Session 17: Tissues of the nervous system

Question 1

Describe the process of myelinization in the PNS.

Answer 1

1. The axon penetrates an existing groove of Schwann cell cytoplasm
2. The edges of the groove fuse to
   form a mesaxon, so that the plasma membranes of
   the two edges fuse together
3. Mesaxon wraps itself around the axon several times
4. The clefts of the Schmidt-lanterman are areas in
   which the cytoplasm of the Schwann cell is present
   within the myelin layer
5. Each axon is surrounded by a myelin formed by a
   series of Schwann cells
6. The myelin sheath forms gaps along its path, called
   nodes of Ranvier

Question 2

Describe the process of myelinization in the CNS.

Answer 2

1. There are no Schwann cells in the CNS
2. The myelin sheath are formed by the processes of
   oligodendrocytes
3. Oligodendrocytes differ from Schwann cells in that
   branches of one cell can envelop segments of
   different several axons

Question 3

Classify neurons according to the # of processes that branch from it and state where each type is found.

Answer 3

1. Unipolar- most neurons in the body (SC &
   sympathetic ganglion)
2. Bipolar- retina, vestibular-cochlea ganglion, olfactory
   mucosae
3. Pseudo-unipolar- spinal ganglion and most cranial
   ganglia

Question 4

Name the 6 different types of encapsulated peripheral nerve terminals (receptors) and the stimuli detected by each.

Answer 4

1. Pacinian corpuscles
   - Touch and pressure
2. Merkel
   - Touch
3. Meissner’s corpuscles
   - Touch
4. Ruffini’s corpuscles
   - Heat
5. Free nerve endings
   - Pain
6. End organs of Krause
   - Cold

Question 5

Name the 5 types of neuroglia (supporting tissue) found in the CNS, their location and each of their function.

Answer 5

1. Oligodendrocytes
   * CNS white matter
   * Production of myelin sheath of CNS
2. Fibrous astrocytes
   * CNS white matter
   * Structural support to neurons
   * Metabolic support to neurons
   * Participate in repair process of nervous tissue
   * Participate in formation of blood-brain barrier
   * Cover bare areas of neurons
   * Forms Glia limitans
3. Protoplasmic astrocytes
   * CNS grey matter
   * Same as fibrous astrocytes
4. Ependyme
   * Canalis centralis of SC & brain ventricles
   * Produce, circulate, and absorb CSF
5. Microglia
   * CNS white and grey matter
   * Phagocytic cells

Question 6

Name the 2 types of neuroglia found in the PNS, their location and the function of each.

Answer 6

1. Schwann cells
   * PNS
   * Produce myelin sheath of PNS
2. Satellite cells
   * PNS
   * Same as astrocytes in the CNS

Question 7

Name the 3 connective tissue of nerve fibers.

Answer 7

1. Epineurium
2. Endoneurium
3. Perineurium

Question 8

Describe the process of axonal injury (regeneration of nerve tissue).

Answer 8

• Chromatolysis
• increase in the volume of the perikaryon
• Migration of the nucleus to the perikaryon
• proximal segment axon degenerates close
to the wound , but growth starts
as soon as debris is removed by macrophages

• In the nerve stub distal to the injury
– The axon swells and disintegrate completely
– The myelin sheath degenerate completely
– The remnants of the axon and the myelin sheath,
are removed by macrophages

• While these regressive changes take place,
Schwann cells proliferate within the
remaining connective tissue sleeve, giving rise to
solid cellular columns
• These rows of Schwann cells serve as guides
to the sprouting axons formed during the
reparative phase

• After these regressive changes, the proximal
segment of the axon grows and branches,
forming several filaments that progress in the
direction of the columns of Schwann cells.
• Only fibers that penetrate these Schwann cell
columns will continue to grow and reach an
effector organ

• When there is an extensive gap between the
distal and proximal segments, or when the
distal segments disappears altogether (as in
the case of amputation of a limb), the newly
grown nerve fibres may form a swelling or
neuroma that can be the source of
spontaneous pain

• Regeneration is functionally efficient only when
the fibres and the columns of Schwann cells are
directed to the correct place
• The possibility is good, however, since each
regenerating fibre gives origin to several
processes, and each column of Schwann cells
receives processes from several regenerating
fibres

• In an injured mixed nerve, however, if
regenerating sensory fibres grow into columns
connected to motor end-plates that were
occupied by motor fibres, the function of the
muscle will not be re-established

Question 9

Describe the blood-brain barrier.

Answer 9

• A functional barrier that prevents the passage of certain substances from the blood to the nerve tissue
• Interaction between astrocytes and capillary
  endothelium:
• Continuous capillaries w/ tight junctions between
  endothelial cells
  ~ no fenestrations
  ~ no pinocytotic vesicles
• Close association between astrocyte foot process
  and endothelial basal lamina
• Reduced permeability