Just about everyone has experienced myalgia, commonly known as muscle pain. Most people have experienced muscle pain that goes away over time, but many people live with chronic pain. People living with chronic myalgia experience deep muscle pain daily. In chronic myalgia, the pain is often described as a deep, muscular ache that can hurt at rest or with movement. Trigger points and tender points are common symptoms. Common injuries and medical conditions affecting the nervous system can lead to chronic muscle pain. Muscle pain is also commonly experienced due to chronic overuse or repeated misuse of a muscle. This can be most insidious in presentation because the pain is generated over time and your nervous system integrates compensations. After some time, pain can become activated by the way you sit, stand, move and even breathe. It is reinforced into “muscle memory” with poor posture and work ergonomics. Usually people are unconscious of their bad posture and only an outside observer can objectively see the extent of their bad habits. Acute physical trauma and even emotional or environmental stress can also lead to a lifetime of tense and painful muscles.
What’s wrong with me Doc?
Pain can come and go without any rhyme or reason or in other cases, it is replicated on demand with a particular movement. Doctors focus on pathological muscle pain linked with disease and not idiopathic pain of unknown origin. The 2017 American Medical College guidelines even stated that common practices like steroid shots are antiquated and may be detrimental, conceding that massage was a better first option. When drugs and physical therapy don’t resolve pain, people often become depressed and their outlook on life can be altered. Fear and protection become primary drives as pain avoidance shapes how we preform daily life activities. Anxiety activates limbic system involvement as your psychology and chemical balance is altered. For most sufferers, chronic pain is a downward cycle that affects every aspect of your life.
Pain in the Brain
The goal of this four part series is to explain the mechanics of muscle function and dysfunction so as to demystify chronic pain. I plan to elucidate the neurological phenomenon behind chronic muscle pain and why nothing you are doing helps consistently. Pain is simply a recognition of chemical and electrical signals terminating in the Insula of the brain. Pain is a purely neurological phenomenon which is why it can be so hard to treat it by only Manually focusing on the tissues of the body. Refer to my blog on pain for more info. The intricate neurophysiological mechanisms that coordinate muscle function in the body are like software programs in the most amazing living computer. If the nervous system is the software, the muscles and organs are the hardware system. Our body’s hardware is definitely susceptible to wear and tear, but the real culprit in most chronic pain cases is dysfunctional communication between the tissues and the brain. It is important to focus on function in order to understand dysfunction in the musculoskeletal system. First, lets explain how muscles work.
How Muscles Work
Skeletal muscle is what most people call their muscles. We use muscles to move around and support our body weight. Skeletal muscles are under the voluntary control of the somatic nervous system which actualizes our desired movement. Skeletal muscles attach to the thin outer tissue wrapping of bones called periosteum. They attach via bundles of collagen fibers known as tendons.
Anatomy of a muscle
Skeletal muscle is made of individual muscle cells grouped into bundles called fascicles. The fibers and muscles develop within connective tissue layers called fasciae. Muscle fibers are long and cylindrical and have more than one nucleus. Muscle fibers are composed of myofibrils. The myofibrils are composed of actin and myosin filaments, repeated in units called sarcomeres. They are the basic functional units of the muscle fiber. The sarcomere is responsible for the striated appearance of skeletal muscle and forms the basic mechanism for muscle contraction.
Fascia functionally separates each muscle in three layers known as the epimysium, perimysium, and endomysium. We encounter these glassy tissue separations when we eat meat on the bone. When cooked meat is “Pulled” or “Shredded” it breaks down the fascial walls leaving the long, tightly packed fibers of the muscle cells exposed.
Muscle spindles are sensory receptors (proprioceptors) embedded inside your muscles. The muscle spindle is enclosed in a connective tissue capsule located inside the center or belly of a muscle. The muscle spindle is comprised of specialized skeletal muscle cells called Intrafusal muscle fibers and neurons that run between the spindles, muscles and the spinal cord. Intrafusal muscle fibers serve as sensory organs that measure and respond to the amount and rate of change in length of a muscle. Like regular muscle fibers, spindle fibers have the ability to contract and relax but muscle spindles’ main job are to detect when they are being stretched. The muscle spindle detects stretch and sends a signal to the spinal cord. The response is carried back to the muscle by motor neurons that make the muscle fibers contract.
