Neurophysiology 05: Brain Structure, Function, and Clinical Dysfunction

CNS Anatomical and Functional Subdivisions

• Anatomical Subdivisions of the CNS: The Central Nervous System (CNS) is anatomically subdivided into 7 major areas:

  • 1. Telencephalon (or Cerebrum)
  • 2. Diencephalon (consisting of the Thalamus and Hypothalamus)
  • 3. Mesencephalon (or Midbrain)
  • 4. Pons
  • 5. Medulla oblongata
  • 6. Cerebellum
  • 7. Spinal cord
  • Brain components: Areas 1 through 6 constitute the brain.

• Functional Brain Divisions: For clinical and physiological purposes, the brain is divided into 5 major areas:

  • 1. Telencephalon (or Cerebrum)
  • 2. Thalamus
  • 3. Hypothalamus
  • 4. Brainstem
  • 5. Cerebellum

• Clinical Definition of the Brainstem:

  • Anatomically, the brainstem includes the Diencephalon, Mesencephalon, Pons, and Medulla oblongata.
  • Functionally and clinically, most practitioners restrict the brainstem to the Mesencephalon, Pons, and Medulla. This is because clinical signs of diencephalic lesions typically relate more closely to those involving the telencephalon.

• Embryonic Nomenclature:

  • Prosencephalon (Forebrain) = Telencephalon + Diencephalon
  • Hindbrain components involve the Pons, Medulla, and Cerebellum.

The Diencephalon: Thalamus and Reticular Formation

• The Thalamus:

  • Localized within the diencephalon as a large nucleus extending into each cerebral hemisphere.
  • Acts as the "Gateway to the cerebral cortex."
  • Functions:
    • Preprocesses most information reaching the cerebral cortex.
    • Integrates and processes information from the sensory systems, governing selective attention.
    • Integrates signals from non-sensory areas, such as the basal nuclei (basal ganglia) and the cerebellum.
    • Essential for maintainance of consciousness and alertness.

• The Reticular Formation:

  • A network of interconnected nuclei spread throughout the Thalamus, Midbrain, Pons, and Medulla oblongata.
  • It is not a single anatomical structure, but a network of different areas.
  • Functional Divisions:
    • ARAS (Afferent part): Ascending Reticular Activating System.
    • Motor nuclei/tracts (Efferent part).

• Ascending Reticular Activating System (ARAS):

  • Mechanism: Keeps the brain "switched on" and ready for stimuli by sending activating signals to the cerebral cortex through the thalamus.
  • Key Functions:
    • Consciousness: Sustains cortical and behavioral arousal.
    • Sleep-Wake Transitions: Regulates the shift between states. Activity is suppressed during sleep to reduce the flow of signals to the cortex.
    • General Alertness: Modulates levels of wakefulness, allowing movement from relaxed states to heightened focus.
  • Pathology: Damage to the ARAS can result in reduced arousal or coma.

• Efferent Component of Reticular Formation:

  • Consists of motor nuclei giving rise to descending motor pathways.
  • Reticulospinal Tracts: Arise from the pontine and medullary reticular nuclei and project axons to the spinal cord.
  • Functions: Modulate muscle tone, posture, and reflex activity.

The Diencephalon: Hypothalamus and Limbic System

• The Hypothalamus (Center of Homeostasis):

  • Homeostasis Definition: The process by which the body maintains stable internal conditions despite external changes.
  • The "4 F's": Fighting, Fleeing, Feeding, and Sex.
  • Anatomy: Composed of more than $12$ nuclei and areas.
  • Regulatory Functions:
    • Endocrine system
    • Body temperature
    • Appetite
    • Sexual behavior
    • Defense reactions
    • Circadian rhythm
    • Autonomic nervous system

• Axosecretory Synapses in the Hypothalamus:

  • Type A: Neurons release hormones directly into the blood vessels of the neurohypophysis (posterior pituitary).
  • Type B: Neurons release hormones into the hypophyseal portal system; these molecules are then transported to modulate the anterior pituitary.

