Skeletal Muscle
Tone
Skeletal muscle tone refers to baseline contraction or “tautness” the skeletal muscles have at rest2.
Skeletal muscles do not contract without the action potential from the fibers, this tone is the product of low rate nerve impulses from the spinal cord2. These low level nerve impulses are modulated partially by the brain and partially by the muscle spindles2. The brain sends afferent signals to the anterior motoneurons of the spinal cord2. The muscle spindle within the skeletal muscle sends signals to the muscle belly itself2.
Dysfunction
Tone dysfunction can be divided into hypertonia, paratonia, or dystonia.
Spasticity and rigidity are the two types of hypertonia. These forms of hypertonia result from dysfunction in the supraspinal pathways and the interaction between spinal cord and muscle spindle6.
Dystonia and paratonia are not related to physiological dysfunction in tone pathways6. The other two disorders of altered tone, namely dystonia and paratonia, are not exactly related to the physiological dysfunction of the tone pathways6.
In the motor control system, spasticity and rigidity are primarily an “output system” problem, whereas dystonia is a “system level processing” problem6.
Dystonia and paratonia have altered tone secondary to network disruption in the basal ganglia, the thalamocortical circuits, and their connections.
Motor control
Intrinsic motor control of a muscle is almost entirely dictate by information from muscle spindles and golgi tendon organs
Sensory endings
| Type | Axon | Myelin | Receptor | Sensitivity to |
|---|---|---|---|---|
| Ia | 12-20µm | Myelinated | Primary spindle ending | Muscle length & Rate of change |
| Ib | 12-20µm | Myelinated | Golgi tendon organ | Muscle contraction (tendon tension) |
| II | 6-12µm | Myelinated | Secondary spindle ending | Muscle length (negligble rate of change) |
| II | 6-12µm | Myelinated | Non-spindle ending | Deep pressure |
| III | 2-6µm | Myelinated | Free nerve endings | Pain Chemical stimuli Temperature (Important in physiological responses to exercise) |
| IV | 0.5-2µm | Unmyelinated | Free nerve endings | Pain Chemical stimuli Temperature |
Type Ia afferent
- AKA “primary ending” or “annulospiral ending”2
- Refers to a large sensory nerve fiber that encircles the central (receptor) portion of each intrafusal fiber2
- Found in the center of the muscle spindle2
- ~17µm in diameter2.
- Transmission velocity: 70-120 m/s (fast as any type of nerve fiber in the body)2.
Type Ia afferents are excited by all 3 intrafusal sensory fibers: static nuclear bag fibers, dynamic nuclear bag fibers, and nuclear chain fibers2. This allows type Ia afferents to receive and transmit information about both length of receptor (static response) and rate of change of receptor length (dynamic response)2.
Type Ib afferent
Type II afferent
- Type II afferent AKA “secondary afferent ending”2
- Sometimes encircles intrafusal fibers similar to type Ia fibers but more often spreads out like branches known as “flowerspray endings”2.
- ~8µm diameter2
Type II afferents receive information only from nuclear chain fibers2. As a result, type II afferents can only receive and transmit information regarding length of a receptor (static response)2.
Type III afferent
Type IV afferent
Muscle spindle
Muscle spindles are special sensory receptors located in the muscle belly that transmit muscle length and rate of change of muscle length to the nervous system2.
- Muscle spindles are ~3-10mm in length2
- Muscle spindles are surrounded by 3-12 intrafusal fibers8
- The intrafusal fibers are pointed at the ends of the muscle spindle and attach to the glycocalyx of the extrafusal fibers2.
The midportion of the muscle spindle contains its sensory “stretch” receptors2. There are 2 ways a muscle spindle can be excited:
- Lengthening of the entire muscle results in a stretch of the midportion of the muscle spindle2.
- Contraction at both ends of the muscle spindle2.
