Divisions and Functions of the Nervous System: Brain Structures and Higher Mental Functions

Divisions of the Nervous System

Overview

The nervous system is divided into several major regions, each with specialized functions essential for sensation, movement, and higher mental processes. This guide focuses on the diencephalon, brain stem, cerebellum, limbic system, reticular formation, and higher mental functions.

The Diencephalon

Main Structures

• Thalamus

• Hypothalamus

• Epithalamus

These paired gray-matter structures are centrally located and play key roles in sensory relay, homeostasis, and sleep regulation.

Thalamus

• Relay Station: Main function is to act as a relay station for information entering the cerebral cortex.

• Sorting and Editing: Sorts, edits, and relays ascending input such as:

  • Impulses from hypothalamus for emotion and visceral function

  • Impulses from cerebellum and basal nuclei to direct motor cortices

  • Impulses for memory or sensory integration

• Overall Role: Mediates sensation, motor activities, cortical arousal, learning, and memory.

Hypothalamus

• Location: Below the thalamus, connected to the pituitary gland via the infundibulum.

• Endocrine Control: Regulates secretions of anterior and posterior pituitary hormones.

• Visceral Control Center: Vital for homeostasis; controls autonomic nervous system (blood pressure, heart rate, digestive tract motility, pupil size).

• Physical Responses to Emotions: Part of the limbic system, perceives pleasure, fear, rage, biological rhythms, and drives.

• Regulatory Functions:

  • Body temperature (sweating/shivering)

  • Hunger and satiety

  • Water balance and thirst

  • Sleep-wake cycles

• Disorders: Disturbances can cause body wasting, obesity, sleep disturbances, dehydration, and emotional imbalances. Can be damaged by tumors, radiation, surgery, or trauma.

Epithalamus

• Location: Most dorsal portion of diencephalon.

• Pineal Gland: Secretes melatonin, which helps regulate sleep-wake cycles.

Brain Stem

Regions and Functions

• Midbrain

• Pons

• Medulla Oblongata

The brain stem controls automatic behaviors necessary for survival and contains nuclei associated with 10 of the 12 cranial nerves.

Midbrain

• Cerebral Peduncles: Contain pyramidal motor tracts.

• Cerebral Aqueduct: Connects third and fourth ventricles.

• Periaqueductal Gray Matter: Involved in pain suppression and fight-or-flight response.

• Substantia Nigra: Functionally linked to basal nuclei; degeneration leads to Parkinson's disease.

Pons

• Location: Between midbrain and medulla oblongata.

• Conduction Tracts:

  • Longitudinal fibers connect higher brain centers and spinal cord.

  • Transversal/dorsal fibers relay impulses between motor cortex and cerebellum.

• Nuclei: Play roles in reticular formation and help maintain normal rhythm of breathing.

Medulla Oblongata

• Blends into spinal cord at foramen magnum.

• Contains:

  • Fourth ventricle

  • Choroid plexus (forms cerebral spinal fluid)

• Structures:

  • Pyramids: Formed by pyramidal tracts from motor cortex

  • Decussation of pyramids: Tracts cross over to opposite side

  • Olives: Relay stretch information from muscles/joints to cerebellum

  • Vestibular and cochlear nuclei: Mediate equilibrium responses

Functions of the Medulla Oblongata

• Autonomic Reflex Center: Many functions overlap with hypothalamus.

• Cardiovascular Center:

  • Cardiac center: Adjusts force/rate of heart contraction

  • Vasomotor center: Adjusts blood vessel diameter

• Respiratory Centers: Generate respiratory rhythm, control rate/depth of breathing.

• Reflexes: Vomiting, hiccupping, swallowing, coughing, sneezing.

Cerebellum

Structure and Function

• 11% of brain mass

• Processes input from cortex, brain stem, and sensory receptors to coordinate skeletal muscle movements.

• Major role in balance

• Cerebellar hemispheres connected by vermis.

Cerebellar Anatomy

• Folia: Transversely oriented gyri.

• Arbor Vitae: Distinctive treelike pattern of white matter.