• Hypothalamic Regulatory Hormones:

  • TRH: Thyrotropin-releasing hormone
  • CRH: Corticotropin-releasing hormone
  • GnRH: Gonadotrophin-releasing hormone
  • GHRH: Growth hormone-releasing hormone
  • Somatostatin (GHIH): Growth hormone inhibiting hormone
  • PIH: Prolactin inhibiting hormone

• The Limbic System:

  • Circuit located at the "limbus" (border) between the cerebrum and thalamus.
  • Interconnected Structures: Includes the hypothalamus, thalamic nuclei, cingulate gyrus, amygdala, hippocampus, mammillary nuclei, and fornix.
  • Functions:
    • Governs individual emotional and social behavior.
    • Related to instinctual behaviors and memory formation.
    • Direct connection to the autonomic nervous system: emotions can influence visceral functions like heart rate ($HR$), blood pressure ($BP$), and gastrointestinal ($GI$) motility.

Function and Structure of the Cerebellum

• Core Function: Coordinates complex skeletal muscle activity and balance.

  • Communicates intimately with the cerebral cortex, brainstem, vestibular system, and spinal cord.
  • Acts as a "fine-tuner," assessing disparities between intended motor actions and actual muscular responses.

• Sensory Input Sources:

  • Skin, joints, and muscles.
  • Vestibular system and visual system.

• Specific Roles:

  • Motor memory and muscular coordination.
  • Control of postural tonus and equilibrium (coordination of posture across left/right and front/back axes).

• Clinical Signs of Cerebellar Damage:

  • Loss of spatial accuracy.
  • Loss of smooth movement execution.
  • Loss of equilibrium.

Anatomical and Functional Details of the Brainstem

• The Midbrain (Mesencephalon):

  • Contains Motor nuclei: Red nucleus and Rostral colliculus.
  • Starting point for brainstem Upper Motor Neuron (UMN) pathways known as Extrapyramidal tracts, vital for posture and gait.
  • Contains the nuclei for Cranial Nerves: $CN\,III$ (Oculomotor) and $CN\,IV$ (Trochlear).
  • Contains part of the ARAS for consciousness and wakefulness.

• The Pons:

  • Contains the Pontine reticular formation (start of UMN extrapyramidal tracts for posture/gait).
  • Cerebellar Peduncles: Pathway for nerve fibers relaying sensory information between the cerebellum and cerebral cortex.
  • Visceral Nuclei: Regulate breathing and micturition (urination).
  • Contains the nucleus for Cranial Nerve: $CN\,V$ (Trigeminal).
  • Contains part of the ARAS.

• The Medulla Oblongata:

  • Contains the Medullary reticular formation (extrapyramidal UMN pathways).
  • Visceral Nuclei: Regulate heart rate and contractility, blood pressure, breathing, swallowing, and vomiting.
  • Contains the nuclei for Cranial Nerves: $CN\,VI$, $CN\,VII$, $CN\,VIII$, $CN\,IX$, $CN\,X$, $CN\,XI$, and $CN\,XII$.
  • Contains part of the ARAS.
  • Note: All sensory fibers ascending to the cortex and all motor fibers descending to the spinal cord must pass through the brainstem.

Clinical Signs of Brain Dysfunction

• Cerebral Cortex Dysfunction:

  • Behavioral Changes: Absence (unresponsive), Delirium (agitation, vocalization), Pacing (compulsive walking), Reduced interaction/exploration, Spatial disorientation (getting stuck in corners), Change in sleep-wake patterns, Aggressiveness, House soiling, and loss of learned behaviors.
  • Neurological Abnormalities:
    • Epileptic seizures.
    • Head pressing.
    • Ipsilateral signs (due to reduction of activity on the contralateral side): Circling, Head turn.
    • Contralateral signs: Hemi-neglect, Blindness, Menace response deficits, Nasal stimulation deficits, Proprioceptive deficits, Tetraparesis/hemiparesis.

• Diencephalic Dysfunction signs:

  • Absence (unresponsive).
  • Reduced level of consciousness.
  • Contralateral blindness, proprioceptive deficits, and tetraparesis/hemiparesis.
  • Polyuria, polydipsia, abnormal eating, and thermoregulation issues.
  • Circling, behavior changes, and epileptic seizures.
  • Unspecific pain (Thalamic pain).

• Brainstem Dysfunction signs:

  • Reduced level of consciousness.
  • Ipsilateral signs: Proprioceptive deficits, Tetraparesis/hemiparesis.
  • Visceral dysfunction (respiratory patterns, blood pressure).
  • Cranial nerve deficits and vestibular signs.
  • Dysphagia (swallowing problems).

• Cerebellar Dysfunction signs:

  • Cerebellar ataxia.
  • Intention tremors of the head and trunk.
  • Hypermetria (overstepping).
  • Unbalance and vestibular signs.
  • Menace response deficits.