Central Nervous System and Brain Structure

Central Nervous System (CNS)

  • Comprised of the brain and spinal cord.

  • Regions and Organization of the CNS of Adult Brain:

    1. Two Cerebral Hemispheres: Divided into left and right hemispheres.

    2. Diencephalon:

    • Thalamus: Relay center for sensory information.

    • Hypothalamus:

      • Infundibulum: Connects to the pituitary gland.

      • Functions include autonomic control, endocrine system regulation, and homeostasis (thirst, hunger, temperature).

    • Posterior Pituitary Gland: Releases hormones.

    • Pineal Gland: Secretes melatonin, regulates sleep-wake cycles.

    • Optic Nerve, Chiasma, Tract, Retina, Choroid Plexus, Third Ventricle: Contributes to visual processing.

    1. Brain Stem: Connection between brain and spinal cord, consists of:

    • Midbrain: Integrates sensory information, regulates motor control.

    • Pons: Involved in breathing regulation.

    • Medulla Oblongata: Controls autonomic functions like heart rate and respiration.

    1. Cerebellum: Coordinates voluntary movements, balance, and motor skills.

Brain Structure

  • Cerebral Cortex:

    • Surface area increased by convolutions (folds).

    • Gray Matter:

      • Located on the outer layer (cortex) of cerebrum and cerebellum, consists of neuronal cell bodies and unmyelinated axons.

    • White Matter:

      • Found deeper within the brain, composed of myelinated axons.

  • Spinal Cord:

    • Features:

      • Hollow center (central cavity) surrounded by gray matter.

      • Gray Matter: Found inside, made up primarily of neuron cell bodies.

      • White Matter: Located outside, consisting of myelinated fiber tracts.

Central and Peripheral Nervous System (PNS)

  • CNS Terminology:

    • Nuclei: Collection of cell bodies in the CNS.

    • Tracts: Bundles of axons in the CNS.

  • PNS Terminology:

    • Ganglia: Clusters of cell bodies in the PNS.

    • Nerves: Bundles of axons in the PNS.

  • Lateralization of Function:

    • The left hemisphere typically controls the right side of the body and vice versa.

    • Cerebral Dominance: Left side is usually dominant; responsible for language, math, logic.

    • Right side is responsible for insight, visual-spatial functions, intuition, artistic skills, face recognition.

Communication between Hemispheres

  • Left and right hemispheres communicate through white matter fiber tracts, specifically the corpus callosum.

  • Surface Markings:

    • Gyri: Bulges on the brain's surface.

    • Sulci: Ridges or grooves between gyri.

    • Fissures: Deep grooves in the brain.

  • Lobes of the Brain:

    • Frontal Lobe: Executive functions, voluntary movement.

    • Parietal Lobe: Sensory perception.

    • Temporal Lobe: Auditory processing.

    • Occipital Lobe: Visual processing.

    • Insula: Involved in taste perception and emotional processing.

  • Central Sulcus:

    • Separates the precentral gyrus (frontal lobe) and the postcentral gyrus (parietal lobe).

Functions of Dominant Hemisphere

  • Primary Functions:

    1. Voluntary Movement: Regulated by the precentral gyrus.

    2. Sensory Perception: Managed by the postcentral gyrus.

    3. Wernicke’s Area: Involved in understanding written language.

    4. Broca’s Area: Responsible for speech production.

    5. Mathematical Computation and Analytical Skills.

  • Cerebral Cortex Functions:

    • Site of conscious awareness, sensory perception, voluntary motor initiation, communication, memory storage, and understanding.

    • Contains approximately 100 billion neurons and neuroglial cells.

Cellular Components of the Brain

  • Ependymal Cells: Line brain vesicles and contribute to the formation of cerebrospinal fluid (CSF).

  • Oligodendrocytes and Schwann Cells: Produce myelin sheath around neurons.

  • Blood-Brain Barrier (BBB):

    • Formed by the synergistic action of choroid ependymal cells, endothelial cells of the choroid plexus, and pia mater, protecting the brain from harmful substances.

Functions of the Cerebrum

  • Motor Areas: Control voluntary movement.

  • Sensory Areas: Responsible for conscious awareness of sensory inputs.

  • Association Areas: Integrate and process diverse information.

  • Conscious Behavior: Engages the entire cortex.

Specific Brain Areas

  • Primary Somatosensory Cortex:

    • Located in the postcentral gyrus, receives sensory information from the skin, skeletal muscles, and joints.

