MUSCLE FIBERS, REFLEXES & MOTOR FUNCTION

Posture & movement depend on contraction of some skeletal muscles while, simultaneously, keeping other muscles relaxed.

​

This muscle contraction & relaxation is under the control of motoneurons that innervate them.

​

The motor system is designed to execute this coordinated response largely through reflexes integrated within the spinal cord.

MOTOR UNIT: A single motoneuron & the muscle fibers it innervates.

Motoneuron pool is a set of many motoneurons innervating many fibers within the same muscle.

​

The number of muscle fibers innervated can vary from a few fibers to thousands depending on the nature of its motor activity.

Thus, an eye movement that requires fine control, it’s motoneurons innervate few muscle fibers only.

Alternately, for postural muscles involved in gross movements, motoneurons innervate thousands of fibers.

MOTOR UNITS

Force of muscle contraction is graded by recruitment of motor units. Eg. small motoneurons innervate few fibers, they have the lowest thresholds, they fire first and generate the smallest amounts of force.

​

On the other hand, large motoneurons innervate many fibers. They have the highest thresholds to fire AP; thus, they fire last, but since they innervate many muscle fibers, they also generate the greatest force.

​

The size principle states (as more motor units are recruited, progressively, larger motoneurons are involved and greater tension will be generated.

Types of Motoneurons

α Motoneurons innervate extrafusal skeletal muscle fibers & hence, an AP in α motoneurons will lead to AP in extrafusal muscle fibers they innervate, which results in its contraction.

γ Motoneurons innervate specialized intrafusal muscle fibers (part of muscle spindle) which functions mainly to sense muscle length; the function of γ motoneurons is to adjust the muscle spindles sensitivity (so they respond appropriately as extrafusal fibers contract and shorten).

​

α and γ motoneurons are coactivated (activated together) so that spindles remain sensitive to changes in muscle length even as the muscle contracts & shortens.

Types of Muscle Fibers

2 types of fibers:

Extrafusal fibers constitute the bulk of skeletal muscle, innervated by α motoneurons, has origin & insertion and are used to generate force.

​

Intrafusal fibers: special fibers that run parallel to extrafusal fibers, encapsulated in sheaths to form spindles.

They are innervated by γ motoneurons with no origin/ insertion but connected to intramuscular connective tissue. too small to generate significant force.

Proprioceptors & Muscle spindles

• Proprioceptors are distributed throughout our joints & musculoskeletal system.
• They detect body position & relay data back to CNS.
• Proprioception is a keystone to the understanding of the motor control.

​

• Muscles & tendons have receptors that can detect muscle length (muscle spindles) and muscle strength (Golgi tendon organs). They also contain free nerve endings that detect muscle pain & possibly extracellular fluid during muscle activity.

Muscle spindles (MSs)

Called fusiform body & made of few intrafusal fibers that lack actin and myosin in their central regions (Noncontractile), surrounded by a capsule that can change length at its ends.

​

• Innervated by motor & sensory fibers. It detects & regulates muscle length via participation in the gamma reflex loop. It is the proprioceptive organ of skeletal muscle.

​

• MSs are attached to extrafusal fibers in parallel and thus become stretched as extrafusal fibers are stretched, W/C activates sensory endings in the muscle spindle.

Types of Intrafusal Fibers

There are 2 types:

1.The largest intrafusal fiber has all of its nuclei bunched up in the middle of the fiber and is called a nuclear bag fiber.

2. The other intrafusal fibers have their nuclei arranged in a line and are called nuclear chain fibers.

​

Sensory & Motor endings in muscle spindle

• There are 2 types of sensory endings in muscle spindle.
• Type Ia (primary) sensory endings innervate the middle portion of all intrafusal fibers, type II (secondary) sensory endings innervate the nuclear chain fibers.
• Together, these nerves detect muscle length and relay this proprioceptive information to spinal cord.
• Motor innervation to intrafusal fibers comes via γ Gamma motoneurons, which cause intrafusal muscle cells to contract and remain responsive.

MUSCLE SPINDLES (MSs) DENSITY

• The density of muscle spindles varies among different skeletal muscles.

​

• Muscles that require precise & delicate movement (Eg. Small extraocular eye & finger muscles) have a high density of muscle spindles (120 spindles/gm).
• Whereas those responsible for gross motor movements (Eg. Large antigravity & leg muscle) have a low density of muscle spindles. (5 spindles/gm)

​

MS is the 3rd most complex sensory organ in the body after the eye and ear.�

Reflexes

• A quick, automatic stereotyped reactions of muscles or glands in responses to stimuli.
• Neuronal reflex involves sensory fibers to CNS & motor fibers to effectors and for it to occur, all elements must be functional and intact. 
• Reflexes are very valuable for clinical evaluation of nervous system functions. 
• If reflexes are absent or abnormal--- physician can diagnose the interrupted pathway or the lost function.

​

Reflex arc: It is the different steps involved in a neural reflex loop. 5 steps of a Reflex arc

• Arrival of stimulus and activation of receptor
• Activation of sensory (Afferent) neuron
• Information processing
• Activation of motor (Efferent) neuron
• Response by effector

Example of Reflex Arc components

Spinal Reflexes

Range from simple monosynaptic to complex polysynaptic or Intersegmental reflex where many segments interact to form complex response.

