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... ws the cross-bridge to detach and re-attach to another active site on an actin molecule. This contraction cycle is repeated as long as free calcium is available to bind the troponin and ATP is available to provide the energy. The signal to stop contraction is the absence of the nerve impulse at the neuromuscular junction. When this occurs, an energy requiring calcium pump located within the sarcoplasmic reticulum begins to move the calcium back into the sarcoplasmic reticulum. This removal of calcium from troponin causes tropomyosin to move back to cover the binding sites on the actin molecule and cross-bridge interaction ceases.
It is possible for skeletal muscle to exert force without the joint angle changing. This might occur when an individual pushes against the wall of a building. Muscle tension increases bu t the wall does not move, so neither does the body part that applies to the force. This is called an isometric contraction.
Isometric contractions maintains a static body position during periods of standing or sitting. In contrast most types of exercise involve contractions that result in movement of body parts. This is called an is isotonic contraction. Tension within the muscle increases but the joint angle changes as the body parts move. Skeletal muscle can be divided into three types of fibers. These are: fast-twitch fibers (fast-glycolysis), low-twitch fibers (slow oxidative), and intermediate fibers (fast oxidative glycolysis).
Fast-twitch fibers have a small number of mitochondria, a limited capacity for aerobic metabolism, and are less resistent to fatigue than slow-twitch fibers. However, fast-twitch fibers are rich in glycogen stores and glycolysis enzymes, which provide them with a large anaerobic capacity. In addition, fast-twitch fibers contain more myo fibrils and ATPase than slow-twitch fibers, and are therefore able to contract more rapidly and develop more force than the slow-twitch fibers. Slow-twitch fibers contain larger numbers of mitochondria and are surrounded by more capillaries than fast-twitch fibers. In addition, slow-twitch fibers contain higher concentrations of the red pigment myoglobin. The high concentration of myoglobin, and the high content of mitochondrial enzymes provide slow-twitch fibers with a high capacity for aerobic metabolism and a high resistance to fatigue.
Intermediate fibers contain biochemical and fatigue characteristics that are somewhere between fast-twitch and slow-twitch fibers. The amount of force exerted during muscular contraction is dependent on a number of factors. These include the types and the number of motor units recruited, the initial length of the muscle, and nature of the neural stimulation of the motor units. Variations in the strength of contraction within an entire muscle depends on the number of muscle fibers that are stimulated to contract. If only a few motor units are recruited, the force is small. If more motor units are stimulated the force is increased.
As the stimulus is increased, the force of contraction is increased due to the recruitment of additional motor units. The peak force generated by muscle decreases as the speed of movement increases. However, the amount of power generated by a muscle group increases as a function of movement velocity. The muscle spindle functions as a length detector in muscle. Golgi tendon organs continuously monitor the tension developed during muscular contraction. In essence, Golgi tendon organs serve as safety devices that help prevent excessive force during muscle contractions.
The 206 bones of your body protect and support your organs and allow movement. Bones are living, changing structures that require adequate calcium and weight-bearing exercise to build and maintain their density and strength. Bones are joined together by different types of joints: fixed joints (as in the skull), hinged joints (as in the fingers), and ball-and-socket joints (as in the shoulders and hips). The bones function as a lever. The bones of the upper and lower limbs push and pull, with the help of muscles.
Bones are also a calcium store. 97 % of the body's calcium is stored in bone. Here it is easily available and turns over fast. In pregnancy the demands of the fetus for calcium require a suitable diet and after menopause hormonal control of calcium levels are impaired which can cause brittleness and a chance for osteoporosis to occur. In addition, bones are a marrow holder. This is secondary to produce maximum strength for minimum weight. The cavities produced in unstressed areas are used for marrow, or in some places just for storage.
Around the outside is a layer of strong, hard, heavy compact bone. In the middle is a branching network of trabecular bone which usually follow lines of force. Marrow sits in the interconnecting cavities between those plates or rods of bone. A joint is formed by the meeting of two or more bones. A joint can allow full movement (synovial), little movement (cartilage nous), or no movement (fibrous). With immovable joints, bones are joined by cartilage (ex: rib meets sternum) or a series of dove tailed edges (ex: skull).
