Spinal Cord Motor Activity
Classification of Reflexes "
1- Somatic reflexes : that you are aware of them
2- Autonomic Reflexes : that control visceral organs.
Examples of spinal reflexes, involving spinal nerves and the spinal cord, include:
1- extensor reflex: leg proprioceptors trigger limb extension
2- myotatic (stretch) reflex: muscle stretch is resisted by reflex contraction of the muscle
3- withdrawal (flexion) reflex: limb flexes to withdraw from a noxious stimulus
A spinal reflex is a stereotyped, automatic motor reaction to an input signal. The monosynaptic myotatic stretch reflex is the most crucial reflex for the maintenance of the erect body posture in humans. For the well-known knee-jerk response, there is latency of around 30 ms between striking (stretching) the tendon of the quadriceps and the muscle contraction. The reflex involves a number of segments in the SC.
Factors that contribute to this latency in the reflex arc are:
1- Speed of transduction at the sensory receptor. This is most rapid when the receptor is spontaneously active and is tuned to the dynamic range.
2- Conduction speed in afferents to the CNS. Speed depends on fiber size and myelination. Larger fibers conduct more rapidly, but at the expense of space.
3- Synaptic delay and number of synapses involved in the pathway. While the interval between arrival of the presynaptic AP and start of EPSP is typically 0.5 ms, it takes a few ms before an AP is evoked in the postsynaptic neuron.
4- Central integration.
a. Spatial summation. 90% of the synapses are on the MN dendrites.
The remaining 10% are on the soma, which have the highest priority. The unitary EPSP at a single synapse is about 0.2 mV in amplitude. Depending on the MN, sufficient of these must be activated to cause a depolarization of 5-10 mV at the axon hillock and thereby evoke an AP.
b. Temporal summation results from activity arriving at different
latencies. Temporal summation is very dependent upon the passive membrane properties (time constant etc.) of the MN.
5- Speed of conduction in the motor axon efferents (size and myelination).
NOTE:
1- Reflex responses are determined by interneurons which “hard-wire” afferent input to efferentoutput. Interneurons organize efferent neurons (motor units) into meaningful movement components, which can be utilized by either spinal input or descending pathways.
2-Since "voluntary movement" and "involuntary reflex/reaction" compete for control of the same interneurons circuits, they cannot be independent on one another. Thus, brain activity will influence spinal reflex responses, making reflex evaluation an interpretive art. (see the end of this lecture)
Diagram of A reflex Arc
Withdrawal Reflex (Flexion Reflex) and Crossed Extensor Reflex
The po1ysynaptic flexor reflex serves important protective functions. One of its purposes is to achieve a rapid withdrawal of a limb in response to painful cutaneous stimuli. To maintain position the flexor withdrawal reflex is usually accompanied by extension of the opposite limb through action of the crossed extensor reflex. Receptors: free nerve endings in the skin. Afferent arch : Adelta and C fibers which terminate in the marginal zone (Lissauer) and in the dorsal part of the central gray matter. Central mechanism: the central processes of the primary sensory neurons synapse with interneurons and funicular neurons that in turn innervate ipsilateral flexor and crossed extensor muscles.
Like the other reflex pathways, interneurons in the flexion reflex pathway receive converging inputs from several different sources, including cutaneous receptors, other spinal cord interneurons and descending pathways. Although the functional significance of this complex pattern of connectivity is uncertain, changes in the character of the reflex following damage to descending pathways provide a clue. Under normal conditions, a noxious stimulus is required to evoke the flexion reflex; following damage to descending pathways, however, other types of stimulation, such as moderate squeezing of a limb, can produce the same response. Thus, the descending projections to the cord may function, at least in part, to gate the responsiveness of interneurons in the flexion reflex pathway to a variety of sensory inputs.
