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 circadian rhythms.

Thalamus

• Relay Station: Acts as a major relay for information entering the cerebral cortex.

• Functions:

  • Sorts, edits, and relays ascending input (e.g., emotion, visceral function, motor control, memory, sensory integration).

  • Receives impulses from hypothalamus, cerebellum, and basal nuclei.

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

Hypothalamus

Location and Structure

• Located below the thalamus.

• Connected to the pituitary gland via the infundibulum.

Functions

• Endocrine System Control: Regulates pituitary gland secretions.

• Homeostasis: Main visceral control and regulating center.

• Autonomic Nervous System: Controls blood pressure, heart rate, digestive tract motility, pupil size.

• Emotional Response: Part of limbic system; perceives pleasure, fear, rage, biological rhythms, and drives.

• Regulation:

  • Body temperature

  • Hunger and satiety

  • Water balance and thirst

  • Sleep-wake cycles

• Disorders: Disturbances can cause obesity, sleep issues, dehydration, emotional imbalances.

Epithalamus

• Most dorsal portion of diencephalon.

• Contains pineal gland which secretes melatonin to regulate sleep-wake cycles.

Brain Stem

Regions

• Midbrain

• Pons

• Medulla Oblongata

The brain stem controls automatic behaviors necessary for survival and connects higher and lower neural centers.

Midbrain

• Located between diencephalon and pons.

• 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: Linked to basal nuclei; degeneration leads to Parkinson’s disease.

Pons

• Located between midbrain and medulla oblongata.

• Composed of conduction tracts:

  • Longitudinal fibers connect higher brain centers and spinal cord.

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

• Some nuclei play a role in reticular formation and breathing rhythm.

Medulla Oblongata

• Blends into spinal cord at foramen magnum.

• Contains fourth ventricle and choroid plexus (forms CSF).

• Pyramids: Formed by pyramidal tracts from motor cortex.

• Decussation of pyramids: Tracts cross over to opposite side.

• Olives: Relay stretch information from muscles and joints.

• Vestibular and cochlear nuclei: Mediate equilibrium responses.

Functions of the Medulla Oblongata

• Autonomic reflex center (overlaps with hypothalamus).

• Cardiovascular center: Adjusts heart contraction and blood vessel diameter.

• Respiratory centers: Generate respiratory rhythm.

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

Cerebellum

Structure and Function

• 11% of brain mass.

• Processes input from cortex, brain stem, and sensory receptors.

• Coordinates precise movements and balance.

• Cerebellar hemispheres connected by vermis.

Cerebellar Anatomy

• Folia: Transversely oriented gyri.

• Arbor vitae: Distinctive treelike pattern of white matter.

• Purkinje fibers: Project from cortex for motor coordination.

• Cerebellar homunculi: Sensory maps of the 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 movement to cerebral motor cortex and brain stem nuclei.

• May also play roles 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.

• Interacts with prefrontal cortex for emotional awareness and response.

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

• Involves association cortex of left hemisphere.

• 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.

Stages of Declarative Memory

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

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

Factors Affecting Memory 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

• Anterograde amnesia: New inputs not associated with old memories; person lives in the present.

• Retrograde amnesia: Loss of memories formed in the distant past.

Brain Wave Patterns and the EEG

• EEG records electrical activity of brain function.

• Used for diagnosing epilepsy, sleep disorders, brain death.

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

Types of Brain Waves

Wave Type	Frequency (Hz)	Description
Alpha	8–13	Regular, rhythmic, low-amplitude; "idling" brain
Beta	14–30	Rhythmic, less regular; mentally alert
Theta	4–7	Irregular; common in children, uncommon in awake adults
Delta	4 or less	High-amplitude; deep sleep, suppressed in awake adults

Seizures

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

• Not associated with intellectual impairments.

• Types:

  • Absence seizures: Mild, brief blanking of expression.

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

Consciousness

• Involves perception, voluntary movement, higher mental processing.

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

• Requires simultaneous activity of large cortical areas.

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; ethical/legal issues.

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.

  • 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 reversible by normal stimulation and involves lower oxygen consumption.