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 kneejerk 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 efferent output. Interneurons organize efferent neurons
(motor units) into meaningful movement components , which can be utilized
by either spinal input or descending pathways .
1
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
2
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
3
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)
4
5
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
6
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 run parallel to the individual muscle fibers.
2. Each encapsulated spindle contains:
7
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 them from skeletal muscle fibers called extrafusal fibers.
a. Intrafusal fibers include Static and Dynamic Nuclear Bag fibers as
well as Nuclear 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 and chain fibers of the spindle.
b. The secondary ending is a group II axon wrapped around the static
nuclear bag 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 causes the primary and secondary endings to depolarize. Stretch of
intrafusal fibers causes increased firing rate of the afferent axons.
d. The primary endings are sensitive to the rate of change in muscle
length which is referred to as velocity sensitivity. Higher firing rates of the
primary endings occur 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 primary ending. This indicates
that the primary endings are not only sensitive to the length of the muscle but
also to the rate of change of the length. Primary nerve endings are especially
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
cord terminate 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 of the intrafusal muscle fibers which causes them to become more taut.
c. The more tight or stretched the intrafusal fibers, the higher the firing
rate of the sensory axons thus increasing spindle sensitivity.
8
9
The muscle spindle is also sensitive during muscle contraction
1. It is clear that the muscle spindle is capable of conveying information to the
CNS about the muscle when it is stretched. However, during states of muscle
contraction the muscle spindle would be rendered incapable of assessing the
status of the muscle because 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.
The intrafusal fibers are prevented from going slack because the gamma
motor neurons cause the intrafusal fibers to contract and remain tight.
4. Alpha-gamma coactivation is an important component of normal
movement because it enables the muscle spindle to convey information about
the rate of change of the muscle 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 fibers thereby tightening the muscle spindle and ensuring its
sensitivity
b. gamma MNs are most responsive to descending input from the brain
and show little 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.
c. alpha gamma coactivation occurs under most conditions but level of
activity may 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:
d.
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 are activated before a
movement which results in increased sensitivity of both primary and
secondary endings dynamic gamma bias increases the sensitivity of only the
dynamic nuclear bag fiber, therefore only the primary ending will be sensitive
as we have already seen the primary ending detects rapid changes in length
and are especially sensitive to small length changes. Thus, dynamic gamma
bias is best suited for maintaining spindle sensitivity during small postural
adjustments. Static gamma bias: primarily the static gamma MNs are
activated prior to and during a movement. because static gamma MNs supply
most of the intrafusal fibers in the spindle, all but the dynamic bag fiber, they
will affect the sensitivity of both primary and secondary endings.
static gamma bias keeps shortened spindles active during small
length changes as well as prevents their slackening during large changes.
10
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.
11
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
12
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
the myotendinous junction.
2. Each GTO is innervated by a group Ib axon which intertwines with the
collagen fascicles.
3. Action potentials are elicited from the group Ib axons when the collagen
is deformed by tension developed during muscle contraction.
The muscle spindle is sensitive to stretch, muscle length, and rate of stretch
while the GTO is sensitive to tension and force of muscle contraction.
13
Diagram of Inverse Stretch Reflex
Why do we need GTO?
1. To prevent muscle damage due to excessive force generation.
2. To prevent fatigue of a motor unit.
How Muscle Tone Is Maintained?
1. Muscle tone is the normally existing, continuous tension found in all resting
muscles.
2. Maintenance of Muscle tone:
i. most of the tension is of a reflex nature and maintained by
proprioceptive impulses arising in the muscle spindles.
ii. these impulses are transmitted via the peripheral and central
processes of the spinal or certain cranial ganglion cells to the
CNS where they activate alpha motor neurons to cause
14
contraction of the extrafusal muscles to oppose the stretching.
3. Tonic stretch reflex: normal muscle tone is maintained by the length of the
intrafusal muscle fibers.
4. Role of muscle tone in maintaining posture:
i. during standing, the quadriceps femoris muscle is stretched by
gravitational pull on the knee.
ii. the resultant afferent discharge plays on the alpha motor neurons
which then cause contraction of the stretch muscle, thus
maintaining an erect posture.
