MedicoPlexus
Human Anatomy & Histology
28. Skeletal muscle. structure, types and the muscle as an organ
Structure
Individual muscle fibers are formed during development from the fusion of several undifferentiated immature cells known as myoblasts into long, cylindrical, multi-nucleated cells. Differentiation into this state is primarily completed before birth with the cells continuing to grow in size thereafter. Skeletal muscle exhibits a distinctive banding pattern when viewed under the microscope due to the arrangement of cytoskeletal elements in the cytoplasm of the muscle fibers. The principal cytoplasmic proteins are myosin and actin (also known as “thick” and “thin” filaments, respectively) which are arranged in a repeating unit called a sarcomere. The interaction of myosin and actin is responsible for muscle contraction.
Every single organelle and macromolecule of a muscle fiber is arranged to ensure form meets function. The cell membrane is called the sarcolemma with the cytoplasm known as the sarcoplasm. In the sarcoplasm are the myofibrils. The myofibrils are long protein bundles about 1 micrometer in diameter each containing myofilaments. Pressed against the inside of the sarcolemma are the unusual flattened myonuclei. Between the myofibrils are the mitochondria.
While the muscle fiber does not have a smooth endoplasmic reticulum, it contains a sarcoplasmic reticulum. The sarcoplasmic reticulum surrounds the myofibrils and holds a reserve of the calcium ions needed to cause a muscle contraction. Periodically, it has dilated end sacs known as terminal cisternae. These cross the muscle fiber from one side to the other. In between two terminal cisternae is a tubular infolding called a transverse tubule (T tubule). T tubules are the pathways for action potentials to signal the sarcoplasmic reticulum to release calcium, causing a muscle contraction. Together, two terminal cisternae and a transverse tubule form a triad.
Another group of cells, the myosatellite cells are found between the basement membrane and the sarcolemma of muscle fibers.[3] These cells are normally quiescent but can be activated by exercise or pathology to provide additional myonuclei for muscle growth or repair.
Connective tissue is present in all muscles as fascia. Enclosing each muscle is a layer of connective tissue known as the epimysium; enclosing each fascicle is a layer called the perimysium and enclosing each muscle fiber is a layer of connective tissue called the endomysium.
- Slow twitch fibres (Type I): Slow twitch fibres contract slowly but can contract repeatedly over long periods. They have a good blood supply, hence they are ‘red fibres’, and are suited to endurance activity using the aerobic energy system which relies on oxygen from the blood for the supply of energy.
Slow twitch fibres are smaller and develop less force than fast twitch fibres. Walking or cycling for 30 minutes at low intensity would use mostly slow twitch fibres.
- Fast twitch fibres (Type IIa): Fast twitch IIa fibres have a fast contraction speed and can use aerobic (oxygen dependent) energy sources as well as anaerobic (no oxygen used) energy sources. Fast twitch IIa fibres are ‘white fibres’ as they are less reliant on oxygen supplied by the blood for energy and therefore fatigue faster than slow twitch fibres.
Fast twitch type IIa fibres are suited to speed, strength and power type activities, such as moderately heavy weight training (8-12 reps) and fast running events such as the 400metres.
- Fast twitch fibres (Type IIb): Fast twitch IIb fibres contract extremely rapidly, create very forceful muscle contractions and fatigue quickly. Fast twitch IIb fibres are also ‘white fibers’ but unlike IIa fibres they can only use anaerobic energy sources.
Like type IIa fibres the fast twitch type IIb fibres are also suited to speed, strength and power type activities. Heavy weight training (1-3 reps), power lifting, and 100metre sprints are examples of activities that predominantly require IIb fibres
Diseases of skeletal muscle are termed Myopathies, while diseases of nerves are called Neuropathies. Both can affect muscle function and/or cause muscle pain and fall under the umbrella of Neuromuscular disease. Myopathies have been modeled with cell culture systems of muscle from healthy or diseased tissue biopsies.
Another source of skeletal muscle and progenitors is provided by the directed differentiation of pluripotent stem cells
29. Connective tissue formations of skeletal muscles and tendons/ internal and external mechanisms of skeletal muscle.
Fascicular Arrangement
A skeletal muscle is arranged much like a can of Vienna sausage. If you have ever opened a can of Vienna sausage, you have noticed the links are packed tightly together, and you can see the ends of the links.
