Unit 5: Muscular and Nervous Tissues

1. Muscular Tissue: Types and Functions
2. Ultra structure of skeletal muscle
3. Neurons: Structure, types and functions
4. Glia Cells: Structure, types and functions
5. Synapse: Structure and types

1. Muscular Tissue: Types and Functions

Muscular tissue is composed of elongated cells (muscle fibers) that are specialized for contraction, producing movement.

Feature	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Structure	Long, cylindrical, multinucleate cells with obvious striations (stripes).	Branching, striated, generally uninucleate cells. Joined by intercalated discs.	Spindle-shaped (fusiform) cells, uninucleate, with no striations.
Control	Voluntary (conscious control).	Involuntary (unconscious control).	Involuntary (unconscious control).
Location	Attached to bones (forming the "flesh" of the body).	Only in the walls of the heart.	Walls of hollow organs (stomach, bladder, intestines, blood vessels).

[Image of the three types of muscle tissue]

2. Ultra structure of skeletal muscle

The "ultrastructure" refers to the fine detail of the muscle fiber, visible with an electron microscope. The striations are due to a highly organized arrangement of contractile proteins.

Sarcolemma: The plasma membrane of the muscle fiber.
Sarcoplasm: The cytoplasm of the muscle fiber.
Myofibrils: Rod-like contractile elements that fill most of the cell. They are made of repeating units called sarcomeres.
Sarcomere: The basic functional unit of contraction, running from one Z-disc to the next.
Myofilaments: The proteins within the sarcomere.
- Thick Filaments (Myosin): Contain the motor protein myosin, which has "heads" that bind to actin. Found in the A band.
- Thin Filaments (Actin): Contain the protein actin, along with regulatory proteins (troponin and tropomyosin). Anchored to the Z-disc. Found in the I band and part of the A band.
Sarcomere Zones:
- A Band: Dark band, where thick and thin filaments overlap.
- I Band: Light band, contains only thin filaments.
- Z Disc (or Z line): A dark line in the middle of the I band; the boundary of the sarcomere.
- H Zone: A lighter region in the middle of the A band; contains only thick filaments.

Sliding Filament Model: During contraction, the myosin heads pull the actin filaments toward the center of the sarcomere. The I bands and H zones shorten, and the Z discs move closer together, causing the entire muscle to shorten. The A band remains the same length.

3. Neurons: Structure, types and functions

Nervous tissue is the main component of the nervous system (brain, spinal cord, nerves). It is composed of two main cell types: neurons and glia.

Neurons are the structural and functional units of the nervous system, specialized to generate and transmit electrical signals (nerve impulses or action potentials).

Structure of a Neuron

[Image of a typical multipolar neuron labeled]

Soma (Cell Body): The "head" of the neuron, containing the nucleus and most organelles (like Nissl bodies, which are RER).
Dendrites: Short, branching extensions that receive signals from other neurons and transmit them *toward* the cell body.
Axon: A single, long extension that transmits signals *away* from the cell body to other neurons, muscles, or glands.
Axon Hillock: The conical region where the axon joins the cell body; this is where action potentials are typically generated.
Myelin Sheath: A fatty, insulating layer that covers the axons of many neurons (formed by glia). It speeds up signal transmission.
Nodes of Ranvier: Gaps in the myelin sheath.
Axon Terminals: The branched endings of the axon that form synapses with other cells.

Types of Neurons (by Function)

Sensory (Afferent) Neurons: Transmit impulses from sensory receptors (in skin, eyes, etc.) *toward* the central nervous system (CNS).
Motor (Efferent) Neurons: Transmit impulses *from* the CNS to effector organs (muscles and glands).
Interneurons (Association Neurons): Found entirely within the CNS; they connect sensory and motor neurons and are involved in processing information.

4. Glia Cells: Structure, types and functions

Glia (or Neuroglia) are the non-neuronal "support cells" of the nervous system. They are more numerous than neurons and play critical roles in insulation, support, and protection.

Glia Type	Location	Structure and Function
Astrocytes	CNS	Star-shaped cells. Most abundant. Functions: Form the blood-brain barrier (BBB), provide structural support, regulate the chemical environment (e.g., mop up excess ions and neurotransmitters).
Microglia	CNS	Small, thorny cells. Functions: Act as the resident "immune cells" or macrophages of the CNS; they engulf cellular debris and pathogens.
Oligodendrocytes	CNS	Cells with few branches. Function: Form the myelin sheath around axons within the CNS. One oligodendrocyte can myelinate multiple axons.
Schwann Cells	PNS	Cells that wrap tightly around axons. Function: Form the myelin sheath around axons outside the CNS (in the Peripheral Nervous System). One Schwann cell myelinates only one segment of one axon.

5. Synapse: Structure and types

A synapse is the specialized junction where a neuron communicates with another cell (another neuron, a muscle cell, or a gland cell).

Structure of a Chemical Synapse

[Image of a chemical synapse labeled]

Presynaptic Neuron: The neuron *sending* the signal. Its axon terminal contains:
- Synaptic Vesicles: Small sacs filled with neurotransmitters (the chemical signal).
Synaptic Cleft: The narrow, fluid-filled gap *between* the two cells.
Postsynaptic Neuron: The neuron *receiving* the signal. Its membrane contains:
- Receptors: Proteins that specifically bind to the neurotransmitters. This binding triggers a response in the postsynaptic cell.

Types of Synapses

Chemical Synapse:
- Mechanism: An electrical signal (action potential) in the presynaptic neuron is converted into a chemical signal (neurotransmitter release), which diffuses across the synaptic cleft and is converted back into an electrical (or chemical) signal in the postsynaptic cell.
- Properties: This is the most common type. It is slower, unidirectional (one-way), and allows for complex signal integration (e.g., inhibition or excitation).
Electrical Synapse:
- Mechanism: The two cells are directly connected by gap junctions. Ions flow directly from one cell's cytoplasm to the next.
- Properties: Allows for very fast, bidirectional (two-way) transmission. Good for synchronizing groups of cells (e.g., in heart muscle, some parts of the brain).