Nervous System and Brain Protection Vocabulary

Fundamentals of the Nervous System and Environmental Detection

  • Definition of Stimulus:

    • Stimulus refers to the changes in environmental conditions that are detected by specialized receptors present in the body.

  • Definition of Reflex Arc:

    • A reflex arc is the pathway taken by a nerve impulse to accomplish a reflex action.

  • Conditioned Reflexes:

    • Conditioned reflexes require conscious training and effort.
    • Example: Playing the harmonium by striking a particular key upon seeing a music note.

  • Link Between Nervous and Endocrine Systems:

    • The hypothalamus of the forebrain functions as the physiological link connecting the nervous system and the endocrine system.

Protection and Structure of the Brain

  • Structures Involved in Brain Protection:

    • The brain is encapsulated and protected by three connective tissue membranes known as meninges:
      • Duramater: The outermost protective membrane layer.
      • Arachnoid membrane: The middle membrane layer.
      • Piamater: The innermost membrane layer.
    • Primary Function: The meninges collectively protect the delicate tissue of the brain from mechanical injury.

  • Anatomy of the Hindbrain:

    • The hindbrain is composed of three main structures:
      • Cerebellum
      • Pons
      • Medulla oblongata

Detailed Structure and Anatomy of a Neuron

  • Overview of Neuron Structure:

    • A neuron is the functional unit of the nervous system and typically consists of three basic parts: Cyton, Dendrites, and Axon.

  • Cyton (Cell Body / Perikaryon):

    • Contains a central nucleus embedded in abundant cytoplasm called neuroplasm.
    • The neuroplasm contains large granular structures known as Nissl's granules.
    • Contains standard cellular organelles, including mitochondria, ribosomes, lysosomes, and endoplasmic reticulum.
    • Neurons lack the cellular mechanisms needed to divide and do not possess the ability to undergo cell division.
    • Contains several thread-like neurofibrils that assist directly in the transmission of nerve impulses to and from the cell body.

  • Dendrites:

    • These are numerous, highly branched cytoplasmic processes that project outward from the surface of the cyton.
    • They function to conduct nerve impulses inward toward the cyton.
    • The extensive branching of dendrites serves to significantly increase the available surface area for receiving incoming signals from neighboring nerve cells.

  • Axon:

    • The axon is a single, elongated, slender cytoplasmic projection.
    • The plasma membrane enclosing the axon is designated as the axolemma, while the cytoplasm inside is termed the axoplasm.
    • The distal terminal of the axon terminates in specialized knob-like swellings known as synaptic knobs (or axon-terminals).
    • Axons are often covered by a protective sheath known as the myelin sheath.
    • The myelin sheath is further encased by an outer layer of specialized cells called Schwann cells, forming the neurilemma.
    • Gaps or interruptions in the myelin sheath along the axon length are designated as Nodes of Ranvier.
    • Functional Role of Myelin Sheath: Acts as an electrical insulator to ensure the rapid and efficient transmission of nerve impulses.

Structural Classification and Distribution of Neurons

  • Structural Categories of Neurons:

    • Neurons are classified structurally based on the total number of nerve processes (extensions) arising directly from the cyton:

  • Unipolar Neurons:

    • Structural Feature: Only a single nerve process arises from the cyton.
    • Functional Feature: This single process acts functionally as both an axon and a dendron.
    • Anatomical Occurrence: Unipolar neurons are found exclusively in early embryos and are not present in adult organisms.

  • Bipolar Neurons:

    • Structural Feature: The cyton gives rise to exactly two distinct nerve processes.
    • Functional Feature: One process functions as an axon, while the second process functions as a dendron.
    • Anatomical Occurrence: Found in specialized sensory structures, including:
      • Retina of the eye
      • Olfactory epithelium of the nasal chambers

  • Multipolar Neurons:

    • Structural Feature: The cyton gives rise to numerous dendrons alongside a single long axon.
    • Anatomical Occurrence: Found predominantly in the cerebral cortex of the brain.