Similarly, when you look at muscle, you can see several fascicles that are packed tightly together in a transverse section. Each fascicle is basically a package of muscle fibers.
While each sausage is surrounded by a jelly-like substance, each fascicle is surrounded by a connective tissue layer that is referred to as the perimysium, as in ‘perimeter.’ The perimysium is important, as it contains lots of blood vessels and nerves that provide nutrients and regulate contraction, respectively.
The epimysium is the connective tissue that surrounds the entire muscle organ. That is easy to remember, as ‘epi-‘ means ‘above or on top’.
The perimysium contains many blood vessels and nerves
Perimysium Diagram
From this perspective, you can see the endomysium is a connective tissue that surrounds individual muscle fibers (also referred to as muscle cells). The prefix ‘endo-‘ means ‘below or within.’ The endomysium contains tiny capillaries and individual neurons providing nutrients and innervation of the muscle fiber.
It is important to note that each layer of connective tissue contains collagen fibers, and they come together to form the tendon on each end of the muscle organ. Again, the tendon attaches the muscle to the bone.
Tendon
A tendon (or sinew) is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae; all three are made of collagen. Ligaments join one bone to another bone; fasciae connect muscles to other muscles. Tendons and muscles work together to move bones.
30. muscles of facial expression
Orbital Group
The orbital group of facial muscles contains two muscles associated with the eye socket. These muscles control the movements of the eyelids, important in protecting the cornea from damage. They are both innervated by the facial nerve.
Orbicularis Oculi
This muscle surrounds the eye socket and extends into the eyelid. It can be functionally split into two parts: the outer orbital part and inner palpebral part.
Actions: The palpebral part of the muscle performs gentle closure of the eyelid, whereas the orbital portion closes more forcefully.
Innervation: Facial nerve.
Corrugator Supercilii
The corrugator supercilii is a much smaller muscle and is located posteriorly to the obicularis oculi muscle.
Attachments: It originates from the superciliary arch, running in a superolateral direction. It inserts into the skin of the eyebrow.
Actions: It acts to draw the eyebrows together, creating vertical wrinkles on the bridge of the nose.
Innervation: Facial nerve.
Nasal Group
The nasal group of facial muscles are associated with movements of the nose, and the skin around it. There are three muscles in this group, and they are all innervated by the facial nerve. They serve little importance in humans. Nasalis
The nasalis is the largest of the nasal muscles. It is split into two parts: transverse and alar.
Attachments: Both portions of the muscle originate from the maxilla. The transverse part attaches to an aponeurosis across the dorsum of the nose. The alar portion of the muscle attaches to the alar cartilage of the nasal skeleton.
Actions: The two parts have opposing functions. The transverse part compresses the nares, and the alar part opens the nares.
Innervation: Facial nerve.
Procerus
The procerus is the most superior of the nasal muscles. It also lies superficially to the other muscles of facial expression.
Attachments: It originates from the nasal bone, inserting into the lower medial forehead.
Actions: Contraction of this muscle pulls the eyebrows downward to produce transverse wrinkles over the nose.
Innervation: Facial nerve.
Depressor Septi Nasi
This muscle assists the alar part of the nasali in opening the nostrils.
Attachments: It runs from the maxilla (above the medial incisor tooth) to the nasal septum.
Actions: It pulls the nose inferiorly, opening the nares.
Innervation: Facial nerve.
Oral Group
These are the most important group of the facial expressors – the are responsible for movements of the mouth and lips. Such movements are required in singing and whistling and add emphasis to vocal communication. The oral group of muscles consists of the orbicularis oris, buccinator, and various smaller muscles.
Orbicularis Oris
The main oral muscles of facial expression. Note how the fibers of buccinator and obicularis oris blend together
The fibres of the orbicularis oris enclose the opening to the oral cavity.
Attachments: Arises from the maxilla and from the other muscles of the cheek. It inserts into the skin and mucous membranes of the lips.
Action: Purses the lips.
Innervation: Facial nerve. Buccinator
This muscle is located between the mandible and maxilla, deep to the other muscles of the face.
Attachments: It originates from the maxilla and mandible. The fibres run in an inferomedial medial direction, blending with the orbicularis oris and the skin of the lips.
Actions: The buccinator pulls the cheek inwards against the teeth, preventing accumulation of food in that area.
Innervation: Facial nerve.
