General Anatomy (Nervous Tissue)

Question 1

What are the basic properties of nervous tissues?

Answer 1

1. To receive stimuli from within and outside the body
2. To conduct stimuli from Receptor to CNS (afferent)
3. To transmit impulses from CNS to (efferent)
   i) muscles for contraction
   ii) glands for secretion

Question 2

What are the subdivisions of the nervous tissues?

Answer 2

1. CNS - Central nervous system
   Brain and Spinal cord
2. PNS - Peripheral nervous system
   i) Cranial nerves (12 pairs) from brain
   ii) Spinal nerves (31 pairs) from spinal cord
3. ANS – Autonomic nervous system
   i) Sympathetic and
   ii) Parasympathetic

Question 3

What are the two types of cells in the nervous tissue?

Answer 3

1. Neurons (or ) Nerve cells
2. Nervous connective tissue – Neuroglial cells

Question 4

What are neurons?

Answer 4

• Neurons are structural and functional units of nervous tissue.
• Adult neurons do not undergo mitosis

Nerve cells or neurons are responsible for the reception, transmission, and processing of stimuli, the triggering of
certain cell activities, and release of neurotransmitters.

Question 5

What are the three parts of the neurons?

Answer 5

1. Cell body or perikaryon - which is the trophic center for the whole cell and also receptive to stimuli.
2. Dendrites - Multiple elongated processes specialized in receiving stimuli from environment, sensory epithelial cells, or other neurons. They possess Nissl granules
3. Axons - Single process specialized in conducting nerve impulse to other cells (nerve, muscle and glands).
   Axon hillock- initial segment of an axon, devoid of Nissl granules.

Question 6

What are the three types of neurons?

Answer 6

1. Bipolar neuron – One dendrite and one axon emerging from the cell body at opposite ends. Found in special sense organs example retina, olfactory neuron.
2. Pseudo unipolar or unipolar neuron – single process dividing into two. One goes to peripheral ending and the other to the CNS. Found in posterior root ganglion.
3. Multipolar neuron – more than one dendrite and single axon . Most neurons of the brain and spinal cord.

Question 7

What are motor neurons?

Answer 7

Send motor impulses to muscles and glands and bring about movements of muscles and secretion of glands.
Found in:
i)Brain –Cranialmotornuclei
ii) Spinal cord – Ventral horn of the grey matter

Question 8

What are sensory neurons?

Answer 8

receive impulses from peripheral receptors
Found in:
i) Brain – Cranial sensory nuclei
ii) Spinal cord- 1. Posterior root ganglia
2. Dorsal horn grey matter

Question 9

What are connector neuron?

Answer 9

connect and integrate the motor and sensory
neurons
i)- gray mater of spinal cord

Question 10

What are the types of neurons according to the size of the axon found in the CNS?

Answer 10

• golgi type 1 neurons
• golgi type 2 neutons

Question 11

What are golgi type 1 neurons?

Answer 11

Have long axon. Axons form long fiber tracts (ascending and descending) of the brain and spinal cord and nerve fibers of peripheral nervous system.
Examples are
i) Pyramidal cells of the cerebral cortex
ii) Purkinje cells of cerebellar cortex and
iii) Motor cells of the spinal cord.

Question 12

What are golgi type 2 neurons?

Answer 12

have a short axon that terminates near the cell body.
They are more numerous than the Golgi type I neurons. They are numerous in the
i) cerebral cortex
ii) cerebellar cortex and are often inhibitory in function.

Question 13

What are synapses?

Answer 13

• Site of functional contact between two neurons at which nerve impulses pass from one neuron to another.
• There is no anatomical continuity.
• The function of the synapse is to convert an electrical impulse from the presynaptic cell into a chemical signal that acts on the post synaptic cell.
• This is done by releasing neurotransmitters.

Question 14

What are neurotransmitters?

Answer 14

Neurotransmitters are chemicals that when combined with a receptor protein initiate second–messenger cascades.

Question 15

How are synapses formed?

Answer 15

• The synapse is formed by an axon terminal(presynaptic terminal) that delivers the signal.
• A region of another cell where a new signal is generated is postsynaptic terminal.
•The thin intercellular space between them is the synaptic cleft.

Question 16

What is the presynaptic terminal?

Answer 16

• The presynaptic terminal always contain synaptic vesicles with neurotransmitters and numerous mitochondria.
• Neurotransmitters are synthesized in the cell body and then stored in vesicles in the presynaptic region.
• During transmission they are released into the synaptic cleft by exocytosis.
If an axon forms a synapse
• With a cell body it is called an –
Axosomatic.
• With a Dendrite – Axodendritic
• With an axon - Axoaxonic

Question 17

What are the two types of synapses?

Answer 17

1 Chemical synapses (use chemical messengers)- commonest
2 Electrical synapses (transmits ionic signals through gap junctions).

The two types differ in structure and in the mechanism of impulse transmission.
Impulse transmission is faster at electrical synapses. Eg: CNS

Question 18

What are neuroglial cells or glial cells?

Answer 18

Non-neuronal nervous connective tissue cells

• Consists of cell and processes
• Their number is more than the neurons
• They provide a microenvironment suitable for the neuronal activity.

Question 19

What are the types of neuroglial cells?

Answer 19

• Astrocytes
• Microglia
• Oligodendrocytes

Question 20

What are Astrocytes?

Answer 20

Astrocytes are star shaped cells with multiple radiating processes. There are 2 types of astrocytes

Question 21

What are the two types of Astrocytes?

Answer 21

1. Fibrous astrocytes located in the white matter
2. Protoplasmic astrocytes with many short branched
   processes found in the grey matter.

