Neural Tissue Part I Notes

Nervous Tissue Part I

Overview of Nervous System

• The nervous and endocrine systems coordinate internal functions.
  • Endocrine system: Uses chemical messengers (hormones) delivered via the bloodstream.
  • Nervous system: Involves three steps:
    • Sense organs receive information.
    • Brain and spinal cord determine responses.
    • Brain and spinal cord issue commands to glands and muscles.

Subdivisions of Nervous System

• Two major anatomical subdivisions:
  • Central Nervous System (CNS):
    • Brain and spinal cord.
    • Enclosed in bony coverings.
  • Peripheral Nervous System (PNS):
    • Nerve: Bundle of axons in connective tissue.
    • Ganglion: Swelling of cell bodies in a nerve.

Functional Divisions of PNS

• Sensory (afferent) divisions (receptors to CNS)
  • Visceral sensory division.
  • Somatic sensory division.
• Motor (efferent) division (CNS to effectors)
  • Visceral motor division (ANS)
    • Effectors: cardiac, smooth muscle, glands
      • Sympathetic division (action).
      • Parasympathetic division (digestion).
  • Somatic motor division
    • Effectors: skeletal muscle

Fundamental Types of Neurons

• Sensory (afferent) neurons: Transmit signals to the CNS.
  • Detect changes in body and external environment.
  • Information transmitted into brain or spinal cord.
• Interneurons (association neurons):
  • Lie between sensory and motor pathways in CNS.
  • Make up 90% of our neurons.
  • Process, store, and retrieve information.
• Motor (efferent) neuron:
  • Send signals out to muscles and gland cells.
  • Organs that carry out responses are called effectors.

Properties of Neurons

• Excitability (irritability):
  • Ability to respond to changes in the body and external environment, called stimuli.
• Conductivity:
  • Produce traveling electrical signals.
• Secretion:
  • When electrical signal reaches the end of a nerve fiber, a chemical neurotransmitter is secreted.

Structure of a Neuron

• Cell body = perikaryon = soma
  • Single, central nucleus with large nucleolus.
  • Cytoskeleton of microtubules and neurofibrils (bundles of actin filaments).
  • Compartmentalizes RER into Nissl bodies.
  • Lipofuscin: product of breakdown of worn-out organelles, increases with age.
• Vast number of short dendrites for receiving signals
• Single axon (nerve fiber) arising from axon hillock for rapid conduction
  • axoplasm and axolemma and synaptic vesicles

Variation in Neural Structure

• Multipolar neuron
  • Most common.
  • Many dendrites/ one axon.
• Bipolar neuron
  • One dendrite/one axon.
  • Found in olfactory system, retina, ear.
• Unipolar neuron
  • Sensory from skin and organs to spinal cord.
• Anaxonic neuron
  • Many dendrites/no axon.
  • Help in visual processes.

Axonal Transport

• Many proteins made in soma must be transported to axon and axon terminal.
  • Repair axolemma, for gated ion channel proteins, as enzymes, or neurotransmitters.
• Fast anterograde axonal transport
  • Either direction up to $400$ mm/day for organelles, enzymes, vesicles and small molecules.
• Fast retrograde axonal transport
  • For recycled materials and pathogens.
• Slow axonal transport or axoplasmic flow
  • Moves cytoskeletal and new axoplasm at $10$ mm/day during repair and regeneration in damaged axons.

Types of Neuroglial Cells

• Oligodendrocytes
  • Form myelin sheaths in CNS.
  • Each wraps around many nerve fibers.
• Ependymal cells
  • Line cavities and produce CSF.
• Microglia (macrophages)
  • Formed from monocytes.
  • In areas of infection, trauma, or stroke.
• Astrocytes
  • Most abundant glial cells.
  • Form framework of CNS.
  • Contribute to BBB (Blood-Brain Barrier) and regulate composition of brain tissue fluid.
  • Convert glucose to lactate to feed neurons.
  • Secrete nerve growth factor promoting synapse formation.
  • Electrical influence on synaptic signaling.
  • Sclerosis: damaged neurons replaced by hardened mass of astrocytes.
• Schwann cells
  • Myelinate fibers of PNS.
• Satellite cells
  • With uncertain function.

Myelin

• Insulating layer around a nerve fiber
  • Oligodendrocytes in CNS and Schwann cells in PNS
  • Formed from wrappings of plasma membrane
  • $20\%$ protein and $80 \%$ lipid (looks white)
  • All myelination completed by late adolescence
• In PNS, hundreds of layers wrap axon
  • The outermost coil is Schwann cell (neurilemma)
  • Covered by basal lamina and endoneurium
• In CNS - no neurilemma or endoneurium
• Oligodendrocytes myelinate several fibers
  • Myelination spirals inward with new layers pushed under the older ones
• Gaps between myelin segments = nodes of Ranvier
• Initial segment (area before 1st Schwann cell) and axon hillock form trigger zone where signals begin

Myelination

• Myelination in PNS begins during fetal development but proceeds most rapidly in infancy.

Unmyelinated Axons of PNS

• Schwann cells hold small nerve fibers in grooves on their surface with only one membrane wrapping

Speed of Nerve Signal

• Diameter of fiber and presence of myelin.
  • Large fibers have more surface area for signals.
• Speeds
  • Small, unmyelinated fibers = $0.5 - 2.0$ m/sec
  • Small, myelinated fibers = $3 - 15.0$ m/sec
  • Large, myelinated fibers = up to $120$ m/sec
• Functions
  • Slow signals supply the stomach and dilate pupil
  • Fast signals supply skeletal muscles and transport sensory signals for vision and balance

Regeneration of Peripheral Nerves (Wallerian Degeneration)

• Occurs if soma and neurilemmal tube are intact
• Stranded end of axon and myelin sheath degenerate
  • Cell soma swells, ER breaks up, and some cells die
• Axon stump puts out several sprouts
• Regeneration tube guides lucky sprout back to its original destination
  • Schwann cells produce nerve growth factors
• Soma returns to its normal appearance

Nerve Growth Factor

• Protein secreted by gland and muscle cells
• Picked up by axon terminals of growing motor neurons, prevents apoptosis
• Isolated by Rita Levi-Montalcini in 1950s
• Won Nobel Prize in 1986 with Stanley Cohen
• Use of growth factors is now a vibrant field of research