Intrafusal muscle fibers
The intrafusal muscle fibers are walled off from the rest of the muscle by a collagen sheath that has a spindle or “fusiform” shape, hence the names spindle and “intrafusal”. Extrafusal muscle fibers are what we recognize as the rest of the muscle that contracts on command and generates skeletal movement. The prefix “extra” simply means they lie outside the fusiform sheath. Intrafusal muscle cells’ don’t have tendons like regular muscles. Both ends exit the spindle capsule and integrate into the extrafusal muscles’ connective tissue.
Nuclear Bags and Chains
There are technically three types of intrafusal muscle fibers. Nuclear bags have both static and dynamic fibers that respond to different stressors and there are nuclear chain fibers. They are named for the arrangement of the nuclei of their cells. Nuclear bag senses the onset of a stretch while the nuclear chain is stimulated by sustained stretching. Both are stimulated by rapid stretch. Extrafusal muscle creates volitional movement whereas muscle spindles respond reflexively without our knowledge.
Nerves of the muscle spindle
Sensory information from the spindle to the CNS (afferent) comes from three nerves
- Ia : Type Ia fibers connect to both nuclear bag fibers and nuclear chain fibers. These connections are also called “annulospiral endings”, deriving from the Latin word annulus which means “a ring-shaped area or structure
- Goes from the chain and bag nuclear fibers (static and dynamic) of the muscle spindle to the spinal cord.
- Sensitive to muscle length and rate of change of length
- II: Flower-spray endings
- Goes from the chain and bag nuclear fibers (static) of the muscle spindle to the spinal cord.
- Sensitive to muscle length
- A person is able to know the position of their muscle, and the rate at which it is changing due to the sensory information from these two intrafusal fiber types.
- Ib: (Not in muscle spindle)
- Goes from the Golgi Tendon Organs to the spinal cord.
- Sensitive to muscle tension
Muscle Function and the Spindle
Tonus is the name for the contraction of skeletal muscles that maintains our posture and body structure while we are at rest. All muscular contractions including this constant involuntary postural contraction called tonus are automated by the nervous system. The level of tension within a muscle is set by its muscle spindles. The muscle spindle’s intrafusal fibers are innervated by gamma motor neurons and beta motor neurons. Extrafusal muscles are innervated by alpha motor neurons. Spindles’ sensitivity settings are determined by the gamma motor system in the brain, which can order them to contract and tighten or allow them to be relaxed and loose. The tighter the spindles are set, the more sensitive they are to stretch. The looser they are, the more tolerant they are to being stretched.
The Myotactic Stretch reflex
When a muscle gets stretched with too much velocity or amplitude, stimulates the intrafusal muscle fibers. The stretch transmits a signal that goes to the spinal cord then back out via a lower motor neuron. Lower motor neurons create skeletal muscle movement and in this case, make the stretching skeletal muscle contract to reduce its length. A gamma motoneuron contracts the spindle to ensure a variable relative tension between the intra and extrafusal fibers so the system stays sensitive to increasing tension. Because of the muscle spindle, we can adapt to external stress and can transfer weight throughout the body in all ranges of motion. This muscle spindle reflex aka stretch reflex (or myotatic reflex) is protective in nature. The muscle, joint and ligaments are protected from being overstretched or torn. It is a failsafe feedback system that keeps the muscles and joints safe as it maintains muscle tone.
Not just a knee jerk reflex
The stretch reflex occurs when you fall asleep and your head falls forward only to have your neck jerk it back. However, the stretch reflex is also responsible for setting resting muscle tone. When the gamma motor system directs the spindle fibers within a muscle to contract, they shorten. This is why people can remain chronically inflexible or “tight”. As soon as the muscle stretches more than the length of its spindles, the stretch reflex causes the muscle’s fibers to contract to the tension level set by the spindles. Any overstimulation of dysfunctional receptors is enough to cause muscles to lock down and restrict movement. The muscle spindle is the reason we can run, jump, accelerate and stop. Without it we would likely look more like a puddle of muscle.