  • Wernicke's Area: Essential for understanding written language.

Somatosensory Association Cortex

  • Analyzes and interprets sensory information, assists with memory, reasoning, emotions, and judgment.

  • Positioned posterior to the primary somatosensory cortex, responsible for interpreting size, texture, and relational aspects of objects without visual input.

Visual Areas of the Occipital Lobe

  • Primary Visual Cortex:

    • Receives and processes visual information from the retinas, translates it into images.

  • Visual Association Area:

    • Involved in recognizing and recalling familiar objects such as flowers and faces.

Auditory Areas

  • Primary Auditory Cortex:

    • Situated in the temporal lobes, processes auditory information from the inner ear regarding pitch, loudness, and location.

  • Auditory Association Area:

    • Allows comprehension of sounds and stores auditory memories, enabling differentiation of various sounds.

Olfactory and Gustatory Cortex

  • Olfactory Cortex:

    • Located in the medial aspect of the temporal lobes near the hippocampus; processes smell and associates it with memories.

  • Gustatory Cortex:

    • Positioned in the insula; involved in taste perception.

Other Sensory Areas

  • Visceral Sensory Area: Responsible for conscious perceptions of visceral sensations (e.g., upset stomach, full bladder).

  • Vestibular Cortex: Engaged in the conscious awareness of balance.

Motor Areas in the Frontal Lobe

  1. Primary Somatic Motor Cortex:

    • Located in precentral gyrus, responsible for controlling fine movements in skeletal muscles.

  2. Broca’s Area:

    • Associated with speech production, primarily in the left hemisphere, directs the muscles involved in speech.

  3. Frontal Eye Field:

    • Manages voluntary eye movements.

  4. Premotor Cortex:

    • Aids in planning movements; controls learned fine motor skills and provides conscious control of precise voluntary movements (e.g., playing an instrument, typing).

  5. Prefrontal Cortex:

    • Involved in intelligent thought, planning, sense of responsibility, and rationalization; necessary for judgment, reasoning, persistence, and conscience; development influenced by social environment feedback.

Basal Nuclei

  • Comprised of nuclei deep within the white matter of the cortex, including:

    • Caudate Nucleus

    • Putamen

    • Globus Pallidus

  • Their function relies on dopamine produced in the substantia nigra of the midbrain, inhibiting unnecessary movements.

  • Parkinson’s Disease: Results from overactivity of basal nuclei due to dopamine deficiency.

  • Huntington’s Disease: A genetic disorder causing “flapping” movements as basal nuclei deteriorate.

White Matter

  • Composed of myelinated fibers and tracts that facilitate communication within the brain:

    1. Association Fibers: Connect different areas within the same hemisphere.

    2. Commissures: Join gray matter of both hemispheres, exemplified by the corpus callosum.

    3. Projection Fibers: Link hemispheres to lower brain structures or spinal cord, such as the internal capsule and corona radiata.

Thalamus

  • Known as the “gateway to the cortex”:

    • Serves as the initial relay station for sensory information, with the exception of olfactory signals.

    • Edits, sorts, and groups information for cortical relay; synapses here yield a “crude awareness” of sensations.

    • Connected via the intermediate mass.

Hypothalamus

  • Positioned directly inferior to the thalamus; involved in various key functions:

    1. Autonomic Control Centers: Regulating visceral activities.

    2. Endocrine Master Control: Overseeing hormonal secretions.

    3. Hormone Production: Synthesizes numerous hormones.

    4. Regulation of Vital Functions: Controls blood pressure, heart rate, respiration, sleep-wake cycles (via the suprachiasmatic nucleus), thirst, hunger, and electrolyte balance.

    5. Temperature Regulation.

Epithalamus

  • Contains optic tracts and the retina; includes the pineal gland which produces melatonin, affecting sleep-wake patterns by regulating light exposure.

Brain Stem

  • Functions as the bridge connecting higher brain regions to the spinal cord:

    1. Midbrain: Located between the diencephalon and the pons; contains the cerebral aqueduct linking the third and fourth ventricles.

    2. Pons: Contains nuclei essential for rhythmical breathing, acting as a respiratory center.

    3. Medulla Oblongata: Hosts cardiovascular and respiratory centers that regulate breathing rate and depth; also contains centers coordinating reflexes like vomiting and sneezing.

Cerebellum

  • Known as the “little brain”, it fine-tunes skeletal muscle activity subconsciously by timing muscle contractions and controlling balance; characterized by arbor vitae, a tree-like arrangement of white matter.

