Human Biology & Histology: Nervous Tissue and Digestive Tract Structure

Nervous Tissue: Myelination and Axonal Conduction

Myelination of Axons

Myelination is a process in which axons are wrapped by specialized glial cells, resulting in the formation of a myelin sheath. This sheath is essential for rapid and efficient transmission of electrical impulses along nerve fibers.

• Schwann cells (in the Peripheral Nervous System, PNS) and oligodendrocytes (in the Central Nervous System, CNS) are responsible for forming the myelin sheath.

• The myelin sheath consists mainly of plasma membrane with a high lipid content, making it an excellent electrical insulator.

• Myelinated axons are separated by gaps called nodes of Ranvier, where the axon membrane is exposed.

Definition: Myelin sheath is a multilayered lipid-rich covering that insulates axons and increases the speed of nerve impulse conduction.

Saltatory Conduction

Saltatory conduction is the process by which action potentials "jump" from one node of Ranvier to the next, greatly increasing the speed of impulse transmission in myelinated axons.

• Action potentials are generated only at the nodes of Ranvier, not along the entire axon.

• This allows for rapid transmission of signals over long distances.

• Saltatory conduction is much faster than conduction in unmyelinated fibers.

Example: In myelinated axons, the impulse can travel at speeds up to 120 m/s, compared to 1 m/s in unmyelinated axons.

Speed of Impulse Conduction

The speed at which nerve impulses travel depends on several factors related to the axon's structure and myelination.

• Fiber diameter: Thicker fibers conduct impulses faster.

• Myelin sheath thickness: Thicker myelin sheaths result in faster conduction.

• Node spacing: Greater distance between nodes of Ranvier increases conduction speed.

Fiber Type	Myelination	Conduction Speed
A fibers	Strong	100–120 m/s
B fibers	Weak	10–20 m/s
C fibers	None	1 m/s

Myelinated vs. Unmyelinated Fibers

Axons can be classified based on the presence or absence of myelin sheaths.

• Myelinated fibers: Wrapped by multiple layers of glial cell membrane, allowing for saltatory conduction.

• Unmyelinated fibers: Lack a myelin sheath; impulses travel more slowly and continuously along the axon.

Example: Most sensory and motor neurons are myelinated, while some autonomic fibers are unmyelinated.

Glial Cells and Their Functions

Glial cells, also known as neuroglia, are non-neuronal cells that provide support, protection, and nutrition to neurons.

• Astrocytes (CNS): Maintain homeostasis, form the blood-brain barrier.

• Oligodendrocytes (CNS): Form myelin sheaths around multiple axons.

• Microglial cells (CNS): Act as immune defense cells.

• Ependymal cells (CNS): Line fluid-filled cavities, circulate cerebrospinal fluid.

• Schwann cells (PNS): Form myelin sheaths around axons.

• Satellite cells (PNS): Surround neuron cell bodies in ganglia.

Blood-Brain Barrier

The blood-brain barrier is a selective barrier that protects the brain from harmful substances in the blood while allowing essential nutrients to pass through.

• Formed by endothelial cells of brain capillaries and astrocyte processes.

• Maintains the brain's microenvironment.

Digestive Tract: Structure and Function

Why Do We Eat and Drink?

Food provides essential nutrients required for energy, growth, and maintenance of body functions.

• Carbohydrates

• Proteins

• Fats

• Vitamins

• Minerals

• Trace elements

• Water

• Dietary fibers

Steps of Digestion

Digestion is a multi-step process that breaks down food into absorbable components.

1. Food uptake

2. Mechanical crushing of food

3. Chemical cleavage of nutrients

4. Resorption (absorption)

5. Excretion of remnants (faeces)

Organization of the Digestive Tract

The digestive tract is a continuous tube extending from the mouth to the anus, with associated organs aiding in digestion.

• Oral cavity

• Pharynx

• Esophagus

• Stomach

• Small intestine

• Large intestine

• Associated organs: salivary glands, pancreas, liver

Oral Cavity Structure

The oral cavity is the entry point for food and is divided into the vestibule and the oral cavity proper.

• Vestibule: Space between lips/cheeks and teeth.

• Oral cavity proper: Area behind the teeth, bounded by the palate above and the tongue/floor below.

Mucosa of the Oral Cavity

The mucosa lines the oral cavity and provides protection against mechanical stress.

• Stratified squamous epithelium: Mainly nonkeratinized, except in the gingiva and hard palate (masticatory mucosa).

• Rapid epithelial turnover (8–10 days).

• Lamina propria: Connective tissue layer beneath the epithelium.

Section Through the Lip

The lip consists of three distinct regions, each with specialized tissue types.

• Outside: Skin (keratinized stratified squamous epithelium).

• Inside: Mucosa (nonkeratinized stratified squamous epithelium).

• In between: Muscle and glands.

The Palate

The palate forms the roof of the oral cavity and separates it from the nasal cavity.

• Hard palate: Bony anterior portion, covered by masticatory mucosa.

• Soft palate: Muscular posterior portion, covered by nonkeratinized mucosa.

Floor of the Oral Cavity

The floor is composed of muscles covered by mucosa, providing support and movement for the tongue.

• Muscular structure allows for manipulation of food during mastication and swallowing.

*Additional info: Some details on axonal transport, neuron regeneration, and histological staining were present in the original notes but not fully included above due to focus on main topics. For exam preparation, students should also review mechanisms of axonal transport (anterograde and retrograde), the role of microtubules, and differences in regeneration between CNS and PNS.*