Introduction to the Nervous System

Nervous System Overview

• The nervous system, weighing about 2 kg (approximately 3% of total body weight), stands out as one of the smallest yet most intricate of the 11 body systems.
• It orchestrates and regulates essential bodily functions, harmonizing other systems to sustain homeostasis.
• The brain serves as the primary hub for sensation processing, correlating it with memory, making decisions, and driving actions.
• The brain acts as the core for intellect, emotions, behavior, and memory.
• The adult brain consists of approximately 85 billion neurons and 10 trillion to 50 trillion neuroglia, weighing approximately 1300 g (almost 3 lb).
• Each neuron creates an average of 1000 synapses with other neurons, resulting in a total synapse number exceeding a thousand trillion, larger than the number of stars in our galaxy.

Functions of the Nervous System

• Sensory Function: Sensory neurons gather information from within the body and the external environment.
• Integrative Function: Information processing and transmission occur within the brain.
• Motor Function: The brain sends signals via motor neurons to muscles, glands, and organs, enabling appropriate responses.
• Complex Functions: The nervous system governs perceptions, emotions, behaviors, and memories.

Classification of the Nervous System

• The nervous system is organized into the Central Nervous System (CNS) and Peripheral Nervous System (PNS).

Central Nervous System (CNS)

• The CNS comprises the brain and spinal cord.
• Enclosed within the skull, the brain has an estimated 85 billion neurons.
• The brain connects to the spinal cord, which communicates with the peripheral nervous system (PNS).
• The spinal cord facilitates information flow between the body and brain.

Nervous Tissues

• Two primary types of nervous tissue are Neurons which are excitatory, and Neuroglia which are non-excitatory.
• Neurons respond to specific stimuli.
• Neuroglia are more numerous than neurons, possibly up to 25 times more.
• Neuroglia nourish and protect neurons and maintain the interstitial fluid surrounding them.

Neuron Parts

• Dendrites accept stimuli and convey impulses to the cell body.
• The Cell body contains the nucleus.
• Axons are fibers that carry impulses away from the cell body.
• Schwann cells produce myelin sheaths.
• Myelin sheaths are dense lipid layers that insulate the axon.
• Nodes of Ranvier are gaps in the myelin sheath.
• Impulses travel from dendrites to the cell body, then to the axon.

Glial Cells in the CNS

• Astrocytes:
  • Star-shaped cells that are the most abundant and largest neuroglia.
  • They wrap around blood capillaries in the CNS, creating a blood-brain barrier.
  • They maintain the chemical environment within the brain (CNS).
• Microglia:
  • Tissue macrophages that are spider-shaped.
  • They engage in phagocytosis to remove debris.
• Ependymal cells:
  • Line the brain and spinal cord cavities.
  • They produce and circulate cerebrospinal fluid.
  • They contribute to the blood-CSF barrier.
• Oligodendrocytes:
  • Facilitate the creation of myelin sheaths around nerve fibers within the CNS.

Glial Cells in the PNS

• Schwann Cells: They encircle PNS axons and also myelinates
• Satellite Cells: Assist with the transportation of materials between neuronal cell bodies and interstitial fluid while proving structural support.

Meninges

• The meninges consist of membranes that protect the brain and spinal cord.
• There are three tissue layers:
  • Dura mater: outer layer.
  • Arachnoid mater: middle layer containing cerebrospinal fluid.
  • Pia mater: inner layer.

