OE

Week 1 ELM 2: Cells in the Nervous System

Introduction to Nervous Systems

Why Have a Nervous System?

  • The nervous system facilitates stimulus processing, decision-making, and movement.

Invertebrate 'Brains'

  • Sponges and jellyfish have a nerve net instead of a centralized brain.
  • Invertebrates possess a ventral nerve cord, whereas vertebrates have a dorsal cord.

Vertebrate Brain Appearance

  • Key brain divisions common to all vertebrates include:
    • Olfactory bulb
    • Cerebral hemispheres
    • Cerebellum
    • Optic tectum
    • Medulla oblongata
  • Mammals uniquely possess a neocortex.
  • Amphioxus (cephalochordate) has a small central collection of neuronal control circuits, representing a primitive brain.

Brain Subdivisions

  • Forebrain
    • Diencephalon
      • Thalamus
      • Hypothalamus
    • Telencephalon
      • Cortex
      • Olfactory bulb
  • Midbrain
    • Mesencephalon
      • Tectum
      • Tegmentum
  • Hindbrain
    • Rhombencephalon
      • Pons
      • Medulla
      • Cerebellum
  • Spinal cord
  • Brainstem includes the midbrain, pons, and medulla.

Development of the Nervous System

Neural Tube Development

  • Three primary cell layers:
    • Endoderm: linings of organs and viscera.
    • Mesoderm: bones and muscles.
    • Ectoderm: nervous system and skin.
  • The neural plate folds and fuses, forming the neural tube.
  • The central nervous system (CNS) develops from the walls of the neural tube.
  • The peripheral nervous system (PNS) derives from the neural crest.

Spina Bifida

  • Spina bifida results from the failure of the posterior neural tube to close.
  • Folic acid supplementation during early pregnancy can reduce the incidence of neural tube defects by 90%.
  • Certain antiepilepsy and bipolar drugs interfere with folate metabolism, increasing the risk of spina bifida.

Developmental Differentiation

  • Three swellings at the rostral end of the neural tube develop into primary vesicles.

Subdivisions of the Brain: Spinal Cord to Cerebellum

Ventricles

  • The brain is hollow and bathed in cerebrospinal fluid (CSF).

Spinal Cord

  • Protected by the spinal column and surrounded by meninges and CSF.
  • Serves as the primary channel for messages between the skin, joints, and muscles to the brain, and from the brain to the periphery.
  • Dorsal roots contain sensory, afferent neurons.
  • Ventral roots contain motor, efferent neurons.
  • Gray matter consists of neuron cell bodies.
  • White matter consists of myelinated axons.

Brainstem

  • Contains the midbrain, pons, medulla, and (sometimes) the cerebellum.
  • The oldest part of the brain, involved in the decision matrix.
  • Controls vital functions.

Pons and Medulla

  • Pons: a swelling on the ventral surface of the brainstem that relays information between the cortex and cerebellum.
  • Medulla: crucial for controlling blood pressure and respiration.

Brain Stem Damage

  • Damage can result from hydrocephalus or hemorrhage.
  • Severe cases can lead to ‘coning’.
  • Damage to the medulla can cause respiratory arrest.

Diencephalon & Mesencephalon

  • Midbrain: responsible for linkages between motor system components (e.g., substantia nigra), eye movements, sleep, and temperature regulation.
  • Diencephalon:
    • Thalamus: involved in relay and gating roles in sleep and conscious movement.
    • Hypothalamus: responsible for homeostasis and reproduction and regulation of body temperature.

Cerebellum

  • An ‘old’ part of the brain.
  • Main function is movement control.
  • Has extensive connections to the cerebrum and spinal cord.
  • Contains approximately the same number of neurons as both cerebral hemispheres combined.
  • Diseases affecting the cerebellum can result in ataxias, characterized by aberrant movement coordination.

Subdivisions of the Brain: The Cerebral Cortex

Cerebral Cortex

  • Clear division between two halves along the sagittal fissure.
  • Controls voluntary actions, cognition, and perception/awareness.
  • Mammals possess a neocortex, characterized by a complex six-layer structure.
  • The number of neurons in the cortex is related to “intelligence.”
  • The cerebral cortex has different sizes but maintains the same general structure across species.

Cortical Folding

  • Problem: Increasing processing power to enhance intelligence requires increasing the number of cortical neurons (located in a thin layer at the surface).
  • Constraint: The skull has a confined structure.
  • Solution: Folding the cortex allows for an increased number of neurons within a smaller volume.
  • A sheet of cortex forms gyri (ridges) and sulci (grooves).

Cerebral Cortex Lobes

  • Frontal lobe
  • Parietal lobe
    • Somatosensory cortex
  • Occipital lobe
  • Temporal lobe
    • Somatomotor cortex

The Homunculus

  • Representation of the body in the somatosensory and somatomotor cortices,mapped to specific body parts, such as the hip,leg,foot,toes, genitals,forearm,arm,wrist,hand,eye,teeth, gums and jaw,tongue and pharynx.