Divisions and Functions of the Nervous System: Diencephalon, Brain Stem, Cerebellum, and Higher Mental Functions

Divisions of the Nervous System

Overview

The nervous system is divided into several major regions, each with distinct anatomical structures and physiological functions. This guide focuses on the diencephalon, brain stem, cerebellum, and higher mental functions, which are essential for sensation, movement, emotion, and cognition.

The Diencephalon

Main Structures

• Thalamus

• Hypothalamus

• Epithalamus

Thalamus

The thalamus contains several bilateral nuclei that project and receive fibers from the cerebral cortex. Its primary function is to act as a relay station for information entering the cortex.

• Relay Station: Sorts, edits, and relays ascending input such as:

  • Impulses from hypothalamus for regulating emotion and visceral function

  • Impulses from cerebellum and basal nuclei to help direct motor cortices

  • Impulses for memory or sensory integration

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

Hypothalamus

Located below the thalamus, the hypothalamus is the main visceral control and regulating center vital to homeostasis.

• Infundibulum: Stalk connecting to pituitary gland, controls endocrine system functions such as:

  • Secretion of anterior pituitary gland

  • Production of posterior pituitary hormones

• Autonomic Nervous System Control: Regulates blood pressure, heart rate, digestive tract motility, pupil size

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

• Regulation of:

  • Body temperature (sweating or shivering)

  • Hunger and satiety

  • Water balance and thirst

  • Sleep-wake cycles

• Disorders: Disturbances can cause severe body wasting, obesity, sleep disturbances, dehydration, emotional imbalances

• Damage: Can be caused by tumors, radiation, surgery, or trauma

Epithalamus

The epithalamus is the most dorsal portion of the diencephalon and contains the pineal gland.

• 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 embedded in white matter. It connects higher and lower neural centers and is associated with 10 of the 12 pairs of cranial nerves.

Midbrain

• Cerebral Peduncles: Contain pyramidal motor tracts

• Cerebral Aqueduct: Channel connecting third and fourth ventricles

• Periaqueductal Gray Matter: Nuclei 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 role 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: crossing over of tracts 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: Many functions overlap with hypothalamus

• Cardiovascular Center:

  • Cardiac center adjusts force and rate of heart contraction

  • Vasomotor center adjusts blood vessel diameter

• Respiratory Centers: Generate respiratory rhythm, control rate and 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 provide precise, coordinated movements of skeletal muscles

• Major role in balance

• Cerebellar hemispheres connected by wormlike vermis

Cerebellar Anatomy

• Folia: Transversely oriented gyri

• Arbor Vitae: Distinctive treelike pattern of white matter

• Purkinje Fibers: Project from cerebellar cortex, synapse within cerebellum for motor coordination

• Cerebellar Homunculi: Sensory maps of entire body

Cerebellar Processing

• Receives impulses from cerebral cortex of intent to initiate voluntary muscle contraction

• Receives signals from proprioceptors and equilibrium pathways

• Calculates the best way to smoothly 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 or fearful facial expressions, assesses danger, elicits fear response

• Cingulate Gyrus: Expresses emotions and behavior regulation

• Interacts 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 and 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 Storage

1. Short-term Memory (STM): Temporary holding, limited to 7-8 pieces of information

2. Long-term Memory (LTM): Limitless capacity

Factors Affecting Transfer from STM to LTM

• Emotional state

• Rehearsal

• Association

• Automatic memory

Memory Consolidation

• 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

• Retrograde Amnesia: Loss of memories formed in the distant past

Brain Wave Patterns and the EEG

Electroencephalogram (EEG)

• Records electrical activity accompanying brain function

• Used for diagnosing epilepsy, sleep disorders, brain death

• Electrodes placed on scalp measure electrical differences between cortical areas

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, brain damage in awake adults

Seizures

Epileptic Seizure

• Torrent of electrical discharges by groups of neurons

• Prevents other messages from getting through

• May cause loss of consciousness, stiffening, jerking

• Not associated with intellectual impairments

Types of Seizures

• Absence Seizures: Mild, brief loss of consciousness

• Tonic-clonic Seizures: Most severe, loss of consciousness, intense convulsions

Consciousness

Definition and Features

• Perception of sensation

• Voluntary initiation and control of movement

• Higher mental processing (memory, logic, judgment)

• Graded on continuum: alertness, drowsiness, stupor, coma

• Involves simultaneous activity of large cortical areas

• Holistic and interconnected

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

Definition and Types

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

• Types:

  • Non-rapid eye movement (NREM) sleep: broken into four stages

  • Rapid eye movement (REM) sleep

Sleep Disorders

• Narcolepsy: Sudden lapse into sleep from awake state

• Cataplexy: Loss of muscle tone while awake

• Insomnia: Inability to obtain or maintain sleep

Discussion Questions

• Damage to the cerebellum may result in loss of coordinated movement and balance.

• Remembering a name involves declarative memory.

• Contrast: Sleeping people can be awakened; comatose individuals cannot, as oxygen consumption is lower and arousal is not possible.

Additional info: Sleep and consciousness are regulated by complex interactions between the reticular formation, hypothalamus, and cerebral cortex. Memory consolidation is enhanced during sleep, particularly during NREM and REM cycles.