7. CNS - Brain and Spinal Cord

Central Nervous System: Brain & Spinal Cord

Learning Objectives

• Describe the general organization of the nervous system.

• Describe the structures protecting the central nervous tissue (CNS) including bone, meninges, cerebrospinal fluid, and the blood-brain barrier.

• List and explain the functions of the glial cells.

• Describe the location, key functions, and general anatomy of the spinal cord.

• List the 5 key functions of the brain.

• Identify the location and main functions of the brain stem, hypothalamus, thalamus, cerebellum, basal nuclei, and cerebrum.

• Identify and describe the main functions of the cortical lobes, including the somatosensory cortex, primary motor cortex, and language areas.

• Describe the location and functions of the limbic system.

General Organization of the Nervous System

• Divided into:

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

  • Peripheral Nervous System (PNS): Made up of nerves extending outside of the CNS.

    • Branches of PNS:

      • Afferent Division: Carries information from the periphery to the CNS, covering sensory stimuli (consciously perceived) and visceral stimuli (unconsciously perceived).

      • Efferent Division: Carries information from the CNS to effector organs in the periphery.

Structures Protecting the CNS

Physical Barriers

• Bony Structures:

  • Cranium (skull)

  • Vertebral column (spine)

• Meninges (protective membranes):

  • Dura Mater: Outermost tough layer.

  • Arachnoid Mater: Spider-like appearance; contains blood capillaries.

  • Pia Mater: Innermost thin layer, follows brain grooves.

• Cerebrospinal Fluid (CSF):

  • Surrounds and cushions the brain and spinal cord, secreted by choroid plexus.

  • Circulates in the subarachnoid space, reabsorbed into blood.

• Blood-Brain Barrier:

  • Formed by tight junctions between endothelial cells and glial cells to shield the brain from harmful blood changes.

Glial Cells Functions

• Four key types in the CNS:

  • Astrocytes: Most abundant; maintain neuronal arrangement, help in homeostasis of extracellular fluid, form the blood-brain barrier.

  • Oligodendrocytes: Form myelin sheaths in the CNS.

  • Microglia: Act as immune cells for the CNS.

  • Ependymal Cells: Line internal cavities and contribute to CSF formation.

The Spinal Cord

Location and Anatomy

• Located between the brain and PNS afferent/efferent fibres.

• Functions:

  • Links brain to body for information transmission.

  • Integrates reflex activity between afferent input and efferent output (spinal reflex).

• Structure:

  • Composed of central grey matter and surrounding white matter.

    • Grey Matter: Contains neuron cell bodies.

    • White Matter: Tracts of nerve fibres; includes ascending (sensory) and descending (motor) tracts.

• Spinal nerves have both afferent and efferent fibres; associated nerves include dorsal roots (afferent) and ventral roots (efferent).

• Nerves are labeled by spinal location: cervical, thoracic, lumbar, sacral, coccygeal.

Key Functions of the Brain

• Five Functions:

  1. Homeostasis

  2. Perception

  3. Voluntary Movement

  4. Emotions

  5. Higher Cognitive Processes (thoughts & memories)

Brain Structures and Their Functions

• Brain Stem: Located at the brain's base, includes midbrain, pons, and medulla oblongata.

  • Functions:

    • Origin of cranial nerves.

    • Maintains vital functions (cardiovascular, respiratory, digestive).

    • Hosts reticular formation for alertness.

• Hypothalamus: Key for homeostasis, regulates temperature, food intake, thirst, urine output, pituitary hormones, and autonomic nervous system.

• Thalamus: Relay station for sensory input, located between brain stem and cerebrum.

• Cerebellum: Controls balance, muscle tone, movement; stores procedural memories.

• Basal Nuclei/Ganglia: Involved in posture, movement facilitation, with Parkinson's impairments noted.

• Cerebrum: Outermost brain structure; consists of left and right hemispheres separated by corpus callosum; grey matter (cortical layer) and white matter (interconnecting fibres).

  • Grey matter for neural integration; white matter for nerve fibre bundles.

Cortical Lobes and Key Functions

• Cerebral Cortex: Divided into four lobes:

  • Frontal Lobe: Voluntary motor activity, speech, thought.

  • Temporal Lobe: Auditory sensation.

  • Parietal Lobe: Somatosensation (touch, pain, pressure).

  • Occipital Lobe: Visual processing.

• Homunculus Diagrams: Map motor and sensory areas correspondingly to body locations, showing distorted areas based on cortical proportions.

• Association Areas: Integrate inputs for higher-level thinking and actions:

  • Prefrontal Association Cortex: Planning, decision-making, creativity, personality.

  • Parietal-Temporal-Occipital and Limbic Association Cortices: Emotion and behaviors.

  • Language Areas:

    • Wernicke’s Area: Understanding communication.

    • Broca’s Area: Producing speech.

Limbic System

• Comprises structures from the cerebrum, thalamus, hypothalamus, amygdala, hippocampus, and olfactory bulb.

• Functions directed towards:

  • Emotions: Amygdala (fear, anxiety), Hypothalamus (autonomic system activation).

  • Motivation: Reward and punishment centers.

  • Sexual Behaviors.

  • Learning and Memory: Hippocampus integral to memory formation.

Key Points

• CNS divided into CNS & PNS, protected by various structures and supported by glial cells.

• Spinal cord crucial for brain-PNS communication and reflex activity.

• Brain organized into structures performing key roles: homeostasis, perception, movement, emotions, and cognition.

• Limbic system contributes to emotions, motivation, and memory formation.

Review Questions

1. Describe the four key structures/adaptations that protect the brain.

2. List the types of glial cells and briefly describe the function of each.

3. Fill out the table linking functions of the brain to key structures:

   • Function: Key Brain Structures: Homeostasis, Perception, Voluntary Movement, Higher Cognitive Processes, Memory, Emotion.