Muscular System

The muscular system is a complex network of muscles that enables the human body to move, maintain posture, and generate heat. Muscles are specialized tissues that are composed of contractile fibers, nerve tissue, and blood vessels. They are responsible for all movements of the body, from simple actions like blinking an eye or smiling to complex movements like running or playing a musical instrument.

Muscles are classified into three types based on their structure and function: skeletal, smooth, and cardiac.

·         Skeletal muscles are attached to bones and are responsible for voluntary movements of the body, such as walking, running, and lifting weights. They are under conscious control and work in pairs, with one muscle contracting while the other relaxes.

·         Smooth muscles are found in the walls of internal organs, such as the stomach, intestines, and blood vessels. They are responsible for involuntary movements, such as peristalsis, which moves food through the digestive system.

·         Cardiac muscles are found only in the heart and are responsible for pumping blood throughout the body.

Muscles are composed of individual muscle fibers, which are arranged in bundles called fascicles. These fascicles are surrounded by connective tissue, which provides support and protection. Each muscle fiber contains many myofibrils, which are long protein fibers that contain the contractile proteins actin and myosin. When a muscle is stimulated by a nerve impulse, the myosin and actin filaments slide past each other, causing the muscle to contract. The greater the number of fibers that contract, the greater the force of the contraction.

Muscles are supplied with blood by a network of blood vessels, including arteries, veins, and capillaries. Blood vessels provide oxygen and nutrients to the muscle tissue, as well as removing waste products such as carbon dioxide and lactic acid. Muscles also require a constant supply of energy in the form of ATP (adenosine triphosphate) to maintain their function. ATP is produced by the breakdown of glucose and other nutrients in a process called cellular respiration.

Muscle Structure

Muscle structure refers to the organization and composition of muscles, which are essential for movement and maintaining body posture. Muscles are composed of different layers and structures that work together to generate force and enable coordinated movement. Let's explore the key components of muscle structure:

Muscles are organized into groups based on their function and location in the body. For example, the muscles of the upper limb form a muscle group responsible for movements of the arm, forearm, and hand.

·         Epimysium is the outermost layer of connective tissue that surrounds the entire muscle. It is a dense, fibrous sheath that provides structural support and protection to the muscle. The epimysium also helps to transmit force generated by the muscle to the bones it is attached to.

·         Perimysium is within the muscle, there are multiple bundles of muscle fibers called fascicles. The perimysium is a layer of connective tissue that surrounds each fascicle, providing support and protection. It contains blood vessels and nerves that supply the fascicles.

·         Fascicles are bundles of muscle fibers (muscle cells) within the muscle. Fascicles are surrounded by the perimysium and contain numerous muscle fibers running in parallel. The arrangement of fascicles within a muscle contributes to its strength, range of motion, and overall function.

·         Endomysium is a layer of connective tissue that surrounds individual muscle fibers within each fascicle. It provides structural support to the muscle fibers and contains capillaries, which supply oxygen and nutrients to the muscle fibers.

·         Muscle Fibers (Muscle Cells) are long, cylindrical cells that make up the functional unit of a muscle. They contain specialized contractile proteins called myofilaments, which enable muscle contraction. Each muscle fiber is wrapped in the endomysium and contains myofibrils.

·         Myofibrils are thread-like structures within muscle fibers that extend the length of the fiber. Myofibrils are composed of repeating units called sarcomeres, which are responsible for muscle contraction. They consist of actin and myosin filaments arranged in a highly organized pattern.

·         Sarcomeres are the basic contractile units of muscles. They are located between two Z-lines (disc-shaped structures) within myofibrils. When muscles contract, the actin and myosin filaments within sarcomeres slide past each other, causing the muscle to shorten and generate force.

Sliding Filament Theory

The sliding filament theory is the most widely accepted explanation for how muscles contract. It describes the molecular events that occur in the sarcomeres, the basic functional units of skeletal muscle, during muscle contraction.

A sarcomere is composed of two types of filaments: thick myosin filaments and thin actin filaments. The myosin filaments are located in the center of the sarcomere and are surrounded by actin filaments, which are anchored to the Z discs at the ends of the sarcomere. Between the z disc and the start of the myosin only actin is seen, this is called the light band or I band. The space from tip of the myosin to the other myosin tip across the M line (which anchors the myosin) is called the dark band or A band.  The space between actin tips, where only myosin is seen is known as the H zone.

