Central Nervous System: Brain Stem, Cerebellum, Functional Systems, and Spinal Cord

Brain Stem

Structure and Function

The brain stem is a critical region of the central nervous system, consisting of the midbrain, pons, and medulla oblongata. It is structurally similar to the spinal cord but contains nuclei embedded in white matter. The brain stem controls automatic behaviors necessary for survival and serves as a conduit for fiber tracts connecting higher and lower neural centers. Nuclei within the brain stem are associated with 10 of the 12 pairs of cranial nerves.

• Midbrain: Located between the diencephalon and pons, contains the cerebral aqueduct connecting the third and fourth ventricles. Major nuclei include the corpora quadrigemina (visual and auditory reflex centers) and substantia nigra (linked to basal nuclei).

• Pons: Chiefly composed of conduction tracts between higher brain centers and the spinal cord, and those connecting the pons to the cerebellum. Nuclei synapse with cranial nerves V (trigeminal), VI (abducens), and VII (facial).

• Medulla Oblongata: Blends into the spinal cord at the foramen magnum, contains the fourth ventricle, and features pyramids formed by pyramidal tracts. Decussation of the pyramids is the point where tracts cross to the opposite side. Nuclei synapse with cranial nerves VIII, IX, X, and XII.

Reticular Formation

The reticular formation is a network of interconnected nuclei throughout the brainstem, playing a crucial role in maintaining behavioral arousal and consciousness. The reticular activating system (RAS) sends impulses to the cerebral cortex to keep it conscious and alert, filtering out repetitive or weak stimuli.

Medulla Oblongata Functions

• Cardiovascular center: Adjusts force and rate of heart contraction and regulates blood vessel diameter.

• Respiratory centers: Generate respiratory rhythm and control rate and depth of breathing.

• Other centers: Regulate vomiting, hiccupping, swallowing, coughing, and sneezing.

Cerebellum

Structure and Function

The cerebellum comprises 11% of brain mass and is located dorsal to the pons and medulla oblongata. It processes input from the premotor cortex, brain stem, and sensory receptors to provide feedback for precise, coordinated movements of skeletal muscles. The cerebellum also plays a major role in balance.

• Cerebellar hemispheres are connected by the vermis.

• Folia: Transversely oriented gyri.

• Arbor vitae: Distinctive treelike pattern of white matter.

• Peduncles: Superior, middle, and inferior cerebellar peduncles connect the cerebellum to other brain regions.

Cerebellar Processing

1. Receives impulses from the cerebral cortex to initiate voluntary muscle contraction.

2. Receives signals from proprioceptors, visual, and equilibrium pathways to inform the cerebellum of body position and momentum.

3. Cerebellar cortex calculates the best way to coordinate muscle contraction.

4. Neurons send a "blueprint" of coordinated movement to the cerebral motor cortex via the thalamus.

Functional Brain Systems

Limbic System

The limbic system is a network of structures on the medial aspects of the cerebral hemispheres and diencephalon, forming a large part of the emotional brain. It is involved in emotional responses, memory, and psychosomatic illnesses.

• Amygdaloid body: Recognizes angry or fearful expressions, assesses danger, and elicits fear responses.

• Hippocampus: Plays a role in memory formation.

• Interacts with prefrontal lobes, allowing emotional reactions to conscious events.

Reticular Formation

Extends through the central core of the brain stem, with connections to the hypothalamus, thalamus, cerebral cortex, cerebellum, and spinal cord. Governs brain arousal and consciousness.

Higher Mental Functions

Language

Language processing involves the association cortex of the left hemisphere. Broca’s area is involved in speech production, while Wernicke’s area is involved in understanding spoken and written words. Corresponding areas on the right side handle nonverbal language components.

Memory

Memory is the storage and retrieval of information. Declarative memory is divided into short-term (STM) and long-term (LTM) memory. STM is limited to seven or eight pieces of information, while LTM has limitless capacity. Memory consolidation involves fitting new facts into categories already stored in the cerebral cortex, with the hippocampus, temporal cortical areas, thalamus, and prefrontal cortex involved.

• Factors affecting transfer from STM to LTM: Emotional state, rehearsal, association, and automatic memory.

Protection of the Brain

Meninges

The meninges are three connective tissue membranes that cover and protect the CNS, enclose venous sinuses, contain cerebrospinal fluid (CSF), and form partitions in the skull. The three layers are:

• Dura mater: Strongest, made of two layers; periosteal layer attaches to the skull, meningeal layer is the true external covering.

