Brain Notes

Brain Development

Cleavage

  • Cleavage: Begins with the zygote undergoing cell division.
  • Stages:
    • Zygote
    • Eight-cell stage
    • Blastula: Characterized by a blastocoel (fluid-filled cavity).
  • Gastrulation:
    • Blastula transforms into a gastrula.
    • Involves the formation of endoderm, ectoderm, and a blastopore.

Neurulation

  • Neuroectodermal Tissue Differentiation:
    • Neuroectodermal tissues differentiate from the ectoderm and thicken into the neural plate.
    • Neural plate border separates the ectoderm from the neural plate.
  • Neural Plate Bending:
    • The neural plate bends dorsally, with the two ends eventually joining at the neural plate borders, now referred to as the neural crest.
  • Neural Tube Formation:
    • Closure of the neural tube disconnects the neural crest from the epidermis.
    • Neural crest cells differentiate to form most of the peripheral nervous system (PNS).
  • Notochord Degeneration:
    • The notochord degenerates and only persists as the nucleus pulposus of the intervertebral discs.
    • Other mesoderm cells differentiate into the somites, the precursors of the axial skeleton and skeletal muscle.

Brain Vesicle Development

  • Three Primary Brain Vesicles (3-4 week embryo):
    • Prosencephalon (Forebrain)
    • Mesencephalon (Midbrain)
    • Rhombencephalon (Hindbrain)
  • Five Secondary Brain Vesicles (5-week embryo):
    • Telencephalon (from Prosencephalon)
    • Diencephalon (from Prosencephalon)
    • Mesencephalon (Midbrain - remains the same)
    • Metencephalon (from Rhombencephalon)
    • Myelencephalon (from Rhombencephalon)

Adult Brain Structures and Ventricle System

  • Forebrain (Prosencephalon):
    • Telencephalon: Cerebrum (cerebral cortex, white matter, basal nuclei); Lateral ventricles
    • Diencephalon: Thalamus, hypothalamus, epithalamus; 3rd ventricle
  • Midbrain (Mesencephalon):
    • Mesencephalon: Midbrain (part of brainstem); Cerebral aqueduct
  • Hindbrain (Rhombencephalon):
    • Metencephalon: Pons (part of brainstem), cerebellum
    • Myelencephalon: Medulla oblongata (part of brainstem); 4th ventricle

Neural Tube Defects (NTDs)

  • Prevention: Folic Acid helps prevent NTDs.
  • Types:
    • Anencephaly
    • Craniorachischisis
    • Open spina bifida
    • Iniencephaly
    • Encephalocele
    • Closed spina bifida

Organization of the Brain

Cerebrum

  • Lobes of the Brain:
    1. Frontal Lobe:
      • Functions: Muscle Movement, Mood, Aggression, Smell, Motivation
      • Lobotomy: Could cause docile passive (submissive) patient; historically used for schizophrenia/psychotic episodes.
    2. Parietal Lobe:
      • Functions: Touch, Taste, Temperature, Pain, Balance
      • Damage: Stroke or Trauma to one side can lead to contralateral hemi spatial Neglect (loss of stimuli awareness on one side).
    3. Temporal Lobe:
      • Functions: Hearing, Smell, Memory, Judgment, Abstract Thought
      • Damage: Could lead to epilepsy.
    4. Occipital Lobe:
      • Function: Vision

Cerebral Cortex

  • Cerebral Cortex: Gray Matter Surface (Outerlayer)
  • Longitudinal Fissure: Separates the hemispheres.
  • Gyrus (Gyri): Outward folds.
  • Sulcus (Sulci): Inward grooves.
  • Corpus Callosum: Connects the hemispheres.
  • Motor Areas: Control muscular movements.
  • Sensory Areas: Interpret sensory impulses.
  • Association Areas: Process emotions and intellect.

Limbic System

  • Hippocampus:
    • Part of the limbic system.
    • Involved in Long-term & Short-term Memory.
    • Consolidation (processing) of information.
    • Spatial Navigation.
  • Structures of the Diencephalon:
    • Thalamus & Hypothalamus
  • Thalamus:
    • Regulate Sleep, Alertness, and wakefulness.
    • Relay Motor signal.
    • Relays Sensory signals.
  • Hypothalamus:
    • Controls Autonomic nervous system and integrates it.
    • Links the Endocrine and Nervous systems by controlling the pituitary gland.
    • Receive sensory impulses from internal organs.
    • Amygdala

Basal Ganglia

  • Components:
    • Caudate nucleus
    • Striatum (Caudate + Putamen)
    • Putamen
    • Globus pallidus (internus and externus)
    • Subthalamic nucleus
    • Substantia nigra
  • Pathways:
    • Direct Pathway (Promote Movement)
    • Indirect Pathway (Inhibit Movement)
  • Neurotransmitters:
    • Glutamate: Excitatory
    • GABA: Inhibitory
    • Dopamine: Enhancer
Direct Pathway (Promote Movement)
  • Excitatory signals are sent by the motor cortex and received by the striatum (Putamen/Caudate).
  • Striatum sends inhibitory (GABA) signals to the Internal Globus Pallidus (GPi) and Subthalamic Nigra par reticulata (SNr).
  • Normally the GPi and SNr send inhibitory signals to the thalamus. Now that the GPi and SNr and inhibited, they will stop sending inhibitory signals to the thalamus
  • The Thalamus is disinhibited, sending excitatory signals (Glutamate) to the motor cortex
  • Dopamine’s Role: Released by the Substantia Nigra pars compacta (SNc) to D1 Receptors in the striatum, Enhancing the activity of the direct pathway.
Indirect Pathway (Inhibit Movement)
  • Excitatory signals are sent by the motor cortex and received by the striatum.
  • Striatum sends inhibitory signals to the Globus Pallidus External (GPe).
  • Normally, the GPe sends inhibitory signals to the Subthalamic Nucleus (STN). Now that the GPe is inhibited the STN become disinhibited.
  • STN is activated and sends an excitatory signal to the GPi and SNr.
  • GPi and SNr send strong inhibitory signals to the thalamus further inhibiting excitatory signals from being sent to the motor cortex.
  • Dopamine’s Role: Released by the Substantia Nigra pars compacta (SNc) to D2 Receptors in the striatum, Inhibiting the inhibitory pathway, reducing inhibitory output from the striatum.

