Central Nervous System and Brain Structure Notes

Structural Organization of the Brain

Central Nervous System (CNS)

  • Composed of the brain and spinal cord.
  • Receives input from sensory neurons.
  • Directs activity of motor neurons to muscles and glands.
  • Association neurons integrate sensory information to maintain homeostasis and respond to environment.
  • Responsible for learning and memory.

Embryonic Development

  • The neural tube forms approximately 20 days post-conception from the ectoderm.
  • Neural crest forms between the neural tube and the epidermis, developing into PNS ganglia.
  • By week 4, the neural tube shows:
  • Prosencephalon (forebrain)
  • Mesencephalon (midbrain)
  • Rhombencephalon (hindbrain)
  • By week 5, these regions further differentiate:
  • Forebrain ➜ telencephalon and diencephalon
  • Midbrain ➜ remains mesencephalon
  • Hindbrain ➜ metencephalon and myelencephalon

Later Development

  • Telencephalon ➜ cerebral hemispheres, lateral ventricles.
  • Diencephalon ➜ thalamus, hypothalamus, third ventricle.
  • Mesencephalon ➜ midbrain and cerebral aqueduct.
  • Metencephalon ➜ pons, cerebellum, upper fourth ventricle.
  • Myelencephalon ➜ medulla oblongata, lower fourth ventricle.
  • Posterior neural tube ➜ spinal cord.

Choroid Plexuses and Cerebrospinal Fluid (CSF)

  • Choroid plexus consists of epithelial cells and blood capillaries that secrete CSF into the ventricles.
  • Contributes to blood-brain barrier through tight junctions.
  • CSF is produced from blood via secretion, not filtration.
  • Ion transport into ventricles drives water movement.
  • CSF Path of Circulation:
  1. Lateral ventricles ➜ 3rd ventricle (interventricular foramina)
  2. Cerebral aqueduct ➜ 4th ventricle
  3. Central canal of spinal cord and subarachnoid space
  4. Secreted back into blood through arachnoid villi into dural sinuses.

Brain Statistics

  • Gray matter consists of cell bodies; white matter consists of myelinated axons.
  • Adult brain: 100 billion neurons, 1.5 kg (3-3.5 lbs).
  • Receives 15% of total body blood flow per minute.
  • Neurogenesis occurs in adults.
  • Neural stem cells in specific brain regions migrate to areas influencing motor control and cognition, mainly in the striatum and hippocampus.

The Cerebrum

Introduction

  • Derived from telencephalon; largest brain part (80% mass), responsible for higher mental functions.
  • Comprises right and left cerebral hemispheres linked by the corpus callosum.

Cerebral Cortex

  • Composed of 2-4 mm gray matter and underlying white matter.
  • Features gyri (raised folds) and sulci (depressed grooves); collectively known as convolutions.
  • Each hemisphere contains 5 lobes:
  • Frontal
  • Parietal
  • Temporal
  • Occipital
  • Insula (hidden lobe)

Functional Regions of the Cerebrum

  • Frontal and Parietal Lobes:

  • Separated by central sulcus.

  • Precentral gyrus (Frontal Lobe) ➜ motor control.

  • Postcentral gyrus (Parietal Lobe) ➜ somatesthetic sensation (somatosensory cortex).

  • Temporal Lobe:

  • Auditory centers; auditory/visual interpretation.

  • Occipital Lobe:

  • Vision and eye movement coordination.

  • Insula:

  • Memory encoding; integrates sensory info, controls autonomic responses to visceral and cardiovascular systems.

Functions of the Cerebral Lobes

  • Frontal Lobe:
  • Voluntary motor control, personality, higher intellectual processes, verbal communication.
  • Parietal Lobe:
  • Sensory interpretation, speech understand and expression, texture interpretation.
  • Temporal Lobe:
  • Auditory interpretation, memory storage.
  • Occipital Lobe:
  • Eye movement focusing, visual integration.
  • Insula:
  • Memory, sensory integration.

