Nervous System & Nervous Tissue

Nervous System & Nervous Tissue

  • Divisions of the Nervous System:
    • Peripheral Nervous System (PNS)
    • Components: Ganglion, Nerve
    • Central Nervous System (CNS)
    • Components: Brain, Spinal Cord

Student Learning Objectives

  • Main Functions of the Nervous System:
    • List functions
    • Explain structural and functional divisions

Axial Skeleton

  • Purpose:
    • Great triumph of design and engineering
    • Skeleton is strong yet light (compact bone & trabeculae)
    • Adapted for weight bearing (Wolff’s Law)
    • Locomotion (articulates and reinforced)
    • Protective function of the CNS (spinal canal & foramen)

Function of the Nervous System

  • Primary Functions:
    • Sensory Receptors: Monitor changes inside and outside the body (stimuli)
    • Information Gathering: Processes sensory input (sensory input)
    • Integration: Interprets sensory input to dictate a response
    • Motor Output: Activates effector organs; muscles or glands

Function of the Axial Skeleton

  • Functions include:
    • Protect the delicate spinal cord and brain
    • Provide locomotion for mobility and flexibility
    • Expose vulnerable organs to the environment
    • Options: a and b; b and c

Communication Between CNS & PNS

  • CNS:
    • Central Nervous System
  • PNS:
    • Peripheral Nervous System

Basic Division of Nervous System

  • Central Nervous System (CNS):
    • Components: Brain, Spinal Cord
    • Description: Integrating and command center of N.S.
    • Receives incoming signals (integrates) and dictates (relays) motor response
  • Peripheral Nervous System (PNS):
    • Components: Peripheral nerves exiting the vertebral foramen (Dorsal Root), Ganglia
    • Description: Communication lines between all areas of the body to the CNS

Comparative Composition of CNS and PNS

  • The CNS comprises of ____.
  • The PNS comprises of ____.
    • Options: ganglia, dorsal root; Peripheral nerves, spinal cord; Spinal cord, ganglia; a and b; b and c

Structure of Neurons

  • Basic Structures of a Neuron:
    • Cell Body: Single nucleus surrounded by cytoplasm
    • Neurofibrils: Bundles of neurofilaments that keep cells from being pulled apart
    • Dendrite Process: Receptive sites with enlarged surface area for receiving signals from other neurons; conduct electricity TOWARDS the cell body
    • Axon:
    • Axon Hillock: Cone-like projection where axons arise
    • Description: Impulse generators and conductors that transmit impulses AWAY from cell body

Neuroglia Types and Functions

  • Student Learning Objective: To identify six types of neuroglia in nervous tissue and distinguish them by location and function. To describe structure of myelin sheaths.

Neuroglia Types

  • Astrocytes:

    • Most abundant glial cells in CNS
    • Regulate neurotransmitter levels
    • Signal increased blood flow in active regions of brain
    • Control ionic environment around neurons
    • Assist synaptic activity in developing nerve tissue

  • Microglial Cells:

    • Smallest and least abundant neuroglia cells in CNS
    • Elongated cell bodies and processes resembling a thorny bush
    • Derived from monocytes; migrate and engulf microorganisms and injured/dead neurons

Blood-Brain Barrier

  • Formation: By astrocytes
  • Function: Stops substances from entering CNS
  • Absent in:
    • Hypothalamus
    • Pineal gland
    • Choroid plexus of each ventricle

More Neuroglia Types

  • Ependymal Cells:

    • Form an epithelium that lines central cavity of spinal cord and brain
    • Fairly permeable layer between cerebrospinal fluid and tissue that bathes cells of CNS
    • Cilia help to circulate CSF

  • Oligodendrocytes:

    • Form small groups and wrap cell processes around thicker axons in CNS
    • Produce insulating covering (myelin sheaths)

  • Satellite Cells:

    • Surround neuron cell bodies in ganglion of PNS
    • Support and maintain the cell body

Myelin Sheath Structure

  • Oligodendrocytes:
    • Produce myelin sheaths in the CNS
  • Schwann Cells:
    • Produce myelin sheaths in the PNS
  • Myelin Composition:
    • A lipoprotein surrounding thicker axons of the body
  • Segments:
    • Comprise plasma membrane of glial cell rolled in concentric layers around axon
    • Myelin prevents leakage of electrical current from axon and increases conduction speed

Neuron Structures

  • A neuron contains structures: Cell body, myelin sheath, axon.
  • Correct structure order: Axon, cell body, dendrite process;
  • Other options include incorrect structures such as purkinje fibers.