Features of the reflex (look at the diagrammed on the next page) include
1- primary afferent neuron (1) participates in both reflexes (2) and ascending pathways (3)
2- divergent interneuronal circuit propagates to several segments and right and left sides (B)
3- positive feedback prolongs the reflex beyond the time of the stimulus (A)
3- individual interneurons are either excitatory or inhibitory (black cells) in their effect;
4- antagonists are inhibited while agonists are excited ( reciprocal innervation) (D)
5- descending pathways (C) modify reflex circuit (reflex is not independent of brain control)
Diagram of Flexion (Withdrawal) Reflex & Crossed Extensor Reflex
Myotatic Stretch (Proprioceptive) Reflexes: determines muscle length
Muscle spindles are:
1• elaborate proprioceptors positioned in parallel with muscle fibers;
2• designed to signal muscle length.
Morphologically, a muscle spindle consists of a connective tissue capsule enclosing:
1- two kinds of mechanoreceptors,
2- two kinds of intrafusal muscle fibers,
3- two kinds of gamma efferent neurons.
Intrafusal muscle fibers:
vs. extrafusal (typical) muscle fibers
1• very small, anchored in endomysium
2• do not contribute anything to whole muscle tension
3• center of each fiber is packed with nuclei & lacks myofilaments
4• polar regions are striated and innervated by gamma neurons
5• two kinds of intrafusal muscle fibers:nuclear bag fibers— central region is dilated; fiber extends beyond the capsule; nuclear chain fibers— smaller, central region contains chain of nuclei.
Mechanoreceptors within muscle spindle :
They are activated by stretch of the central region, which is stretched either
1) by contraction of polar regions of intrafusal muscle fibers, or
2) by passive stretch of the whole muscle (including the intrafusal fibers)
Monitoring the State of the Muscle
1. The CNS receives information about the state of the muscle from two receptors within the muscle itself.
2.The muscle spindle provides information about the length of the muscle.
3.The Golgi Tendon Organ signals changes in muscle tension.
The Muscle Spindle
1. Muscle spindles are distributed throughout the fleshy part of the muscle and runparallel to the individual muscle fibers.
2. Each encapsulated spindle contains:
a. A group of small specialized muscle fibers called intrafusal fibers
b. Sensory or Afferent axons
c. Motor or Efferent axons
3. The intrafusal fibers do not contribute to force production which distinguishes themfrom skeletal muscle fibers called extrafusal fibers.
a. Intrafusal fibers include Static and Dynamic Nuclear Bag fibers as well asNuclear chain fibers.
4. There are two sensory axons that wrap around the intrafusal fibers.
a. The primary ending is a group Ia axon wrapped around the nuclear
bag andchain fibers of the spindle.
b. The secondary ending is a group II axon wrapped around the static
nuclearbag fiber and the nuclear chain fibers but NOT the dynamic nuclear bag fiber.
c. When the muscle is stretched, the intrafusal fibers are elongated which causesthe primary and secondary endings to depolarize.Stretch of intrafusal fiberscauses increased firing rate of the afferent axons.
d. The primary endings are sensitive to the rate of change in muscle length whichis referred to as velocity sensitivity. Higher firing rates of the primary endingsoccur during faster stretches.
5. We can see the differences in the primary and secondary endings by recording their firing rates during various types of stretches. A linear stretch increased the firing rate of both primary and secondary endings. A brisk tendon tap only increases the firing rate of the primaryending. This indicates that the primary endings are not only sensitive to the length ofthe muscle but also to the rate of change of the length. Primary nerve endings areespecially sensitive to very small stretches.
6. If an intermittent stretch in the form of vibration is applied to the muscle, only the
primary afferents have an increase in firing rate. The intermittent stretch is
occurring too fast to affect the steady state firing of the secondary endings.
7. The motor endings regulate the sensitivity of the muscle spindle. Principles of
Gamma Activation
a. Axons from gamma motor neurons in the ventral horn of the spinal cordterminate near the ends of the nuclear bag and chain fibers where the
contractile elements are located.
b. Input from the gamma motor neuron stimulates a contraction at the ends ofthe intrafusal muscle fibers which causes them to become more taut.
c.The more tight or stretched the intrafusal fibers, the higher the firing rateof the sensory axons thus increasing spindle sensitivity.
