The body systems that are involved in typing this assignment are the skeletal system, the respiratory system, the cardiovascular system, the nervous system, the integumentary system, and the muscular system.
The skeletal system is comprised of the bones, the cartilages, ligaments, and other connective tissues. It produces blood cells, and provides protection, leverage, structure, and support for the body. It works with the muscular system and the nervous system to enable body movements. The bones in the arms work with the muscles in the upper arm and forearm to contribute to the movement required to perform the action of typing.
The respiratory system includes the nose, the trachea, and the lungs. This system takes in oxygen and releases carbon dioxide. The trachea branches into two bronchial tubes, each connecting to a lung. The bronchial tubes divide into bronchioles (smaller tubes). The bronchioles end in air sacs (alveoli.) When a person breathes, the oxygen passes through this system and enters the blood stream through the walls of the air sacs. The respiratory system contributes by helping a person breathe while typing an assignment.
The heart, veins, arteries, capillaries and blood make up the cardiovascular system. The arteries, veins, and capillaries act as the body’s transport system, and deliver oxygen to the cells. The oxygen delivered to the muscle cells in the arms and fingers provide the necessary movement that enables the fingers to type. The oxygen delivered to other body groups also helps with the motion of typing. For example, the brain needs oxygen to determine the information that needs to be typed.
The integument system consists of the skin and its accessory structures such as hair and nails. The skin forms a tough barrier that enables the fingers to be able to type. It also protects the tissues from abrasion caused by typing.
The nervous system constitutes the brain, the spinal cord, and nerves. It is the master controlling and communications system of the body. It is organized into two main divisions: the Peripheral Nervous System (PNS) and the Central Nervous System (CNS). CNS consists of the spinal cord and brain, and the PNS consists of the nerves and the ganglia. The nervous system has three overlapping functions. It monitors the sensory input regarding the changes in body from the sensory receptors via “afferent” neurons. It performs sensory integration, which includes processing, interpreting, and acting on sensory information. In response to sensory input, it provides motor output, which involves activation of effector organs via efferent neurons.
When a person begins to type (when fingers touch the keyboard), a stimulus is created that produces a graded change in the membrane potential of the PNS sensory receptor cells. This stimulus depolarizes the receptor cells to a threshold, generating an action potential. Axons of the sensory neurons carry information about the type of stimulus (touch, pressure, temperature) as action potentials to the CNS. Information processing occurs at every synapse and information is distributed to multiple nuclei and centers in the spinal cord and brain.
The largest portion of an adult brain is the cerebrum. It is divided into two hemispheres that receive sensory information, and send motor commands to the opposite side of the body (contralateral). The cerebral cortex (outer gray matter) has domains that have discrete cortical areas where sensory or motor functions are localized. In the brain, the general somatic sensory information is relayed to the Primary Somatosensory Cortex neurons. The Premotor Cortex or Somatic Sensory Association area, monitors the activity of the Primary Somatosensory Cortex, and allows a person to recognize the action of fingers touching the keyboard. The Premotor Cortex then relays the instructions to perform the act of typing to the Primary Motor Cortex. The Primary Motor Cortex neurons (pyramidal cells) then direct voluntary movements by controlling somatic motor neurons in the brain stem and spinal cord.
The basal cell nuclei are masses of gray matter that are located deep inside the white matter of the cerebrum. The basal cell nuclei and cerebellum are responsible for coordination and feedback control over muscle contractions. Specialized neurons from the Primary Motor Cortex extend their axons all the way to the striatum portion of the basal nuclei. These cortical neurons release the neurotransmitter glutamate that excites the cells in the basal cell striatum. The excited striatum cells project in two different directions giving rise to two major pathways: the “direct” and the “indirect” pathways. It is believed that the direct pathway selectively facilitates certain motor (or cognitive) programs in the cerebral cortex that are adaptive for the present task such as typing, whereas the indirect pathway simultaneously inhibits the execution of competing motor programs (Knierim, 1977). The indirect pathway uses Gamma Aminobutyric acid (GABA) as a neurotransmitter to inhibit the competing motor activities. The levels of action and inhibition are determined by the levels of dopamine produced by the substantia nigra of the basal cell nuclei.
