CNS 3

Central Nervous System Physiology Lecture III

Brain Stem

  • Brainstem Components
      - Brainstem
      - Cerebrum
      - Thalamus
      - Midbrain
      - Pons
      - Medulla Oblongata
      - Cerebellum
      - Spinal Cord (Cerebrum removed)

Pons

  • Location: Just above medulla oblongata.

  • Function: Acts as a bridge connecting the spinal cord with the brain and various brain parts with each other.

  • Pons Functions:
      1. Contains two centers that regulate breathing:
        - Pneumotaxic Center: Controls the rate and rhythm of respiration.
        - Apneustic Center: Controls the depth of respiration.
      2. Contains nuclei affecting sleep and bladder control.

Midbrain

  • Structure: Formed of 4 lobes above and behind the pons (Corpora Quadrigemina).
      - A) Superior Colliculi (upper 2 lobes):
        - Contains control centers for some visual reflexes:
          - Pupillary Reflex: Responses associated with changes in light.
          - Reflex centers for coordinating eye movement with head and neck in response to visual stimuli.
      - B) Inferior Colliculi (lower 2 lobes):
        - Contains control centers for some auditory reflexes:
          - Reflex centers for movements of head and trunk to locate sounds.
          - Startle Response: Reaction to loud noises.
      - C) Also contains Substantia Nigra, which suppresses unwanted muscle contractions.

Reticular Formation (Reticular Activation System)

  • Definition: A diffuse system of interconnecting fibers extending through several areas of the brain, including the brain stem, comprising a large portion of the entire brainstem, extending into the spinal cord and diencephalon.

  • Functions of RAS: (Both sensory and motor function)
      1. Regulates sleep and consciousness cycle.
      2. Helps control muscle tone, balance, and posture during body movements.
      3. Filters sensory input (habituation).

Diencephalon

  • Components:
      - Hypothalamus
      - Thalamus
      - Pineal Gland (part of the epithalamus)

  • Diagram Parts:
      - Pituitary Gland
      - Spinal Cord
      - Central Canal

Epithalamus

  • Definition: Includes roof of the 3rd ventricle and mainly includes the pineal gland.

Thalamus

  • Structure: 4/5ths of diencephalon volume, approximately 1.2 inches long, forms lateral walls of the 3rd ventricle and the intermediate mass.

  • Function: Acts as a sensory relay center, often referred to as the "Rome of the Nervous System" or the "gateway to the cerebral cortex."
      - Main relay station for sensory impulses reaching the cerebral cortex from the spinal cord, brain stem, and cerebellum, such as taste, touch, heat, cold, pain, and some smell.
      - Exception: The only sensory signals that can reach the cortex without going through the thalamus are for the sense of smell.

Hypothalamus

  • Definition: Part of the brain primarily involved in regulating the internal environment, has no blood-brain barrier, and forms the floor and part of the lateral walls of the 3rd ventricle.

  • Functions of Hypothalamus:
      - Links “mind” and “body.”
      - Relays reflexes related to smell.
      - Produces oxytocin and vasopressin, transporting them to the posterior pituitary gland.
      - Produces releasing and inhibiting hormones controlling the anterior pituitary (“Master Gland”).
      - Regulates body temperature.
      - Regulates food and water intake and thirst.
      - Manages sleep/wake cycle.
      - Contains autonomic regulatory centers.

Cerebellum

  • Definition: Second largest part of the brain, located just below and posterior to the cerebrum. It is the only part of the brain that is highly folded.

  • Structure: Consists of two hemispheres with gray matter outside and white matter inside, known as arbor vitae (tree of life).

Functions of Cerebellum:

  • Helps coordinate voluntary muscles without directly sending impulses to muscles.
      1. Works with cerebrum to coordinate different muscle groups.
      2. Controls skeletal muscles to maintain balance.
         - Receives input from proprioceptors in muscles, tendons, joints, and equilibrium receptors in eyes, comparing intended movement with actual movement.
      3. Learning and storing motor skills
         - Examples include playing a musical instrument, riding a bike, and typing.
      4. Roles in awareness, emotions, and judging the passage of time.
      - Diseases of the cerebellum can cause Ataxia (characterized by tremors, speech problems, and lack of equilibrium), not paralysis.

Cerebrum

  • Total Structure: Largest portion of the brain (approximately 60% of brain mass).

  • Hemispheres: Two hemispheres joined by tracts called the Corpus Callosum.

  • Appearance: Heavily convoluted with gyri and sulci for greater cortical area in a small space, also possesses larger grooves (fissures) that divide each hemisphere into four main regions named after the bones they lie under:
      1. Frontal
      2. Parietal
      3. Occipital
      4. Temporal

Structure of Each Hemisphere

  • Outer Gray Matter: Cerebral cortex (2-4 mm thick).
      - Neurons in the cortex are organized into six cellular layers (neocortex), differing in composition, functional properties, and connection sets.
      - Cortex is systematically subdivided into more than 40 functionally distinct areas.

  • Inner White Matter: Comprising tracts (bundles of myelinated axons).

  • Nuclei: Islands of gray matter in the brain's interior (cell bodies and sometimes dendrites).

Basal Nuclei (Basal Ganglia)

  • Definition: Clusters of gray matter surrounding the thalamus, responsible for directing movements.

  • Clinical Note: Overactivity due to lack of dopamine can lead to Parkinson’s disease.

