Posted by angel on Sunday, January 31, 2010 at 10:44am.
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Situation: You need to step up on a step to reach an object on a high shelf over your head. Your starting point should be anatomical position and your finishing point should be the position you are in once your hand has reached the object overhead.
(Part 1) Trace the nerve impulse, listing the “macroscopic” structures and steps in as much detail as possible, from which your brain sends the message to the appropriate muscles to step up one step.
(Part 2) Include the steps involved in transmitting the impulse “microscopically” through an individual neuron, from one neuron to another, and then from the neuron to the muscle fiber.
(Part 3) Include in this the steps involved in the actual muscle fiber contraction – a.k.a. the Sliding filament theory.
(Part 4) Specify the movements, muscles, bones, and joints involved in stepping up on the step. Explain any and all movements individually across each joint involved specifying the actions involved (i.e. flexion, extension, etc.), the muscles causing such actions, the bones being pulled on by said muscles, the types of joints involved, and how these movements collectively relate to the overall scenario/movements in this application.
(Part 5) Next, specify the movements, muscles, bones, and joints involved in reaching up above your head to the object on the shelf. Again, explain any and all movements individually across each joint involved specifying the actions involved (i.e. flexion, extension, etc.), the muscles causing such actions, the bones being pulled on by said muscles, the types of joints involved, and how these movements collectively relate to the overall scenario/movements in this application..
*Remember that all answers should be in your own words.
Motor neurons with in the spinal cord are used to move the muscle. Motor neurons create an electrical impulse with in the spinal cord. This impulse moves down \
The axons of the motor neurons transmits from the brain along the spinal cord to the nerve that extends the tip of the axons that comes to the muscle fiber...
Part 1: The movement begins in the brain with a voluntary thought. The motor areas of the brain are located in the cerebral cortex. To be more specific, the primary motor area is in the precentral gyrus of the frontal lobe. Once the motor area of the cerebral cortex (the anterior of each hemisphere) sends the signal to move the arm overhead, it transmits the signals via action potential.
The signals then go to the thalamus (relays motor information) and cerebellum (coordinates movements).
This all takes place in the central nervous system.
The action potentials are then sent via neurons that control the muscle fibers which control the arm.
The neurons which relay the information to the muscles pass through the spinal cord via nerves.
The neurons involved in our desired actions belong to radial, axillary, sciatic, and femoral nerves. Subsequently, the information is transmitted to the muscle fibers, which cause the contractions of the appropriate muscles. This latter part of the signaling takes place within the peripheral nervous system.
The movement begins in the brain as a voluntary thought, which results in the production of action potentials that are sent to the nervous system and then to the particular neurons that control the muscle fibers that tell those muscles when to work; in this case, that you would like to step up. The action potential starts in the central nervous system until it reaches an alpha neuron, which in turn transmits the signal down its axon. The action potential is continued as the signal moves along the length of the axon until it reaches the neuron end. When the impulse reaches the end of the axon, it causes a release of calcium. This calcium causes the release of acetylcholine in to the space between the neuron end and motor portion that is ahead of it. The acetylcholine binds to acetylcholine receptors on the motor portion of the cell. This binding activates an inherent ion channel for sodium and potassium, allowing an influx of positively charged sodium to enter the cell. This results in an action potential. The potential spreads along the muscle fiber in a forward motion, so that each successive motor neuron is activated by the last one it had contact with. These steps take advantage of the fact that calcium can be recycled back to the cell’s sarcoplasmic reticulum to allow the movement to continue forward. The released calcium binds to troponin C that is on the actin associated thin filaments of the myofibrils. Tropomysin is normally there to prevent myosin binding; however, in this case, troponin T permits the movement of tropomysin, and frees it from the binding. Ready-state myosin binds to the newly freed binding sites on the thin filament. Myosin is now bound to actin in the strong binding state. The Z-bands are now oriented towards each other, which shortens the sarcomere and the I-band of the muscle. Next, myosin binds ATP, which lets it release the previously bound actin. These steps continue repeatedly, provided ATP is available and calcium is present.