When a muscle contracts, the myosin heads interact with the actin filaments, forming cross-bridges that cause the filaments to slide past each other, shortening the sarcomere and thus the muscle. The sliding filament theory proposes that this process occurs through a series of molecular events that involve the binding and hydrolysis of ATP (adenosine triphosphate), the energy currency of the cell. At rest, the myosin heads are in a relaxed state, with their ATP-binding sites blocked by the protein tropomyosin. Calcium ions (Ca2+) are stored in the sarcoplasmic reticulum, a specialized network of membrane-bound channels and sacs that surrounds each myofibril (bundle of sarcomeres) within the muscle fiber. When a nerve impulse reaches the muscle fiber, it triggers the release of Ca2+ from the sarcoplasmic reticulum into the sarcoplasm (the cytoplasm of the muscle cell).

The Ca2+ ions bind to a protein called troponin, which is located on the actin filaments. This binding causes a conformational change in the troponin-tropomyosin complex, which exposes the myosin-binding sites on the actin filaments after the tropomyosin is removed from the actin binding sites.

The myosin heads then bind to the exposed sites on the actin filaments, forming cross-bridges. This binding triggers the release of the inorganic phosphate (Pi) from the ATP molecule bound to the myosin head, which causes a conformational change in the myosin head, shifting it to a high-energy position. This energy is used to move the actin filament past the myosin filament, shortening the sarcomere. The myosin head then undergoes another conformational change, which releases the ADP (adenosine diphosphate) molecule and allows a new ATP molecule to bind to the myosin head. The binding of ATP causes the myosin head to detach from the actin filament and return to its relaxed state. This cycle of myosin head binding, Pi release, power stroke, ADP release, and ATP binding is repeated as long as Ca2+ ions are present in the sarcoplasm and ATP is available to fuel the process. When the nerve impulse ceases, Ca2+ ions are actively transported back into the sarcoplasmic reticulum, and the troponin-tropomyosin complex returns to its blocking position, preventing the myosin heads from binding to the actin filaments and allowing the muscle to relax.

Muscles work together in groups to produce movement. For example, when a person bends their elbow, the biceps muscle contracts while the triceps muscle relaxes. When the person straightens their elbow, the biceps muscle relaxes while the triceps muscle contracts. Muscles also work in opposition to each other to maintain posture. For example, the muscles in the front of the neck work in opposition to the muscles in the back of the neck to keep the head upright.

End Motor Plate

Muscle are innovated by a nerve at the end motor plate.  The end motor plate, also known as the neuromuscular junction, is a specialized region where the motor neuron and muscle fiber come into contact. It is an essential component of the muscular system responsible for the transmission of electrical impulses from the nervous system to the muscle fiber, ultimately leading to muscle contraction.

The neuromuscular junction consists of three main components: the presynaptic terminal, the synaptic cleft, and the postsynaptic membrane.

·         The presynaptic terminal is located on the axon of the motor neuron, and it contains small membrane-bound sacs called synaptic vesicles that store neurotransmitters. These neurotransmitters are chemical messengers that are released into the synaptic cleft in response to an action potential traveling down the axon of the motor neuron.

·         The synaptic cleft is the narrow gap between the presynaptic terminal and the postsynaptic membrane. It is filled with extracellular fluid, and it acts as a barrier that separates the two cells.

·         The postsynaptic membrane is the specialized region on the muscle fiber where the neurotransmitter binds to specific receptors, leading to the generation of an action potential in the muscle fiber.

When an action potential reaches the presynaptic terminal, it triggers the release of the neurotransmitter acetylcholine (ACh) from the synaptic vesicles. ACh diffuses across the synaptic cleft and binds to the ACh receptors on the postsynaptic membrane of the muscle fiber. This binding causes the opening of ion channels that allow the influx of positively charged ions, leading to the depolarization of the muscle fiber membrane.

The depolarization of the muscle fiber membrane, through the t-tubules, triggers the release of calcium ions from the sarcoplasmic reticulum, a specialized organelle in the muscle fiber that stores calcium ions. The calcium ions bind to regulatory proteins called troponin, which leads to a conformational change in another protein called tropomyosin, exposing the binding sites for myosin on the actin filaments initiating the sliding filament theory.

Naming Muscles

Muscles are named based on several characteristics, including their location, shape, size, direction of fibers, and function. Understanding the naming convention used for muscles is an essential part of studying human anatomy and physiology, as it allows for accurate communication about the location and function of different muscle groups.

·         Location: Muscles are often named based on their location in the body. For example, the pectoralis major muscle is located in the chest (pectorals) and is a large (major) muscle that covers much of the upper chest region. Similarly, the gluteus maximus muscle is located in the buttocks (gluteus) and is the largest (maximus) muscle in this region.

·         Shape: Muscles can also be named based on their shape. For example, the deltoid muscle, located in the shoulder region, is named for its triangular (delta) shape. The trapezius muscle, located in the upper back and neck, is named for its trapezoidal shape.