There are 2 sensory endings in the midportion of the muscle spindle: type Ia afferent and type II afferent
Intrafusal fibers
Intrafusal fibers are tiny skeletal muscle fibers. Intrafusal fibers are distinct from extrafusal fibers since intrafusal fibers have little to no actin and myosin filaments between its two ends2. As a result, when activated, the central portion of the intrafusal muscle fiber does not contract when the ends contract, but rather functions as a sensory receptor2.
The central portion of the intrafusal fiber is encircled by a large type Ia afferent which forms the primary ending (annulospiral ending) and transmits sensory afferent signals to the spinal cord2.
Motor signals from the small type A γ-motor neurons in the anterior horn of the spinal cord are sent through γ-motor neurons to activate the intrafusal fibers causing its ends containing actin and myosin to contract2,6.
There are 3 types of intrafusal fiber: Dynamic and Static Nuclear bag fibers and nuclear chain fibers2.
Dynamic Nuclear bag fibers
Static Nuclear bag fibers
- 1-3 per muscle spindle2
- These get their name since the nuclear are congregated into “bags” in the central portion of the intrafusal fiber2.
Nuclear bag fibers can only activate type Ia afferents and cannot activate type II afferents2.
Nuclear bag fibers contract when activated by γ-dynamic motor efferents2. This contraction enhances the dynamic response2.
Nuclear chain fibers
- 3-9 per muscle spindle2
- 50% smaller than nuclear bag fibers2.
- 50% shorter than nuclear bag fibers2.
- The name “chain” comes from the way the nuclei are aligned in a chain through the receptor area2.
Nuclear chain fibers can activate both type II afferents and type Ia afferents2.
Nuclear chain fibers contract when activated by γ-static motor efferents2. Contraction of nuclear chain fibers enhances the static response ## Responses
There are 2 responses detected by the muscle spindle:
- Static response: Length of the receptor changes
- Dynamic response: Rate of change of receptor length
Static response
The static response refers to a receptor response to change in the length of the receptor.
The static response occurs when the central (receptor) portion of the intrafusal fibers are stretched slowly2. During this, the nuclear chain fibers are activated and transmit signals to the type Ia and II afferents. The number of signals transmitted by type Ia and II afferents is directly proportional to the amount of stretch2.
The type Ia and II afferents will continue to send these signals for several minutes2.
Dynamic response
The dynamic response refers to the rate of change in receptor length2.
The dynamic response occurs when the central (receptor) portion of the intrafusal fibers are stretched suddenly2. The nuclear bag fibers detect this rapid and sudden stretch and transmit this signal to only the type Ia afferent2.
This is a powerful activation, much stronger than the static response2.
This can be activated even when the intrafusal fiber is stretched <1µm for a fraction of a second and will still result in a powerful activation, sending a high number of impulses through the type Ia afferent2. This powerful stimulation only occurs while the length is actually increasing and it stops and returns to the static response2.
Shortening of the intrafusal fiber can also incur a response, notifying the spinal cord of a decrease in intrafusal length2.
Motor efferents
γ-motor efferents are important in controlling the intensity of the static and dynamic responses2.
There are two types:
Gamma-dynamic
Mainly excites nuclear bag intrafusal fibers2.
When the γ-d efferents excite the nuclear bag fibers, this enhances the dynamic response and negligibly impacts the static response2.
Gamma-static
Mainly excites the nuclear chain intrafusal fibers2.
γ-s excitation of the nuclear chain fibers enhances the static response and negligibly impacts the dynamic response2.
Extrafusal fibers
The α-motor neurons (type A&alpha-nerve fiber) activate the extrafusal fibers2,6.