• Purkinje Fibers: Project from cerebellar cortex for motor coordination.

• Cerebellar Homunculi: Sensory maps of the entire body.

Cerebellar Processing

• Receives impulses from cerebral cortex to initiate voluntary muscle contraction.

• Receives signals from proprioceptors and equilibrium pathways.

• Calculates best way to coordinate muscle contraction.

• Sends blueprint of coordinated movement to cerebral motor cortex and brain stem nuclei.

• Plays a role in thinking, language, and emotion.

Limbic System

Functions

• Emotional or affective brain

• Amygdala: Recognizes angry/fearful expressions, assesses danger, elicits fear response.

• Cingulate Gyrus: Expresses emotions and regulates behavior.

• Interaction with Prefrontal Cortex: Allows emotional reactions to conscious understanding, awareness of emotional richness.

Reticular Formation

Reticular Activating System (RAS)

• Sends impulses to cerebral cortex to keep it conscious and alert.

• Filters out repetitive, familiar, or weak stimuli.

• Inhibited by sleep centers, alcohol, drugs.

• Severe injury can result in permanent unconsciousness (coma).

Higher Mental Functions

Overview

• Language

• Memory

• Brain waves and EEGs

• Consciousness

• Sleep and sleep-wake cycles

Language

• Broca's Area: Speech production; lesions cause inability to speak.

• Wernicke's Area: Understanding spoken/written words; lesions cause nonsensical speech.

Memory

• Declarative Memory: Facts (names, faces, words, dates).

• Procedural Memory: Skills (playing piano).

• Motor Memory: Motor skills (riding a bike).

• Emotional Memory: Experiences linked to emotion.

Two stages of declarative memory storage:

• Short-term memory (STM): Temporary, limited to 7-8 items.

• Long-term memory (LTM): Limitless capacity.

Factors affecting STM to LTM transfer:

• Emotional state

• Rehearsal

• Association

• Automatic memory

Memory consolidation involves fitting new facts into categories already stored in cerebral cortex. Sleep is important for consolidation.

Memory Loss

• Damage to hippocampus or temporal lobe: Slight or widespread amnesia.

• Anterograde amnesia: Cannot form new memories.

• Retrograde amnesia: Loss of old memories.

Brain Wave Patterns and the EEG

• EEG: Records electrical activity of brain; used for diagnosing epilepsy, sleep disorders, brain death.

• Measures wave frequency in Hertz (Hz): $1 \text{ Hz} = 1 \text{ peak per second}$

Wave Type	Frequency (Hz)	State
Alpha	8–13	Awake but relaxed
Beta	14–30	Awake, alert
Theta	4–7	Common in children
Delta	4 or less	Deep sleep

Seizures

• Epileptic seizure: Electrical discharges by groups of neurons; may cause loss of consciousness, jerking.

• Absence seizures: Mild, brief blanking of expression.

• Tonic-clonic seizures: Severe, loss of consciousness, convulsions.

Consciousness

• Perception of sensation

• Voluntary movement

• Higher mental processing

• Defined on a continuum: alertness, drowsiness, stupor, coma

Loss of Consciousness

• Fainting (syncope): Brief loss, often due to low blood pressure or emotional stress.

• Coma: Extended unconsciousness, not the same as deep sleep.

• Brain death: Irreversible coma.

Sleep and Sleep-Wake Cycles

• Sleep: State of partial unconsciousness; can be aroused by stimulation.

• Types:

  • Non-rapid eye movement (NREM) sleep: Four stages

  • Rapid eye movement (REM) sleep

• Disorders:

  • Narcolepsy: Sudden lapse into sleep

  • Cataplexy: Loss of muscle tone while awake

  • Insomnia: Inability to obtain sufficient sleep

Discussion Questions

• What functions would likely be lost by a patient who had a stroke in the cerebellum?

• What kind of memory is involved with remembering my name?

• Contrast sleeping and a coma.

Example: A stroke in the cerebellum may result in loss of coordinated movement and balance. Remembering a name involves declarative memory. Sleep is a reversible state of unconsciousness, while coma is not.