The muscle spindle is also sensitive during muscle contraction
1. It is clear that the muscle spindle is capable of conveying information to the CNSabout the muscle when it is stretched. However, during states of muscle contractionthe muscle spindle would be rendered incapable of assessing the status of the musclebecause the intrafusal fibers would be put on slack.
2. The CNS compensates for this through a process called alpha-gamma coactivation.The CNS stimulates alpha and gamma motor neurons simultaneously.
3.The extrafusal fibers contract due to firing of the alpha motor neuron. Theintrafusal fibers are prevented from going slack because the gamma motorneurons cause the intrafusal fibers to contract and remain tight.
4. Alpha-gamma coactivation is an important component of normal movement because itenables the muscle spindle to convey information about the rate of change of themuscle length. The CNS can then adjust or correct the movement trajectory
Principles of Gamma Activation
a. stimulation of gamma MNs (motor neurons) results in contraction of the intrafusal fibersthereby tightening the muscle spindle and ensuring its sensitivity
b. gamma MNs are most responsive to descending input from the brain and showlittle or no response to peripheral input. This means that higher centers can control the sensitivity of the muscle spindle by activating or biasing the gamma MNs.
- alpha gamma coactivation occurs under most conditions but level of activitymay be higher in gamma MNs. This is called gamma biasing. Gamma biasing occurs in one of three ways based on type of gamma MNs there are two types of gamma MNs:
Static and Dynamic.
Dynamic supply the dynamic nuclear bag fiber and static supply the static nuclear bag fiber and the nuclear chain fibers static and dynamic gamma biasing means that both types of MNs areactivated before a movement which results in increased sensitivity of bothprimary and secondary endings dynamic gamma bias increases the sensitivity of only the dynamic nuclearbag fiber, therefore only the primary ending will be sensitive as we have already seen the primary ending detects rapid changes inlength and are especially sensitive to small length changes. Thus, dynamic gamma bias is best suited for maintaining spindlesensitivity during small postural adjustments.Static gamma bias: primarily the static gamma MNs are activated prior toand during a movement.because static gamma MNs supply most of the intrafusal fibers in thespindle, all but the dynamic bag fiber, they will affect the sensitivity ofboth primary and secondary endings.
static gamma bias keeps shortened spindles active during small lengthchanges as well as prevents their slackening during large changes.
The most famous stretch reflex is the quadriceps reflex (knee jerk reflex), produced by tapping the patellar tendon, which in turn stretches the quadriceps. The reflex is initiated by special muscle receptors called muscle spindles, which are sensitive to stretch. Muscle spindles are composed of 8-10 modified muscle fibers called intrafusal fibers arranged in parallel with the ordinary (extrafusal) fibers that make up the bulk of the muscle. Sensory fibers (Ia) are coiled around the central part of the spindle. Streching the muscle deforms the intrafusal muscle fibers, which lead to increased activity of the sensory fibers that innervate each spindle. The impulses are transmitted through Ia afferent fibers to the spinal cord, where the fibers establish synaptic contact with alpha motor neurons, which in turn produce contraction of quadriceps and extension of the leg at the knee. At the same time as the quadriceps contracts there is a reciprocal inhibition of the antagonistic muscles, the flexors of the knee. The inhibition of the flexors is mediated by polysynaptic reflex arcs, and since the motor neurons for the flexors are located in more caudal segments than the motor neurons for quadriceps, the inhibitory reflex is intersegmental, in contrast with the stretch reflex, which is intrasegmental (reciprocal innervation).
