2.1.2 Spinal Reflexes Flashcards
Term: an alpha motor neuron and all of the muscle cells it innervates
Motor Unit
How many alpha motor neurons innervate a given muscle cell?
Only ONE
With that being said, alpha motor neurons can innervate multiple muscle cells
What is the innervation ratio? What are the characteristics of high and low ratios?
of muscle cells / alpha motor neuron
Highly variable and dependent on the region of the body:
- Small ratio: fine control, capable of small loads
- Large ratio: coarse control, capable of large loads
What are the two mechanism by which the motor neuron force can be increased?
- Increased force with increased firing rate of a single motor neuron
- Increased force with an increase in number of motor neurons firing
What are the two types of muscle receptors and what do they sense?
Primary muscle spindle affferents: muscle position and velocity
Golgi Tendon organ: tension at tendon
What is referred to as the final common pathway?
The motor unit
Motor units are referred to clinically as ____
Lower motor neurons
What is meant by the motoneuron and muscle cell synapse is obligatory?
That every action potential of the motoneuron causes in an action potential in every muscle cell innervated by that motoneuron
Term: all motor neurons that innervate a single muscle
Motor neuron pool
What is a motor unit?
: an alpha motor neuron and all of the muscle cells it innervates
Every motor neuron AP causes?
-an AP in every muscle cell innervated (due to safety factor)
What is the motor unit pool and where are they located?
- all motor neurons that innervate a single muscle
- Occupy localized longitudinal columns in the ventral horn of the spinal cord
What type of pattern does the motor neuron pool exhibit in the ventral horn?
- Exhibit a medial to lateral organization in the ventral horn
- Distal muscles: dorsolateral
- Proximal/Axial muscles: ventromedial
Increasing firing rate of a motor neuron will?
Increase force by increasing firing rate of an individual motor unit
How will fast rate compare to slow rate in AP firing on a motor?
-Frequency of APs determines force generated by muscle
- -Slow rate: muscle has time to relax between APs
- -Fast rate: tension does not reach baseline before next AP
- Firing rate will eventual reach a maximum force capable of being generated
- -Termed “tetanic tension” (typically 50-60hz)
What is the other way to increase motor neuron force other than increasing firing rate?
Increase force by increasing number of motor units firing
- Can increase number of motor units via recruitment
- Small motor units are recruited first, followed by large
- -Low tension = small units only
- -High tension = large units activated also
Describe the motor neuron property of orderly recruitment by size. How will this change with an external stimulus?
- Motor unit will only be activated when a recruitment threshold (tension) is reached
- Small motor neurons are activated first because their recruitment threshold is easier reached due to a smaller resistance
- -I.e. for the same synaptic current, depolarization of the small cell will be greater than in the larger (more resistant) cell
- -EXTERNAL stimulation (electrodes) will activate large motor units first
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What is the pathway of Axon projections of Ia afferent fibers?
What is a spinal reflex?
- Involuntary response to stimuli
- Automatic and sterotyped
- Does not require cognitive input
- Simplest form of neural integration in the nervous system
What are the components of a spinal reflex?
- Components:
- Receptor
- Interneuron
- Effector Organ: skeletal muscle, smooth muscle, or gland
- -Somatic Reflexes: skeletal muscle
- -Autonomic Reflexes: smooth muscle and glands
Abnormal reflexes are?
characteristic of pathologies and CNS damage
What is a stretch reflex?
- Deep tendon monosynaptic reflexes
- Function to help maintain tone and set background tension of postural muscles needed for the performance of voluntary movements
What are the two components of a stretch reflex?
- Phasic – corresponding to dynamic component of stretch
- Tonic – corresponding to static phase of stretch
What is the sequence of events for a stretch reflex?
- 1) Tendon of a muscle is tapped, causing it to stretch
- 2) Sensory endings (1a and II axons) in muscle spindle are stimulated
- 3) Central afferent processes synapse in the spinal cord onto motorneurons of the corresponding stretched muscle
- 4) Excitatory synapses excite the motorneurons, causing muscle contraction
What is the Jendrassik maneuver?
A procedure to heighten stretch reflexes. Patient links fingers of the hand together and makes a slight outward pull. It functions to disinhibit the segmental pool of inhibitory neurons.