·         Size: Muscles can also be named based on their size. For example, the quadriceps femoris muscle is named for its four (quad) distinct heads and its location in the thigh (femoris). The biceps brachii muscle, located in the upper arm, is named for its two (bi) distinct heads.

·         Direction of fibers: Muscles can also be named based on the direction of their fibers. For example, the rectus abdominis muscle, located in the abdominal region, has fibers that run vertically (rectus) along the midline of the body. The oblique muscles, located in the abdominal and back regions, have fibers that run diagonally (oblique) across the body.

Function: Muscles can also be named based on their function. For example, the adductor muscles, located in the inner thigh, are responsible for adducting or bringing the legs together. The abductor muscles, located in the outer hip region, are responsible for abducting or moving the legs away from the midline of the body. 

Select Muscles of the Body

Head and Neck Muscles

·         The orbicularis oculi is a circular muscle that surrounds the eye. It is responsible for closing the eyelids and protecting the eye from external stimuli.

·         The orbicularis oris is a circular muscle located around the mouth. It is responsible for puckering and closing the lips, as well as controlling the movements of the mouth during speech and facial expressions.

·         The buccinator is a thin, flat muscle located in the cheeks. It plays a crucial role in chewing and helps to compress the cheeks against the teeth, allowing for actions such as blowing, whistling, and sucking.

·         The masseter is a powerful muscle located in the jaw. It is the primary muscle responsible for closing the jaw during chewing and biting. It helps to elevate the mandible and is one of the strongest muscles in the human body.

·         The temporalis is a large muscle located on the side of the head, above the ear. It is responsible for the movement of the jaw, including chewing and biting. It works in conjunction with the masseter muscle to elevate and retract the mandible.

·         The frontalis muscle is located on the forehead and extends from the eyebrows to the hairline. It is responsible for raising the eyebrows and wrinkling the forehead, allowing for various facial expressions such as surprise or concern.

·         The occipitalis muscle is located at the back of the head. It works in conjunction with the frontalis muscle to move the scalp backward and raise the eyebrows.

·         The mentalis is a small muscle located on the chin. It allows for various movements of the lower lip and chin, including protrusion and wrinkling. It plays a role in facial expressions such as pouting or expressing doubt.

·         The stylohyoid muscle is a slender muscle that runs from the styloid process of the temporal bone to the hyoid bone in the neck. It helps in the movements of the hyoid bone during swallowing and speaking.

·         The mylohyoid is a flat, triangular muscle located in the floor of the mouth. It forms the base of the oral cavity and assists in swallowing, elevating the hyoid bone, and controlling the position of the tongue.

·         The sternohyoid muscle is a thin, strap-like muscle located in the neck. It runs from the sternum to the hyoid bone and depresses the hyoid bone and larynx, aiding in actions such as swallowing and speaking.

·         The thyrohyoid muscle is a small muscle located in the neck. It connects the thyroid cartilage to the hyoid bone and helps in the movements of the larynx during swallowing and speaking.

·         The sternocleidomastoid muscle is a large muscle located in the neck. It runs diagonally from the sternum and clavicle to the mastoid process behind the ear. It allows for the rotation and flexion of the head and neck and also assists in raising the sternum during deep inhalation.

·         The platysma is a thin, sheet-like muscle located in the neck and upper chest. It covers the front of the neck and assists in various facial movements, such as lowering the jaw or pulling down the corners of the mouth in expressions of sadness or tension.

·         The pectoralis major is a large, fan-shaped muscle located in the chest. It covers the upper part of the chest and is responsible for various movements of the shoulder joint, including flexion, adduction, and medial rotation.

·         The serratus anterior is a muscle located on the lateral surface of the ribcage. It connects the scapula (shoulder blade) to the ribs and is responsible for stabilizing the scapula, allowing for movements such as protraction and upward rotation.

·         The external intercostals are a group of muscles located between the ribs. They are responsible for elevating the ribs during inhalation, thereby aiding in expanding the chest cavity and facilitating breathing.

·         The internal intercostals are a group of muscles located between the ribs. They run in the opposite direction to the external intercostals and are responsible for depressing the ribs during forced exhalation.

·         The rectus abdominis is a paired muscle that runs vertically along the front of the abdomen. It is commonly referred to as the "six-pack" muscle and is responsible for flexing the lumbar spine, as well as assisting in various movements involving the trunk and pelvis.

·         The external abdominal oblique is a broad, superficial muscle that forms the outermost layer of the lateral abdominal wall. It allows for the flexion and rotation of the trunk and assists in compressing the abdominal contents.