Ventricles and Cerebrospinal Fluid

• A network of interconnected cavities located within the cerebral hemispheres and brainstem are called ventricles.
• The brain has four ventricles:
  • Two lateral ventricles (first and second ventricles).
  • One third ventricle.
  • One fourth ventricle.
• Interventricular foramen and cerebral aqueduct.
• Cerebrospinal Fluid (CSF):
  • Production occurs mainly in the choroid plexuses, which are networks of blood capillaries inside the ventricles.
  • Is a clear, colorless watery fluid that buffers, protects and delivers nutrients.
  • Delivers small quantities of oxygen, glucose, and other vital substances to neurons and neuroglia.
  • Continuously circulates through the brain's cavities, spinal cord, and subarachnoid area.
  • The adult CSF volume is 80–150 mL (3 to 5 oz).
• Composition Includes water, glucose, protein, nitrogen substances, electrolytes (Na, K, Ca, Chloride), and a few cells

Process of CSF Secretion and Circulation

• Choroid plexus secretes CSF.
• From the right and left lateral ventricles, it flows to the third ventricle then fourth ventricle.
• Next it flows to the subarachnoid space and is then absorbed into the sinus.

Functions of CSF

• Supports the brain and spinal cord.
• Protects brain and spinal cord.
• Maintains pressure.
• Keep the brain and spinal cord moist
• Provides nutrition.
• Waste removal.

Spinal Meningeal Spaces

• Epidural space: Located above the dura mater.
• Subdural space: Exists between the dura and arachnoid mater
• Subarachnoid space: Located between the arachnoid and pia mater, it contains CSF and supplies blood to the brain.

Major Parts of Brain

• Telencephalon: Cerebrum (Cerebral hemispheres)
• Diencephalon: Includes Thalamus, Hypothalamus, and Epithalamus
• Brain stem: Includes Medulla Oblangata, Pons, and Midbrain (mesencephalon)
• Cerebellum

Cerebral Hemispheres (Cerebrum/Cerebral Cortex)

• Folds of the cerebrum:
  • Larger folds (Sulci) divide the cerebrum into lobes.
    • Central sulcus: Separates the frontal and parietal lobes.
    • Lateral sulcus: Separates the frontal and temporal lobes.
    • Parieto-occipital sulcus: Separates the parietal and occipital lobes.
  • Smaller folds (Gyri) increase the brain's surface area.
• Lobes of cerebrum:
  • Frontal lobe.
  • Parietal lobe.
  • Occipital lobe.
  • Temporal lobe.
• Layers of cerebrum:
  • Gray matter:
    • Outer layer consisting of neuron cell bodies.
  • White matter:
    • Nerve fiber tracts within the gray matter.
    • The corpus callosum connects the hemispheres deep within the brain.
• Functional areas of the cerebrum:
  • Sensory Area:
    • Somatic sensory area gets impulses from skin receptors.
    • Visual area in the occipital lobe gets impulses from the eyes.
    • Auditory area near the lateral sulcus gets impulses from the ear.
    • Olfactory area- In the temporal lobe, receives impulses from the nose.
    • Taste area located near the parietal lobe gets impulses from the tongue.
  • Motor Area:
    • Primary motor area sends impulses to skeletal muscles.
    • Broca’s area is involved in speech.
  • Interpretation areas:
    • Wernicke's area: interprets the meaning of speech recognizing spoken words.
    • The prefrontal cortex is concerned with the makeup of a person's personality, intellect, complex learning abilities, recall of information, reasoning, conscience, intuition, mood, and future planning.
    • General interpretation area: receives impulses from the areas described above

Diencephalon

• The diencephalon makes tissue core that is surrounded completely by the cerebral hemispheres.
• 3 Parts:
  • Thalamus
  • Hypothalamus
  • Epithalamus

Thalamus

• Makes up 80% of the diencephalon and gray matter.
• It it’s the relay station for sensory impulses to the cerebrum.
• Plays a role with consciousness.