What are the characteristics of the phasic component of the stretch reflex? What do the type Ia afferents sense? What do the type II afferents sense?
What are the characteristics of the tonic component of the stretch reflex?
What is the principle of recriprocal innervation?
- Contraction of a muscle is accompanied by inhibition of its antagonist
- Ia fibers make monosynaptic excitatory connections with homonymous motor neurons
- -On the same muscle that was tapped
- Branches of the Ia fibers excited inhibitory interneurons that inhibit motor neurons of antagonistic muscles
- -Relieves inhibition of muscle jerk in response to tap
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What is an inverse stretch reflex?
- Opposite of the stretch reflex – here muscle stretch DECREASES muscle contraction
- Activated by stretching a rigid or spastic muscle
What are the two phases of the inverse stretch reflex?
- Contraction initially increases because of the stretch reflex (lengthened muscle ⇒ increased Ia activation) until a threshold is reached
- At threshold, rigidity “melts away” and limb extends freely as inverse stretch reflex occurs
What is the sequence of events of an inverse stretch reflex and what is it mediated by?
- Mediated by GTO and Ib afferents
- Sequence of events
- Stretch of rigid/spastic muscle ⇒ increased muscle tension ⇒ increased Ib afferents
- At threshold, inhibitory interneuron is switched on
- Inhibition of alpha mn ⇒ decreased muscle contraction ⇒ muscle lengthens
How does the muscle spindle compare to the golgi tendon reflex?
What is muscle tone?
- state of contraction of muscle, dependent on the excitability of alpha mns
- -Assessed clinically via the deep tendon reflexes
What are the characteristcs of Lower motor neuron lesions?
1) Lower Motor Neuron Lesions ⇒ Hypotonia/Hyporeflexia
- Diminished deep tendon reflexes, flaccid muscles
- Early stage spinal cord transection
* -Alpha motorneuron excitability is diminished
- Following peripheral nerve injury (ventral horn)
What are the characteristics of upper motor neuron injury?
1) Upper Motor Neuron Lesions ⇒ Hypertonia/Hyperreflexia
- Heightened deep tendon reflexes, rigid muscles (spasticity/clonus)
- Occurs following upper motor neuron lesion
- Inhibition to lower motorneurons is lost
What is rigidity associated with hypertonia/hyperreflexia?
-Sustained contraction at rest (in the absence of voluntary movement)
- -Increased tonic stretch reflex (tension when holding muscle constant)
- -Increased muscle stiffness in some cases
-Examples: decerebrate rigidity, Parkinson’s
What is spasticity associated with hypertonia/hyperreflexia?
Spasticity
-Little or not contraction at rest
- -Increased phasic stretch reflex (tension when moving muscle)
- -Resistance to stretch is proportional to velocity of stretch ⇒ faster a spastic muscle is stretch, the more it resists
-Examples: late phase spinal cord transection, lesions in motor cortex, decerebrate rigidity
What is clonus associated with hypertonia/hyperreflexia?
Clonus
- Rhythmic series of contractions brought about by sustained stretch of a spastic muscle
- Sequence of events:
- -Quick stretch of spastic muscle ⇒ intense burst of Ia afferents ⇒ activation alpha MN
- -Reflex contraction ⇒ muscle shortening ⇒ spindle activity decreases
- -Removes excitation of alpha mns ⇒ decreased contraction ⇒ rebound lengthening
- -Spindle stretch ⇒ Ia afferent burst ⇒ contraction
- -Cycle repeats
-Clonic cycle is interrupted only by holding joint in a constant position
What are the characteristics of a flexion (withdrawl) reflex?
-Initiated by cutaneous stimuli
- Functions to remove body from potentially damaging stimuli
- Protects against slow transmission of noxious information to CNS via C fibers
- Uses flexor reflex afferents (FRAs)
What is the crossed-extensor reflex?
- Contralateral components of the flexion reflex
- Helps to maintain balance
- Example:
- -Right leg steps on something sharp ⇒ initiates flexion to withdrawal leg
- -Excitation of contralateral extensor motor neurons ⇒ weight supported on left leg
-Reflexes can be contralateral – sensory input must cross the midline in spinal cord
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Why can ballerinas stand on their toes for so long!?!?!?