​

The stretch reflex (myotatic reflex)

• Simplest, Prototype Reflex of all spinal cord reflexes
• it is a Monosynaptic reflex (only one synapse between sensory afferent & motor efferent nerves) that provides automatic regulation of skeletal muscle length. (Ex, Patellar (knee jerk) reflex in which Sensory receptors are muscle spindles)
• It monitors skeletal muscle length and tone & in case of muscle stretching there will be a muscle contraction.

​

stretch reflex (myotatic reflex)

• When we tap a tendon, the muscle will be stretched , the muscle spindle will also be streched and hence its nerve activity increases. This increases α motoneuron activity, which will cause muscle fibers to contract & thus resist the stretching.

​

• In the same time, A secondary set of efferent neurons also causes the opposing muscle (hamstring) to relax (reciprocal inhibition). This reflex maintain the muscle at a constant length.

​

• γ motoneurons regulate how sensitive the stretch reflex is by tightening or relaxing the fibers within the spindle. (This reflex has the shortest latency of all spinal reflexes)

• When a muscle is stretched
• stretch receptors in the intrafusal fibres are stimulated via type Ia afferent impulse which is transmitted to the spinal cord
• α motor neuron is stimulated & the muscle is contracted
• Monosynaptic
• Neurotransmitter is glutamate

muscle

contraction

Stretch reflex

Spinal cord level of stretch reflexes�Deep tendon reflexes (DTR)�examples of sterch reflexes

• reflex level
• biceps jerk C5,6
• supinator jerk C5,6
• triceps jerk C7,8
• knee jerk L3,4
• ankle jerk S1,2

γ motor neuron

• Cell body is located in the anterior horn
• motor neuron travels through the motor nerve
• supplies the intrafusal fibres (has some contractile (Elengation) elements at either end) .
• Its active all the time in a mild contraction state
• Maintain the sensitivity of muscle spindle to stretch
• modified by the descending pathways (Excitatory and inhibitory influences)
• Sum effect is generally inhibitory in nature .

​

​

α motor neuron & Intrafusal Fibers

Figure 13.21

• When α motor neuron is active
• extrafusal fibres are contracted & So muscle contracts
• when γ motor neuron is active
• intrafusal fibres are contracted at its ends only

stretch receptors are stimulated (Once Ia afferents are stimulated stretch reflex is initiated & activated

• Impulses will travel through Ia afferents to SC.
• alpha motor neuron is then activated
• muscle then contracts.
• Therefore, the stretch reflex is important for maintaining muscle tone and upright posture.

• Why we need to stimulate the intrafusal fibers first, because it would be easier just to stimulate the extrafusal muscle fiber with the alpha efferent neuron.

​

• But look what actually happens:

Descending pyramidal tracts actually stimulate both alpha & gamma neurons together(Co-activated).�Both intrafusal fibers (at ends only) AND extrafusal fibers contract to same extent. The stretch receptor doesn’t feel any thing abnormal!

Alpha-Gamma Co-activation.

• gamma motoneurons are activated in parallel with alpha motoneurons to maintain the firing of spindle afferents when the extrafusal muscles shorten and this will sets up a situation in which there will be an automatic increase in tension of the muscle if the load is too heavy.

​

​

• This combined stimulation of the two neuron types is called alpha-gamma co-activation.

What happens to the stretch receptor if the load is too big for the muscle to lift? stretch receptor (MS) becomes too stretched (before the muscle is shorten) & it increases the frequency of APs along the stretch receptor afferent fibers

This increases in the frequency of APs along the stretch receptor afferent fiber will stimulate only the alpha efferent, causing the extrafusal muscle fiber to recruit more fibers and generate greater amounts of tension to counter stretch.

Supraspinal control of stretch reflex

• CNS can influence stretch reflex via γ motoneurons, which control the sensitivity of the reflex receptor. (MS).
• Facilitatory signals arrive through ventral reticulo-spinal tract from area 4, neocerebellum & the vestibular nucleus.
• Neocerebellum (lateral zone of cerebellum) receives input exclusively from parietal lobe via pontine nuclei (forming cortico-ponto-cerebellar pathways), and sends output mainly to VLN (thalamus) (W/C in turn is connected to premotor cortex & primary motor area) and to red nucleus.
• Neocerebellum is involved in planning movement that is about to occur, in evaluating sensory information for action plus maintenance of equilibrium & muscle tone.
• Inhibitory signals arrive at γ motoneurons through lateral reticulospinal tract from area 6 and red nucleus.

​

Self Control of reflexes�(Jendrassik manoeuver)

• The Jendrassik maneuver is a medical maneuver wherein the patient flexes both sets of fingers into a hook-like form and interlocks fingers together.

​

• Often a larger reflex response will be observed when the patient is occupied with this maneuver, as the maneuver may prevent the patient from consciously inhibiting or influencing his or her response to the hammer.

​

End of Part 1

• Next time – More on reflexes classification plus other polysynatic reflexes.
• Common Motor disorders