Hypothalamus

• A small part of the diencephalon located inferior to the thalamus.
• A major regulator of homeostasis that controls many activities in the body.
• Functions:
  • Controlling the ANS
  • Hormone Production
  • Regulates behavioral and emotional pattern
  • Feeding and drinking regulation
  • Regulates body temperature
  • Circadian rhythm regulation

Epithalamus

• Location is superior and posterior to thalamus and has the pineal gland.
• Melatonin is secreted from the pineal gland.
• Regulates sleepiness

Brain Stem

• Connects the spinal cord and diencephalon.
• Parts:
  • Midbrain
  • Pons
  • Medulla oblongata

Midbrain

• A mesencephalon that extends from the diencephalon to the pons containing 2 nuclei.
• Substantia nigra: Dopamine is released helping control subconscious muscle moments while it extends from the substantia nigra to the basal nuclei and loss of these neurons is associated with Parkinson’s disease
• Red nuclei: Looks reddish due to a rich blood supply and contains pigment while also helping control muscle movements through synapses in the cerebellum and the cerebral cortex

Pons

• A bridge that helps connect parts of the brain.
• Nuclei help act as rely stations.
• Pneumotaxic and apnoustic centers in pons helps help with respiration and work in combination with the medulla oblongata respiratory center.

Medulla Oblongata

• Lowest part of brain stem.
• Has all ascending and descending tracts for sensation.
• In the right pyramid 90% of them cross to the left.
• The the cardiovascular center- regulates the rate and force of the heartbeat and the diameter of the blood vessel.
• The medullary respiratory center- adjusts the basic rhythm of breathing.
• The vomiting center of the medulla causes vomiting.
• The deglutition center of the medulla promotes deglutition (swallowing) of a mass of food.
• Sneezing and coughing center involves spasmodic contraction of breathing muscles that forcefully expel air through the nose and mouth.

Cerebellum

• Second largest part of the brain.
• Ovoid in shape and has to hemispheres.
• Gray matter surface and white matter lies deep.
• Functions:
  • Smooths and coordinates muscle contractions.
  • Helps posture, keeps balance and equilibrium.
  • Impulses from eyes and semicircular canals provide info and head position.
  • Partakes in role of learning and language

Spinal Cord

• It is protected by the spinal meninges.
• Extends from medulla oblongata to first lumbar vertebra and is 45 cm long.
• Located below the lumbar vertebra is the cauda equina.
• Link between brain and rest of the body.
• Nerves provide the communication between the body and spinal cord but appears segmented from spinal nerve pairs.

Spinal Cord Internal Anatomy

• Composed of gray matter and white matter that is supported by neuroglia in a cross section. -A shaped Grey Matter containing clusters of neuronal cells. -Two posterior -Two anterior. -Two lateral columns.
• Posterior gray horns contain cell bodies and axons for inter neurons.
• Anterior gray contains motor nuclei for cell bodies.
• Activity of cardiac and smooth muscle is regulated through lateral gray horns.
• White Matter organization includes anterior column, posterior, and lateral.
• Information is commonly shared through having distinct axon bundles.

Spinal Cord Physiology

• The spinal cord serves two vital roles:
  • Sensory/Motor Tracts: Gathers nerve impulses travel back and forth from the brain.
  • Reflexes/Arcs: Performs rapid, unplanned actions. Sensory neurons transport messages to the gray spinal cord matter.
    • Monosynaptic reflex: one synapse runs between the sensory and motor neurons.
    • Polysynaptic reflexes: consists of 1 or more interneurons. Nerve impulses are transported out of the motor neuron and stimulate a part of the body. The effector is a muscle or gland that performs its action.

Spinal Nerves

• Parallel bundles of axons wrapped in layers of connective tissue connecting the central nervous system (CNS) to receptors, muscles, and glands.
• 31 nerve pairs are named and numbered based on the level of the vertebral column from the emerge.
• Spinal Nerve Types:
  • 8 cervical nerves
  • 12 Thoracic Nerves
  • 5 Lumbar Nerves
  • 5 Sacral Nerves
  • 1 Coccygeal Nerve.

Cranial Nerves

• There are 12 pairs emerging from the brain.
• Cranial nerve classification: -Sensory nerves (I, II, VIII) carry the sensory neurons. -Motor neurons (III, IV, VI, XI, XII) are based on if they can contain motor neurons. -Mixed nerves (V, VII, IX, X) have sensory/motor neurons.