbs show a velocity-dependent increase in tonic stretch reflexes).
Describe co-contraction
- Co-Contraction of muscles at a joint means that both the prime mover at a joint and its antagonists are contracted simultaneously.
- This action stiffens the joint
- Co-contraction results from Co-activation of agonist and antagonist muscles at a joint. Coactivation and co-contraction are often used interchangeably.
- Term coactivation is also used in term alpha-gamma coactivation where it means the simultaneous activation of alpha and gamma motor neurons
When would co-contraction be useful?
In order to stabilize the joint
What is the Ib reflex? What is another name for it?
Golgi tendon organ reflex or Group Ib reflex – a disynaptic inhibitory reflex that can cause inhibition of the GTO’s muscle (autogenic inhibition) and excitation of its antagonist
What is the result of a normal Ib reflex?
Result of reflex - increased tension in muscle causes muscle to be inhibited. Decreased tension in muscle (example, as the result of fatigue) causes inhibition by Ib interneuron to decrease.
So GTO system can be a feedback system that monitors and maintains muscle force
Be able to draw the neuronal circuit that includes the Ib interneuron
See print out
Flexor withdrawal reflex
A LOCAL sign
- Example: when you step on a tack and you immediately flex your foot upward and away from the stimuli to avoid harm
Crossed extension reflex
Some authors consider part of flexion withdrawal reflex, others consider it to be an associated reflex
- Go back to the example about stepping on a tack
- The flexor withdrawal reflex is to pull your foot away
- The crossed extension reflex is to contract the muscles on the other side of the body so you don’t fall over
This contralateral stimulation of motor neurons to stabilize the body is called the crossed extension reflex, and is a result of the withdrawal reflex (usually in the lower extremities)
Be able to compare and contrast the output of a muscle spindle and a Golgi tendon organ during muscle contraction and passive muscle stretch
Muscle spindles
- Found within muscles parallel to the extrafusal muscle fibers
- Measure muscle length
Golgi tendon organ (GTO)
- Found between muscle and tendon (in series with muscle)
- Measures muscle tension
Describe the feedback system for regulating muscle tension
Ib Reflex Circuit
- Result of reflex - increased tension in muscle causes muscle to be inhibited.
- Decreased tension in muscle (example, as the result of fatigue) causes inhibition by Ib interneuron to decrease. - So system can be a feedback system that monitors and maintains muscle force.
Clasp-knife response
- Most easily demonstrated in the knee extensors of spestic patients
- Clinician tries to quickly bend the patient’s knee
- Resistance to flexion seems to build up gradually and then at a certain point, resistance suddenly decreases.
- The hyperactive stretch reflexes of the spastic patient causes the increase in resistance to stretch found in the knee extensors.
- The sudden decrease is the clasp-knife phenomenon.
- The biceps is an anti-gravity muscle in humans and the clasp-knife reflex can be demonstrated in a spastic biceps when you try to extend the elbow.
What type of patients will show the clasp-knife response?
A spastic limb might show the clasp knife response
Homonymous muscle
Homonymous muscle – the working muscle of the motor unit (i.e. flexors)
Agonist muscle
Agonist muscle – muscles that perform the same/similar function to the homonymous muscle (i.e. flexors)
Synergist muscle
Synergist muscle – muscles that perform the same/similar function to the homonymous muscle (i.e. flexors)
Antagonist muscle
Antagonist muscle – muscles that perform the opposite function to the homonymous muscle (i.e. extensors)
Hypertonia
- Exaggerated reflex
- Several different types; most common is spasticity
Atonia
NO muscle tone (found during spinal shock)
Hypotonia
Weak reflex - Found with pyramidal tract lesions (in monkeys) and cerebellar lesions
Spasticity
A motor disorder that can be caused by brain trauma (including cerebral palsy), spinal cord injury, multiple sclerosis (variable)
Characteristics of spasticity
- A velocity-dependent increase in tonic stretch reflexes
- Resistance to tonic stretch increases as speed of stretch increases
- An increase in muscle tone - A type of hypertonia
In general, spasticity is more prominent in which muscles?
Antigravity muscles
- Biceps
- Quadriceps
Fusimotor neuron
Gamma motor neuron