Mirror Neurons

  • Found in frontal/parietal lobes; responsible for goal-directed actions and learning social skills.
  • Connect to emotion centers, potentially linked to autism.

Visualizing the Brain

  • CT (Computed Tomography): Soft tissue imaging.
  • PET (Positron Emission Tomography): Tracks brain metabolism using radioisotopes.
  • MRI (Magnetic Resonance Imaging): Differentiates brain tissue based on water content.
  • fMRI (Functional MRI): Measures blood flow to active brain regions.
  • MEG (Magnetoencephalography): Maps brain activity by measuring magnetic fields.
  • EEG (Electroencephalogram): Detects synaptic potentials, identifies brain wave types associated with activity levels.

Sleep

  • Sleep is genetically controlled but influenced by the environment.
  • Neurotransmitters: Histamine (wakefulness), Adenosine & GABA (sleep).
  • Two primary types of sleep:
  • REM (Rapid Eye Movement): Dreaming state, EEG shows theta waves.
  • Non-REM: Divided into four stages (slow-wave sleep).
  • Sleep patterns involve cycling between non-REM and REM every 90 minutes.
  • Benefits of sleep: Consolidates declarative and spatial memories, regulates proteins related to Alzheimer’s Disease.

Basal Nuclei

  • Masses of gray matter in the cerebrum controlling voluntary muscle movement.
  • Commonly referred to as basal ganglia, with significant structures:
  • Caudate Nucleus
  • Lentiform Nucleus (Putamen & Globus Pallidus)
  • Functions included influencing motor control pathways, and disturbances lead to disorders like Parkinson's and Huntington's disease.

Cerebral Lateralization and Dominance

  • Each hemisphere of the brain controls contralateral functions.
  • Right Hemisphere: visuospatial tasks; Left Hemisphere: language processing and analytical skills.

Language Areas

  • Key centers:
  • Broca’s Area (motor speech)
  • Wernicke’s Area (language comprehension)
  • Broca’s Aphasia: Difficulty with speech production; understanding intact.
  • Wernicke’s Aphasia: Fluent but incoherent speech; comprehension lost.

Limbic System and Emotion

  • Integrates emotional responses (including aggression and fear), centers include:
  • Cingulate Gyrus
  • Amygdala
  • Hippocampus
  • Hormones influence emotional memory, affecting consolidation and retrieval.

Memory

  • Memory regions: Hippocampus (critical for memory formation, consolidation), temporal lobe, and regions related to specific memory tasks.
  • Types: Long-term (structural changes needed), short-term, working memory.
  • Long-term memory can further classified into declarative (facts/events) and nondeclarative (skills).
  • Alzheimer’s Disease: Characterized by cognitive decline, neurofibrillary tangles, and amyloid plaques affecting memory.

Diencephalon

  • Comprises epithalamus, thalamus, hypothalamus, pituitary gland.
  • Thalamus: sensory relay center.
  • Hypothalamus Functions: Homeostasis, regulating sleep, temperature, hunger, emotional responses, and endocrine functions.

Midbrain and Hindbrain

  • Midbrain: Visual/auditory reflexes, motor coordination via substantia nigra (affected in Parkinson's).
  • Hindbrain: Comprises pons (breathing regulation), cerebellum (motor coordination), and medulla oblongata (autonomic functions).

Spinal Cord Tracts

  • White matter: ascending (sensory) and descending (motor) tracts.
  • Ascending tracts include lateral spinothalamic (pain, temp) and dorsal column (touch, proprioception).
  • Descending (pyramidal and extrapyramidal) control voluntary movement and reflexes.

Reflex Arc

  • Comprises sensory receptor, neuron, interneuron (optional, depending on the arc's complexity), motor neuron, and effector muscle/gland.
  • Types include somatic (skeletal muscle) and autonomic (smooth/cardiac muscle or glands).