Notes on the Muscular and Nervous Systems

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Notes on the Muscular and Nervous Systems

Chapter 11: The Muscular System

The Musculoskeletal system of the human body is made up of muscles that provide support and stiffness to the skeleton joints. The muscles act as motors to the musculoskeletal system that allows movement of the skeleton. They can actively produce tension, and that gives it the capacity to stiffen the joints enabling change. 

Types of Muscles

There are three types of muscle tissues in the human body, namely:

The smooth or visceral muscle

This type of muscle is soft in appearance and are exceedingly extensible. They include the muscles in hollow organs like the uterus, bladder, intestines, and the stomach. Also, the walls of blood vessels, that is, veins and arteries, are made up of these muscles.

Cardiac Muscle

They include the walls of the heart. Just like the smooth muscles, cardiac muscles are innervated by an autonomic nervous system.

Skeletal muscle

These muscles have a striated appearance. They have parallel stripes that are regularly spaced, giving it its appearance. Unlike the cardiac and smooth muscles, skeletal muscles are multinucleated. The voluntary or somatic nervous system controls the muscles, and hence, their contractions are voluntary.

Functions of Muscles

Muscles are involved in controlling the movement and posture of humans as well as heat production and protection. As for stability, muscles serve as shock absorbers to outside pressures, thus protecting the body. Also, vital body organs such as the heart and abdomen are covered by muscles that protect them from harm. Similarly, through the contraction, most of the energy used is released to the body as heat, thus helps in keeping the body warm. Another function includes controlling blood pressure. The skeletal muscles alter the pressure in veins during contraction leading to the venous return of blood.

Skeletal Muscles’ Structure

Microstructure:

 Muscle cells look like a long, threadlike fiber that is 10-100 micrometers in diameter and 30 centimeters long. The thread is covered by a thin membrane known as the sarcolemma. The sarcolemma is then covered by the endomysium, which connects different muscle fibers to the tendon.

Each muscle fiber is made up of smaller threadlike structures, the Myofibrils that runs through its entire length. The Myofibrils are crossed with transverse and dark bands of light that give the skeletal muscle its striated appearance. The repeating unit between the stripes in a myofibril is called the Sarcomere, within which protein filaments overlap. Protein filaments are either thin (actin) or thick (myosin), and they overlap with each other at one end of the Sarcomere and join to other sarcomeres at the other end. The point of joining is referred to as a ‘Z line’ or band.

The ‘I band’ is the area that contains only actin filaments, and it appears like a light band. On the other side, the region that contains myosin filaments is darker and is known as the ‘A band.’ The band is known for its refraction of more than one wavelength of light.

Within the A band is an area where the satin and myosin do not overlap, and it’s called the H zone. M zone, or the middle disk, lies in the middle of the H band, and it connects various myosin.

Macrostructure.

A fascicle, also known as a fasciculus, is a combination of a hundred or more muscle fibers. The endomysium of each strand is connected to connective tissue, the perimysium that covers the fascicle. An epimysium is a connective tissue that binds different fascicles together to form a complete muscle. A muscle fiber contains myofilaments that run its entire length and are attached to the endomysium at the ends. The muscle fibers are joined through a series of connective tissues and are finally attached to a bone. Tendons are the cords that connect muscles to the bones.

Muscles are attached to movable limbs, and thus, the tendon move during contraction and relaxation of the flesh. The movement may create friction between adjacent muscles and, therefore, damage. Thus, there is a loose connective tissue between the muscles that help in reducing friction. A sac that lubricates soft tissues on bones and known a bursa is also available to assist in friction minimization.

Muscle action.

The contraction ability of muscles is used to differentiate them. Contraction refers to the development of tension within a tissue that causes it to pull on its attachments. Muscle action may involve shortening, lengthening, or muscles maintaining the same length.

Muscle actions include:

Concentric action- it occurs when an active muscle draws its attachments closer together, especially when the movement is in the same direction with that of the attached limb.

Eccentric contraction- it occurs when an active muscle draws its attachments further apart from each other. Mostly, it occurs when the muscle activity is in the opposite direction with the rotation of the attached limb.

Isometric contraction- occurs when a muscle is active, but its attachments do not move relative to each other. Mostly, the action occurs when the attached limb to the tissue does not rotate and thus no change in the kinetic and potential energy.

