Nervous tissue

Describe the difference between anterograde and retrograde transport

Anterograde transport: Carries materials from the soma or cell body to the axon and dendrites. Retrograde transport: Carries materials from the axon and dendrites to the soma or cell body.

Describe the axon segments

Axon segments: Initial: Short segment originating from the axon hillock. Conductive: Longest an may or may not be myelinated. Transmissive: Terminal end that divides into telodendria with terminal boutons.

Describe blood supply, innervation and repair process of cardiac muscle

Blood Supply - Provided by the coronary arteries and their branches. Innervation - By both of the portions of the autonomic nervous system ( sympathetic and parasympathetic ). Repair: Destroyed cardiac muscle cells are not replaced by new cardiac muscle cells but rather by fibrous connective tissue. Myocardial infarct is a common cause of cardiac cell death. A single large infarct or repeated small infarcts can be fatal.

Describe innervation, blood supply and repair of skeletal muscle

Blood supply: Derived from small arteries in the surrounding connective tissue. Innervation: Most smooth muscle is innervated by both the parasympathetic and sympathetic portions of the autonomic nervous system; however some smooth muscle is only innervated by only one portion; and some smooth muscle contracts in response to hormones. Repair: Some smooth muscle responds to injury by undergoing mitosis; and in other smooth muscle new cells are derived from undifferentiated cells in blood vessels, endothelial cells, and pericytes. Note: The uterus has regularly replicating populations of smooth muscle cells.

What is the nissl substance

Free ribosomes and RER in basophilic clumps. Extends throughout the cell and dendrites but not into the axon. (in cell body)

Describe the gross structure of skeletal muscle

Gross structure: Skeletal muscle cells are also called fibers. Specialized for rapid, forceful, voluntary contraction. Connective tissue: Endomysium: Delicate connective tissue around individual fibers. Perimysium: Connective tissue that surround a group of fibers called a fascicle. Epimysium: Dense irregular connective tissue that surrounds the entire muscle. Note: The epimysium, perimysium, and endomysium are continuous with the tendons and/or aponeuroses of the muscle.

Describe the histological structure of cardiac muscle cells

Histologic structure of cardiac muscle cells (fibers): Cylindrical cells 10-20um in diameter and 100-150um in length. One or two centrally located nuclei. Ends of fibers branch and meet branches of other fibers giving the appearance of a syncytium. Not a syncytium because there are discrete cell boundaries, but it functions like one. Fibers have the same arrangement of contractile proteins and a similar framework of other proteins. In contrast to skeletal muscle the T-tubules are larger, the sarcoplasmic reticulum is not as extensive, and there are diads located at the Z line. Fibers accumulate lipofuscin pigment with age.

Are mature neurons capable of undergoing mitosis?

NO

Describe the blood supply and innervation of skeletal muscle

Blood supply: Has a rich blood supply that is usually derived from at least a couple of arteries. Innervation: Motor innervation: Derived from lower motor neurons in the brainstem (motor nuclei of cranial nerves), and lower motor neurons in the spinal cord (α-motor neurons or ventral horn cells). Sensory innervation: Derived from pseudounipolar neurons in dorsal root ganglia and some cranial nerve ganglia. Their peripheral process connects to muscle spindles and Golgi tendon organs. Note: The sensory and motor fibers to a muscle course in one nerve.

Describe the histologic structure of skeletal muscle cells

Each skeletal muscle fiber (myofiber) is a multinucleated cell (syncytium): Diameter is from 10-100um and length is from 1mm to 35cm. Nuclei are peripherally located. Contain myofibrils composed of actin and myosin filaments (myofilaments). A sarcolemma encloses the nuclei and myofibrils. There are periodic invaginations of the sarcolemma called transverse tubules (T-tubules). Along with the normal organelles the sarcoplasm contains numerous glycogen granules, myoglobin, and sarcoplasmic reticulum (SER). The SER forms a system of membrane bound tubules that extend throughout the cytoplasm and expands into terminal cisterna next to a T-tubule. The two terminal cisternae and T-tubule arrangement is called a triad and is located at the junction of the A and I bands.

Describe intercalated discs

Intercalated discs: Located between the ends of adjacent cardiac muscle fibers. Function: Provide points of anchorage for the myofibrils. Provide for attachment to neighboring cells. Permit extremely rapid spread of contractile stimulus from one cell to another. Cell junctions: Desmosomes. Gap junctions. Fascia adherens.

Describe neuron and nerve fiber regenration

Local Changes or Reaction: The severed ends of the axon retract away from each other and the cut ends close. Macrophages and fibroblasts infiltrate the area,secrete cytokines and growth factors, and phagocytose the debris. Anterograde changes or Reaction: Terminal portion of axon (portion distal to the injury) and its myelin sheath degenerates and is phagocytosed by proliferating Schwann cells and macrophages (termed Wallerian degeneration). Proliferating Schwann cells form Schwann tubes within the original endoneurium. Retrograde Changes or Reaction: The soma of the neuron swells, Nissl bodies disperse and the nucleus becomes eccentric (all of this is chromatolysis). Proximal portion of the axon degenerates, sprouts form and are guided by Schwann cells to the endoneurium to reach the target cell. Schwann cells, macrophages, fibroblasts, and endoneurium must be present for regeneration to occur. Schwann cells form myelinated and unmyelinated fibers. Regeneration proceeds at about 3-4 mm per day.

