Spinal Reflexes Flashcards
Components of Muscle Spindle
- muscle spindle senses muscle length and velocity.
- Stretch receptor w/ specialized muscle fibers
- connective tissue capsule surrounding middle 1/3 fibers.
- fibers of the spindle are intrafusal fibers.
- function is to act as sensors.
- two types of intrafusal fibers
1. Nuclear bag fibers:- nuclei clustered in center of fiber in a bulging bag area.
- sensitive to both changes in muscle length and velocity.
- Nuclear chain fibers
- nuclei arranged in single file chain near center of fiber.
- only sensitive to changes in muscle length
- do not signal changes in muscle velocity.
- Intrafusal fibers located in the surrounding extrafusal skeletal muscle fibers.
- intrafusal fibers are parallel to extrafusal fibers.
- ends of intrafusal fibers anchored to extrafusal fibers.
- When muscle is stretched, muscle spindle fibers also stretched
components of a reflex and the neural pathways involved.
- reflex is an involuntary stereotyped response to a sensory input.
- Involuntary, stereotypical response
- involves activation of peripheral sensory receptors, alpha motor neurons & skeletal muscle
- The spinal reflex is an independent circuit modulated by supraspinal and segmental input.
- spinal reflexes can operate w/o normal descending control
- leads to abnormal muscle tone.
- Reflexes used clinically to assess integrity of the spinal cord & its peripheral connections
Muscle Spindle Fiber Innervation
- afferent innervation: group Ia and II fibers.
- Ia fibers innervate all intrafusal muscle fibers (bag and chain).
- detect and signal both muscle length and muscle velocity.
- Group II fibers innervate nuclear chain fibers & possibly some of the nuclear bag fibers.
- detect and signal only muscle length.
- don’t make a significant contribution to the myotatic reflex
- motor innervation consists of a gamma motor axons.
- small multipolar motor neurons in anterior horn of spinal cord
- axons exit spinal cord in ventral root along w/ α-motor axons
- run in spinal nerves & innervate contractile ends of intrafusals
- There are several types
- innervate the distal ends of the intrafusal fibers
- regulate the sensitivity of the muscle spindle
- prevent loss of signal when muscle is voluntarily contracted.
Function of Muscle Spindle
-Muscle stretch: extrafusal & intrafusal fibers also lengthened.
*distorts primary ending & it sends a volley of APs to the CNS
-Increase in firing is most while the muscle is changing length
*signals the velocity of lengthening.
-Muscle attains new length
*the firing decreases but remains elevated
*signals the new, longer muscle length.
-Muscle contraction: length of the extrafusal fibers decreases
*intrafusal fibers would become slack or flaccid.
*1˚ endings would no longer be distorted & would cease firing
*Would deprive CNS of info about muscle’s length & velocity.
-alpha and gamma motor neurons co- activate to prevent the loss of afferent information
-Co-activation causes intrafusal fibers to shorten along with the extrafusal fibers.
*maintains intrafusal tension & distortion of the primary endings
-1˚ ending continues to send length & velocity to CNS during the contraction.
-gamma motor neuron activity referred to as gamma bias/gain
*adjusted by UMN pathways & local reflex activity.
Higher gamma bias or gain = more sensitive muscle spindle.
Components of Golgi Tendon
- bulb-shaped force/tension receptor
- found a muscle-tendon junctions
- narrow, encapsulated structure 1mm length & 0.1mm diameter
- Consists of finely braided collagen fibers
- links the distal ends of the muscle fibers to the tendon.
- Connected in series with the extrafusal fibers
Innervation of Golgi Tendon
- innervated by a single Ib afferent axon.
- The Ib afferent branches into many fine, endings
- Lose myelin & intertwine in braided collagen fibers in capsule.
- Increased muscle tension from stretch or contraction activates the Ib afferent.
- tension straightens braided collagen fibers
- distort the intertwined Ib nerve endings.
- distortion opens mechanically activated membrane ion channels
- producing depolarization.
- If depolarization reaches threshold, APs are initiated
- propagated to the CNS.
- Action potential frequency is correlated with muscle tension.
