Lesson 4: Histology - Muscle & Nervous Tissues

Skeletal (muscle tissue)

- Cell structure: Long cylindrical fiber, striated, many peripherally located nuclei - Function: Voluntary movement, produces heat, protects organs - Location: Attached to bones and around entrance points to body, makes possible locomotion, facial expressions, posture, and other voluntary movements of the body. Another function of skeletal muscle is to generate heat as a byproduct of their contraction and thus participate in thermal homeostasis. Shivering is an involuntary contraction of skeletal muscles in response to perceived lower than normal body temperature.

Cardia (muscle tissue)

- Cell structure: Short, branched, striated, single central nucleus - Function: Contracts to move blood in the heart - Location: Heart, forms the contractile walls of the heart.

Smooth (muscle tissue)

- Cell structure: Short, spindle-shaped, single nucleus in each fiber - Function: Involuntary movement of many materials including food, air during respiration, secretions, and the flow of blood through blood vessels. Smooth muscle tissue contraction is responsible for involuntary movements in the internal organs. It forms the contractile component of the digestive, urinary, and reproductive systems as well as the airways and arteries - Location: Walls of major organs and passageways

Connective Tissue Layers of Skeletal Muscle

- Each skeletal muscle has three layers of connective tissue that enclose it, provide structure to the muscle as a whole, and also compartmentalize the muscle fibers within and around other muscles - Each muscle is wrapped in a sheath of dense, irregular connective tissue called the epimysium, which allows a muscle to contract and move powerfully while maintaining its structural integrity independent of surrounding structures. - Inside each skeletal muscle, muscle fibers are organized into individual bundles, each called a fascicle, by a middle layer of connective tissue called the perimysium. - it allows the nervous system to trigger a specific movement of a muscle by activating a subset of muscle fibers within a fascicle of the muscle - Inside each fascicle, each muscle fiber is encased in a thin connective tissue layer of collagen and reticular fibers called the endomysium. The endomysium contains the extracellular fluid, nutrients, blood vessels, and nerves needed to support the muscle fiber.

Muscle Tissue Types

- Muscle cells are excitable meaning that they can respond to a stimulus. They are also contractile, meaning they can shorten and generate a force - classified into three types according to structure and function: skeletal, cardiac, and smooth

Skeletal Muscle & Body Movement

- To move the skeleton a skeletal muscle must be attached to a fixed part of the skeleton - The movable end of the muscle that attaches to the bone being pulled is called the muscle's insertion, and the end of the muscle attached to a fixed (stabilized) bone is called the origin. - In many cases the origin is the proximal attachment point while the insertion is the distal attachment point.

Unipolar cells (neuron)

- Unipolar cells have only one process emerging from the cell. True unipolar cells are only found in invertebrate animals, so the unipolar cells in humans are more appropriately called "pseudo-unipolar" cells. Invertebrate unipolar cells do not have dendrites. Human unipolar cells have an axon that emerges from the cell body, but it splits so that the axon can extend along a very long distance - Unipolar cells are exclusively sensory neurons and have two unique characteristics. First, their dendrites are receiving sensory information, sometimes directly from the stimulus itself. Secondly, the cell bodies of unipolar neurons are always found in ganglia. - Sensory reception is a peripheral function (those dendrites are in the periphery, perhaps in the skin) so the cell body is in the periphery, though closer to the CNS in a ganglion. The axon projects from the dendrite endings, past the cell body in a ganglion, and into the central nervous system.

Multipolar cells (neurons)

- are all the neurons that are not unipolar or bipolar - Have one axon and two or more dendrites usually more, with the exception of unipolar sensory ganglion cells, and the two specific bipolar cells.

Neuron Classification

- first way to classify them is structurally by the number of processes attached to the cell body. Using the standard model of neurons, one of these processes is the axon, and the rest are dendrites. Because information flows through the neuron from dendrites or cell bodies toward the axon, these names are based on the neuron's polarity - Neurons can also be functionally classified on the basis of the role they play in the nervous system. Sensory, or afferent, neurons carry information about the environment towards the central nervous system. Interneurons are found exclusively within the central nervous system and receive information either from sensory neurons or other interneurons. Motor, or efferent, neurons receive information from interneurons or directly from sensory neurons in order stimulate responses in tissues throughout the body.

Bipolar cells (neurons)

- have two processes, which extend from each end of the cell body, opposite to each other. - one is the axon and the one the dendrite. - not very common, found mainly in the olfactory epithelium (where stimuli are sensed), and as part of the retina.

