Overview
The brainstem consists of three parts (midbrain, pons, medulla oblongata) and controls basic autonomic survival behaviors essential for life, such as breathing and heart function.
Brainstem Structure & General Functions
- The brainstem has three main parts: midbrain, pons, and medulla oblongata
- Controls basic autonomic survival behaviors—functions that keep you alive while sleeping
- Regulates breathing, heartbeat, and cardiovascular functions
- Damage to brainstem can be fatal because it controls vital life functions
- Whiplash injuries involving the dens (C2 vertebra) can impact the medulla oblongata through foramen magnum
- Contains the reticular formation throughout, responsible for consciousness
- Serves as pathway for fiber tracts (axons) traveling from spinal cord to brain regions, especially thalamus
Cranial Nerves Overview
| Cranial Nerve Pair | Name | Brainstem Attachment |
|---|
| I | Olfactory | Not attached to brainstem |
| II | Optic | Not attached to brainstem |
| III | Oculomotor | Midbrain |
| IV | Trochlear | Midbrain |
| V | Trigeminal | Pons |
| VI | Abducens | Pons |
| VII | Facial | Pons |
| VIII | Vestibulocochlear | Pons & Medulla oblongata |
| IX | Glossopharyngeal | Medulla oblongata |
| X | Vagus | Medulla oblongata |
| XI | Accessory | Medulla oblongata |
| XII | Hypoglossal | Medulla oblongata |
- Twelve pairs of cranial nerves total; must know Roman numerals for all
- Pairs 3–12 attach to brainstem; pairs 1–2 do not
- Pair VIII is special: attaches to both pons and medulla oblongata
Medulla Oblongata
- Most inferior portion of brainstem with the most functions
- Contains autonomic nuclei (cell bodies of autonomic neurons)
- Controls conscious proprioception, pressure, vibration, and touch sensory information
- Houses the cardiovascular center: regulates heart rate, force of heartbeat, blood vessel diameter
- Contains respiratory center: controls rhythm of breathing (gradual increases/decreases, not sudden changes)
- Manages life-saving reflexes: vomiting, coughing, sneezing, hiccupping
- Controls deglutition (swallowing reflex)
- Cranial nerve pairs VIII–XII (8 through 12) originate from this region
Pons
- Intermediate structure; name means "bridge" between other brainstem parts
- Contains cell bodies that inhibit inspiration and others that promote inspiration
- Controls breathing depth (deep versus shallow breathing)
- Breathing depth changes based on body's oxygen/carbon dioxide needs
- Carbon dioxide sensors (not oxygen sensors) trigger breathing depth changes
- Body monitors blood pH (normal range: 7.35–7.45) via CO₂ levels
- High COâ‚‚ makes blood acidic; triggers faster, deeper breathing to expel excess
- Low COâ‚‚ leads to shallower breathing to accumulate more and maintain pH
- Cranial nerve pairs V–VIII (5 through 8) originate from pons
Midbrain
- Contains cerebral aqueduct connecting third and fourth ventricles for cerebrospinal fluid flow
- Controls startle reflexes in response to sudden visual or auditory stimuli
- Manages automatic movement of head, eyes, and trunk toward unexpected stimuli
- Regulates eye tracking movements for reading (left-to-right scanning)
- Controls repetitive eye movements for tracking moving objects
- Cranial nerve pairs III and IV (3 and 4) originate from midbrain
Key Terms & Definitions
- Proprioception: Ability to sense position of body and limbs in space without seeing them
- Deglutition: Technical term for the act of swallowing
- Fiber tracts: Bundles of axons passing through brainstem from spinal cord to brain
- Reticular formation: Structure throughout brainstem responsible for consciousness
- Autonomic nuclei: Collections of cell bodies controlling involuntary functions