Other Oral Muscles
There are other muscles that act of the lips and mouth. Anatomically, they can be divided into upper and lower groups:
The lower group contains the depressor anguli oris, depressor labii inferioris and the mentalis.
The upper group contains the risorius, zygomaticus major, zygomaticus minor, levator labii superioris, levator labii superioris alaeque nasi and levator anguli oris.
31. muscles of mastication
Masseter
The masseter muscle is the most powerful muscle of mastication. It is quadrangular in shape and can be split into two parts: deep and superficial.
The entirety of the muscle lies superficially to the pterygoids and temporalis, covering them.
Attachments: The superficial part originates from maxillary process of the zygomatic bone. The deep part originates from the zygomatic arch of the temporal bone. Both parts attach to the ramus of the mandible.
Actions: Elevates the mandible, closing the mouth.
Innervation: Mandibular nerve (V3).
Temporalis
The temporalis muscle originates from the temporal fossa – a shallow depression on the lateral aspect of the skull. The muscle is covered by tough fascia, this can harvest surgically, and used to repair a perforated tympanic membrane (an operation known as a myringoplasty).
Attachments: Originates from the temporal fossa. It condenses into a tendon, which inserts onto the coronoid process of the mandible.
Actions: Elevates the mandible, closing the mouth. Also retracts the mandible, pulling the jaw posteriorly.
Innervation: Mandibular nerve (V3).
Medial Pterygoid
The medial pterygoid muscle has a quadrangular shape, with two heads: deep and superficial. It is located inferiorly to the lateral pterygoid.
Attachments: The superficial head originates from the maxilla. The deep head originates from the lateral pterygoid plate of the sphenoid bone. Both parts attach to the ramus of the mandible, near the angle of mandible.
Actions: Elevates the mandible, closing the mouth.
Innervation: Mandibular nerve (V3).
Lateral Pterygoid
The lateral pterygoid muscle has a triangular shape, with two heads: superior and inferior. It has horizontally orientated muscle fibres, and thus is the major protractor of the mandible.
Attachments: The superior head originates from the greater wing of the sphenoid. The inferior head originates from the lateral pterygoid plate of the sphenoid. The two heads converge into a tendon, which attaches to the neck of the mandible.
Actions: Acting bilaterally, the lateral pterygoids protract the mandible, pushing the jaw forwards. Unilateral action produces the ‘side to side’ movement of the jaw.
Innervation: Mandibular nerve (V3).
32. superficial muscles of back
Trapezius
The trapezius is a broad, flat and triangular muscle. The muscles on each side form a trapezoid shape. It is the most superficial of all the back muscles.
Attachments: Originates from the skull, ligamentum nuchae and the spinous processes of C7-T12. The fibres attach to the clavicle, acromion and the scapula spine.
Innervation: Motor innervation is from the accessory nerve. It also receives proprioceptor fibres from C3 and C4 spinal nerves.
Actions: The upper fibres of the trapezius elevates the scapula and rotates it during abduction of the arm. The middle fibres retract the scapula, and the lower fibres pull the scapula inferiorly.
Latissimus Dorsi
The latissimus dorsi originates from the lower part of the back, where it covers a wide area.
Attachments: Has a broad origin – arising from the spinous processes of T6-T12, iliac crest, thoracolumbar fascia and the inferior three ribs. The fibres converge into a tendon that attaches to the intertubercular sulcus of the humerus.
Innervation: Thoracodorsal nerve.
Actions: Extends, adducts and medially rotates the upper limb.
Levator Scapulae
The levator scapulae is a small strap-like muscle. It begins in the neck and descends to attach to the scapula.
Attachments: Originates from the transverse processes of the C1-C4 vertebrae and attaches to the medial border of the scapula.
Innervation: Dorsal scapular nerve.
Actions: Elevates the scapula.
Rhomboids
There are two rhomboid muscles – major and minor. The rhomboid minor is situated superiorly to the major.
Rhomboid Major
Attachments: Originates from the spinous processes of T2-T5 vertebrae. Attaches to the medial border of the scapula, between the scapula spine and inferior angle.
Innervation: Dorsal scapula nerve.
Actions: Retracts and rotates the scapula.
Rhomboid Minor
Attachments: Originates from the spinous processes of C7-T1 vertebrae. Attaches to the medial border of the scapula, at the level of the spine of scapula.
Innervation: Dorsal scapula nerve.
Actions: Retracts and rotates the scapula.
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