Session 10 - Nervous Tissue

Question 1

What are the anatomical divisions of the nervous system?

Answer 1

Central Nervous System (CNS):

• Composed of Brain and Spinal Cord
• Contains Relay Neurons

Peripheral Nervous System:

• Composed of cranial, spinal and peripheral nerves
• Contains sensory and motor neurons

Question 2

What is the difference between the grey and white matter and where is it located?

Answer 2

Grey Matter:
- Found peripherally in the brain and also in areas called “nuclei” but centrally in the spinal cord (butterfly shaped)
- It consists of the nerve cell bodies, axon, terminal, non myelinated axons and neuroganglia (support cells)
White matter:
- Central in brain, peripheral in spinal cord
- It consists of myelinated material

Question 3

Describe the structure of a cross-section of the spinal cord

Answer 3

• The central grey matter roughly forms a butterfly shape with each prong forming the ventral and dorsal horn respectively. Each side is connected by the grey commissure
• The white matter forms ascending and descending tracts (myelinated)

Question 4

What are the 4 types of neurons?

Answer 4

• Motor: CNS to periphery, that send signals to effector tissue. (large motor neurons, pre/post-synaptic autonomic neurones)
• Sensory: Periphery towards the CNS, sends signals to integrative center (pseudo unipolar neuron, bipolar neuron)
• Integrative: CNS, collate all information (pyramidal, interneurons, purkinje cells)
• Anaxonic: Retina, act as relays

Question 5

Describe the different types of neuron structure

Answer 5

Multipolar Neuron:
- one axon and multiple dendrites
- most common
- most neurons in the brain and spinal cord
Dipolar Neuron:
- One axon and one dendrite
- Olfactory (smell) cells. retina, inner ear
Unipolar neuron:
- Single process leading away from the soma
- Sensory from skin to organs to spinal cord
Anaxonic Neuron:
- Many dendrites but no axon
- Help in visual processes

Question 6

Describe the process of neurotransmitter synthesis

Answer 6

Most of the cell machinery (Golgi, ER etc.) is found in the cell body away from the axon terminal. As such it must be transported to the terminal.

1) Neurotransmitter synthesised and then packaged into an anterograde vesicle
2) Vesicle transported down axon via microtubules
3) Release of neurotransmitter when stimulated
4) Inactivated neurotransmitter is resorbed
5) Retrograde vesicle transported back up axon to the cell body

Question 7

What are the 5 main types of synapse?

Answer 7

1) Axosomatic - directly to the plasma membrane of nerve or cell
2) Axodendritic - Axon terminal synapses with a dendritic spike
3) Axoaxonic - Synapse at the axonic bouton my enhance or inhibit the action at the synapse
4) Dendodrendritic - Dendrite to dendrite
5) Axo-axonal - Axon on the axon usually inhibits inputs

Question 8

Neurons are bundled into groups separated by connective tissue. How is this done?

Answer 8

Endoneurium:
 - Loose connective tissue
 - Surrounds single nerve cells
Perineurium:
 - Loose connective tissue
 - Maintains ionic composition
 - Surrounds clusters of axons (fascicle)
Epineurium:
 - Dense irregular connective tissue
 - Separates different types of nerves and fills spaces between fascicles
Paraneurium:
- Fascia that separates nerves from surrounding structures (often contains lots of adipose tissue)

Question 9

How does myelination occur?

Answer 9

1) The axon sitting in a groove is surrounded by a Schwann cell
2) The mesaxon membrane initiates myelination by surrounding the embedded axon.
3) A sheet-like extension of the mesaxon membrane then wraps around the axon, forming multiple membrane layers
4) Cytoplasm is extruded from between 2 opposing plasma membranes of the Schwann cell, which then becomes compacted to form myelin (19-20 rounds)

Question 10

Describe the structure of an unmyelinated nerve cell

Answer 10

The individual axons (multiple per Schwann cell) are engulfed by the cytoplasm of a Schwann cell. This surrounds the axon apart from a small gap, called a mesaxon allowing for (slower) continuous conduction

Question 11

Name some of the Glia (support) cells in the CNS

Answer 11

• Oligodendrocytes (like Schwann cells for the CNS)
• Astrocyte
• Satellite cells
• Microglial cells
• Ependymal cells

Question 12

What is the role of an oligodendrocyte?

