Nervous Tissue

Chapter 12: Nervous Tissue

Nervous System Overview

  • Two Major Anatomical Subdivisions:

    • Central Nervous System (CNS):

    • Comprises the brain and spinal cord.

    • Enclosed in bony coverings for protection.

    • Peripheral Nervous System (PNS):

    • Defined as nerves, which are bundles of axons in connective tissue.

    • Contains Ganglion: swelling of cell bodies in a nerve.

Nervous & Endocrine Systems Connection

  • Coordination Mechanisms:

    • Nervous and Endocrine systems are involved in maintaining internal coordination.

    • Nervous System Process:

    1. Sense organs receive information.

    2. Brain and spinal cord determine responses.

    3. Brain and spinal cord issue commands to glands and muscles.

    • Endocrine System Process:

    • Utilizes chemical messengers (hormones) delivered to the bloodstream.

Functional Divisions of PNS

  • Subdivisions of the Nervous System:

    • Central Nervous System:

      • Consists of the brain and spinal cord.

    • Peripheral Nervous System:

      • Further divided into sensory division and motor division.

    • Sensory Division:

    • Includes visceral sensory and somatic sensory divisions.

    • Motor Division:

    • Further divided into visceral motor division (Automatic Nervous System - ANS) and somatic motor division.

    • Visceral effectors: cardiac and smooth muscle, glands.

    • Sympathetic Division: associated with "fight or flight" responses.

    • Parasympathetic Division: associated with digestion.

    • Somatic Motor Division: effectors involve skeletal muscles.

PNS: Sensory & Motor Divisions

  • Sensory (Afferent) Divisions:

    • Transmit signals from receptors to CNS.

    • Includes both visceral sensory and somatic sensory divisions.

  • Motor (Efferent) Division:

    • Conduct signals from CNS to effectors (muscles and glands).

    • Includes visceral motor and somatic motor divisions.

Properties of Neurons

  • Key Properties:

    • Excitability (Irritability):

    • Ability to respond to changes (stimuli) in the body and the external environment.

    • Conductivity:

    • Ability to produce traveling electrical signals between cells.

    • Secretion:

    • When an electrical signal reaches the end of a nerve fiber, it triggers the release of chemical neurotransmitters across synaptic gaps.

Fundamental Classes of Neurons

  • Peripheral Nervous System:

    • Sensory (Afferent) Neurons:

    • Conduct signals from receptors to the CNS.

  • Central Nervous System:

    • Motor (Efferent) Neurons:

    • Conduct signals from the CNS to effectors (muscles and glands).

    • Interneurons:

    • Reside exclusively in the CNS; they account for about 90% of neurons.

    • Functions include processing, storing, and retrieving information.

Structure of a Neuron

  • Components:

    • Cell Body (Perikaryon/Soma):

    • Contains the nucleus and organelles.

    • Dendrites:

    • Vast number of short branches designed for receiving signals from other neurons.

    • Axon (Nerve Fiber):

    • Singular long extension arising from the axon hillock, facilitating rapid conduction of electrical impulses.

Neuron Classification

  • Categories of Neurons Based on Structure:

    • Multipolar Neurons:

    • Most common type, primarily located in the brain and spinal cord.

    • Bipolar Neurons:

    • Found in sensory organs such as the ear and nose, and in the retina.

    • Unipolar Neurons:

    • Feature a single process extending from the soma, typically transmitting signals related to touch or pain to the spinal cord.

    • Example: present in skin.

    • Anaxonic Neurons:

    • Lack an axon, consisting of multiple dendrites, incapable of producing action potentials and communicate locally through dendrites.

Axonal Transport

  • Definition:

    • The mechanism for the movement of proteins, organelles, and other essential materials along the axon.

  • Direction of Transport:

    • Anterograde:

    • Transport away from the soma and down the axon.

    • Retrograde:

    • Transport moving up the axon towards the soma.

Supportive Cells in Nervous Tissue

  • Neuronal Population:

    • Approximately 1 trillion neurons compose the nervous system; a significant portion is made up of neuroglia, estimated to outnumber neurons by a factor of 10:1.

    • Neuroglia (Glial cells):

    • Serve to support, nourish, and maintain nervous tissue, often referred to as the 'glue' of the nervous system.

Supportive Cells: 6 Types

  • Neuroglia of CNS:

    • Astrocytes:

    • Function to cover brain surfaces, support framework, regulate the composition of extracellular fluid (ECF), form the blood-brain barrier, nourish neurons, and produce growth factors promoting the formation of synapses.

    • Oligodendrocytes:

    • Form myelin around axons in the brain and spinal cord, aiding in conduction speed.

    • Ependymal Cells:

    • Line the cavities of the brain and spinal cord; involved in secreting and circulating cerebrospinal fluid.

    • Microglia:

    • Act as phagocytes, destroying microorganisms and dead nervous tissue.

  • Neuroglia of PNS:

    • Schwann Cells:

    • Produce the neurilemma around PNS nerve fibers and myelinate the majority of them; they facilitate nerve fiber regeneration.

    • Satellite Cells:

    • Surround neuron somas in ganglia, providing electrical insulation and regulating the chemical environment.

Myelin

  • Definition:

    • An insulating layer surrounding a nerve fiber, playing a critical role in the conduction of electrical impulses.

  • Formation:

    • Composed of layers of plasma membrane from glial cells that wrap around the neuron.

    • Myelination is usually completed by late adolescence.

Myelin in PNS

  • Composed of hundreds of layers wrapping each axon.

  • The outermost coil is referred to as the Schwann cell (neurilemma).

Myelin in CNS

  • Lacks both neurilemma and endoneurium structures typical of PNS.

  • Oligodendrocytes can myelinate multiple axons, providing insulation that enhances conduction speed.

Myelination Process

  • Begins during fetal development, with rapid progression typically occurring during infancy.

Unmyelinated Nerve Fibers

  • Present in both CNS and PNS.

  • In the PNS, even "unmyelinated" fibers are enveloped by a single Schwann cell that wraps around them, often forming channels through which small fibers can traverse.

Speed of Nerve Signal

  • Factors Influencing Signal Speed:

    • Diameter of the fiber and the presence of myelin.

    • Larger diameter fibers exhibit more surface area for signal conduction.

  • Speed Ranges:

    • Small, unmyelinated fibers: 0.5−2.00.5 - 2.0 m/sec.

    • Small, myelinated fibers: 3−15.03 - 15.0 m/sec.

    • Large, myelinated fibers: up to 120120 m/sec.

  • Functions of Signal Speed:

    • Slow signals are associated with supplying the stomach and dilating pupils, while fast signals are connected to muscle function and sensory signal transport involving vision and balance.

Nerve Regeneration: PNS

  1. Normal nerve fiber is observed to exhibit standard features at the neuromuscular junction (NMJ).

  2. When a nerve fiber is cut, protein synthesis ceases.

  3. Degeneration Phase:

    • Distal fibers begin to degenerate, and local Schwann cells follow.

    • The soma reacts by swelling; notable degeneration indicates that some neurons may die.

  4. Early Regeneration:

    • A regeneration tube forms as Schwann cells initiate the production of growth molecules (cell-adhesion molecules).

  5. Late Regeneration:

    • The regenerating tube guides the growth toward original damage, reestablishing synaptic contact with target cells or structures (e.g., muscle).

  6. Final Regrowth:

    • The regenerated fiber successfully reconnects with original fibers, restoring function.