Unit 3: The Biology of Emotion, Motivation, and Homeostasis - Brain Stem Neurobiology Notes

Unit 3: The Biology of Emotion, Motivation, and Homeostasis

Chapter 40: The Brain Stem Neurobiology (BIOL 4279/5279)

Schedule and Exams

  • Upcoming Exam Schedule:

    • Exam 3: March 6, covering Chapters 40-44

    • Review and catch-up sessions scheduled leading up to Exam 3.

    • Quiz #6 due on specific dates.

Unit 3 Learning Objectives

By the end of this unit, you should be able to:

  • Identify the major components of the brainstem and their primary functions.

  • Describe how the hypothalamus maintains body homeostasis.

  • Discuss how the brain processes complex emotions, such as fear.

  • Explain the role of dopamine in regulating reward and addiction processes.

  • Locate the thalamus and describe its role in sleep/wake cycles.

Chapter 40 Learning Objectives

By the end of this lecture, you should be able to:

  • Label the three regions that make up the brainstem and list their major functions.

  • Differentiate between the somatic vs autonomic, sympathetic vs parasympathetic nervous systems.

  • Match the distinct cranial nerve column groups with their function.

  • Discuss a neuronal reflex and how they are used in neurologic exams.

  • Describe the neural components that control respiration.

Anatomical Overview of the Central Nervous System (CNS)

The CNS comprises the following seven main parts:

  1. Spinal Cord: Receives and processes sensory information from skin, joints, and muscles from limbs and trunk.

  2. Medulla Oblongata: Autonomic functions include digestion, breathing, and heart rate control.

  3. Pons: Relays information about movement from the cerebrum to the cerebellum.

  4. Midbrain: Regulates sensory and motor functions, including eye movements and coordination of visual and auditory reflexes.

  5. Cerebellum: Modulates the force and range of movement; critical for motor skills and functionally connected to the brainstem.

  6. Diencephalon: Comprises thalamus and hypothalamus; processes information from the rest of the brain and regulates autonomic, endocrine, and visceral functions.

  7. Cerebrum: Contains two cerebral hemispheres; integrates complex sensory and neural functions and is responsible for initiating and coordinating voluntary activity.

Reticular Formation

  • The reticular formation is a network of brainstem nuclei that serves as a major integration and relay center for vital brain functions necessary for survival.

  • Does not have distinct borders, but encompasses general areas with specific functions, including:

    • Attention

    • Arousal

    • Consciousness

    • Sleep-wake cycles

    • Habituation

  • Consequences of dysfunction:

    • Lack of signaling can lead to coma, narcolepsy, hypersomnia, sudden infant death syndrome (SIDS).

    • Overactive signaling can result in hallucinations, flashbacks, and sleep-wake disturbances.

Brainstem Structure and Functions

  1. Medulla Oblongata:

    • Most caudal region; connects spinal cord with pons.

    • Regulates autonomic functions: heart rate, respiration, digestion, and blood pressure.

  2. Pons:

    • Middle part of the brainstem; conveys movement information from the cerebrum to the cerebellum.

  3. Midbrain:

    • Rostral-most portion of the brainstem; controls sensory and motor functions including eye movement and visual/auditory reflex coordination.

Peripheral Nervous System (PNS) Overview

  • Components:

    • Central Nervous System (CNS): Encompasses the brain and spinal cord.

    • Peripheral Nervous System (PNS): Comprises nerves outside of the brain and spinal cord.

  • Divisions:

    • Somatic Nervous System: Comprises sensory nerves from skin, muscles, and joints.

    • Autonomic Nervous System: Mediates visceral sensations related to internal organs (e.g., hunger, nausea, heart palpitations).

  • The CNS integrates information from both divisions to direct movement.

Autonomic Nervous System Functionality

  • Final effectors include:

    1. Smooth Muscle: Controls movements within digestive, respiratory, vascular, excretory, and reproductive systems.

    2. Cardiac Muscle: Regulates heartbeat.

    3. Glands: Secrete fluids (exocrine through ducts; endocrine hormones into the bloodstream).

Sympathetic and Parasympathetic Systems

  • The sympathetic and parasympathetic systems regulate the same effector organs but with opposing effects:

    • Sympathetic System:

    • Facilitates energy expenditure for enhanced activity.