Nerves

- in the periphery are different than the central counterpart, tracts - Nerves are composed of more than just nervous tissue. They have connective tissues included in their structure, as well as blood vessels supplying the tissues with nourishment, very similar to what was described for skeletal muscle tissue - The outer surface of a nerve is a surrounding layer of fibrous connective tissue called the epineurium - Within the nerve, axons are further bundled into fascicles, which are each surrounded by their own layer of fibrous connective tissue called perineurium. - Finally, individual axons are surrounded by loose connective tissue called the endoneurium - associated with the region of the CNS to which they are connected, either as cranial nerves connected to the brain or spinal nerves connected to the spinal cord.

Neuron Anatomy

- main part of the neuron is the cell body, aka the soma (=body) - extensions of the cell membranes are called processes - typically only have one axon (a fiber that emerges from the cell body and projects to target cells). Can branch repeatedly to communicate with target cells. The axon is what propagates the nerve impulse, which is communicated to one or more cells. - The other processes of the neuron are dendrites, which receive information from other neurons at specialized areas of contact called synapses. Dendrites are highly branched processes, providing locations for other neurons to communicate with the cell body. - Information flows through a neuron from the dendrites, across the cell body, and down the axon. This gives the neuron a polarity—meaning that information flows in this one direction - Where the axon emerges from the cell body, there is a special region referred to as the axon hillock. This is a tapering of the cell body toward the axon fiber. - Many axons are wrapped by an insulating substance called myelin, which is made up of glial cells. Myelin acts as insulation much like the plastic or rubber that is used to insulate electrical wires - At the end of the axon is the axon terminal, where there are usually several branches extending toward the target cell, each of which ends in an enlargement called a synaptic end bulb. These bulbs are what make the connection with the target cell at the synapse.

Nervous Tissue

- organized into two major regions: the central and peripheral nervous systems - central nervous system (CNS) is the brain and spinal cord - peripheral nervous system (PNS) is everything else - Nervous tissue, present in both the CNS and PNS, contains two basic types of cells: neurons and glial cells.

Neurons

- the primary type of cell that most anyone associates with the nervous system. - considered to be the basis of nervous tissue - responsible for the computation and communication that the nervous system provides. They are electrically active and release chemical signals to target cells - Glial cells, or glia, are known to play a supporting role for nervous tissue - Neurons are cells and therefore have a soma, or cell body, but they also have extensions of the cell; each extension is generally referred to as a process - one important process that every neuron has called an axon, which is the fiber that connects a neuron with its target. - Another type of process that branches off from the soma is the dendrite. Dendrites are responsible for receiving most of the input from other neurons. - Looking at nervous tissue, there are regions that predominantly contain cell bodies and regions that are largely composed of just axons. These two regions within nervous system structures are often referred to as gray matter (the regions with many cell bodies and dendrites) or white matter (the regions with many axons). - Gray matter is not necessarily gray. It can be pinkish because of blood content, or even slightly tan, depending on how long the tissue has been preserved. But white matter is white because axons are insulated by a lipid-rich substance called myelin. - A localized collection of neuron cell bodies in the CNS is referred to as a nucleus. - In the PNS, a cluster of neuron cell bodies is referred to as a ganglion - A bundle of axons, or fibers, found in the CNS is called a tract whereas the same thing in the PNS would be called a nerve. When those axons are in the PNS, the term is nerve, but if they are CNS, the term is tract.

Circular

Also called sphincters. Surrounds an opening to control the size of the opening. - ex: orbicularius oculi

Fascicle Organization Patterns

Based on the patterns of fascicle arrangement, skeletal muscles can be classified in several ways: - Parallel (fusiform): - Parallel (non-fusiform) - Circular - Convergent (triangular) - Unipennate - Bipennate - Multipennate

Parallel (non-fusiform)

Fibers are arranged in the same direction along the long axis of the muscle with no belly. - ex: Sartorius

Parallel (fusiform)

Fibers are arranged in the same direction along the long axis of the muscle. Spindle-shaped with a central, large belly. - ex: biceps brachii

Bipennate

Tendon runs through the central region of the muscle with muscle fibers located on both sides of the tendon. - ex: rectus femoris

Unipennate

Tendon runs through the central region of the muscle with muscle fibers located on one side of the tendon. - ex: extensor digitorum

Multipennate

Tendon runs through the central region of the muscle with muscle fibers wrapping the tendon on all sides to form separate fascicles. - ex: deltoid

Convergent (triangular)

Widespread muscle fibers come to a single, slender attachment point, often in the shape of a triangle. - ex: pectoralis major