Describe the muscle tissue in the heart

Located in the heart and at the base of large vessels entering and leaving the heart. Within the heart it is located in the myocardium: Epicardium: Covers the outer surface of the heart and is composed of a mesothelium (visceral layer of serous pericardium) and underlying connective tissue containing the blood vessels and nerves to the heart. Myocardium: Cardiac muscle held together by connective tissue. The inner portion of the myocardium contains specialized cardiac muscle fibers and Purkinje fibers that are part of the impulse conduction system of the heart. Endocardium: Covers the inner surface of the heart and consists of an endothelium and subendothelial connective tissue.

Myofibril structure

Myofibril structure: Composed of thick myosin and thin actin myofilaments. Observation with the microscope reveals a banding pattern consisting of : Alternating light bands (I bands) and dark bands (A bands). Fine dark lines called Z bands (disks or lines) in the middle of the I band. The region between two Z bands is a sarcomere which is the contractile (functional) unit in skeletal muscle. The A band contains thick myosin filaments and a portion of the actin filaments on each side. The center of an A band is devoid of actin filaments and thus forms an H band. At the center of the H band is a M line which is the attachment point of the myosin filaments. The I band is composed of actin filaments and the Z line marks their attachment. There are a number of proteins associated with myofilaments (nebulin and titin), and with myofibrils (desmin and plectin).

Describe location and structures of smooth muscles

Located in the wall of GI tract, ducts of glands, respiratory passages, ducts of the urinary and genital tracts, walls of arteries/veins/lymphatic vessels, arrector pili muscles, areola of nipple, dartos tunic of scrotum, iris and ciliary body of eye. Fibers are nonstriated and control of contraction is involuntary. Fusiform-shaped cells ranging from 20um to .5mm on length. Single, central, elongated nucleus with two or more nucleoli. Longitudinal section: Nuclei are staggered. Cross section: Nuclei are only seen in some cells. Cytoplasmic organelles are bunched on either side of the nucleus. Commonly observed in groups (sheets or bundles) held together by connective tissue, but may be single (myoepithelial cell). Contractile proteins are actin, myosin, and intermediate filaments. The contractile proteins criss-cross the cell and attach to dense bodies (adhesion densities containing α-actinin) in the cell membrane and within the cytoplasm. Tension generated by contraction is transmitted through the dense bodies to other cells allowing a group of smooth muscle cells to function as a unit. Smooth muscle cells also secrete connective tissue elements: Type III and IV collagen. Laminin and elastin. Proteoglycans (GAG's). Specialized for continuous contractions of low force. Influenced by the autonomic nervous system, hormones, and local metabolites. Note: Peristalsis of the GI tract and enteric nervous system.

Describe the location and structure of skeletal muscle

Major tissue of the body and is controlled by the somatic nervous system. Forms the muscles of the musculoskeletal system and some muscles that do not move bones (diaphragm, extraocular muscles, muscles of facial expression). Motor unit: A lower motor neuron (motor neuron in a cranial nerve nucleus, or a motor neuron in the ventral horn of the spinal cord) and the skeletal muscle fibers it innervates. Each skeletal muscle has many motor units and the ratio of neuron to muscle fibers varies from 1/1, to 1/100. The nerve supply to a skeletal muscle is essential for contraction and muscle tone. Note: Denervation results in flaccid paralysis, areflexia, atonia, and atrophy of a muscle (LMN lesion).

Microglia vs ependyma

Microglia: Macrophages of the CNS. Found in both gray and white matter. They are mesodermal in origin. Migrate to the sites of dead neurons and glial cells and phagocytize them. Regarded as immune protectors of the CNS. Ependyma: Simple cuboidal epithelium that lines the ventricles of the brain and the central canal of the spinal cord. Modified ependymal cells and associated capillaries form choroid plexus which produces cerebrospinal fluid (CSF).

Describe the classification of muscle

Muscle is a tissue that is specialized for contraction in order to move the body or individual components of the body. Classification: Appearance of cells (striated or nonstriated). Controlled by somatic or autonomic nervous system (voluntary or involuntary). Based on the classification there are three types of muscle tissue: Skeletal (striated voluntary). Cardiac (striated involuntary). Smooth (nonstriated involuntary). Nomenclature: Sarcolemma - Cell membrane. Sarcoplasm - Cytoplasm. Sarcoplasmic reticulum - Smooth endoplasmic reticulum. Sarcomere - Functional unit within a myofiber.