    • Communicates via acetylcholine (ACh) and norepinephrine.

    • Activates glands producing epinephrine (adrenaline).

    • Associated with the fight or flight response.

    • Parasympathetic System:

    • Facilitates energy conservation and returns to a non-emergency state.

    • Communicates primarily via acetylcholine (ACh).

    • Promotes the rest and digest response.

Ascending and Descending Pathways

  • The brainstem contains all ascending tracts transferring sensory information to the cerebral cortex and descending tracts sending motor commands.

  • Sensory Decussation/Pyramidal Decussation:

    • This is the point where afferent/efferent neurons cross the midline, allowing contralateral body region sensing and control.

Cranial Nerves in the Brainstem

  • Cranial nerves provide sensory and motor innervation for the head:

    • Glossopharyngeal and vagus nerves supply visceral innervation to neck, chest, and abdominal organs (not including pelvis).

    • Neurological exams can assess cranial nerve function to localize damage.

Functional Columns of Adult Cranial Nerve Nuclei

  • Cranial nerve nuclei organized into six functional columns on the rostrocaudal axis:

    • Sensory vs. Motor:

    • Sensory: Afferent neurons retrieve sensory information and relay it to the CNS.

    • Motor: Efferent neurons transmit commands from the CNS to muscles/glands.

    • Somatic vs. Visceral:

    • Somatic: Controls voluntary actions/transmits external sensory info.

    • Visceral: Controls involuntary actions/transmits internal sensory info.

    • General vs. Special:

    • General: Processes common sensory/motor functions.

    • Special: Processes sensory inputs unique to specific senses (e.g., hearing, balance).

Reflexes Involving Cranial Nerves

  • Pupillary Light Reflex:

    • Automatic response to light mediated by parasympathetic innervation of the iris.

    • Important for maintaining basic bodily functions.

    • Autonomic Reflex: Involves visceral system and does not require conscious thought.

  • Functional Mechanisms:

    • Retinal ganglion cells function as luminance detectors.

    • Project to the olivary pretectal nucleus (OPN) in the midbrain/thalamus junction.

    • OPN neurons project to Edinger-Westphal nucleus, which contains parasympathetic preganglionic neurons.

    • These neurons exit with the oculomotor (III) nerve to contact ciliary ganglion cells that control pupilloconstrictor muscles.

    • Damage along this pathway results in impaired reflex response.

Other Cranial Nerve Reflexes

  1. Vestibulo-Ocular Reflex:

    • Stabilizes image on the retina during head movements.

    • Impaired by damage to the pons.

  2. Corneal Reflex:

    • Causes eyelid closure and upward eye movement when the cornea is stimulated.

    • Impaired if there is damage along the sensory pathway or the facial nerve.

  3. Gag Reflex:

    • Protects the airway when stimulating the posterior oropharynx.

    • Loss of reflex indicates injury to the medulla or cranial nerve X on the affected side.

  4. Complex Behaviors and Reflexes:

    • Includes actions like vomiting and baroreceptor reflex, which are coordinated through multiple reflex pathways in the brainstem.

Rhythmic Breathing Regulation

  • Generated within the medulla through neuronal firing:

    • Neuronal firing → Phrenic nerve firing → Diaphragm contraction (breathing).

  • The Pre-Bötzinger Complex is crucial for generating rhythmic breathing, modulated by sensory input from the hypoglossal nucleus.

  • Dorsal Respiratory Group: Receives sensory input to prevent lung over-inflation and responds to low oxygen levels.

  • Ventral Respiratory Group: Coordinates motor output for respiration and generates rhythm.

Implications of Injuries in the Brainstem

  • Dorsal Midbrain Injury:

    • Can impair pupillary light responses due to damage to the Oculomotor Nerve nucleus (III).

  • Dorsal Medulla Injury:

    • Can impair respiration due to damage to the Hypoglossal nerve nucleus (XII).

Conclusion

  • For the next class, complete the mid-semester feedback survey (Quiz #6).

  • Read Chapter 41: The hypothalamus: Autonomic, Hormonal, and Behavioral Control of Survival.