bio lecture 12 for exam 2 notes (copy)

Striations: Present in skeletal and cardiac muscle cells due to organized actin and myosin fibers. Absent in smooth muscle cells, which contract differently without shortening in the same manner as striated muscle due to a less organized arrangement of actin and myosin.
Control:
- Cardiac and smooth muscle cells are under involuntary control, contracting without conscious thought.
- Skeletal muscle cells are under voluntary control, meaning we can contract and relax them at will.
Nuclei:
- Skeletal muscle cells are multinucleated; they form embryologically from the fusion of multiple cells that retain their nuclei.
- Cardiac and smooth muscle cells are uninucleated, each containing a single nucleus.

Nervous tissue is the central component of our nervous system.
It consists of two main types of cells: neurons and neuroglia (or neuroglial cells).

Highly specialized cells responsible for conducting nerve impulses.
Structure:
- Cell Body (Soma): Contains the nucleus and most of the cell's cytoplasm. Neurons have very few other organelles, as they are highly specialized.
- Dendrites: Branching extensions that receive signals from sensory receptors or the axon terminals of other neurons. Some dendrites can also interface directly or indirectly with the environment to receive stimuli.
- Axon: A single, long extension from the cell body that conducts nerve impulses away from the cell body.
- Axon Terminals: The ends of the axon that connect to the dendrites of other neurons, transmitting impulses from one cell to the next.
Myelination:
- Most axons are covered by a myelin sheath, which acts as an electrical insulator, similar to rubber coating on electrical wires.
- The myelin sheath is formed by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system.
- The myelin sheath has brief breaks called Nodes of Ranvier along its length, which are crucial for speeding up nerve impulse conduction through saltatory conduction, allowing messages to travel across the body rapidly (in a fraction of a fraction of a second).
Specialization: Neurons are so specialized that they shed many organelles and cannot perform all basic cellular functions, making them reliant on support cells.

Specialized nervous cells that support and nourish neurons.
They compensate for the limited organelles in neurons, performing essential tasks. For example, neuroglial cells have extra mitochondria to meet their own energy needs and those of the neurons.
Types and Functions:
- Astrocytes: Star-shaped cells that support neurons, regulate the chemical environment, and form the blood-brain barrier.
- Oligodendrocytes: Form myelin sheaths around axons in the central nervous system.
- Schwann Cells: Form myelin sheaths around axons in the peripheral nervous system and aid in nerve regeneration.
- Microglia: Act as the immune cells of the central nervous system, phagocytizing cellular debris and pathogens.
- Ependymal Cells: Line the ventricles of the brain and central canal of the spinal cord, producing cerebrospinal fluid.
Ratio: Current research suggests a ratio of approximately 1 neuron for every 9 neuroglial cells, meaning roughly 90\% of nervous tissue is composed of neuroglial cells.
Size: Neuroglial cells are relatively small compared to neurons.
Neuron Size: Neurons are among the largest cells in the body; the longest neuron can stretch from the lower back to the tip of the toes.

Composed of tightly packed cells that form continuous layers, primarily functioning as barriers.
Functions: Beyond forming barriers, epithelial tissues are heavily involved in secretion, absorption, excretion, and filtration – essentially, the gated transfer of materials across surfaces.
Location: Cover most body surfaces (internal and external) and line body cavities. They act as **protective