Neurobiology of Pain and Myelination

Myelination of Nerve Fibers

• In the CNS, oligodendrocytes myelinate multiple axons, forming an insulating myelin sheath that speeds up impulse conduction.
• In the PNS, Schwann cells myelinate a single segment of one axon, also enhancing impulse velocity.

Wallerian Degeneration and Nerve Fiber Regeneration

• Wallerian degeneration occurs after nerve injury, leading to degeneration of the axon segment distal to the injury site and breakdown of the myelin sheath.
• Regeneration in the PNS involves surviving Schwann cells forming a regeneration tube, guiding the regrowth of the axon towards its target.

Non-Adapting Nature of Pain Receptors

• Pain receptors (nociceptors) do not adapt, continually responding to the presence of harmful stimuli.
• This characteristic ensures pain serves as a persistent warning signal for ongoing tissue damage.

Slow vs. Fast Pain Pathways

• Fast pain pathways are transmitted by A-delta fibers, causing sharp, immediate, localized pain.
• Slow pain pathways utilize C fibers, resulting in dull, aching pain with a slower onset and more diffuse sensation.

Chemicals Exciting Pain Nerve Fibers

• Pain nerve fibers can be excited by bradykinin, histamine, prostaglandins, substance P, and ATP, all of which are related to tissue damage or inflammation.

Tissue Ischemia and Pain

• Ischemia results in a lack of oxygen, leading to metabolic waste accumulation like lactic acid, which stimulates pain receptors.

Muscle Spasm and Pain

• Muscle spasms create sustained muscle contraction, compressing blood vessels and leading to ischemia and pain due to reduced oxygen flow.

Analgesia System in Pain Suppression

• The analgesia system includes brain and spinal cord networks that modulate pain perception through neurotransmitters like endorphins, inhibiting pain signal transmission.

Concept of Referred Pain

• Referred pain occurs when pain from an internal organ is perceived in a different body area due to convergence of sensory nerves at the spinal cord level.

Gate Control Theory of Pain

• Gate Control Theory posits that non-painful input can inhibit pain signals in the spinal cord, effectively "closing the gate" to pain perception.

Meninges and Dural Venous Sinuses

• The meninges consist of three layers: dura mater (outermost), arachnoid mater (middle), and pia mater (innermost).
• Dural venous sinuses, formed between dura mater layers, drain venous blood from the brain to the internal jugular veins.
• Dural extensions include falx cerebri (separating cerebral hemispheres), tentorium cerebelli (separating cerebrum from cerebellum), and falx cerebelli (separating cerebellar hemispheres).

Cerebrospinal Fluid (CSF)

• CSF is secreted by choroid plexuses, primarily in brain ventricles, and is a clear fluid containing water, glucose, proteins, ions, and some white blood cells.
• It drains through ventricles into the subarachnoid space, absorbed by arachnoid villi into venous sinuses.
• Functions include cushioning the brain, maintaining a stable environment, and removing metabolic waste.

Ventricles of the Brain

• Lateral ventricles are two large cavities in the cerebral hemispheres.
• Third ventricle lies between the thalamic halves.
• Fourth ventricle is situated between the brainstem and cerebellum, connecting with the spinal canal.

Brainstem and Medulla Oblongata

• The brainstem regulates essential life functions, including heart rate, breathing, and consciousness.
• The medulla oblongata, at the brainstem's lower end, controls cardiovascular and respiratory functions and reflexes like coughing.

Structure and Functions of the Pons

• The pons lies above the medulla and connects different brain areas with its nerve tracts.
• It is involved in breathing, sleep regulation, sensory information relay, and movement coordination.

Structure and Functions of the Midbrain

• The midbrain, above the pons, consists of the tectum and tegmentum, influencing sensory processing and motor control.
• It regulates visual and auditory info processing and sleep-wake cycles.

Functions of the Hypothalamus

• Key roles include regulating body temperature, hunger/thirst, circadian rhythms, autonomic nervous system responses, and endocrine system control via the pituitary gland.

Explore the intricate processes of myelination in nerve fibers, covering the roles of oligodendrocytes and Schwann cells. Understand Wallerian degeneration and the regenerative capabilities of the PNS. Additionally, learn about the pain receptors' behavior and the distinctions between slow and fast pain pathways.