Topic 8

EXS 207 Topic 8: Central Nervous System Functions

Functions of the Nervous System

  • Sensory Input: Gathering information through sensory receptors.

  • Integration: Processing and interpreting sensory data to create appropriate responses.

  • Motor Response: Execution of actions in response to integrated sensory information.

  • Component Systems:

    • PNS (Peripheral Nervous System)

    • CNS (Central Nervous System)

CNS Protection

Protective Structures

  • Bone:

    • Skull: Protects the brain.

    • Vertebral Column: Protects the spinal cord.

  • Dura Mater:

    • Described as "tough," this is the thick, strong, and durable outer layer of the meninges.

  • Arachnoid Mater:

    • Given the nickname "spider web-like" due to extensions that secure it to the pia mater.

    • Contains blood vessels that form capillaries in the pia mater.

  • Pia Mater:

    • Known as the "gentle" layer, it is delicate and adheres to the surface of the brain and spinal cord.

  • Subarachnoid Space:

    • Contains cerebrospinal fluid (CSF).

  • Clinical Consideration:

    • Lumbar Puncture: Also known as a lumbar tap, performed between L4-L5 to collect CSF for analysis.

Meninges and Cerebrospinal Fluid (CSF)

Meningitis

  • Definition: Inflammation of the meninges, potentially induced by bacterial or viral infection.

  • Clinical Risk: Very dangerous and can lead to complications involving the CNS.

  • Encephalitis: Inflammation of the brain.

Composition of CSF

  • Characteristics:

    • Watery solution, similar to blood plasma but lower in protein.

    • Maintains different ion concentrations crucial for brain function.

    • Constantly replenished with a volume of approximately 150 mL (or ½ cup) keeping enough CSF in circulation.

Functions of CSF

  • Buoyancy: Supports and cushions the brain and spinal cord, decreasing effective weight.

  • Chemical Stability:

    • Regulation of ion concentrations.

    • Nutrient delivery and waste removal.

    • Carries chemical signals including hormones.

Locations of CSF

  • Ventricles: Spaces within the brain where CSF is housed.

  • Central Canal: Long channel within the spinal cord.

  • Subarachnoid Space: Area surrounding both the brain and spinal cord.

CSF Production and Flow

  • Production:

    • Choroid plexuses are networks of capillaries that produce CSF, surrounded by ependymal cells.

    • Ependymal cells utilize blood from surrounding capillaries to synthesize about 500 mL of CSF per day.

  • Flow Process:

    1. CSF is produced by the choroid plexuses.

    2. It flows through the brain's ventricles and the central canal of the spinal cord.

    3. Enters subarachnoid space through apertures in the fourth ventricle.

    4. Absorbed into blood of the dural venous sinuses through arachnoid granulations.

Clinical Consideration: Hydrocephalus

  • Occurs when CSF drainage or circulation is impaired, leading to accumulation and increased brain pressure.

  • Symptoms include visible swelling in infants due to pressure within the cranial cavity, potentially resulting from conditions such as meningitis.

Brain Blood Supply

Brain's Oxygen Requirements

  • Oxygen Dependency: The brain requires a constant oxygen supply, with interruptions over 4 minutes likely resulting in permanent brain damage.

  • Glucose Dependency: The brain uses glucose as its primary energy source for ATP production.

Blood-Brain Barrier (BBB)

  • Characteristics:

    • Brain capillaries are the least permeable in the body due to tightly joined endothelial cells.

    • Maintains a constant extracellular environment critical for neural function.

Substances Crossing the BBB

  • Permissible Substances Include:

    • Wastes (e.g., urea), O2, glucose, proteins, essential amino acids, some ions, and CO2.

    • Brain also has selective permeability for substances such as caffeine, alcohol, nicotine, heroin, and certain antibiotics, though it remains selective, allowing only specific passage.

Exceptions to BBB

  • Vomiting Center: Located in the brain stem, monitors for poisons in the blood.

  • Hypothalamus: Involved in regulating body temperature, water balance, and various metabolic processes, has more permeability.

BBB and Disease Connection

  • Alzheimer's Disease: Potential entry of amyloid beta into the brain, correlating with disrupted BBB function.

  • Multiple Sclerosis: May be linked to BBB dysfunction allowing lymphocyte passage, leading to damage to myelin sheaths.

  • Impact of Meningitis: May disrupt BBB, complicating treatment efforts by allowing neurotoxins to enter when certain antibiotics are administered.

Brain Overview

Basic Functions

  • Sensory Input Reception: Processing sensory information.

  • Correlation with Sensations: Relating sensations with stored information for interpretation.

  • Decision Making: Formulating responses based on sensory integration.

  • Motor Output: Sending directives for action.

  • Roles: Source of intelligence, emotions, behavior, and memory.

