Fundamental Body Tissues and Nervous System Functions
Understanding Body Tissues
Tissues are sets of similar cells that work together to perform a specific function.
The extracellular matrix is the material that forms the intercellular spaces between cells.
Epithelial Tissues: Linings and Surfaces
Epithelial tissues, such as the endometrium, form linings for cavities and surfaces throughout the body.
- Simple Epithelium: Consists of a single layer of cells, found in external organs like lung alveoli. It specializes in the exchange of substances, and some types have cilia (e.g., in nasal passages).
- Stratified Epithelium: Composed of several layers of cells, along with a basal cell layer. Cells are formed from the bottom up, as seen in the skin.
Muscle Tissue: Movement and Contraction
Muscle tissue is specialized for movement and changing length.
- Smooth Muscle: Found in internal organs, its contractions are involuntary. It consists of spindle-shaped cells, each containing a single nucleus and no striations (e.g., stomach).
- Skeletal Muscle: Attached to bones, its contractions are controlled voluntarily. It features long, multinucleated cells with visible striations.
- Cardiac Muscle: Located in the heart and major arteries, its contractions are involuntary. It is characterized by branching, multinucleated cells with striations.
Connective Tissues: Support and Structure
Connective tissues maintain and fill the organs of the body, providing support and structure.
- Adipose Tissue: Acts as a filler, accumulating lipids as energy reserves. Under the skin, it serves as insulation and a cushion.
- Bone Tissue: Can be dense or spongy. It has a calcium-rich matrix, with cells embedded within protein fibers.
- Dense Bone: Found in flat bones and the surface of other bones; provides strength and can be broken.
- Spongy Bone: Located in long bones, containing numerous blood vessels (marrow) within its matrix.
- Cartilage: A flexible tissue that forms structural components. Its cells are immersed in a protein matrix that provides firmness and strength. Found in joints, tendons, and ribs; it can ossify (turn into bone).
The Nervous System: Communication and Control
The nervous system communicates between different organs and body systems. It receives external and internal stimuli, coordinating body functions, and is directed by the brain.
It is a crucial coordination and control system found in the animal kingdom. Invertebrates have a neural net or cerebral ganglia, while vertebrates possess a dorsal brain and spinal cord.
Components of the Nervous System
The nervous system is primarily composed of glia and neurons.
Glia: Support Cells of the Nervous System
Glia (neuroglia) accompany, nourish, protect, and help regenerate neurons. They are connected by tight junctions.
- Astrocytes: Create the blood-brain barrier, which prevents the passage of toxic substances to the nervous system while allowing soluble substances to pass.
- Schwann Cells: Wrap around axons, providing insulation for nerve impulses by forming the myelin sheath. They also aid in axon regeneration and are found in the peripheral nervous system.
- Oligodendrocytes: Insulate the central nervous system by forming the myelin sheath, but unlike Schwann cells, they do not regenerate.
Neurons: Transmitting Nerve Impulses
Neurons are specialized nervous tissue cells responsible for the transmission of nerve impulses. They consist of:
- Dendrites: Branching extensions that capture environmental changes or stimuli and transmit them to the soma.
- Soma (Cell Body): Contains the nucleus and organelles. It primarily uses glucose for energy.
- Axon: A long prolongation extending from the soma that carries nerve impulses. It is often surrounded by myelin sheaths. Small spaces between myelin segments are called Nodes of Ranvier. A longer separation between nodes can lead to faster nerve impulse transmission.
- Axon Terminal: The end of the axon where nerve impulses are transmitted to another cell, gland, or muscle.
Synaptic Transmission: Neuron Communication
A synapse is the point where one neuron communicates with another. It is both a space and a process occurring at the synaptic buttons at the end of the axon. At rest, neurons exhibit specific ion distributions: Na+ and Cl– are primarily outside, while K+ and negative organic anions (Io–) are primarily inside.
Types of Neurons
- Efferent Neurons: Send stimuli away from the central nervous system (motor neurons).
- Interneurons: Process and relay stimuli within the central nervous system.
- Afferent Neurons: Receive stimuli and send them towards the central nervous system (sensory neurons).
Resting Potential: The Neuron’s Baseline State
The resting potential describes the neuron’s state when not transmitting an impulse. The cytoplasm is rich in negative organic ions (anions) and has a high concentration of K+ and low Na+. K+ slowly leaks out, resulting in a net negative charge inside the cell.
Conversely, the extracellular fluid is rich in Na+ and Cl–, with low K+, resulting in a net positive charge outside. These charge differences across the membrane determine its polarization (typically around -70mV). This concentration gradient and charge separation are maintained by the sodium-potassium pump, which actively removes Na+ from the neuron and brings K+ into the cell from the extracellular fluid.
Action Potential: The Nerve Impulse
An action potential is generated when a stimulus arrives. This causes voltage-gated sodium channels to open, allowing Na+ to rapidly enter the neuron. A rapid depolarization occurs, making the inside of the cell positive (reaching approximately +40mV).
Following depolarization, sodium channels inactivate, and potassium channels open, allowing K+ to exit the neuron. This leads to repolarization and a brief period of hyperpolarization before the membrane returns to its resting potential.