Brain Stem

  • Midbrain:
    • Relays Sensory and Motor information
    • Reflexes in head, eye, and torso
  • Pons Varolii:
    • Connects brain and spinal cord
    • Connects parts of the brain with each other
    • Controls breathing
  • Medulla Oblongata:
    • Responsible for the cardiovascular control (symp + parasymp NVS)
    • Monitors blood through baroreceptors
    • Reflexes such as vomiting, coughing, sneezing, and swallow
    • Connects brain and spinal cord
    • Ventilation

Hemispheric Lateralization

Left Hemisphere

  • General interpretive and speech center
  • Language-based skills
  • Dominant hemisphere in 80% of people
  • Controls the right side of the body

Right Hemisphere

  • Spatial relationship and analyses
  • Controls Left side of the body

Broca’s and Wernicke’s Areas

  • The Broca’s and Wernicke’s areas are the language formation areas in the left hemisphere of right-handed people.
  • Any Lesions (Damage) to these areas will cause language comprehension difficulties and speech disorders
  • The equivalent area of the Broca’s area in the right hemisphere is responsible for Emotional Responses to languages. Lesions to these areas will lead to emotionless speech.

Corpus Callosum

  • The largest group of nerve fibers that communicate between the left and right hemispheres are in the Corpus Callosum.
  • 200 million axons communicate between the hemispheres
  • The Corpus Collosum is cut as treatment for epilepsy (Last Resort)
  • This causes a “split-brain” personality
  • Most CNS structures are symmetrical/bilateral
  • Some cortical functions are asymmetrical (Superior motor-coordination of the right hand)
Split-Brain Patient
  • If shown an image in their left visual field, they can’t name it
    • Because:
      • The image is only sent to the left side of the brain.
      • The speech-control center is located on the left side of the brain
      • Communication between the two sides of the brain is inhibited
  • If touching an object on their left hand, they can’t name it
    • Because:
      • Each hemisphere has a perception of representation of the other side of the body
      • The speech-control center is located on the left side of the brain
      • Communication between the two sides of the brain is inhibited

Brain Waves

  • Brain waves are patterns of neuronal activity in the brain generated by synaptic activity in the cortex, coordinated by thalamus
  • Measured by Electroencephalogram (EEG)
  • Grouped based on Hz
  • Changes based on age, stimuli, diseases of the brain, and chemical state of the body

Types of Brain Waves (Groups)

  • Alpha waves (8-13 Hz):
    • Regular and Rhythmic
    • Relaxed “Idling”
  • Beta waves (14-30 Hz):
    • Less Regular but Rhythmic
    • Alert / concentrated / anxious / thinking
  • Theta waves (4-7 Hz):
    • More Irregular
    • Common in children
    • Idle / suppressing an action
  • Delta waves (4Hz or less):
    • High amplitude (Large Peaks)
    • Deep Sleep / an inhibition in the reticular activating system (Responsible for sleep) / Anesthesia
    • Could indicate brain damage

Consciousness

  • Consciousness is:
    • Perception of sensation
    • Voluntary initiation and control of movement
    • Capabilities associated with high mental processing power (memory, judgement, logic)
  • Loss of consciousness is a signal of impairment in brain function
  • Clinical definitions of a continuum used to measure consciousness:
    • Alertness
    • Drowsiness (Lethargy)
    • Stupor (lighter comma that responds to intense stimuli)
    • Coma

Sleep

  • The 4 stages of sleep:
    1. Sleep
    2. Deepening sleep
      • Brain waves begin to slow down
    3. Slow wave sleep (Delta waves)
    4. Deepest Sleep
      • Hard to wake from
      • May lead to grogginess & confusion
  • Each Sleep Cycle lasts around 90 minutes (30-45 minutes in stage 4)
  • The Cycle repeats 4-6 a night
  • Between each cycle, the brain enters a special stage called Rapid Eye Movement (REM) Sleep
  • REM Sleep is called that because of the way the eyes move behind the eye lid during this stage
  • REM Sleep is where dreams occur

Sleep Cycles reflect Circadian (24-hour) Rhythm (Internal Clock)

  • A sleep pattern alternates between REM and NREM (Non-REM) sleep
  • Slow-wave Sleep (Stage 3 & 4) is believed to be the restorative stage (healing/glymphatic function increase)
  • During Slow-wave Sleep memories are strengthened by connecting them to existing neuronal networks (Memory Consolidation)
  • Stage 4 sleep Declines steadily and may disappear after age 60
  • Daily Sleep Requirements Decline with age
  • REM Sleep integrates memories into existing memories to create new perspectives
  • REM Sleep is associated with creativity and emotional processing
  • REM sleep deprivation causes people to become Moody and Depressed
  • Sleep deprivation:
    • Reduce Immune Function
    • Impair Functions of Working Memory

Sleep Disorders

  • Narcolepsy:
    • Lapsing suddenly into sleep from awake state
    • Difficulty regulating Sleep-wake cycle
  • Insomnia:
    • Chronic inability to get enough sleep and/or quality sleep
  • Sleep Apnea:
    • Temporarily Lossing the ability to breath during sleep