The signals, which can accumulate via temporal or spatial summation, are transmitted from the synaptic terminal of the presynaptic neuron to the postsynaptic membrane of the dendrites of the postsynaptic neurons. A typical neurotransmitter used between two neurons is acetylcholine. Binding of neurotransmitter at the postsynaptic membrane can cause graded potentials, via either depolarization or repolarization. In case of acetylcholine, binding of acetylcholine causes graded potential via depolarization. If depolarization reaches a threshold, an action potential is generated in the axon hillock. At this point, voltage gated sodium channels open, and sodium ions rush into the cell. The action potential then is transmitted down the axon, further opening voltage gated sodium channels (once enough sodium enters the cell, voltage gated sodium channel closes and potassium channel opens, which allow potassium ions to exit the cell, which allows the cell to return to the resting state). In myelinated neurons, salutatory propagation may occur. In either case, once the action potential reaches the synaptic cleft, calcium ions enter the synaptic cleft, which trigger exocytosis of neurotransmitter (in our case acetylcholine). Acetylcholine, then, can bind to the postsynaptic membrane of the postsynaptic neuron to generate graded potential. If enough graded potential is generated, another action potential results.
When the neuron is connected to a muscle, postsynaptic membrane is replaced with neuromuscular junction. In this case, the neurotransmitter used is always acetylcholine. Acetylcholine, like before, binds to the neuromuscular junction causing a depolarization. Enough depolarization (threshold) causes the sarcoplasmic reticulum to release calcium ions. Then, the calcium ions activate the muscle and cause them the contract.
Sliding filament theory is based on the observation that the muscle fibers are composed of two kinds of filaments: thick filaments and thin filaments. Muscle fibers are composed of repeating units of sarcomere. Each sarcomere contains overlapping thick and thin filaments. In the middle of the sarcomere are thick filaments, which makes up the A band. The length of the A band does not change during contraction. The thin filaments are interlaced between the thick filaments and also overlap with the lateral portion of the thick filaments. The area of overlap is called the zone of overlap. The myosin head of the thin filament can bind to the active site of the thick filament. The contraction of the muscle requires, then, movement of the thin filaments toward the thick filament medially. This is where the calcium ions are required. Calcium ions can bind to troponin, a molecule on the thick filament near the active sites where myosin head of the thin filament can potentially bind. Due to binding of calcium, troponin changes shape and moves the active site so that it can interact with myosin head of the thin filament. Further, ATP is required for the “power stroke” of the myosin head, which can then move toward the center of the sarcomere. When another ATP binds to the myosin head, the active site and myosin head of the thin filament detach. Because of this, the active site is able to form another cross bridge, allowing further muscle contraction. Hence, during the muscle contraction the zone of overlap lengthens while the H zone, the part of sacromere composed of thick filament only shortens.
The iliacus muscle contracts, pulling on the femur distal to lesser trochanter (the insertion point of the iliacus), causing the flexion of the hip joint, which is a type of ball-and-socket diarthrosis joint. Also, contraction of tensor fasciae latae and psosas major pull on the iliotibial tract (the insertion point of tensor fasciae latae) and lesser trochanter (the insertion point of psosas major), respectively, causing the flexion of the hip joint (again, a ball-and-socket joint). Further, Sartorius muscle pulls on the medial surface of the tibia near the tibial tuberosity (the insertion point of Sartorius), causing flexion of the knee joint, which is a hinge type of synovial joint. These muscle contractions are involved in lifting of the leg above the step. Next, the leg needs to extend toward the ground surface of the step to lift the rest of the body above the step. So the contraction of the gluteus maximus, pulling on the iliotibial tract and gluteal tuberosity of femur (the insertion points of gluteus maximus), causes the extension of the hip joint, which is a ball-and-socket type of diarthrosis joint.
When stepping up on the step, the soleus, quadriceps femoris, hamstrings and gluteus maximus are attached to and ‘pull’ the hip bone, femur, patella, tibia, and fibula, As for the involved joints, these include the hip joint, knee joint, ankle joint, and vertebral column.
The deltoid muscle contracts, pulling on the deltoid tuberosity of humerus (the insertion point of deltoid muscle) to cause extension of the shoulder (glenohumeral) joint which is a ball-and-socket type of diarthrosis joint. Also teres major and latissimus dorsi pull on medial lip of intertubercular groove of humerus (the insertion point of teres major) and the floor of intertubercular groove of the humerus (the insertion point of the latissimus dorsi), respectively, to cause extension of the shoulder joint (again, ball-and-socket, diarthrosis).
Also, contraction of biceps brachii pulls on the tuberosity of radius (the insertion point of the biceps brachii) to supinate the elbow joint, which is a hinge type of diarthrosis joint. Similarly, Supinator pulls on the anterolateral surface of radius distal (the insertion point of the supinator), to cause a supination of the elbow joint (hinge type of diarthrosis).
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