Exercise
“The final common determinant of success in athletic events is what the muscles can do for you—that is, what strength they can give when it is needed, what power they can achieve in the performance of work, and how long they can continue their activity.”2
Muscular Strength
“The strength of a muscle is determined mainly by its size, with a maximal contractile force between 3 and 4 kg/cm2 of muscle cross-sectional area. Thus, a person who has enlarged his or her muscles through an exercise training program will have correspondingly increased muscle strength.”2
“To give an example of muscle strength, a world-class male weight lifter might have a quadriceps muscle with a cross-sectional area as great as 150 square centimeters. This measurement would translate into a maximal contractile strength of 525 kilograms (or 1155 pounds), with all this force applied to the patellar tendon. Therefore, one can readily understand how it is possible for this tendon at times to be ruptured or actually to be avulsed from its insertion into the tibia below the knee. Also, when such forces occur in tendons that span a joint, similar forces are applied to the surfaces of the joint or sometimes to ligaments spanning the joints, thus accounting for such happenings as displaced cartilages, compression fractures about the joint, and torn ligaments.”2
Holding Strength
“The holding strength of muscles is about 40% greater than the contractile strength. That is, if a muscle is already contracted and a force then attempts to stretch out the muscle, as occurs when landing after a jump, this action requires about 40% more force than can be achieved by a shortening contraction. Therefore, the force of 525 kilograms previously calculated for the patellar tendon during muscle contraction becomes 735 kilograms (1617 pounds) during holding contractions, which further compounds the problems of the tendons, joints, and ligaments. It can also lead to internal tearing in the muscle. In fact, forceful stretching of a maximally contracted muscle is one of the surest ways to create the highest degree of muscle soreness.”2
Mechanical Work
“Mechanical work performed by a muscle is the amount of force applied by the muscle multiplied by the distance over which the force is applied. The power of muscle contraction is different from muscle strength because power is a measure of the total amount of work that the muscle performs in a unit period of time. Power is therefore determined not only by the strength of muscle contraction but also by its distance of contraction and the number of times that it contracts each minute. Muscle power is generally measured in kilogram meters (kg-m) per minute. That is, a muscle that can lift 1-kilogram weight to a height of 1 meter or that can move some object laterally against a force of 1 kilogram for a distance of 1 meter in 1 minute is said to have a power of 1 kg-m/min. The maximal power achievable by all the muscles in the body of a highly trained athlete with all the muscles working together is approximately the following:”2
Muscular Power
Skeletal Muscle Degredation
Facilitory
Inhibitory
- Activin Receptor 2B (ACVR2B)9
Dysfunction
- Tone dysfunction
- Guarding
- Overactivity
- Passive stiffness
- Tightness
Myofascial Trigger Points (TrP)
See more at the Myofascial trigger point page.
Tightness
Muscle Tightness refers to a pathological state of a muscle10.
Although muscle tightness is frequently used interchangeably with terms like “Muscle tension” or “muscle tone” these are distinct terms10.
Some definitions of muscle tightness include “decreased range of motion” but this is faulty since it implies that muscle tightness is mainly in the extremities and does not include throat, chest, abdomen, etc10.
Definition:
- Range of motion of associated segments10
- Loss of Daily functioning: Loss of function such as activities of daily living was considered a sign of muscle tightness10.
- Change in muscle tissue texture: Sometimes tight muscles are dense, ropey, and lack pliability10.
- Change in sensation: Patients may complain of change in sensation, such as increased irritation, soreness, and a swollen feeling10.
- Pain:10
- Asymmetry: Could be asymmetry from left to right, or during postural observation, or gait10.
- Contracted muscle state: A “tight” muscle is in a shortened and contracted state10.
Final definition: “Muscle tightness is an inability of a muscle(s) or muscle groups to move through a full range of motion for a specified body part. The muscle(s) may undergo a change in tissue quality and result in a semi-contracted state in which the muscle(s) are unable to relax or lengthen. The tightness in the muscle produces and causes asymmetry and restricts flexibility. This may result in loss of partial or full function for the affected muscle(s) or muscle group. Muscle tightness may or may not include uncomfortable sensations or pain experiences.”10.
Guarding
Guarding is a phrase used commonly at my clinical with Brad Jones11. We used the term guarding to refer to a muscle that had increased tension.
A guarded muscle was seen as a muscle that was compensating for other dysfunction and being overused11.