Dominic Stone
The human spine is a complex structure of vertebrae, discs, joints, ligaments, and muscles. The spine is designed to provide stability and smooth movement, as well as protection for the spinal cord. The discs in the spine, known as intervertebral discs, act as shock absorbers and provide flexibility to the spine. They are composed of a tough outer ring and a soft, gel-like inner core. The spine is supported by various muscles, including the extensors, flexors, and oblique muscles, which help stabilize and move the spine. These muscles attach to the spine at specific points, such as the transverse processes, and play a crucial role in maintaining spinal health and function.
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What You'll Learn
The lumbar spine and its muscles
The lumbar spine, located in the lower back, consists of five vertebrae (known as L1 to L5) that provide stability for the back and spinal column. The lumbar spine supports the upper two sections of the spine and bears most of the body's weight. The muscles attached to the lumbar spine allow the body to walk, run, sit, lift, and move the body in all directions.
There are three main types of back muscles that help the lumbar spine function: extensors, flexors, and obliques. Extensors are attached to the back of the spine and enable actions like standing and lifting objects. The extensor group consists of the erector spinae and the multifidi. The erector spinae muscles include the longissimus thoracis and iliocostalis lumborum. The contraction of this group results in an extension moment at the lumbar spine. The flexor group lies in front of the lumbar spine and allows for trunk and hip flexion. The psoas major joins the iliacus in the thigh to become the iliopsoas. The oblique muscles are attached to the sides of the spine and help rotate the spine and maintain proper posture.
The paraspinal muscles are central to spinal stability, and their weakness contributes to back pain. The multifidus muscle is a deep, thin muscle situated adjacent to the lumbar spine. It originates from the sacrum and the spinous processes of the lumbar and thoracic vertebrae, with its fibers running upward and inward to insert into the cervical vertebrae. The rhomboid major and rhomboid minor muscles originate from the spinous processes of the upper thoracic vertebrae and attach to the medial border of the scapula. These muscles bring the shoulder blades closer to the spine, aiding in posture maintenance. The latissimus dorsi muscle originates in the lower thoracic and lumbar vertebrae and plays a crucial role in maintaining an erect posture and providing support for various upper-body movements.
The lumbar spine is also supported by the longissimus muscles, which extend along the spine, inserting into the thoracic and cervical vertebrae and ribs. These muscles assist in spinal extension, lateral flexion, and rotation, aiding in movements like twisting and bending. The spinalis muscles originate along the sacrum and the spinous processes of the lumbar and thoracic vertebrae, providing support to the lumbar region and spinal cord during postural adjustments.
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Intervertebral discs as shock absorbers
The human spine is a complex structure composed of vertebrae and intervertebral discs that sit between the vertebrae bones. There are 23 discs in the human spine: 6 in the cervical region (neck), 12 in the thoracic region (middle back), and 5 in the lumbar region (lower back). Intervertebral discs (IVDs) are important for the normal functioning of the spine. They are cushions of fibrocartilage that act as the principal joints between two vertebrae in the spinal column.
IVDs provide a cushioning effect for the vertebrae and help reduce the stress caused by impact. They act as shock absorbers for the spine, allowing it to be flexible without sacrificing strength. This shock-absorbing function is critical for protecting the nervous system, including the brain and spinal cord. By providing cushioning, IVDs prevent the vertebrae from grinding together.
As we age, intervertebral discs lose their hydration and elasticity, making them more susceptible to tearing and minor strains. This condition, known as disc desiccation, is caused by the death of cells that produce and maintain the extracellular matrix (ECM). The nucleus pulposus (NP) shrinks as its gelatinous form is replaced by fibrotic tissue, reducing its functionality. This increased stress can lead to spinal nerve issues, causing pain and weakness.
To address degenerative disc diseases, recent studies have proposed hydrogel injection as a novel therapy. Hydrogel, made from hyaluronic acid and/or collagen-hyaluronan, has proven effective in restoring spine motion and intervertebral disc height. However, it is still unknown if this technique can maintain the shock-absorbing property of the treated disc. Further research is needed to understand the impact of hydrogel injection on the shock-absorbing function of degenerative discs fully.