Principal Parts of the Brain

  1. Brain Stem: Continuous with the spinal cord, housing nuclei for most cranial nerves.

  2. Cerebellum: Posterior to brain stem; critical for motor control.

  3. Diencephalon: Central part of the brain, situated just above the brain stem.

  4. Cerebrum: The largest and most developed region of the brain, responsible for higher brain functions.

Brain Stem Functions

  • Contains white matter tracts for information relay, and reflex centers that manage automatic survival behaviors.

Brain Stem Regions

  • Midbrain: Houses reflex centers for visual and auditory information.

  • Pons: Connects the cerebellum and provides reflex centers for breathing.

  • Medulla Oblongata: Contains centers for cardiovascular control, respiratory functions, and reflexes for vomiting and swallowing.

Diencephalon Functions

Hypothalamus Functions

  • Autonomic Nervous System Control: Regulates various involuntary bodily functions.

  • Endocrine Function Control: Links hormonal processes with physical responses, such as temperature regulation.

  • Hunger and Thirst Regulation: Hunger and satiety centers manage appetite and thirst regulation.

  • Sleep-Wake Cycle: In conjunction with the pineal gland, influences sleep patterns.

Epithalamus

  • Pineal Body: Secretes melatonin, a hormone responsible for sleep regulation and circadian rhythms.

Thalamus

  • Acts as a relay station for sensory and motor information between the cerebral cortex and other parts of the CNS.

Cerebellum Overview

Anatomy

  • The second largest brain region divided into left and right hemispheres.

  • Composed of an outer layer of gray matter and inner white matter (arbor vitae).

Functions

  • Movement Coordination: Coordinates skeletal muscle movements for smooth execution.

  • Posture and Balance Maintenance: Adjusts to sensory input for balance.

  • Monitoring and Integration: Constantly tracks motor cortex intentions and body positions through proprioceptors, integrating data subconsciously for movement planning.

Cerebrum Overview

Anatomy

  • Structure: Largest brain component, characterized by ridges (gyri) and grooves (sulci).

  • Composed of two hemispheres separated by a longitudinal fissure, each with five lobes.

Cerebral Layers

  • White Matter: Internal layer containing myelinated fibers, facilitating communication within and between hemispheres.

    • Association Fibers: Connect within the same hemisphere.

    • Commissural Fibers: Connect the two hemispheres.

    • Projection Fibers: Connect cerebrum with other CNS parts.

Gray Matter**: The cerebral cortex responsible for integration and decision-making processes.

  • Defined as the "executive suite" of the nervous system.

Functional Regions of Cerebral Cortex

  • Sensory Areas: Responsible for awareness of sensory stimuli.

    • Primary Somatosensory Cortex: Processes sensory input and identifies body areas stimulated.

    • Somatosensory Association Cortex: Interprets and integrates sensory information.

    • Primary Visual Cortex and Visual Association Area: Processes visual stimuli, including color and motion.

    • Primary Auditory Cortex and Auditory Association Area: Interprets sound, allowing differentiation between types of auditory input.

Additional Sensory Areas

  • Primary Olfactory Cortex: Processes odors, located in the medially positioned temporal lobe.

    • 5 Taste

      • Salty

      • Savory/Umami

      • Sour

      • Sweet

      • Bitter

  • Gustatory Cortex: Responsible for taste perception, situated in the insula.

  • Visceral Sensory Area: Provides conscious awareness of visceral sensations like hunger or an upset stomach.

Motor Areas

  • Primary Motor Cortex: Initiates impulses for skeletal muscle movement.

  • Premotor Cortex: Assists in planning and organizing motor functions into complex tasks.

  • Frontal Eye Field: Controls voluntary eye movements.

  • Broca’s Area: Linked to speech production, primarily located in the left hemisphere.

  • Somatotopy: Organization of the nervous system reflects the map of the body.

Multimodal Integration Areas

  • Function: Facilitate complex integrations of sensory and motor information, memory storage, and action decision-making.

  • Notable Area: Prefrontal Cortex: Responsible for higher cognitive functions like judgment, reasoning, and aspects of personality and working memory.

The Limbic System

  • Description: Located in both cerebral hemispheres and diencephalon, it is often referred to as the emotional brain, regulating emotions such as pleasure, fear, and sorrow.

  • Interconnectivity: Communicates closely with the hypothalamus and prefrontal cortex to influence physical expressions of emotions and rational decisions.

Spinal Cord Overview

External Anatomy

  • Continuity with the Medulla: Exits the skull via the foramen magnum.

  • Enlargements: Cervical and lumbar enlargements correlate with limb nerve origins.

  • End Point: Concludes between L1 and L2 vertebrae.

  • Anchoring: Secured to the coccyx via the filum terminale.

  • Cauda Equina: Comprises a bundle of nerve roots at the lower end of the vertebral canal.

Internal Anatomy

  • Composition:

    • Central canal surrounded by CSF.

    • Inner gray matter (butterfly-shaped).

    • Outer white matter encasing the gray area.

Functions of the Spinal Cord

  • White Matter Tracts: Contains ascending (sensory) and descending (motor) pathways.

  • Gray Matter Participation: Integral in reflex actions.