Function of Golgi Tendon
- responds primarily to increases in tension by muscle contraction.
- Less sensitive to tension from passive stretch
- b/c of the mechanical properties of its collagen fibers.
neural circuitry of the myotatic reflex.
- monosynaptic reflex involving 2 neurons (afferent & efferent) & a single intervening synapse.
- The reflex is initiated by muscle stretch.
- Group Ia endings in muscle spindle are stimulated
- Ia fibers run in the dorsal root to the spinal cord ventral horn
- synapse directly on a-motor neurons in the ventral horn.
- a-motor neurons innervate extrafusal fibers of the muscle containing the stimulated muscle spindle
- cause contraction of the extrafusal muscle fibers.
- Muscle spindles, since they lie in parallel with the extrafusal fibers, are passively shortened or “unloaded”.
Example of Myotatic Reflex: The Knee Jerk:
Tapping the patellar tendon (below kneecap)
- stretches the quadriceps
- excites the quadriceps muscle spindle primary endings
- increases firing of Ia afferent fibers
- monosynapticly activates the alpha-motor neuron for quadriceps
- alpha-motor nerve impulses course in the femoral nerve to quadriceps
- contraction of quadriceps muscle
- decreases firing of Ia afferent fiber
Reciprocal Inhibition of Antagonist
- Ia afferent fiber from the agonist muscle sends a collateral excitatory synapse w/ inhibitory interneuron.
- allows inhibition of the alpha motor neurons of the ipsilateral antagonist muscle
- easier to shorten muscle if ipsilateral agonist muscle excited & ipsilateral antagonist muscle inhibited
- reflex results negative feedback loop utilized to resist changes in muscle length.
- Afferents from muscle spindles also branch to relay neurons that give rise to the spinocerebellar tracts.
- tracts ascend to the cerebellum for postural adjustments
3 functions of the Golgi Tendon Organ
- Regulation of muscle tension
- Equalization of muscle fiber tension
- Coordination of voluntary and reflexive movements.
Regulation of muscle tension by Golgi Tendon
- produces a negative feedback system
- regulates amount of tension produced by the muscle.
- When muscles contracting, GTO is firing relative to amt of tension produced by the agonist muscle
- If force in agonist muscle unexpectedly increases, its GTO firing rate also increases.
- sends volley of APs up GTO’s Ib afferents toward spinal cord
- Excitatory synapse on Ib inhibitory interneuron increases its firing rate.
- sends volley of APs up GTO’s Ib afferents toward spinal cord
- Inhibitory synapse of Ib interneuron on á-motor neuron of agonist
- Decreases firing rate & decreases muscle tension.
- provides negative feedback system
- keeps muscle tension constant
- often called the “inverse myotatic reflex”
- Net effect is to dampen the tendency of moving limb parts to oscillate back and forth.
2. Excitatory synapse on an excitatory interneuron increases its firing rate. - Interneuron excites á-motor neuron of the antagonist muscle
- increases antagonist á-motor neuron firing rate
- increases the antagonist’s tension.
- This increase in antagonist tension augments the decrease in agonist tension
Equalization of Muscle Fiber Tension by Golgi Tendon
- Fibers that exert excess tension are inhibited by the reflex
- fibers that exert too little tension are more excited by the absence of reflex inhibition
- allows the spread of muscle load over all the fibers.
Coordination of Voluntary and Reflex Movements by Golgi Tendon
- inhibitory interneurons receive modifying inputs from:
- cutaneous receptors, joint receptors, muscle spindles
- descending pathways such as the corticospinal and reticulospinal tracts
- Allows a range of actions like picking up a can to crushing a can
Flexor Withdrawal Reflex
- initiated by cutaneous receptors (noxious stimuli)
- produces a general flexion of the entire limb.
- Ex. withdrawal of extremity using flexor muscles after touching a hot stove or stepping on a sharp object
- magnitude of the reflex varies depending on the intensity of the stimulus and the muscles involved
- Because the flexor reflex involves an entire limb, its pathway must spread over several spinal segments
- ascending and descending collaterals of the primary sensory fibers and spinal interneurons.