Answer 12

Same role as Schwann cell only each cell wraps around more than one axon simultaneously

Question 13

What is the role of an astrocyte?

Answer 13

• They have on on capillaries (perivascular) and on nerves (perineural) that contain gap junctions. This allows for the transport of nutrients from blood to nerve cells.
• They can also regulate nerve impulses, by the release of GABA (inhibitory neurotransmitter) near nodes of Ranvier
• Contribute to blood-brain barrier

Question 14

What is the role of a microglial cell?

Answer 14

• Act as resident macrophage, immune response, remove damaged nerve cells, sense increased K+ ions (as damaged nerves release K+ ions)

Question 15

What is the role of a satellite cell?

Answer 15

1) only found on sensory neuron cell bodies
2) only in dorsal root ganglion
3) cover almost the entire surface
4) may have similar functions to an astrocyte

Question 16

What is the function of an ependymal cell?

Answer 16

• Synthesis and secretion of CSF (cerebrospinal fluid) in the ventricles
• Cilia move the fluid through ventricles to the spinal cord
• Microvilli absorb CSF for removal of pathogens
• Control fluid release into brain

Question 17

How is the spinal cord segmented?

Answer 17

Cervical - 8 segments/7vertebrae
Thoracic - 12 Segments and vertebrae
Lumbar - 5 segments and vertebrae
Sacrum - 5 segments and 1 fused bone segment
Coccyx - 1 segment and 1 bone segment

Question 18

At what level does the spinal cord stop?

Answer 18

The end of the spinal cord is at the L1-L2 vertebrae. The collection of nerves that run below this are called the cauda equine and leave via their corresponding segments

Question 19

What is a ganglion?

Answer 19

A collection of cell bodies in the peripheral nervous system

Question 20

Which nerve roots and horns are associated to either sensory sensory or motor neurons?

Answer 20

Dorsal - nerve root and horn is associated to sensory nerves only
Ventral - nerve root and horn is associated to motor nerves only

Question 21

Each spinal nerve innovates specific regions what are they called?

Answer 21

Dermatome - discrete cutaneous distribution to segment segment of skin
Myotome - Distribution of muscles associated to a certain group of muscles

Question 22

Describe the structure of a reflex arc in the SNS

Answer 22

Sensory neurone passes through the spinal nerve to the dorsal root where it passes through the dorsal root ganglion (where it’s cell body is situated)
Motor neurone cell body at terminal of sensory neurones found in the ventral horn. Motor neurone then passes down the ventral root to the spinal nerve where it continues directly to innervate the skeletal muscle

Question 23

What is the sympathetic chain?

Answer 23

This is a chain of sympathetic post-ganglionic cell bodies that are all linked together making a structure that runs beside the spine.

Question 24

What is the lateral horn and where is it found?

Answer 24

The lateral horn is only found between T1 and L2 of the spinal cord segment. It is where the cell bodies of the sympathetic pre-ganglionic neurones are found.

Question 25

Where do all the neurons and supporting (glia) cells derive from?

Answer 25

They all derive from the ectoderm under the actions of the notochord. The ectoderm forms the neural tube from which the CNS is derived.

Question 26

Describe the structure of how the sympathetic autonomic system communicates through the spinal nerves.

Answer 26

The preganglionic sympathetic neurons body is found in the lateral horn and progresses down the ventral root into the sympathetic ganglion via the white ramus communicans (distal to the grey Remus communicans). This then changes after the synapse to the postganglionic sympathetic neurone (doing a loop de loop).