Nervous and Musculosketal Systems

neurons

sensory neurons, relay neurons, motor neurons

Resting Potential is the difference in charge between inside and outside of a neuron created by the imbalance between K+ and Na+

• Action potential is an electrical impulse that travels down an axon. The response is triggered by a stimulus strong enough to produce a depolarization that reaches the threshold. Once a threshold is reached, all gates open.

Because the sodium-potassium pump moves three sodium out for every two ions, there is a higher concentration of sodium on the outside and a higher concentration of potassium on the inside. This, together with negatively charged proteins and chloride ions inside the cell, creates a negative charge on the inside, and a positive charge on the outside. When a rapid change in the membrane potential surpasses the threshold, caused by a stimulus strong enough to trigger depolarization, voltage-gated sodium ion channels open, followed quickly by the opening of the potassium channels. When the gates open, sodium and potassium go down their concentration gradients. The sodium gates open first, creating the rising phase. Then the pottasium gates open, which leads to the falling phase, culminating in an undershoot. This has to be resolved by the sodium-pottasium pump in order to restore the resting potential (repolarizing)

When the action potential reaches the synaptic terminal of an axon, it causes the membrane to become more permeable to Ca2+. This causes an influx of calcium ions which causes the synaptic vesicles to fuse with the presynaptic cleft by exocytosis. Then, the neurotransmitters diffuse across the cleft and bind to receptors on the postsynaptic membrane which also function as channels. This opens the channels, increasing the membrane to permeability to sodium, triggering the action potential in the postsynaptic cell. The neurotransmitter is broken down by a specific enzyme in the synaptic cleft (ex) acetylcholinesterase breaks down the neurotransmitter acetylcholine. The breakdown products are absorbed by the pre-synaptic neuron by endocytosis and used to re synthesize more neurotransmitter, using energy from the mitochondria. This stops the synapse from being permanently on.

• Bones: act as a rigid framework for muscles to pull against, muscles attachments and joint configuration determine direction of movement
• Muscles: Pull on the bones, producing movement
• Ligaments: join bone to bone
• Tendons: attach muscles to bones
• Nerves: stimulate and coordinate muscle contractions

• a. humerus
• b. synovial membrane (joint capsule)
• c. synovial fluid
• d. ulna
• e. cartilage
• f. ligaments

• 1) humerus
• 2) biceps
• 3) tendon
• 4) elbow joint
• 5) ulna

Humerus forms a hinge joint with the ulna and the radius forms a pivot joint with the ulna. The bicepts and triceps are attached to the humerus near the shoulder. The bicep is attached to the radius, and the triceps is attached to the ulna. The capsule wraps around the joint and prevents leakage of the synovial fluid. The synovial fluid provides a joint lubricant, which reduces friction and damage to the bone. The cartilage provides a smooth surface that reduces friction as well. Ligaments attach bone to bone to keep the joint stable. The tendons attach the triceps and biceps to bones.

• The hip joint is a ball and socket joint, able to emovie in many planes and in several directions (an arc)
• The knee joint is a hinge joint, able to move in a single plane

• Striated muscle fibers contain many myofibrils, and each myofibril is composed of units called sarcomeres. Protein filaments form the contractile unit of the sarcomere. Thin filaments are composed of actin and thick filaments are myosin. Within each sarcomere there are light and dark bands due to the partial overlap of filaments. The dark regions are regions of overlapping thick and thin filaments, and the light regions are only actin or myosin.
• Each muscle cell has many nuclei, because of the fusion of many cells into one. It also has many mitochondria to produce ATP required for the sliding filament action
• The sarcoplasmic reticulum stores calcium ions. When an action potential reaches a motor endplate (synapse with the muscle cell), it s transferred via the neurotransmitter acetylcholine to the sarcolemma. It travels along the sarcolemma down the T-tubule until it reaches the sarcoplasmic reticulum. This triggers the release of calcium ions, which free up the myosin binding sites on actin. This allows the myosin head to bind to the actin, allowing the sliding-filament action
• Once the myosin binding sites are exposed, myosin heads bind to the actin forming a crossbridge. ATP attaches to the myosin head.