PG1006
Lecture 3
Nerve Packaging, Support and Protection
Dr. Neil Docherty
My Teaching Objec/ves 1.  Clarify the cellular structural basis of neuron organisa5on into nerves in the periphery 2.  Outline the role of myelin in insula5on of neuronal signals in the periphery and centrally 3.  Introduce the histological appearance and general func5on of support cells (glia) in the C.N.S. 4.  Introduce the concept and importance of the blood-­‐
brain-­‐barrier Afferent
Interneurone
Concept Check from
Previous Lecture
Efferent
Effector Organ
Neuronal Ultrastructure Excita5on This requires signal insulation
to propagate over long
distances
Conduc5on -What is the insulator?
-Where is it produced from?
-How does the insulation
Influence conduction?
Transmission Wheater’s Func5onal Histology – chapter 7 The Peripheral Nervous System •  Subdivisions –  Afferent – carries informa5on to the CNS (sensory receptor at peripheral end) –  Efferent – carries informa5on to the effector organ (muscle or gland) Efferent Division •  Further subdivided •  Soma5c nervous system •  Autonomic nervous system Peripheral Nerve Packaging (Axons) •  Axons of both afferent and efferent neurones are bundled into nerves (afferent only, efferent only, mixed). •  Bundles of axons = fascicles (F) •  no. of fasicles can vary •  Endoneurium surrounds each axon •  Perineurium surrounds each fascicle (P) Wheater’s Func5onal Histology – chapter 7 •  Epineurium surrounds all the fascicles (E) •  on if >1 fascicle exists •  Rich blood supply H & E staining Individual Fascicle Wheater’s Func5onal Histology – chapter 7 The Peripheral Nervous System (Ganglia) •  Ganglia –  Bundle of nerve cell bodies surrounded and supported by satellite cells •  Subdivisions –  Dorsal root ganglia (spinal) – contain cell bodies of sensory nerves –  Autonomic ganglia – contain cell bodies of autonomic nerves Dorsal root ganglia (spinal)
•  Dendrites – Input Zone •  Receive s5muli •  Produce a graded poten5al •  Spread of poten5al •  Axon Hillock – Triggering Zone •  Ac5on poten5als triggered •  Axon – Conduc1ng Zone •  Spread of ac5on poten5al •  Local current flow •  Synapse – Output Zone •  Sends signal to another neurone •  Sends signal to an effector organ Myelina/on Note that the myelin appears
to be “sheathing” the axon
Structure Function Relationships
Myelination Helps Achieve Saltatory
Conduction
•  Aps occur at Nodes of Ranvier • Local current flow •  Rapid conduc5on Up to 80m/s in large motor neurones Cellular Basis of Myelina/on of the PNS •  Schwann cells –  Myelin forming cells of the PNS –  Forma5on of myelin sheath –  Concentric wrapping and cytoplasmic exclusion •  Each Schwann cell makes up a single segment of myelin sheath •  Many Schwann cells along axon Wheater’s Func5onal Histology – chapter 7 •  Aid in; debris removal and regenera5on of peripheral nerves as well as providing for fast conductance Rou/ne Histological Features of Myelina/on of the PNS Peripheral Nerve (scia5c) (H&E longitudinal sec5on) Features -­‐Schwann cell nuclei, axons, bubbly demyelina5on artefact, Nodes of Ranvier Further reading suggestion
DEMYELINATION AND
MULTIPLE SCLEROSIS
Note and consider
the functional significance of
cytoplasmic exclusion
Unmyelinated Fibres •  Cytoplasm of Schwann cell only •  One or more axons encased by cell •  Slow conductance •  Autonomic and Sensory fibres Wheater’s Func5onal Histology – chapter 7 Peripheral Nerve Cross Sec/on (TEM 5000x) Note myelinated and unmyelinated fibres
(S-Schwann Cell, F=Fibroblast)
The Central Nervous System Organisa/on •  Brain and spinal cord •  Subdivisions –  Grey macer – contains most of the neuronal cell bodies and dendrites –  White macer – contains most of the axons (myelinated) •  Neurones through-­‐out the brain communicate extensively –  Tend to be organised within discrete loca5ons –  -­‐Run in tracts –  Coordinated in nuclei. •  Composed of neurons and the suppor/ng cells of the CNS (glia) and blood vessels. Support Cells of the CNS -­‐ Neuroglia •  Comprises all the non-­‐neural cells of the CNS •  Form almost half the total mass of the CNS –  i.e. Occupy space between neurones •  Provide mechanical and metabolic support to neurones •  4 principle types of neuroglia –  Oligodendrites, astrocytes, microglia and ependymal cells Myelina/on of the CNS – Oligodendrocytes •  A single oligodendrocyte can contribute to the myelina5on of up to 50 axons •  Different oligodendrocytes are responsible for forming myelin sheath Human Physiology by Lauralee Sherwood, chapter 4 Annotate from Wheater’s Func5onal Histology – chapter 7 Astrocytes • 
• 
• 
• 
Most abundant glial cell Metabolic exchange between blood and neurons Uptake and degrada5on of neurotransmicers Induce forma5on of blood brain barrier Concept check
The image on the left shows immunohistochemical detection of glial fibrillary
acidic protein (GFAP), the intermediate filament of astrocytes.
What do you remember with regard to the words in bold?
Microglia •  Immune defence cells of the CNS •  Release nerve growth factors in res5ng state (sta5onary) •  Ac5vated by infec5on or injury –  Alter change (amoeboid) –  Become phagocy5c –  Release chemicals Ependymal •  Line ventricles and spinal canal of the spinal cord •  Cerebrospinal fluid (CSF) producing cells •  Possess cilia which contribute to the flow of CS Simple, cubiodal, ciliated
C.N.S. Neurons and Glia Together Human Physiology by Lauralee Sherwood, chapter 4 The Blood Brain Barrier (BBB) In some areas of the brain the
barrier is weaker
Class Question
Why would this be?
What would it allow the brain to do?
Further reading suggestion
BBB INTEGRITY AND THE PROGRESSION
OF ALZHEIMER’S DISEASE
Your Learning from Today Should focus on being able to;
1.  Describe how peripheral nerves are packaged
2. Explain the cellular basis and physiological importance
of myelination
3. Discuss the structure and importance of the blood brain
barrier