Difference between unmyelinated and myelinated nerve fibers

Nerve fibers are either dendrites or axons. Peripheral nerves and the white matter of the CNS are composed of nerve fibers. Unmyelinated nerve fibers: Not surrounded by a myelin sheath. In the CNS they are bare. In the PNS a number of fibers are embedded in individual recesses (mesaxon) in a single Schwann cell and its basal lamina. They are very slow conducting fibers. Myelinated nerve fibers: Are surrounded by a myelin sheath (varies in thickness) and are found in the CNS and PNS. In the CNS the myelin sheath is formed by oligodendrocytes which can myelinate several axons. In the PNS Schwann cells wrap their membranes around a single axon to form a myelin sheath. There are several Schwann cells or oligodendrocytes along the length of a single axon, and the junction between two cells is called a node of Ranvier.

Describe peripheral nerves

Peripheral nerves: These are bundles of unmyelinated and myelinated nerve fibers in the peripheral nervous system (PNS). Peripheral nerves have the following connective tissue components: Epineurium : Connective tissue that surrounds an entire cranial, or spinal nerve. Perineurium: Connective tissue that divides the nerve into fascicles. Endoneurium: Very thin connective tissue that surrounds individual nerve fibers and their associated Schwann cells. White matter: Within the CNS the unmyelinated and myelinated nerve fibers course in the white matter of the spinal cord and brain, and are not enclosed in connective components.

Oligodendrocytes

Produce the myelin of the CNS. Can myelinate up to 50 axons. Nodes of Ranvier are bare compared to those in the PNS that are covered with endoneurium and interdigitating Schwann cells.

Difference between bipolar and pseudo unipolar neurons

Pseudounipolar Neurons: True unipolar neurons are found only in development. Pseudounipolar neurons are found in the adult and have one process (which most consider to be an axon) that bifurcates with one serving an afferent function (dendritic) and the other serving an efferent function (axon). These neurons function in bringing sensory information into the CNS. Found: Ganglia of cranial nerves V, VII, IX and X, and spinal or dorsal root ganglia. Bipolar Neurons: Have an axon and dendrite at opposite poles. These neurons function in bringing sensory information into the CNS. Found: Olfactory epithelium (CNI), retina (CNII), and vestibulocochlear ganglion (CNVIII).

Describe difference between red and white fibers

Red fibers: Small fibers with large amounts of myoglobin (oxygen storage molecule and accounts for red color) and many mitochondria (highly aerobic cells that burn oxygen to produce ATP - use the enzyme succinate dehydrogenase). They are slower and more sustained in contracting (slow twitch motor units which are more resistant to fatigue). White fibers: Large fibers with less myoglobin, fewer mitochondria and high levels of glycogen and glycolytic enzymes (highly anaerobic cells whose ATP comes from metabolism of glycogen into glucose). Exhibit rapid sporadic and intense contraction (fast twitch motor units that fatigue rapidly). Intermediate fibers: Exhibit intermediate characteristics between red and white fibers.

Describe repair and how exercise affects muscle tissues

Repair- Satellite cells within skeletal muscle proliferate after injury, form myoblasts which fuse to form myotubes that mature into skeletal muscle fibers. Exercise- Increases the size of the muscle fibers (hypertrophy) not the number of fibers by increasing the number of myofilaments and myofibrils within a myofiber or by the fusion of satellite cells with an existing myofiber. Note: Atrophy of a skeletal muscle is usually due to a pathological condition or disuse. Hyperplasia is an increase in the number of muscle fibers and is usually an abnormal condition.

What are lipofuscin granules

Residues in the soma that are representative of lysosomal activity that accumulate with age

Difference between schwann and satellite cells

Schwann cells: Produce the myelin sheath of myelinated nerves and engulf axons of unmyelinated nerves in the PNS. Myelin sheath is multiple layers of the Schwann cell membrane wrapped around an axon. Neurilemma is the small amount of cytoplasm left in the outermost layer. Myelinated nerves vary from lightly myelinated to heavily myelinated. Satellite cells: Layer of small cuboidal cells that surround the cell bodies of ganglia (pseudounipolar neurons located in cranial, spinal, and autonomic ganglia). Provide insulation and serve as a pathway for metabolic exchange.

Astrocytes

Star-shaped cells found in the gray matter (protoplasmic astrocytes) and white matter (fibrous astrocytes) of the CNS. They have end feet (perivascular feet) that sit on blood vessels, neurons, and the pia mater. They probably function in metabolic transport.

Describe neuron and nerve fiber regeneration

When neurons are destroyed there is minimal replacement by some stem cells. Injured cells and their processes in the CNS are phagocytosed by microglia, and glial cells fill the space with a glial scar. Limited regeneration of nerve fibers occurs in the CNS by replacement neurons forming new nerve fibers or remaining neurons forming collateral branches. The absence of endoneurial sheaths also contributes to the limited regeneration of nerve fibers. In the PNS an injured or transected nerve fiber undergoes regeneration by a process called the axon reaction. The extent of damage is important in determining the amount of functional loss.