Musculoskeletal System Flashcards
Define the myofibril, sarcomere, and thick and thin filaments, as well as light and dark bands. Describe their structures. What are their purposes?
Skeletal muscle has a striated pattern due to myofibrils - the building blocks of muscle fibres. The striation patterns that these myofibrils form is due to thick and thin filaments of myosin and actin, respectively. These thick and thin filaments appear as dark and light bands under the microscope. Sarcomeres are the repeated arrangement of these protein (individual units) which comprise the myofibril. Many myofibrils together held within a sarcolemma (cell membrane), along with sarcoplasm, are what comprise an individual muscle fibre.
Thick filaments reside in the middle of the sarcomere, and overlap with thin filaments - they appear dark due to the high levels of thick myosin protein. Myosin molecules consist of a helical tail with two heads at the end, their functional units. The tails orientate to the centre of the sarcomere, and the heads point towards the Z-lines surrounding the collection of tails in 360 degrees, allowing for smooth and progressive transduction of contraction.
Thin filaments are made of globular actin, which is arranges into helices. In the groove of the helical structure is tropomyosin, an inhibitor of binding between actin and myosin.
What is the purpose of the fibrous sheet which wraps around muscle fibres?
The fibrous sheet which wraps around individual muscle fibres allows for them to move independently of one another, contracting without causing friction.
Define the A-band, H-hand, I-band, M-line, and Z-lines of a sarcomere.
1) The A-band is the dark band of a sarcomere. It is composed of the thick filaments, including where they overlap with the thin filaments.
2) The H-band is the part of the thick filaments which do not overlap with the thin (centre of the sarcomere).
3) The I-band is the thin filaments which extend to the outside of the sarcomeres, where they do not overlap with the thick filaments.
4) The M-line is the middle line of a sarcomere (centre of the H- and A-bands).
5) The Z-lines are dark bands which run perpendicular to the fibres, indicating the junction between to adjacent sarcomeres.
Describe the physiology of skeletal muscle contraction.
When the action potential of a motorneuron reaches the neuromuscular junction, excitatory acetylcholine (acting on nicotinic receptors) causes the influx of sodium into muscle fibres from the T-tubules of the sarcoplasmic reticulum. These calcium ions diffuse into the myofibrils and fuse to troponin C, one of the components of the troponin complex of the thin filaments. Troponin T then changes conformation due to this, and pulls out tropomyosin from the groove of the globular actin helix.
The myosin heads of the thick filaments are then able to bind to the globular actin of the thin filaments, and ATP present on the heads is converted to ADP and Pi (by myosin ATPase), which are released. The release of ADP from the myosin head causes it to chanage conformation such that it pivots and bends, pulling on the actin filament (applies load), and sliding the Z-lines towards the M-line (I-bands contract but the A-bands do not).
When this occurs within one pair of myosin heads within a single sarcomere, the effect is negligible - this occurs many times to give a net effect of muscular contraction.
At the ends of sarcomeres, elastic (titin) filaments hold the myosin to the Z-lines, transducing the work done by myosin ATPase. This titin is substituted for dystrophin where the sarcomere connects to a tendom, in order to transduce the net work done onto the tendon, and subsequently, the skeleton.
Define the axial and appendicular skeletons, and the intermediate points.
The axial skeleton includes the skull, spinal column, thorax, and pelvis (the core of the skeleton), and the appendicular skeleton includes the limbs and appendages. The shoulder and pelvis girdles are the intermediate points between the axial annd appendicular skeletons.
Describe the structure, histology, and functions of bones, joints, cartilage, ligaments, and tendons.
1) Bones give basic structure and support to the body - as well as protection. They act to absorb the work done by locomotion, including posture, exercise, etc. They also store minerals and chemical energy, and are responsible for the production of blood.
Spongy red marrow is mostly located towards the heads of long bones, it is vascularised, and is the site where red and white blood cells (and platelets) are formed. Yellow marrow is found in the shaft of long bones, and is primarily used for the storage of fats.
The epiphyseal line is the point at which growth of bones take place - there is a proximal epiphysis and a distal epiphysis. The former is closest to the axial skeleton with the latter further away.
Flat and curved bones tend to be very resistant to shock, and so are used for protection. Short, square bones are good for absorbing stress in many directions (typically used in bony joints), and irregular bones have shapes which are necessary for their functions.
Osteocytes are mature bone cells, osteoblasts are bone-forming cells, and osteoclasts are bone-destroying cells (remodel the bone matrix and release calcium).
2) Joints are fundamental to the function of an organism, as they are pivoting points which provide the parameters of movement.
3) Cartilage is important in absorbing the shock applied to joints, in order to protect them from injury.
4) Ligaments join bone to bone to keep our body morphologically stable, and tendons join muscle to bone, in order to transduce work done onto the skeleton.
Detail the structures of the 3 main kinds of joints.
1) Fibrous joints are immovable - they connect bones to be rigidly held together (e.g. in the skull and pelvis)
2) Cartilaginous joints are slightly movable - here, the bones are attached by cartilage (e.g. spine and ribs).
3) Synovial joints are freely movable - cavities between the bones are filled with synovial fluid (to lubricate and protect). There are many kinds of synovial joints, with structures related to their functions. For example, pivot joints in the C1-2 vertebrae, hinge joints in the elbows, ball-and-socket joints in the hips, etc.
Describe the 3 classes of levers (joints).
Joints can be classed as different types of levers based on the fulcrum - the position of the joint with respect to the force exerted by the external environment and the muscles used to resist that.
