Muscle Tissue PDF

Slide 3 Excitability: muscle tissue has the ability to respond to a stimulus. The stimulus for muscle contraction is usually a chemical. For example, in skeletal muscle, the chemical signal is the neurotransmitter acetylcholine. Contractility: muscle is unique because it has the ability to shorten...

Slide 3 Excitability: muscle tissue has the ability to respond to a stimulus. The stimulus for muscle contraction is usually a chemical. For example, in skeletal muscle, the chemical signal is the neurotransmitter acetylcholine. Contractility: muscle is unique because it has the ability to shorten when stimulated properly. In cardiac and smooth muscle, this shortening modifies the diameter of tubing so that the movement of substances is modified. In skeletal muscle, shortening of muscle is used as the force to create dynamic movement. Extensibility: muscle has the ability to stretch. Many tissues would be damaged if deformed, but muscle tissue can extend beyond its resting length. Elasticity: muscle fibers have the ability to recoil to their original shape after being stretched.\ The ability of muscle tissue to perform actions in the body occurs because of these four properties. Though we often think of movement with contractility only, the ability to selectively excite, stretch, and recoil works together with contractility to carry out functions in the body and allow movements in the environment.  Slide 4 The casual mention of muscle will often bring skeletal muscle to mind. But the human body contains three types of muscle that are all important to maintaining homeostasis. Smooth muscle- Function: smooth muscle helps change the size of a structure to alter delivery dynamics. For example, muscular arteries have the ability to vasoconstrict or vasodilate because smooth muscle contraction or relaxation changes the vessel diameter. Subsequently, the flow of blood can be manipulated throughout the body. Location: smooth muscle is found in hollow organs like respiratory bronchioles, arteries, and the gastrointestinal tract. Cell Morphology: each smooth muscle cell is a single cell, which is different than the other muscle types. Many contain gap junctions so intracellular fluid is shared. Regulation of contraction: (involuntary)- autonomic nerves, hormones, chemicals, and sometimes stretch stimulate smooth muscle. Speed of contraction: very slow Cardiac muscle- Function: contraction of the heart exclusively. Location: atria and ventricle walls.  Cell morphology: branching chains of cells. One or two nuclei per cell. Gap junctions between cells allow sharing of intracellular fluid. Regulation of contraction: (involuntary)- pacing by the intrinsic conduction system of the heart. Modification by the autonomic nervous system. Speed of contraction: slow Skeletal muscle- Function: dynamic movement. Location: attached to bones or skin. Cell morphology: single, long, cylindrical, multinucleated, striated cells. Muscle fibers are arranged in a logical format to optimize force for the joint they are a part of. Regulation of contraction: (voluntary)- stimulation from efferent neurons of the somatic nervous system. Speed of contraction: slow to fast based on modulation by the somatic nervous system.  Slide 7 Excitable membranes are unique because they have the ability to trigger an action. In the context of skeletal muscle, the action is contraction. Membrane potential is established by separating charges on the inside and outside of a cell. The inside of an excitable cell is primarily negative. This is because the proteins within a cell have a negative charge. The outside of an excitable cell is primarily positive. This is because of positively charges ions outside the cell. The inside of the cell is not exclusively negative, nor is the outside exclusively positive. For an excitable membrane, we are only saying that the majority of charge is negative inside and positive outside. Because a difference in charge exists inside and outside the cell, the term polarity is used. Polar is used to describe opposites, such as the polar ends of the globe. Depolarization is when less of a difference is created. For example, if the positive charges outside a cell are allowed to flow inside less of a difference exists between both sides. Membrane potential is another way of describing polarization. The term potential is used because the polarization establishes the potential for something to occur. In this case, to trigger muscle contraction.  Depolarization occurs due to the movement of ions into or out of the cell. When part of an excitable membrane is depolarized, it can trigger an event that depolarizes nearby membrane as well. This is called an action potential. An action potential is like a domino effect on the sarcolemma of a muscle cell; depolarization of one area causes depolarization of the next, until the wave of depolarization has traveled the length of the cell. Following an action potential, active transport forces ions to move back to their original positions. Action potentials on muscle cells trigger intracellular events at the sarcomere that will lead to contraction.  Slide 8 The excitable nature of a muscle cell is created by the interaction of a motor neuron and the sarcolemma. When a motor neuron is excited, an action potential travels down its axon and eventually reaches the axon terminal. The nerve terminal contains vesicles containing the neurotransmitter acetylcholine. The action potential stimulates the release of acetylcholine into the synaptic cleft between the neuron and the muscle cell. Acetylcholine diffuse through the synapse and binds to acetylcholine receptors on the sarcolemma. When these receptors bind with the neurotransmitter, they allow the movement of ions. This movement of ions depolarizes a section of the sarcolemma. Depolarization of the post-synaptic surface cause an action potential to travel across the sarcolemma. This action potential eventually moves into the muscle cell by structures that run transversly trough the cell, thus the name T-tubules. The T-tubule connects to the muscle cell’s endoplasmic reticulum, which is called the sarcoplasmic reticulum. The action potential triggers calcium channels on the sarcoplasmic reticulum to open. As a result, calcium flows out of the sarcoplasmic reticulum by facilitated diffusion and into the muscle cell. Calcium triggers the interaction or proteins that cause contraction. The processes outlined above result in a single muscle twitch. Following the action potential and subsequent muscle twitch, acetylcholine in the synapse is degraded by the enzyme acetylcholinesterase. The membrane is returned to its original potential by active transport; pumps return the ions to their original configurations. The calcium in the muscle is returned to the sarcoplasmic reticulum by active transport. The events of excitation contraction coupling occur in milliseconds. The combined actions of several muscle twitches, in several cells is required to make the overall muscle belly contact.