• Arachnoid mater: Middle layer with spiderweb-like extensions; subarachnoid space contains CSF and blood vessels.

• Pia mater: Delicate layer that clings tightly to the brain, containing many blood vessels.

Cerebrospinal Fluid (CSF)

CSF forms a liquid cushion around the brain, providing buoyancy, protection, and nourishment. It is produced by the choroid plexus and filtered at a constant rate. Ependymal cells control its composition and help cleanse it by removing wastes.

Blood Brain Barrier

The blood brain barrier maintains a stable environment for the brain by restricting the passage of substances from the blood. Tight junctions between endothelial cells, astrocyte feet, and pericytes form the barrier. Substances move through endothelial cells via simple diffusion, facilitated diffusion, or transcytosis.

Spinal Cord Gross Anatomy and Protection

Structure and Function

The spinal cord is enclosed in the vertebral column, beginning at the foramen magnum and ending at the L1 or L2 vertebra. It provides two-way communication between the brain and body and serves as a major reflex center.

• Cauda equina: Collection of nerve roots at the inferior end of the vertebral canal.

• Conus medullaris: Cone-shaped terminal end of the spinal cord.

• Filum terminale: Fibrous extension anchoring the spinal cord to the coccyx.

Protection

The spinal cord is protected by bone, meninges, and CSF. The epidural space contains fat and veins, while the subarachnoid space contains CSF. Dural and arachnoid membranes extend to the sacrum.

Spinal Cord Cross-sectional Anatomy

Gray matter is located in the core, surrounded by white matter. The central canal runs the length of the cord and is filled with CSF. The cross section resembles a butterfly or letter "H" and is divided into dorsal, ventral, and lateral horns.

• Dorsal horns: Interneurons receiving somatic and visceral sensory input.

• Ventral horns: Somatic motor neurons.

• Lateral horns: Sympathetic neurons (only in thoracic and superior lumbar regions).

• Gray commissure: Bridge connecting masses of gray matter.

• Ventral roots: Motor neuron axons.

• Dorsal roots: Sensory input.

• Spinal nerves: Formed by fusion of dorsal and ventral roots.

Functional Groups of Gray Matter

Gray matter is divided into four groups based on somatic or visceral innervation:

• Somatic sensory (SS)

• Visceral sensory (VS)

• Visceral (autonomic) motor (VM)

• Somatic motor (SM)

White Matter

White matter consists of myelinated and nonmyelinated nerve fibers allowing communication between parts of the spinal cord and between the spinal cord and brain. Tracts run in three directions: ascending (sensory inputs), descending (motor outputs), and transverse (commissural fibers). White matter is divided into dorsal, lateral, and ventral columns (funiculi).

Spinal Cord Trauma and Disorders

Types of Trauma

• Paresthesias: Sensory function loss due to damage to dorsal roots or sensory tracts.

• Paralysis: Motor function loss due to damage to ventral roots or ventral horn cells.

• Flaccid paralysis: Severe damage to ventral root or ventral horn cells; no voluntary or involuntary control.

• Spastic paralysis: Damage to upper motor neurons; muscles stimulated by reflex activity, no voluntary control.

• Transection: Total motor and sensory loss below the level of injury; paraplegia or quadriplegia depending on location.

• Spinal shock: Transient period of functional loss caudal to lesion.

Disorders

• Poliomyelitis: Destruction of ventral horn motor neurons by poliovirus; can lead to muscle atrophy and death.

• Amyotrophic lateral sclerosis (ALS): Destruction of ventral horn motor neurons and fibers of pyramidal tract; loss of ability to speak, swallow, and breathe; death typically within 5 years.

Table: Functions of Major Brain Regions

Region	Function
Cerebral Hemispheres (Cortical gray matter)	Localizes and interprets sensory inputs Controls voluntary and skilled skeletal muscle activity Functions in intellectual and emotional processing
Basal nuclei (ganglia)	Subcortical motor centers Help control skeletal muscle movements

Additional info:

Degenerative brain disorders such as Alzheimer’s, Parkinson’s, and Huntington’s are covered in required reading and involve progressive loss of neural function and structure. Clinical terms such as autism, dyslexia, microcephaly, myelogram, neuroses, and psychoses are relevant for further study.