5. Role of gamma motor neurons:
i. the spontaneous activities of gamma motor neuron cause contraction
of the distal contractile parts of the spindles, thus altering the
length of the middle sensory segments.
ii. the alteration in length of the spindle is transmitted to the CNS via
the afferent fibers which activate the alpha motor neurons
monosynaptically and bring about contraction of the extrafusal
fibers.
iii. the continuous presence of muscle tone brought about by the action
of the gamma motor neurons provides the muscles with a
baseline from which to initiate movement.
6. Both the alpha and gamma neurons are acted upon by supraspinal motor
centers like the cerebral cortex, the red nucleus, the vestibular nucleus
and the reticular formation.
7. The pathway acting on alpha motor neurons leads to rapid and forceful
contractions and the pathway acting on gamma motor neurons is used
in ordinary, less rapid and less forceful contractions.
8. Pathological conditions:
i. the muscle tone may be increased (hypertonus), decreased
(hypotonus) or abolished under different pathological
conditions.
ii. when the muscle tone is abolished as a result of the interruption of
the stretch reflex arc, for example, the muscles become flaccid.
iii. when the descending supraspinal pathways facilitating the stretch
reflex are interrupted, hypotonus of the muscle results; the
reverse is true when the descending pathways inhibiting the
stretch reflex are removed.
iv. the last condition is commonly encountered following hemorrhage in
the internal capsule, causing increased briskness in the stretch
reflex and an increased resistance to passive movement at the
joints.
15
v. it is most obvious in the flexor muscles of the upper limb and the
extensor muscles of the lower limb.
Clinical Importance of Muscle Spindles
Muscle spindles are a key factor in determining muscle tone.
1. Muscle tone is the resistance of a muscle to passive stretch.
2. Muscle tone is assessed by passively flexing and extending the limb and
feeling the degree of resistance encountered.
3. Under normal conditions, the muscle spindle and stretch reflex are
making continual adjustments to maintain the optimal length of the
muscle. The optimal length of the muscle is the sum of the excitatory
and inhibitory inputs reaching the motor neuron from the brain.
4. Normal muscle tone allows us to stand erect and overcome the pull of
gravity. It also provides a spring like quality to the muscle which
means that muscles can store energy and release it during movements
such as running.
5. Normally muscle tone is relatively low; however, lesions of the CNS
produce dramatic increases in muscle tone known as hypertonus or
spasticity; or decreases in tone referred to as hypotonus or flaccidity.
6. There are at least three levels within the spinal cord at which alterations in
neural input will result in tone changes.
a. The CNS can change the amount of fusimotor activity; i.e. the firing
rate of the gamma motor neurons, thereby changing the sensitivity of
the muscle spindle.
b. Changes in direct descending inputs to the alpha motor neurons will
reset the optimal length of the muscle, inducing changes in the
activity of the stretch reflex.
c. Presynaptic changes in the effectiveness of inputs to the motor
neurons will drive the firing rate of the motor neuron.
What are the supraspinal structures involved in the control of muscle
tone?
(This will be discussed in detail in motor system lecture)
1. Damage to supraspinal structures results in hypotonia or hypertonia
depending on the structures involved.
2. If only the pyramidal tract is lesioned, hypotonia results indicating that the
tract plays a facilitory role in the control of tone.
3. however, if the primary motor cortex or extrapyramidal structures are
lesioned hypertonia or spasticity is seen. Indicating that they play an
inhibitory role in the control of tone.
16
4. extrapyramidal structures include:
a. Lateral Vestibular Nucleus: direct excitation of alpha MNs to the
extensor muscles of the limbs, normally inhibited by cortex and cerebellum
b. Reticular formation: loss of inhibitory control over the reticular
formation by the cortex, cerebellum, and striatum, leads to excitation of flexor
and extensor MNs of the limbs.
c. Red Nucleus: provides excitatory input to flexor MNs in the cervical
cord
Diagram Illustrating Effect of Higher Brain Centers On Muscle Tone
====================== FINIS ===========================
Dr Mahmoud Ahmad Fora
17