·         The internal abdominal oblique is a deep muscle located beneath the external abdominal oblique. It runs in the opposite direction to the external oblique and is involved in trunk rotation, as well as compressing the abdominal contents.

·         The transversus abdominis is the deepest muscle of the abdominal wall. It runs horizontally across the abdomen and is responsible for stabilizing the trunk, compressing the abdominal contents, and assisting in forced expiration.

·         The trapezius is a large muscle located on the upper back and neck. It is shaped like a trapezoid and is responsible for movements such as elevation, retraction, and depression of the scapula, as well as extension and lateral flexion of the neck.

·         The latissimus dorsi is a large, triangular muscle located on the back. It spans from the lower spine to the upper arm and is responsible for various movements of the shoulder, including extension, adduction, and medial rotation.

·         The teres major is a muscle located on the back, beneath the latissimus dorsi. It assists in movements of the shoulder joint, such as extension, adduction, and medial rotation.

·         The rhomboids are a pair of muscles located in the upper back, between the shoulder blades. They help in retracting and stabilizing the scapula, as well as assisting in downward rotation and elevation.

·         The erector spinae is a group of muscles that runs vertically along the spine. It helps in maintaining the posture and extension of the spine and assists in movements such as bending backward and sideways.

·         The psoas major is a long muscle located in the lower back and hip region. It runs from the lumbar spine to the femur and is involved in flexing the hip joint and stabilizing the lumbar spine.

Muscles of the Arm

·         The deltoid is a large, triangular muscle located on the shoulder. It covers the shoulder joint and is responsible for various movements of the arm, including abduction, flexion, extension, and rotation.

·         The triceps brachii is a three-headed muscle located on the posterior side of the upper arm. It extends the forearm at the elbow joint and assists in actions such as pushing or throwing.

·         The biceps brachii is a two-headed muscle located on the anterior side of the upper arm. It flexes the forearm at the elbow joint and also plays a role in supination (rotation of the forearm to face palm up).

·         The brachialis is a muscle located underneath the biceps brachii. It is the primary flexor of the forearm at the elbow joint and plays a crucial role in lifting and carrying objects.

·         The brachioradialis is a muscle located on the lateral side of the forearm. It helps in flexing the forearm at the elbow joint and also assists in pronation and supination of the forearm.

·         The pronator teres is a muscle located on the anterior side of the forearm. It plays a key role in pronation of the forearm, where the palm of the hand faces downward.

·         The palmaris longus is a slender muscle located in the anterior forearm. It runs from the elbow to the palm of the hand and assists in flexion of the hand at the wrist joint.

·         The flexor carpi ulnaris is a muscle located on the medial side of the forearm. It is responsible for flexing and adducting the hand at the wrist joint.

·         The extensor carpi radialis longus is a muscle located on the lateral side of the forearm. It extends and abducts the hand at the wrist joint.

·         The extensor digitorum is a muscle located on the posterior side of the forearm. It extends the fingers at the metacarpophalangeal joints and assists in extending the hand at the wrist joint.

·         The extensor carpi ulnaris is a muscle located on the posterior side of the forearm. It extends and adducts the hand at the wrist joint.

·         The flexor carpi radialis is a muscle located on the anterior side of the forearm. It flexes and abducts the hand at the wrist joint.

Muscles of the Leg

·         The sartorius is a long, strap-like muscle that runs diagonally across the front of the thigh. It is the longest muscle in the human body and is responsible for flexing, abducting, and laterally rotating the hip joint, as well as flexing the knee joint.

·         The gracilis is a long, thin muscle located on the inner thigh. It runs from the pubic bone to the tibia and is responsible for adducting the thigh, as well as flexing the knee joint.

·         The rectus femoris is one of the four quadriceps muscles located on the front of the thigh. It is the only quadriceps muscle that crosses both the hip and knee joints, and it is responsible for extending the leg at the knee joint and flexing the thigh at the hip joint.

·         The vastus medialis is one of the four quadriceps muscles located on the front of the thigh. It is responsible for extending the leg at the knee joint and stabilizing the patella (kneecap) during movement.

·         The vastus intermedius is one of the four quadriceps muscles located on the front of the thigh. It lies deep to the rectus femoris and is responsible for extending the leg at the knee joint.

·         The vastus lateralis is one of the four quadriceps muscles located on the front of the thigh. It is the largest and most lateral of the quadriceps muscles and is responsible for extending the leg at the knee joint.

·         The biceps femoris is one of the hamstring muscles located on the back of the thigh. It is responsible for flexing the knee joint and extending the hip joint, as well as aiding in the rotation of the leg.