Question 22

What are the functions of astrocytes?

Answer 22

1. Structural support- hold blood vessels and neurons in place
2. Metabolic exchanges- increases blood supply when need arises
3. Blood brain barrier- perivascular feet
4. Repair processes – scar tissue
   can form

Question 23

What are microglial cells?

Answer 23

• Are small elongated cells with short irregular processes and dense elongated nuclei.
• Found in the CNS.
• Phagocytic cells of the mononuclear
phagocytic system in the nerve tissue.
• They are involved in inflammation and
repair in the adult CNS.

Question 24

What happens microglial cells are activated?

Answer 24

1. Microglia retract their processes and become macrophages which are phagocytic
2. Act as antigen presenting cell
3. Dispose off unwanted cellular debris caused by CNS lesions.

Question 25

What are ependymal cells?

Answer 25

• Are low columnar epithelial cells, lining the
  i) ventricles of the brain and
  ii) central canal of the spinal cord
• Secreting epithelium which produces the
  cerebrospinal fluid.
• In some locations ependymal cells are ciliated which facilitates the movement of cerebrospinal fluid .

Question 26

What are oligodendrocytes?

Answer 26

• Small cell with less number of processes.
• Function is formation of myelin and electric insulation in CNS. Oligodendrocytes can branch and serve one neuron and its processes

Question 27

What are Schwann cells?

Answer 27

• Myelin production and electric insulation in the PNS.
• One Schwann cell forms myelin around a segment of one axon by wrapping of the Schwann cell membrane around the axon.

Question 28

What are nerve fibers?

Answer 28

• Consist of an axon enveloped by a special sheath
- In PNS the sheath cell is the schwann cell.
- In CNS the sheath cell is the oligodendrocyte.

Question 29

What are three distinct components of connective tissue that support the Schwann cells?

Answer 29

• Endoneurium
• Perineurium
• Epineurium

Question 30

What is endoneurium?

Answer 30

loose connective tissue surrounding each individual nerve fiber.

Question 31

What is perineurium?

Answer 31

specialized connective tissue surrounding each nerve bundle or fascicle. Serves as a diffusion barrier that contributes to the formation of the blood-nerve barrier.

Question 32

What is epineurium?

Answer 32

dense irregular connective tissue that surrounds a peripheral nerve and fills the spaces between nerve bundles or fascicle

Question 33

What are the three types of nerve injury?

Answer 33

• Neurapraxia
• Axonotmesis
• Neurotmesis

Question 34

What is Neurapraxia?

Answer 34

• Mildest form of nerve damage.
• No disruption of nerve or its sheath.
• Only interruption in conduction of impulse in the nerve.
• Full recovery with true regeneration within hours to months of injury.
• Eg: compression of nerve, disruption of blood supply, numbness

Question 35

What is Axonotmesis?

Answer 35

• More severe injury
• Disruption of neuronal sheath sparing myelin sheath.
• Can cause paralysis of motor, sensory and autonomic functions.
• Eg: Stretching of fractured bones and joints causing nerve injury

Question 36

What is Neurotmesis?

Answer 36

• Most severe lesion.
• Disruption of axon and myelin sheath.
• Involves perineurium, endoneurium, axons and myelin sheaths.
• Complete loss of motor, sensory and autonomic functions.
• Eg: severe contusion, stretch, laceration, local anaesthetic toxicity.

Question 37

What are the reactions of neurons to injury?

Answer 37

1. Damage of the cell body results in death of the neuron.
   Neurons cannot be replaced postnatally because
   i) neurons cannot undergo cell division
   ii) no neural precursor cells are present after birth.
2. Severing or crushing of an axon (axonal injury) leads to characteristic changes.
   a) Wallerian (anterograde) degeneration occurs in the distal portion of the of the axon
   i) The axon swells and degenerates, the myelin sheath fragments and phagocytes remove cellular debris.
   ii) After 3 weeks Schwann cells proliferate and form a tube of cells distal to the injury.

Question 38

What is the process of the recovery and regeneration of damaged axons?

Answer 38

If the lesion in the peripheral nerve is in a sufficient distance from the cell body, the nerve may recover

i) Axonal sprouts are produced at the distal end of the axon stump using materials synthesized in the cell body.
ii) The axon stump elongates and seek the tubes formed by Schwann cells.
iii) If sprouts penetrate the Schwann cell tube and reestablish contact with the appropriate effector cell, regeneration is likely to occur.

Question 39

What happen if axon injury is close to cell body?

Answer 39

If axonal injury is close to cell body, reactional changes occur in the neuron cell body (retrograde degeneration)- loss of trophic support

i) The cell body swells due to edema and nucleus is displaced peripherally
ii) Nissl bodies disperse known as chromatolysis (absent cytoplasmic basophilia)

Question 40

What are the demyelinative diseases of the nervous system?

Answer 40

• multiple sclerosis
• Pathophysiology of MS

Question 41

What is multiple sclerosis (MS)?

Answer 41

• is an immune-mediated inflammatory disease that attacks myelinated axons and oligodendrocytes in the central nervous system, destroying the myelin and the axon in variable degrees.
• Its counterpart in the peripheral nervous system is inflammatory demyelinative polyradiculoneuropathy (Guillain-Barré syndrome- GBS) and its chronic variants.

Question 42

What is pathophysiology of MS?

Answer 42

BBB disruption (The blood–brain barrier is a protective barrier that denies the entrance of foreign material)→ penetration of the barrier by lymphocytes and activated→result in direct attacks on myelin sheaths and oligodendrocytes within the CNS→characteristic demyelination