Chapter 14: The Brain and Cranial Nerves – Anatomy & Physiology Study Notes

Chapter 14: The Brain and Cranial Nerves

14-1 The Brain

The brain is a large, delicate mass of neural tissue containing internal passageways and chambers filled with cerebrospinal fluid (CSF). It is divided into six major regions, each with specific functions that become more complex and variable as you ascend from the medulla oblongata to the cerebrum. Conscious thought and intelligence are produced in the neural cortex of the cerebral hemispheres.

• Cerebrum: Largest part of the adult brain, responsible for higher mental functions such as conscious thought, intellect, and memory. It is divided into left and right cerebral hemispheres.

• Cerebellum: Second-largest part of the brain, coordinates repetitive body movements and receives proprioceptive information. It consists of two hemispheres and is covered by gray matter called the cerebellar cortex.

• Diencephalon: Located under the cerebrum and above the cerebellum, it links the cerebrum with the brain stem. The three divisions are the left thalamus, right thalamus, and hypothalamus.

• Thalamus: Relays and processes sensory information.

• Hypothalamus: Connects the endocrine and nervous systems, involved in hormone production, emotion, and autonomic function.

• Pituitary gland: Major endocrine gland attached to the hypothalamus via the infundibulum (stalk), interfaces nervous and endocrine systems.

• Brainstem: Processes information between the spinal cord and cerebrum or cerebellum. Includes the midbrain, pons, and medulla oblongata.

• Midbrain (Mesencephalon): Processes sight, sound, and associated reflexes (e.g., responses to loud sounds), and maintains consciousness.

• Pons: Connects the cerebellum to the brainstem, contains tracts involved in somatic and visceral motor control.

• Medulla oblongata: Connects the brain to the spinal cord, relays sensory information, and regulates autonomic functions such as heart rate, blood pressure, and digestion.

Additional info: The pituitary gland and hypothalamus are central to the neuroendocrine system, influencing many target organs through hormone release.

14-2 Brain Protection and Support

The brain is protected physically and biochemically to ensure its proper function and safety from injury and harmful substances.

• Physical Protection:

  • Bones of the cranium

  • Cranial meninges

  • Cerebrospinal fluid (CSF)

• Biochemical Isolation:

  • Blood brain barrier (BBB), regulated by astrocytes

Cranial Meninges

The cranial meninges are three layers that protect the brain and are continuous with the spinal meninges:

• Dura mater: Outer layer, consists of an inner fibrous (meningeal) layer and an outer fibrous (periosteal) layer fused to the periosteum. Dural venous sinuses are located between these layers.

• Arachnoid mater: Middle layer, covers the brain with a smooth sheet and attaches to the dura mater. The subarachnoid space lies between the arachnoid mater and pia mater.

• Pia mater: Inner layer, attached to the brain surface by astrocytes and follows the grooves and structures of the brain.

Dural Folds

Dural folds are extensions of the dura mater into the cranial cavity, stabilizing and supporting the brain. They contain collecting veins (dural venous sinuses). The three largest dural folds are:

• Falx cerebri: Projects between cerebral hemispheres, contains the superior and inferior sagittal sinuses.

• Tentorium cerebelli: Separates cerebrum from cerebellum, contains the transverse sinus.

• Falx cerebelli: Divides cerebellar hemispheres below the tentorium cerebelli.

Cerebrospinal Fluid (CSF)

CSF surrounds all exposed surfaces of the CNS and interchanges with the interstitial fluid of the brain. Its functions include:

• Supporting brain weight

• Cushioning neural structures from physical trauma

• Transporting nutrients, chemical messengers, and wastes

CSF circulates from the choroid plexus through the ventricles, to the central canal of the spinal cord, and into the subarachnoid space via two lateral apertures and one median aperture.

Choroid Plexus

• Specialized ependymal cells and capillaries secrete CSF into ventricles, remove waste products, and adjust CSF composition (proteins, ions, lipids, amino acids).

• Produces about 500 mL of CSF per day.

CSF in Subarachnoid Space

• Arachnoid villi: Extensions of the subarachnoid membrane that penetrate the dura mater and extend into the superior sagittal sinus.

• Arachnoid granulations: Large clusters of villi in adults, where CSF is absorbed into venous circulation.

• Hypercephaly: "Water on the brain" due to excess CSF.

Blood Supply to the Brain

• Supplies nutrients and oxygen to the brain, which has no energy or oxygen storage.

• Delivered by internal carotid and vertebral arteries.

• Most blood is removed from dural venous sinuses by internal jugular veins.

Cerebrovascular Diseases

• Disorders that interfere with or stop blood supply to the brain.

• Cerebrovascular accident (CVA) or stroke: Stops blood flow to a portion of the brain due to blood clot or vessel rupture; affected neurons begin to die within minutes.

Blood Brain Barrier (BBB)

• Isolates CNS tissue from general circulation, formed by tight junctions between endothelial cells of CNS capillaries.

• Lipid-soluble compounds (O2, CO2, steroids, prostaglandins, small alcohols) can diffuse into the interstitial fluid of the CNS.

• Astrocytes regulate BBB by releasing chemicals that control permeability of endothelium.

Blood-CSF Barrier

• Formed by specialized ependymal cells surrounding capillaries of the choroid plexus.

• Limits transfer of compounds to CSF, allowing chemical composition of blood and CSF to differ (ions, hormones, neurotransmitters, amino acids, proteins).

Breaks in the BBB

• Portions of hypothalamus (secrete hypothalamic hormones)

• Posterior lobe of pituitary gland (secretes ADH and oxytocin)

• Pineal gland (secretes melatonin)

• Choroid plexus (specialized ependymal cells maintain blood-CSF barrier)

14-3 The Medulla Oblongata

The medulla oblongata is the most inferior part of the brainstem and is essential for autonomic control and communication between the brain and spinal cord.

• Functions:

  • Controls visceral functions

  • Coordinates complex autonomic reflexes

  • Allows brain and spinal cord to communicate

• Three groups of nuclei:

  1. Control autonomic visceral functions/reflexes

  2. Sensory and motor nuclei of cranial nerves

  3. Relay stations for brain and spinal cord

Autonomic and Reflex Nuclei

• Reticular Formation: Mass of gray and white matter with embedded nuclei, regulates autonomic functions such as blood pressure, breathing, thermoregulation, body position, alertness, and sleep.

• Cardiovascular centers: Cardiac and vasomotor centers control blood flow through peripheral tissues, heart rate, and strength of cardiac contractions.

• Respiratory rhythmicity centers: Set the pace for respiratory movements.

Sensory and Motor Nuclei of Cranial Nerves

• Associated with five cranial nerves (IX, X, XI, XII, VIII).

• Motor commands to neck, pharynx, and back; motor commands to thoracic and peritoneal organs; sensory information from internal ear to vestibule and cochlear.

Relay Stations Along Sensory and Motor Pathways

• Gracile nucleus and cuneate nucleus: Pass somatic sensory information to the thalamus.

• Solitary nuclei: Receive visceral sensory information and forward it to other autonomic centers.

• Olivary complex (Olives): Relay information about somatic motor commands to the cerebellar cortex.

Additional info: The medulla oblongata is vital for basic life functions and is often referred to as the "primitive brain" due to its role in autonomic regulation.