·         The semitendinosus is one of the hamstring muscles located on the back of the thigh. It is responsible for flexing the knee joint and extending the hip joint, as well as aiding in the rotation of the leg.

·         The semimembranosus is one of the hamstring muscles located on the back of the thigh. It is responsible for flexing the knee joint and extending the hip joint, as well as aiding in the rotation of the leg.

·         The gluteus maximus is the largest and most superficial muscle of the buttocks. It is responsible for extending the hip joint, as well as assisting in the lateral rotation and stabilization of the thigh.

·         The gluteus medius is a muscle located on the outer surface of the hip. It is responsible for abduction and medial rotation of the thigh, as well as stabilizing the pelvis during walking or standing on one leg.

·         The gluteus minimus is a small muscle located beneath the gluteus medius. It assists in abduction and medial rotation of the thigh, as well as stabilization of the pelvis.

·         The gastrocnemius is a prominent calf muscle located at the back of the lower leg. It is responsible for plantar flexion of the foot (pointing the toes downward) and assists in flexing the knee joint.

·         The soleus is a deep muscle located beneath the gastrocnemius in the calf. It is responsible for plantar flexion of the foot and works alongside the gastrocnemius to provide power during activities such as walking, running, and jumping.

·         The fibularis longus, also known as the peroneus longus, is a muscle located on the lateral side of the lower leg. It assists in eversion and plantar flexion of the foot and provides stability during standing and walking.

·         The tibialis anterior is a muscle located on the front of the lower leg. It is responsible for dorsiflexion (lifting the foot upward) and inversion (turning the sole of the foot inward), as well as supporting the arch of the foot.

Overview

The muscular system is a complex network of muscles that enables the human body to move, maintain posture, and generate heat. It consists of specialized tissues composed of contractile fibers, nerve tissue, and blood vessels. Muscles are responsible for all movements of the body, ranging from simple actions like blinking an eye or smiling to complex movements like running or playing a musical instrument.

There are three types of muscles: skeletal, smooth, and cardiac. Skeletal muscles are attached to bones and are responsible for voluntary movements, such as walking, running, and lifting weights. They are under conscious control and work in pairs, with one muscle contracting while the other relaxes. Smooth muscles are found in the walls of internal organs, like the stomach, intestines, and blood vessels. They are responsible for involuntary movements, such as peristalsis, which moves food through the digestive system. Cardiac muscles are only found in the heart and are responsible for pumping blood throughout the body.

Muscles are composed of individual muscle fibers arranged in bundles called fascicles. These fascicles are surrounded by connective tissue, providing support and protection. Each muscle fiber contains myofibrils, which are long protein fibers containing contractile proteins called actin and myosin. When a muscle is stimulated by a nerve impulse, the actin and myosin filaments slide past each other, causing the muscle to contract. The force of the contraction depends on the number of fibers that contract.

Muscles require a constant supply of oxygen and nutrients, which are provided by a network of blood vessels, including arteries, veins, and capillaries. Blood vessels also remove waste products like carbon dioxide and lactic acid. Additionally, muscles need ATP (adenosine triphosphate) for energy to maintain their function. ATP is produced through cellular respiration, which involves the breakdown of glucose and other nutrients.

Muscles are innervated by nerves at the end motor plate, also known as the neuromuscular junction. This junction is a specialized region where the motor neuron and muscle fiber come into contact. It plays a vital role in transmitting electrical impulses from the nervous system to the muscle fiber, leading to muscle contraction. The neuromuscular junction consists of the presynaptic terminal, synaptic cleft, and postsynaptic membrane. The presynaptic terminal releases the neurotransmitter acetylcholine, which binds to receptors on the postsynaptic membrane, initiating muscle fiber depolarization and contraction.

The sliding filament theory explains how muscles contract at the molecular level. Muscle contraction occurs in the sarcomeres, the functional units of skeletal muscle. Sarcomeres consist of thick myosin filaments and thin actin filaments. During contraction, myosin heads interact with actin filaments, forming cross-bridges and causing the filaments to slide past each other, shortening the sarcomere and the muscle. This process involves the binding and hydrolysis of ATP, regulated by the release and uptake of calcium ions from the sarcoplasmic reticulum.

Muscles work together in groups to produce movement and maintain posture. They can be named based on their location, shape, size, direction of fibers, and function. Understanding the naming convention helps accurately communicate the location and function of different muscle groups in the human body.

The muscular system is a complex and essential system for human movement and function. It allows us to perform various activities, from basic movements like breathing and blinking to complex movements like dancing or playing sports. Understanding the structure and function of muscles is crucial for comprehending how the body works and how to maintain good health.