When a specific motor neuron (pyramid cell) located in the spinal cord receives an excitatory neuron, it generates an action potential that travels rapidly along the nerve to a special type of synapse called the neuromuscular junction. At this synapse, the action potential initiates an electrochemical process where nitric oxide diffuses into the presynaptic neuron and releases the neurotransmitter Acetylcholine (ACh) into the space between the muscle fiber and presynaptic terminal. The ACh molecules bind to the ion-channel receptors on the muscle membrane causing the cation channels to open. This allows for greater flow of sodium ions into the muscle cell, initiating a series of steps that produce muscle contraction in forearm, hand, and fingers that are involved in typing. The effects of ACh are temporary since the enzyme Acetylcholinesterase (AChe) present in the postsynaptic membrane quickly breaks it down into choline and acetate. The choline is actively absorbed by the axon terminal and is used to synthesize more ACh, using the acetate provided by the coenzyme A and thus the cycle continues.
Simultaneously, one area of the Premotor Cortex, The Frontal Eye Field that controls voluntary eye movements, scans the lines being typed. The Visual Cortex of the occipital lobe receives the visual information which in this case are the words being typed. The Visual Association Area of the cortex monitors the activity in the Visual Cortex, and interprets the information being typed. The Broca’s area present in only one hemisphere of the cerebrum, aids in language comprehension, and motor activities associated with hand movements. With repetition, proper pattern of stimulation becomes stored in the Premotor Cortex, which coordinates learned movements such as typing. Typing can then be performed smoothly and easily by triggering a stored pattern, and not through controlling individual neurons.
The muscular system consists of tissues, and works with the skeletal system and nervous system to control body movements. The three basic types of muscle tissues are: the skeletal muscles that aid in movement, the smooth muscles are found inside organs such as the stomach, and the cardiac muscle that is found in the heart. Most of the muscles that help in typing this assignment are located in the forearm. They move the hand, wrist, and fingers based on the impulses received from the PNS.
Several superficial, deep posterior and anterior muscles move the forearm and hand. The anterior superficial muscles (biceps brachii, short head; triceps brachii, long head; brachialis; brachioradialis) and posterior superficial muscles (triceps brachii, long head; ticeps brachii, lateral head; brachioradialis; anconeus) move the forearm.
The anterior superficial muscles, flexor carpi radialis, palmaris longus, and flexor carpi ulnaris work together to produce flexion, abduction, and adduction at the wrist. The posterior superficial muscles, extensor carpi radialis longus, extensor carpi ulnaris, extensor carpi radialis brevis provide extension, abduction, and adduction. The muscles, pronator teres, supinator, and pronator quadratus provide pronation and supination of the forearm. The middle and deep layer anterior muscles (flexor digitorium superficialis, flexor pollicis longus, and flexor digitorum profundus) flex the fingers and thumb. The posterior deep layer muscles, extensor digitorum, and extensor digiti minimi extend the fingers. The deep layer muscles, extensor pollicis longus, abductor pollicis longus and exntesor pollicis brevis move the thumb.
The large muscles of the forearm end before reaching the wrist, and only their tendons cross the articulation ensuring maximum mobility at both wrist and hand. The muscles in the forearm known as the extrinsic muscles provide strength and crude control of hand and fingers. The intrinsic muscles originate on the carpal and the meta carpal bones and provide fine control of the hand exercised during typing.
All the body systems described above work together and contribute to the action of typing this assignment. The nervous system determines that the action of typing needs to be performed. It sends and receives impulses to and from the skeletal and muscular system to execute the action. The skeletal and muscular systems perform the action of typing. Simultaneously all other body systems such as the cardiac system, endocrine system, respiratory system, work in the background to keep the body positioned and active to aid in the action of typing.
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