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Muscles attached to the articular disc of the temporomandibular joint
The articular disc of the temporomandibular joint (TMJ) is a thin, oval plate made of non-vascular fibrous connective tissue. It is located between the mandible's condyloid process and the cranium's mandibular fossa. The disc divides the joint into two cavities, each with its own synovial membrane. The disc is attached to the condyle medially and laterally by the collateral ligaments. The anterior disc attaches to the joint capsule and the superior head of the lateral pterygoid muscle. The lateral pterygoid muscle is likely to influence the position of the articular disc directly during TMJ movements.
The posterior portion of the articular disc, or retrodiscal tissue, is attached to the mandibular fossa. Unlike the disc, the retrodiscal tissue is vascular and highly innervated. As a result, it often contributes to the pain of Temporomandibular Disorder (TMD). The mandibular fossa is a component of the temporal bone, which forms the TMJ along with the mandible.
The masseter muscle is attached to the articular disc on the front edge. It originates from the zygomatic arch and inserts on the branch of the mandible and the coronoid process. The temporalis muscle also sometimes makes contact with the articular disc anteriorly. It originates from the temporal fossa of the skull and the medial face of the zygomatic process, and inserts on the coronoid mandibular process.
In some cases, the discotemporal bundle, zygomaticomandibularis, and masseter are attached to the anterior surface of the disc. The discotemporal muscle bundles of the temporalis are also attached to the anterior surface of the disc. The total vector of these muscles pulls the disc anteriorly.
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Muscle spasms and their causes
Muscle spasms, also known as muscle cramps, are painful contractions and tightening of muscles. They are common, involuntary, unpredictable, and usually not serious. They can affect anyone and occur in any area of the body but are most common in the thighs, calves, hands, arms, belly, ribcage, and arches of the feet.
Muscle spasms can be caused by various factors, including:
- Dehydration
- Electrolyte imbalance (too much or too little potassium, magnesium, and calcium)
- Muscle fatigue
- Overexercising or overusing muscles
- Exercising in extreme heat
- Poor posture
- Stress
- Lack of stretching
- Age
While muscle spasms usually resolve on their own and are not a cause for concern, they can be a symptom of an underlying health condition such as multiple sclerosis, thyroid disease, or cirrhosis of the liver. If muscle spasms are frequent, severe, or accompanied by other symptoms, it is important to consult a doctor for further evaluation and treatment.
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Cardiac muscle cells and intercalated discs
Cardiac muscle, also called the myocardium, is one of three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. Cardiac muscle fibres are shorter than skeletal muscle fibres and usually contain only one nucleus, which is located in the central region of the cell. Cardiac muscle fibres also possess many mitochondria and myoglobin, as ATP is produced primarily through aerobic metabolism. Cardiac muscle fibres are extensively branched and are connected to one another at their ends by intercalated discs.
Intercalated discs or lines of Eberth are microscopic identifying features of cardiac muscle. Cardiac muscle consists of individual heart muscle cells (cardiomyocytes) connected by intercalated discs to work as a single functional syncytium. By contrast, skeletal muscle consists of multinucleated muscle fibres and exhibits no intercalated discs. Intercalated discs support synchronized contraction of cardiac tissue in a wave-like pattern so that the heart can work like a pump. They occur at the Z line of the sarcomere and can be easily visualized when observing a longitudinal section of the tissue. Intercalated discs are complex structures that connect adjacent cardiac muscle cells.
The three types of cell junction recognized as making up an intercalated disc are desmosomes, fascia adherens junctions, and gap junctions. Fascia adherens are anchoring sites for actin, and connect to the closest sarcomere. Desmosomes are intercellular structures that anchor cardiac muscle fibres together and are vital in maintaining the structural integrity of the heart. Gap junctions connect the cytoplasms of neighbouring cells electrically, allowing cardiac action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle. All of these junctions work together as a single unit called the area composita.
Mutations in the intercalated disc gene are responsible for various cardiomyopathies that can lead to heart failure. Ruptured intercalated discs, when seen on histopathology, have two main causes: microtome sectioning and forceful myocardial contraction, in turn, caused by ventricular fibrillation or electrical injury.
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Frequently asked questions
The three main groups of muscles supporting the spine are the extensor, flexor, and oblique muscles.
Extensor muscles are attached to the back of the spine and enable actions such as standing and lifting objects. They include the large paired muscles in the lower back called erector spinae, which help hold up the spine, and the gluteal muscles.
Flexor muscles are attached to the front of the spine and enable bending forward, lifting, and arching the lower back. They include the abdominal muscles.
Oblique muscles are attached to the sides of the spine and help rotate the spine and maintain proper posture.