Identifying Neuroglia Roles

  • Most Abundant Glial Cell in CNS: A. Astrocytes
  • Glial Cell with “Brush” Surface for CSF Circulation: B. Ependymal cells

Anatomy of a Neuron

  • Cell Body: Contains nucleus; renews membrane of cell and protein components of cytosol
  • Dendrite: Receptive sites for signals from surrounding neurons (conduction TOWARDS cell body)
  • Axon: Attached at the cell body via axon hillock; impulse generators (conduction AWAY from cell body)
  • Axon Terminal: Releases neurotransmitters that excite or inhibit neurons/target organs

Different Types of Neurons

  • Types:
    • Unipolar: Found in sensory ganglia of PNS
    • Processes: Peripheral process extends to receptors; central process runs into CNS
    • Bipolar: Rare; found in special sensory organs (e.g., inner ear)
    • Multipolar: More than 2 processes; numerous dendrites and 1 axon; over 99% of neurons in the body

Types of Reflexes

  • Student Learning Objective: Describe the structural link between PNS and CNS; define reflex, and list components of a reflex arc; distinguish monosynaptic reflexes from polysynaptic reflexes.

Neuronal Integration

  • Structural Connection: PNS & CNS are linked functionally and structurally
  • Components:
    • PNS: Axons of sensory (afferent) and motor (efferent) neurons bundled together as nerves
    • Interneuron: Links afferent to efferent neurons in gray matter

Reflex Arc

  • Description: Reflects basic structural plan of the nervous system
  • Function: Enables rapid motor responses to stimuli; unlearned, premeditated, involuntary actions
  • Types of Reflexes:
    • Somatic Reflex:
    • Involves contraction of skeletal muscles
    • Visceral Reflex:
    • Involves activation of smooth muscles, cardiac muscles, and glands

Types of Reflex Arcs

  • Readily Occurring Reflexes: Somatic and Visceral reflex arcs

Processing Types

Serial Processing

  • Description: One task after another
    • Monosynaptic Reflex:
    • “One synapse” with no interneuron; e.g., knee jerk reflex—fastest reflex
    • Polysynaptic Reflex:
    • “Multiple synapse” with one or more interneurons; e.g., withdrawal reflexes

Neuronal Pathways (Serial processing)

  • Spinal Pathway: Rapid impulse traveling via Spino thalamic Tract; begins from spine to thalamus; pathway for pain and temperature

Parallel Processing

  • Description: Enables execution of multiple tasks simultaneously
    • Neurons joined in parallel, with impulses sent through multiple pathways
    • Integration occurs rapidly across various brain regions for recognition and memory

Neuronal Pathway (Parallel processing)

  • Corticospinal Tract: Impulse travels from motor cortex via multiple interneurons; from ventral horn (spinal cord) to receptors in motor junction of muscle fibers for movement.
  • Spinothalamic Tract: Stimulus at dorsal horn sends impulse to thalamus where pain is relayed after a withdrawal reflex occurs.
  • Cortical Pathway: Continues from thalamus to sensory cortex (postcentral gyrus) for processing somatic sensations.

Neuronal Circuits

Diverging Circuit

  • Description: One presynaptic neuron synapses with several others
    • Example: Stretch reflex stimulates numerous sensory neurons in spinal cord
    • Information is distributed through multiple neuronal pathways.

Converging Circuit

  • Description: Many neurons synapse onto a single postsynaptic neuron
    • Significance: Influences further nerve impulse, found in CNS for information integration.

Reverberating Circuit

  • Description: One neuron receives feedback from another in the same circuit
    • Branch off axon circles back and synapses with previous neuron; signal continues until fatigue/inhibition occurs.
    • Examples: swinging arms while walking, breathing, swimming patterns.

Student Learning Objective: Clinical Aspect

  • Multiple Sclerosis (MS):
    • Description: Progressive disease that destroys patches of myelin in brain and spinal cord; disrupts neuronal signals in CNS—causing sensory disorders & muscle weakness.
    • Symptoms: Numbness or pain, visual disturbances, muscle weakness or paralysis, imbalance, slurred speech, bladder incontinence, fatigue, depression.
    • Etiology: Unknown; suspected autoimmune disorder; potential genetic, environmental factors, vitamin D deficiency, Epstein-Barr virus.
    • Pathophysiology: Lymphocytes break down myelin, macrophages consume remains; inflammatory response can destroy axons; MRI may reveal characteristic lesions.
    • Treatment: Anti-inflammatory steroid drugs help reduce frequency of inflammation and symptoms.

Axon Regeneration in PNS

  • Stage 1: Fragmentation at the injury site
  • Stage 2: Macrophages invade and destroy axon distal to injury (Wallerian degeneration)
    • Surviving Schwann cells engulf the myelin fragments, secrete chemicals that recruit macrophages.
  • Stage 3: Axon filaments grow peripherally from the injured site; regeneration of microtubules formed by Schwann cells surrounding original axons.
  • Stage 4: Eventual reinnervation of target organ (partial recovery of function) may occur; in CNS, neuroglia do not form guiding bands for regrowing axons and can hinder axons with growth-inhibiting chemicals.