Functions of Cerebral Cortex:

  • Responsible for complex human traits such as conscious mind, abstract thought, memory, and awareness.

  • Functional Divisions:
      1. Motor Areas: Control voluntary motor functions.
      2. Sensory Areas: Provide conscious awareness of sensations.
      3. Association Areas: Integrate various types of information from multiple brain areas.

Lateralization of Function:

  • Each hemisphere mainly governs the sensory and motor functions of the opposite side of the body.
      - Example: The left hemisphere controls the right hand.

Lobes of the Cerebrum

  • Fissures: Divide each hemisphere into four regions, each with specific functions:
      1. Frontal Lobe:
         - Associated with personality and voluntary movement control.
      2. Parietal Lobe:
         - Associated with touch, stretch, and perception of somatic sensations.
      3. Occipital Lobe:
         - Associated with processing vision.
      4. Temporal Lobe:
         - Associated with processing sound and speech, awareness of equilibrium.

Spinal Cord

Structure and Location

  • Definition: A cylindrical tube located in the spinal canal of the vertebral column, approximately 17-18 inches long, extending from foramen magnum to the lower border of the first lumbar vertebrae.

  • Subdivisions: Cervical, thoracic, lumbar, sacral regions.

  • Termination: The spinal cord terminates in a bundle of nerves called the cauda equina.

  • Surrounding Structures:
      - Meninges
      - Adipose cushion
      - CSF (Cerebrospinal Fluid)
      - Blood vessels

Importance of Spinal Cord

  • Plays a critical role in integrating visceral and somatic sensory and motor systems essential for survival. It comprises a larger proportion of the total CNS mass in domestic animals compared to humans.

Spinal Nerve Tracts

  • Definition: Numerous tracts exist within the spinal cord, serving as two-way conduction paths between peripheral nerves and the brain.

  • Classification:
      - Ascending Tracts: Carry sensory information to the brain.
      - Descending Tracts: Carry motor response information from the brain to motor neurons.
      - Example Tract: Spinothalamic tract is made up of axons originating from cell bodies in the spinal cord that terminate in the thalamus; these are sensory (ascending).

Functions of Spinal Cord:

  • Serves as the center for many significant reflexes and efferent motor nerves for all body muscles.

  • Contains reflex centers for:
      - Urination
      - Defecation
      - Parturition
      - Acceleration of heart rate
      - Dilation of pupils

  • Conducts impulses upwards towards the brain and downwards from the brain for voluntary movement via spinal tracts.

Spinal Reflexes:

  • The spinal cord can independently respond to certain stimuli.

  • Definition of Reflex: A simple, involuntary, and stereotyped response to a stimulus.

  • Components of the Spinal Reflex Arc:
      1. Sensory Nerve Ending (Receptors): Receive stimuli.
      2. Afferent Sensory Nerve Fiber: Contains cell bodies in the dorsal root ganglion.
      3. Interneurons: Communicate with motor neurons if present.
      4. Efferent Motor Nerve Fiber: Transmits impulses to striated skeletal muscle.
      5. Effector Motor Nerve Ending: Leads to the skeletal muscle fiber forming a motor unit.

Protection of CNS

  • Both the brain and spinal cord are heavily protected by:
      1. Bone: Skull and vertebral column.
      2. Adipose Cushion: Surrounds the spinal cord.
      3. Meninges: Tough, flexible covering.
      4. Liquid Cushion: Cerebrospinal fluid.

Meninges

  • Composition: Composed of three layers:
      1. Dura Mater:
         - Strong fibrous connective tissue.
         - Outer layer in the skull acts as the periosteum of cranial bones.
      2. Arachnoid Layer:
         - Delicate, transparent layer.
         - Subdural space between dura mater and arachnoid membrane.
         - Subarachnoid space between arachnoid layer and pia mater.
      3. Pia Mater:
         - Adheres to the outer surface of the brain and spinal cord.
         - Contains blood vessels.

Extensions of Meninges

  • Extensions form partitions between brain parts:
      1. Falx Cerebri: Largest partition between cerebral hemispheres.
      2. Falx Cerebelli: Separates cerebellar hemispheres (not in sheep brain).
      3. Tentorium Cerebelli: Separates the cerebrum from the cerebellum.

  • Meninges continue around the spinal cord and extend beyond its end, providing a safer site for lumbar puncture to collect CSF.

Cerebrospinal Fluid (CSF)

  • Definition: A cushion of fluid surrounding and within the brain and spinal cord, enabling the brain to “float” in CSF (~140 ml).

  • Functions of CSF:
      - Provides buoyancy and protection to delicate brain tissues.
      - Produces chemical stability.

  • Location of CSF:
      - In brain ventricles and ducts.
      - Central canal of the spinal cord.
      - Subarachnoid space of the meninges (the space between the arachnoid layer and pia mater).

Circulation of CSF

  • Process:
      1. Fluid moves from the lateral ventricles through duct to the 3rd ventricle.
      2. Another duct transfers fluid to the 4th ventricle.
      3. Fluid moves to the central canal of the spinal cord and into the subarachnoid space surrounding the spinal cord and brain.
      4. CSF is reabsorbed from the subarachnoid space into arachnoid granulations.

  • Clinical Note: If circulation is obstructed by a tumor or other means during fetal development, it may lead to hydrocephalus.