The Central Nervous System: Brain, Brainstem, and Spinal Cord
Hypothalamic-Hypophyseal Axis
Adenohypophyseal endocrine cells: Release adenohypophyseal hormones to the systemic circulation through the anterior hypophyseal veins.
Brainstem
External Morphology – Ventral View
The origins of cranial nerves III to XII can be observed.
Mesencephalon
- Cerebral peduncles: Connect the brainstem with the prosencephalon.
- Interpeduncular fossa: A depression.
Pons
- Pontocerebellar fascicles: Running transversally.
- Middle peduncles: Formed by pontocerebellar fibers.
Medulla Oblongata
- Olive: Relays information from different sources of the CNS to the cerebellum.
- Pyramid: Made up of myelinated axons originating in pyramidal cortical neurons of the ipsilateral cortex.
- Decussation of pyramids: At this level, 90% of axons from pyramidal cortical neurons cross to the other side, whereas the rest remain on the same side.
External Morphology – Lateral View
The origins of cranial nerves III to XII can be observed.
Mesencephalon
- Cerebral peduncles: Connect the brainstem with the prosencephalon.
- Superior colliculus: Integrates visual, auditory, and somatosensory inputs for orienting behavior.
- Inferior colliculus: Relays inputs to higher centers.
Pons
- Pontocerebellar fascicles: Running transversally.
- Middle cerebellar peduncle: Formed by pontocerebellar fibers.
- Superior cerebellar peduncle
- Inferior cerebellar peduncle
Medulla Oblongata
- Olive: Relays information from different sources of the CNS to the cerebellum.
- Pyramid: Made up of myelinated axons originating in pyramidal cortical neurons of the ipsilateral cortex.
External Morphology – Dorsal View
Origins of the trochlear cranial nerves.
Mesencephalon
- Superior colliculi: Integrate visual, auditory, and somatosensory inputs for orienting behavior.
- Inferior colliculi: Relay auditory inputs to higher centers.
Pons
The cerebellar peduncles are formed by tracts of myelinated fibers running towards or away from the cerebellum and constitute the lateral walls of the IV ventricle. The anterior two-thirds of the floor of the IV ventricle belongs to the pons. The superior medullary velum is a thin lamina that forms the roof of the rhomboid fossa.
Medulla Oblongata
The caudal one-third of the rhomboid fossa belongs to the medulla oblongata. The most caudal part of the medulla oblongata resembles the spinal cord’s appearance.
Neurotransmitters
Serotonin: Mainly in the raphe nuclei. Ascending projections to the forebrain from the rostral groups of raphe neurons are involved in the regulation of mood, appetite, sleep (activate NREM phase), learning, and memory, whereas descending projections from the caudal groups appear to raise the sensory threshold.
Parts of the Spinal Cord
Cervical Spine
Largest. Large number of motor neurons and sensory afferents, and also because all descending and ascending tracts meet here.
Thoracic Spine
Narrowest part. Gray matter is considerably less developed, due to the lower number of motor neurons and sensory afferents. At this level, there is a lateral horn, where preganglionic sympathetic motor neurons are located.
Lumbar Spine
Large and mostly composed of gray matter, white matter is scarcer because it’s composed of tracts corresponding only to axons descending to and ascending from lumbar and sacral levels.
Sacral Spine
At the level of S2-4, there is a lateral horn, where parasympathetic motor neurons are located. They form the spinal parasympathetic system.
Spinal Nerve
- Dorsal root (afferent/sensory): Composed of sensory fibers. Intercalated along its course, there is a dorsal root ganglion, where primary sensory neurons are located.
- Ventral root (efferent/motor): Composed of motor fibers.
- Sympathetic ganglia: At thoracic levels, they’re connected to the spinal nerve. Sympathetic ganglia contain postganglionic vegetative motor neurons.
Brain Divisions
- Prosencephalon (forebrain): Telencephalon (cerebral hemispheres) + Diencephalon (epithalamus, thalamus, subthalamus, hypothalamus)
- Mesencephalon (midbrain)
- Rhombencephalon (hindbrain): Metencephalon (cerebellum + pons) + Myelencephalon (medulla oblongata)
- Spinal Cord
Functions of Thyroid and Parathyroid Hormones
High levels of Ca in the blood stimulate thyroid gland parafollicular cells to release more calcitonin (CT). CT inhibits osteoclasts, thus decreasing blood Ca levels. Low levels of Ca in the blood stimulate parathyroid gland chief cells to release more parathyroid hormone (PTH). PTH promotes resorption of Ca from bone extracellular matrix into the blood and retards loss of Ca in urine, thus increasing blood Ca levels. PTH also stimulates the kidneys to release calcitriol. Calcitriol stimulates increased absorption of Ca from foods which increases blood Ca levels.
Thyroid Gland
Internal Structure
Composed of thyroid follicles that absorb iodine from the blood to produce thyroid hormones. Inside the follicles, colloid serves as a reservoir of materials for thyroid hormone production.
Cells
- Follicular cells: Form the wall of follicles.
- Parafollicular cells: Scattered among follicular cells and in spaces between the spherical follicles.