Class 1 levers are like seesaws - the fulcrum lies central to the force exerted by the external environment and the muscles which resist. They match resistance with force to keep posture stable (for example, C1-2 pivot joints have force exerted from the neck muscle exerted onto them in order to keep the head stable against gravity).
Class 2 levers are like wheelbarrows - the fulcrum is found on one side, and the resistance is found centrally. The force is applied on the opposite side to balance this (the most mechanically efficient levers). For example, when standing on your tiptoes, the resistance to the force of gravity is applied via the calf muscles.
Class 3 levers are like tongs, where the fulcrum is where they come together, the force applied from the muscle is where you push the tongs together, and the resistance comes from the thick you are picking up. For example, when lifting an object with one arm, the resistance is applied where you are gripping the object, the force comes from the bicep and shoulder muscles, and is transduced to the shoulder and elbow joints.
Explain the two categories of descending efferent pathways motorneuron axons take from the CNS to effectors in the PNS.
1) Lateral pathways descend run down the sides of the spinal cord - some will decussate before reaching the spinal cord, and other will do so within it. For voluntary movements, these pathways begin in the cortex, and cross over at the medulla. An example is rubrospial tracts, which project from the mid-brain, and are associated autonomic motor control (e.g. postural) - brain lesions to lateral corticospinal pathways usually results in paralysis to the opposite side.
2) Ventromedial corticospinal tracts run in front of and behind the spinal cord, and are usually associated with posture and autonomic movements (e.g. walking). These pathways always originate in the brainstem.
The vestibulospinal tract is involved in head balance and turning, the tectospinal tract is involved in orientation, the pontine ventromedial tracts enhance postural reflexes (agonist), and the medullary ventromedial tract liberates postural muscles from reflex (antagonistic)
Explain the brain areas involved in planning and execution of movements.
Planning of movement occurs in brain areas 4 and 6 (primary motor cortex and higher motor area, respectively). As well as these, the premotor area gives sensory guidance, and the supplementary motor area mediates intentional preparation of movement (plans the specific sequences and timings).
1) Electrical stimultion of area 4 results in activity of hundreds of thousands of motor neurons.
2) Each motor neron causes a tiny muscle contraction associated with a particular direction.
3) The net stimulation of many motor neurons gives the resulting motion - In other words, each cell represents a single vote, and the final direction and force of movement is determined by a tally of them.
The cerebellum calibrates and coordinates the sequence of muscle contractions.
Explain how the limbic system can influence skill acquisition.
The limbic system is vital for learning and memory, and integrates emotional and motor information. The basal ganglia of the limbic system projects into area 6 (M1), and M1 projects back into the basal ganglia to form a loop. Dopaminergic neurons form the pathway from the substantia nigra pars compacta into the basal ganglia circuit, inducing movement via D1 (excitatory) and D2 (inhibitory) receptors, giving a more coordinated response for fine control of movement.
Define a motor unit and state Henneman’s size principle.
Motor units consist of a motorneuron along with all the muscle fibres which it innervates. Motor unit ratios are the ratios of motor nerves to innervated muscle fibres. In a large muscle, such as muscles of the back, as many as a hundred muscle fibres may be innervated by each motor nerve. Where you require finer resolution of force, such as in the fingers, there may be 10 muscle fibres innervated by each nerve. In the eyes, the ratio is 1:1.
Henneman’s size principle states than smaller motor units will always be recruited before larger motor units. This is important in order to minimise fatigue.
Describe the 4 main mechanoreceptors found in the skin.
1) Merkel disk receptors are located in the epidermis (near the dermis border), and respond best to steady pressure from small objects. They fire continuously but at low frequencies with small receptive field sizes.
2) Ruffini cylinders are found in the dermis, and respond best to steady pressure and stretching of the skin. They are similar to Merkel receptors, but fire at much higher frequencies and have much larger receptive field sizes (necessary as they are deeper within the skin).
3) Meissner corpuscles are found in the dermis (just below the epidermis), and respond best to rubbing against the skin of skin movements across a surface. They are fast adapting, and tell us when something happens suddenly (respond quickly but not continuously). They fire at low frequencies, and have small receptive field sizes.
4) Pacinian corpuscles are found deep in subcataneous fat of the dermis, and respond to many stimulations, especially vibrations. They are also fast adapating, and tell us when something happens suddenly, reponding quickly but not continuously. They fire at very high frequencies, and have very large receptive field sizes.
Describe the two main proprioceptors found in muscle and tendons.
1) Muscle spindles are stretch receptors - they work via sensory fibres which are wrapped around intrafusal muscle fibres. When these fibres stretch, the coils of dendrites wrapped around them open ion channels to generate an action potential.
2) Golgi tendon organs measure the force output with sensory dendrites which are interwoven with collagen fibrils within the tendon. The amount of pull on the tendon (i.e. the amount of work being transduced from the muscle to bone) is sensed by movement of these collagen fibrils, which move the golgi tendon organs.
Explain how the vestibular-occular system, proprioceptors, and mechanoreceptors inform the body’s position and activity.
All systems - occular, vestibular, somatosensory, and more, feedback to one another such that the brains attention is focused on relevant information to aid movement. Information from the occular system about the external environment is combined with information we have learned in the past (e.g. ice = slippery). Vestibular organs inform our brain of position of our head and movements. Muscle spindles and golgi tendon organs inform us of the position of our limbs, which can be cross-referenced with the mechanoreceptors within the skin and information from visual and auditory inputs. All inputs are integrated in the brain to give us an idea of the position of our body within our environment, and informs us of any potential dangers and necessary movements which are required.
Additionally, internal proprioceptors, such as stretch receptors in blood vessel walls, aid our ANS in regulating homeostasis.