Borrowing a concept from engineering, the stretch reflex arc can be viewed as a negative feedback loop that tends to maintain muscle length at a constant value. The desired muscle length is specified by the activity of descending pathways that influence the motor neuron pool. Deviations from the desired length are detected by the muscle spindles; thus increases or decreases in the stretch of the intrafusal fibers change the level of activity in the sensory fibers that innervate the spindles. These changes, in turn, lead to appropriate adjustments in the activity of the alpha motor neurons, returning the muscle to the desired length.
The gain is adjusted by changing the level of activation of the gamma motor neurons. These small gamma motor neurons are interspersed among the alpha motor neurons in the ventral horn of the spinal cord. An increase in the activity of gamma motor neurons produces an increase in the amount of tension in the intrafusal fibers. Although the intrafusal fibers are much too sparse to generate a net increase in muscle tension, contraction of the intrafusal fibers increases the sensitivity of Ia sensory fibers to muscle stretch. The same stretch can then produce a larger amount of Ia afferent activity, which causes an increase in the activity of the alpha motor neurons that innervate the extrafusal muscle fibers.
Diagram of Stretch (Myotatic) Reflex
The Inverse Myotatic Reflex: limits the muscle tension
Another sensory structure that is important in the reflex regulation of motor unit activity is the Golgi tendon organ. Golgi tendon organs are encapsulated endings located at the junction of the muscle and tendon. Each tendon organ is related to a single group Ib sensory axon (the Ib axons are slightly smaller than the Ia axons that innervate the muscle spindles). In contrast to the parallel arrangement of extrafusal muscle fibers and spindles, Golgi tendon organs are in series with the muscle fibers. When a muscle is passively stretched, most of the change in length occurs in the muscle fibers, since they are more elastic than the fibrils of the tendon. When a muscle actively contracts, however, the force acts directly on the tendon, leading to an increase in the tension of the collagen fibrils in the tendon organ and compression of the intertwined sensory receptors. As a result, Golgi tendon organs are sensitive to increase in muscle tension that arise from muscle contraction and, unlike spindles, are much less sensitive to passive stretch.
The Ib axons from Golgi tendon organs contact inhibitory interneurons in the spinal cord (called Ib inhibitory interneurons) that synapse, in turn with the alpha motor neurons that innervate the same muscle. The Golgi tendon circuit is thus a negative feedback system that regulates muscle tension, decreasing the activation of muscles when exceptionally large forces are generated. This reflex circuit also operates at reduced levels of muscle force, counteracting small changes in muscle tension by increasing or decreasing the inhibition of alpha motor neurons. Under these conditions, the Golgi tendon system tends to maintain a steady level of muscle force, counteracting effects such as fatique, which diminishes muscle force. sIf the muscle spindle system is viewed as a feedback system that monitors and maintains muscle length, then the Golgi tendon system is a feedback system that monitors and maintains muscle force. Like the muscle spindle system, the Golgi tendon organ system is not a closed loop. Ib inhibitory interneurons also receive synaptic inputs from a variety of other sources, including cutaneous receptors, joint receptors, muscle spindles, and descending pathways. Together these inputs regulate the responsiveness of Ib interneurons to activity arising in Golgi tendon organs.
Although there are stretch reflexes in all muscles, they are especially prominent in antigravity muscles, where they form the basis for postural reflexes. Stretching of a muscle does not necessarily elicit a reflex contraction. Many factors influence whether there will be a response, such as the velocity of stretching, how long the stretch is, whether the muscle is active when being stretched, and whether- a reflex contraction is functionally appropriate. Short (30 msec) and long-latency stretch reflex. Fig. 21 shows a theoretical possibility of how the same muscle pair may work synergistically or antagonistically in various situations
The Golgi Tendon Organ
1. The Golgi Tendon Organs (GTO) are located within the collagen at themyotendinous junction.
2. Each GTO is innervated by a group Ib axon which intertwines with the collagenfascicles.
3. Action potentials are elicited from the group Ib axons when the collagen isdeformed by tension developed during muscle contraction.
The muscle spindle is sensitive to stretch, muscle length, and rate of stretch while theGTO is sensitive to tension and force of muscle contraction.