physiology of neuron

Question 1

multipolar

Answer 1

One of the poles gives rise to axon and all others to dendrites.
* in Most vertebrate neurons, especially in the CNS
* Dendrites branch to form the dendritic tree.

Question 2

bipolar neuron

Answer 2

Have two poles, one for axon and other for dendrite.
* Found in vestibular and cochlear ganglia, nasal olfactory epithelium and as bipolar cells in the retina.

Question 3

unipolar neuron

Answer 3

Have a single pole, from which both the processes - axon and dendrite arise.
* True unipolar cells are present only in embryonic stage.

Question 4

Motor neurons

Answer 4

-efferent nerve cells
-Carry the motor impulses from the CNS to the peripheral effector organs
- long axon and short dendrites

Question 5

Sensory neurons

Answer 5

-afferent nerve cells
-Carry the sensory impulses from the periphery to the CNS
-short axon and long dendrites

Question 6

structure of multipolar

Answer 6

anki

Question 7

Cell body

Answer 7

• soma or perikaryon.
• Round, stellate, pyramidal or fusiform in shape.
  ✓ Cytoplasm: Contains numerous Nissl granules, many mitochondria, Golgi apparatus, lysosomes and
  cytoskeletal proteins
  o Nissl granules are composed of rough endoplasmic reticulum studded with ribosomes and polysomes.
  o Neurofilaments (neurofibrils) consist of microfilaments and microtubules.
  ✓ Nucleus: contains one nucleolus; but centrioles are absent.

Question 8

Dendrites

Answer 8

The numerous short extensions from the cell body
* dendritic spines, help in increasing the surface area

Question 9

Axon

Answer 9

Single longer process of the nerve cell.
* Cell membrane continues as axolemma.
* Cytoplasm–> axoplasm that contains mitochondria, Golgi apparatus, and cytoskeletal proteins.
* arises from axon hillock which is
devoid of Nissl bodies.
* Action potential is generated at the initial segment in motor neurons and at the first node of
Ranvier in sensory neurons.
* Axon terminal: The terminal part that divides into telodendria. The endings of it form terminal buttons or synaptic knobs.
-The buttons contain neurotransmitter vesicles.
* Myelin sheath is present around the axon in the myelinated nerve fibres. Each Schwann cell
provides the myelin sheath for a short segment of the axon. At the junction of any two such
segments, there is nodes of Ranvier.

Question 10

Unmyelinated axons

Answer 10

no myelin sheath.
* Somatic nerve fibers- small diameter, postganglionic sympathetic
neurons of the autonomic nervous system, dorsal root fibers and most of
the fibers in invertebrates are unmyelinated.
* Speed of conduction of impulse is slower.

Question 11

Myelinated axons

Answer 11

in PNS is formed by the Schwann cells. The large diameter somatic nerve fibers as well as the preganglionic fibers of the autonomic nervous system are myelinated. The myelin sheath consists of compressed layers of Schwann cells.
* in the CNS, is carried out by oligodendrocytes. Each oligodendrocyte projects several processes that wrap around many axons.
* rich in lipids,excellent insulator.

Question 12

Significance of myelin sheath

Answer 12

1. increases the velocity of conduction of nerve fiber.
2. decreases energy expenditure by the cell.
3. provides a protective covering to axon.
4. acts as an insulator.
5. responsible for the color of the white matter

Question 13

formation of myelin sheaths

Answer 13

• axon near Schwann cell invaginates into it
• mesaxon formed by further invagination of axon
  mesaxon elongates and begins to wind round the axon
  -mesaxon makes many turns around axon.lipids are deposited between the layers of mesaxon
• myelin sheath formed
• thin layer of Schwann cell cytoplasm remains a neurilemma

Question 14

Saltatory conduction

Answer 14

-Since myelin is an insulator, an action potential cannot induce a depolarization where the myelin sheath is present.
* The nearest point where an action potential can induce depolarization electrotonically is the nearest node of Ranvier.
* Voltage-gated Na+ channels are highly concentrated in the nodes of Ranvier.
* The depolarization induced at the node is suprathreshold and leads to an action potential. This action potential then leads to excitation at the next node of Ranvier.
* Thus, the impulse jumps from node to node.

Question 15

Axoplasmic transport

Answer 15

Transfer of substances between cell body and axon terminal is called axoplasmic transport.
1) Anterograde axoplasmic transport
2) Retrograde axoplasmic transport
3) Transneuronal axoplasmic transport

Question 16

Anterograde axoplasmic transport

Answer 16

-from the cell body toward the axon terminals
- mapped by [3H]-leucin.
* may be fast or slow.

Fast axoplasmic transport
-400 mm/day, by kinesin, a microtubule associated protein that transports many organelles, vesicles and membrane glycoproteins

Slow axoplasmic transport
* 0.5–2 mm/day. Various structural proteins like actin, neurofilaments and microtubules get transported by slow transport.
* It has an important role in supplying materials for the regeneration of axons following nerve injury.

Question 17

Retrograde axoplasmic transport

Answer 17

-from the axon terminals to the cell body.
* 200 mm/day, by dynein, microtubule associated protein.
* keeps the soma informed about the synaptic environment.
* mapped by horse-radish peroxidase.

• Examples of retrograde transport are:
  1. Transport of viruses: The chickenpox virus / varicella zoster that causes herpes simplex reaches cell body from nerve terminals in the skin .The virus may remain in a dormant state in nerve root for many years before causing herpes zoster afterwards.
  2. Transport of toxins: Tetanus toxin at motor neuron ending is transported to the cell body
  3. Transfer of nerve growth factor taken up by presynaptic terminal and transferred to soma .
  4. Reuptake of synaptic transmitters: Neurotransmitters like norepinephrine (NE) released at the nerve terminals are rapidly removed from the synaptic cleft by reuptake into the presynaptic
  neuron. In the presynaptic terminal, NE is repackaged into vesicles or deaminated by mitochondrial monoamine oxidase.

Question 18

Transneuronal axoplasmic transport

Answer 18

Trophic substances like nerve growth factors are transported across the synapse to the presynaptic membrane of another neuron.
* Helps in maintenance of the synaptic contacts.

Question 19

neuroglia

Answer 19

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