Muscles are given different terms to describe their roles relevant to other muscles or joint actions. Agonist muscles are those that can create a force in the similar direction of the motion of the joints. In this case, concentrically active muscles are agonistic to the actions occurring at the bones they are attached. An antagonist’s muscle is that which is capable of creating an opposite force to the attached joint. A stabilizer muscle prevents movement in a limb when the reference muscle contracts, especially if the change is unwanted.

A neutralizer is a muscle that creates a force to oppose the actions of other tissues. Synergy, on the other hand, is used to define a muscle that assists agonist muscles or a muscle whose torque adds up to the force of an agonist. The behavior of tissue is affected by the arrangement of muscle fibers or the myofibrils in the muscle. The maximum energy of a muscle depends on factors such as the cross-sectional area, length, and velocity of the tissue. Other factors include prestretch, fiber type, fatigue, and the duration of the stimulus.

Chapter 12: The Nervous System

The musculoskeletal system is managed and controlled by the nervous system. The control is done through the collection of information from the stimuli, processing it and then, initiating a response to it. The system is organized into the central and peripheral nervous systems. The central nervous system consists of the brain and the spinal cord, which are covered by the skull and vertebral column, respectively. The brain acts as the central processor, while the spinal cord transmits signals to and from the peripheral system. The peripheral system consists of 12 cranial nerves and 31 pairs of spinal nerves, which detects information from the external environment and sends stimuli to the muscles.

The nervous system can also be subdivided into the somatic and autonomic systems. The somatic system, also called the voluntary system, is involved in conscious sensations. On the other hand, the autonomic system is engaged with unconscious feelings and actions. The difference between the two is that the somatic nervous system controls movement while the autonomic part regulates the functions of internal organs. Neurons form an essential unit of the nervous system. The cell membrane of a neuron has an electrical potential that can change due to stimulation.

The types of neurons include:

Sensory or afferent neurons- these neurons receive stimuli from the external and internal environment and sends it to the central nervous system. Their cell bodies lie to the spinal cord.

Motor/efferent neurons- they receive stimuli from the other types of neurons and send the signal to the muscles. Their cell bodies are located within the spinal cord.

Interneurons/ connector neurons- they link both the motor and the sensory neurons.

A motor unit is a vital unit of the neuromuscular system, and it is composed of muscle fibers and a single motor neuron. The number of muscle fibers in a motor neuron represents the number of branches at the axon end. The ratio of motor neurons to the muscle fibers in a muscle represents the degree of control over muscle contraction that a person possesses.

The muscle fibers are distributed throughout the area of the tissue, and thus, adjacent fibers form a different motor unit. The potential of an individual muscle fiber membrane is usually small compared to the chemical stimuli of a neural action. Thus, a muscle action potential is always generated by a neural action potential. The force produced by a muscle contraction can be controlled by controlling the number of active motor units in the body. Also, power can be controlled by controlling the stimulation rate of the muscle. The ability of a motor unit to generate an action potential depends on the stimulus it receives from the neurons that makes it.

Receptors and Reflexes.

The interoceptors respond to stimuli from the internal environment or the sources inside the body, such as internal organs. On the other hand, exteroceptors react to external stimuli from external sources such as the five senses of touch, hearing, taste, smell, and sight. Some receptors initiate involuntary responses resulting from the sensory input and, known as reflexes. The reflexes are mostly functional during the development age of humans, although few continue to serve the protective function in adult-life.

Types of Reflexes

There are the Proprioceptors and the Proprioceptive reflexes. The proprioceptors are the sensory organs that monitor the status of the musculoskeletal system. Receptors are located within joints to give feedback regarding the joint position or within muscles to provide feedback on muscle length and tension.

The proprioceptors include:

The muscle spindle detects the changes in muscle length as well as its stretch. It contains several muscle fibers with sensory and motor neurons connected to them. Stimulation in a muscle spindle occurs as a result of the stretches in the muscles.

The Golgi tendon organ is located within the tendons and is in series with the tissue. Its sensory fibers are stimulated by tensions caused by either contractions or stretching of the muscles. The tendon reflex turns off the active development of pressure within a muscle, thus protecting it from rupturing if the tension is excellent. An example of a tendon reflex is the collapse of the leg of a high jumper because of the extreme force subjected to the knee muscles.

The vestibular system works together with the neck joint proprioceptors to detect changes in the head position as related to the neck. The righting reflexes maintain an upright body and head posture while Tonic reflexes facilitate extremity positions when the neck is twisted. Body movement is also influenced by exteroceptors, which include sense receptors. The touch receptors are known as Pacinian corpuscles. The touch receptors are sensitive to changes in pressure beneath the legs and at the palms of the hand.

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