Functional Brain Systems

Limbic System

  • Involved in emotion and memory processes. Key structures include:

    • Amygdala: Associated with fear and response to threats.

    • Hippocampus: Located in temporal regions, crucial for long-term memory consolidation.

Reticular Formation

  • Includes the Reticular Activating System (RAS), which sends impulses to the cerebral cortex, maintaining consciousness and alertness; decreased activity results in sleep, while severe damage may lead to coma.

Stages of Sleep

  1. NREM Stage 1: Light relaxation; easy arousal.

  2. NREM Stage 2: Irregular EEG patterns; slightly harder to arouse.

  3. NREM Stage 3: Deep sleep; vital signs decrease.

  4. NREM Stage 4: Dominated by delta waves; hardest to arouse, associated with bed wetting, night terrors, and sleepwalking.

Memory Processes

  • Short-Term Memory (Working Memory): Temporary holding of information, typically limited to seven or eight pieces.

  • Long-Term Memory Storage: Has an expansive capacity; physical changes in neurons and synapses occur during memory formation.

  • Hippocampus Role: Crucial for the storage and retrieval of information, as well as remembering and recalling past events.

Cerebrospinal Fluid (CSF)

  • A clear solution similar to plasma with lower protein, higher glucose, no red blood cells; created by ependymal cells and choroid plexus.

  • CSF acts to absorb waste and necessary solutes while delivering glucose, oxygen, vitamins, and ions (e.g., Na^+, Cl^-, Mg^{2+}).

Brain Disorders/Conditions

  • Consciousness: Clinically defined as the response behavior to stimuli.

  • Fainting/Syncope: A brief loss of consciousness.

  • Coma: Extended periods of unconsciousness.

  • Stroke (Cerebrovascular Accident, CVA): Resulting from a clot or bleed; Tissue Plasminogen Activator (TPA) is the only approved treatment for stroke.

  • Insomnia: Chronic difficulties in achieving sleep.

  • Concussion: A mild traumatic brain injury from impacting an object.

  • Contusion: Permanent brain damage.

  • Meningitis: Infections of the meninges (can be bacterial, viral, or fungal).

  • Hydrocephalus: Excess cerebrospinal fluid accumulated due to blockage of flow.

  • Parkinson's Disease: Neurodegenerative disorder affecting dopamine-releasing neurons in the substantia nigra, causing movement disturbances.

  • Huntington's Disease: Genetic disorder leading to degeneration of basal nuclei and cerebral cortex, resulting in motor and cognitive decline.

  • Other Disorders: Schizophrenia, aphasia, dyslexia, Alzheimer's disease (progressive degenerative disorder leading to dementia), transient ischemic attacks, sleep apnea, narcolepsy, seizures.

Spinal Cord and White Matter

  • Epidural Space: Cushioning of fat and veins between the vertebrae and spinal dura mater.

  • White Matter Composition: Consists of white columns containing ascending (sensory) and descending (motor) tracts, including lateral, ventral, and dorsal funiculi.

Descending Motor Pathways

  • Pyramidal (Corticospinal): Originates in the cortex; responsible for voluntary movements.

  • Extrapyramidal (Rubrospinal): Arise from other brain structures; responsible for involuntary movements.

  • Motor Neuron Details:

    • Upper Motor Neurons: Pyramidal cells in the primary motor cortex.

    • Lower Motor Neurons: Located in the ventral horn, innervate skeletal muscles directly.

Effects of Spinal Cord Trauma

  • Functional Loss: Observed as paresthesias (sensory loss).

  • Types of Paralysis:

    • Flaccid Paralysis: Severe damage to the ventral root or horn cells. Muscles cannot receive impulses, leading to atrophy and absence of reflexes.

    • Spastic Paralysis: Damage to upper motor neurons; leads to intact spinal neurons and hyperactive reflexes due to lack of voluntary control.

  • Transection Effects:

    • Hemiplegia: Paralysis of one side of the body.

    • Paraplegia: Paralysis affecting the lower limbs (injury between T1 and L1).

    • Quadriplegia: Paralysis of all four limbs (injury in the cervical region).

  • Diseases:

    • Poliomyelitis: Caused by poliovirus, leading to degeneration of ventral horn neurons and muscle atrophy.

    • Amyotrophic Lateral Sclerosis (ALS): Characterized by the progressive degeneration of lower motor neurons and pyramidal tracts, resulting in loss of speech and respiratory function, typically fatal within five years.