Slightly movable joints are where bones are joined by ligaments only (ex: where tibia and fibula meet) or by ligaments and fibrous cartilage (ex: between vertebrae). Freely movable joints are where both ends of the bone are covered with cartilage and surrounded by a fibrous capsule. This capsule is lined with smooth tissue called synovial membrane which secretes a fluid to lubricate the joint. This type of joint is strengthened by ligaments and is the most common type of joint. There are six different types of freely movable joints: 1) Pivot - bone rotates on a fibrous ring; 2) Saddle - thumb joints, the articular surfaces fit together concave 3) Condyloid - convex surfaces fit into concave, free movement, but no rotation (ex: wrist); 4) Gliding - vertebrae of spine, two nearly flat surfaces glide over 5) Hinge - joint movement is in one plane (ex: elbow); 6) Ball-and-socket - the shoulder and hip joints are the only ball and socket joints in the body.
Bone head fits into cup-like cavity, movement is allowed in any direction. They are the most freely movable synovial joints. Types of movement of synovial joints: - flexion - decreasing the angle between two bones - extension - increasing the angle between two bones - abduction - moving the bone away from the midline - adduction - moving the bone towards the midline - rotation - moving the bones around a central axis - circum duction - complete circular movement - elevation - raising a part of the body - depression - lowering a part of the body The nervous system is the body's means of perceiving and responding to events in the internal and external environments. Receptors capable of sensing touch, pain, temperature, and chemical stimuli send information to the central nervous system (CNS) concerning changes in our environment. The CNS responds by either voluntary movement or a change in the rate of release of some hormone from the endocrine system, depending on which response is appropriate. The nervous system is divided into two major divisions, the central nervous system and the peripheral nervous system.
The central nervous system includes the brain and the spinal cord, and the peripheral nervous system includes the nerves outside the central nervous system. Nerve cells are called neurons and are divided anatomically into the cell body, dendrites, and axon. Axons are covered by schwann cells, with gaps between these cells called nodes of ranvier. Neurons are specialized cells that respond to physical or chemical changes in their environment.
At rest, nerve cells are negatively charged in the anterior when compared to the electrical charge outside the cell. This difference in ele clerical charge is called the resting membrane potential. A neuron fires due to a stimulus changing the permeability of the membrane, allowing sodium to enter at a high rate, depolarizing the cell. When the depolarization reaches threshold, an action potential or nerve impulse is initiated. Re polarization occurs immediately following depolarization due to an increase in membrane permeability to potassium, and a decreased permeability to sodium.
Neurons communicate with other neurons at junctions called synapsis. Synaptic transmission occurs when sufficient amounts of a specific neurotransmitter are released from the presynaptic neuron. Upon release, the neurotransmitter binds to a receptor on the post synaptic on the postsynaptic membrane. An excitatory transmitter increases neuronal permeability to sodium and results in excitatory postsynaptic potentials. However, some transmitters are inhibitory and cause the neuron to become more negative or hyper polarized. This hyper polarization of the membrane is called an inhibitory postsynaptic potential.
Propriosceptors are position receptors located in joint capsules, ligaments, and muscles. The three most abundant joint and ligament receptors are free nerve endings, golgi-type receptors, and parisian corpuscles. These receptors provide the body with a conscious means of recognition of the orientation of body parts as well as feedback relative to the rates of limb movement. Reflexes provide the body with a rapid unconscious means of reacting to some stimuli. The vestibular apparatus is responsible for maintaining general equilibrium and is located in the inner ear. Specifically, these receptors provide information about linear and angular acceleration.
The spinal cord plays an important role in voluntary movement due to groups of neurons capable of controlling certain aspects of motor activity. The spinal mechanism by which a voluntary movement is translated into appropriate muscle action is termed spinal tuning. The brain can be divided into three parts: the brain stem, the cerebrum, and the cerebellum. The motor cortex controls motor activity with the aid of input from subcortical areas.
The cerebellum receives feedback from proprioceptor's after movement has begun and sends information to the cortex concerning possible corrections of that particular movement pattern. The basal ganglia are neurons involved in organizing complex movements and the initiation of slow movements. The promoter cortex operates in conjunction with the motor cortex to refine complex motor actions and may be important in acquisition of motor skills. The autonomic nervous sytem is responsible for maintaining the constancy of the body's internal environment and can be separated into two divisions: the sympathetic division and the parasympathetic division. In general, the sympathetic portion tends to excite an organ, while the parasympathetic portion tends to inhibit the same organ. Bibliography:
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