Nervous Tissue & Cell Function

Question 0

Classification of Neuron Morphology

Answer 0

• based on the number of processes extending from the cell body
• Multipolar neurons - have one axon and two or more dendrites.
  
  • Almost all neurons in the CNS are multipolar.
• Bipolar neurons - have one axon and one dendrite.
• Pseudounipolar neurons - have one axon with 2 long processes.
• Most bipolar & pseudounipolar neurons are sensory neurons
  
  • transmit impulses of sensory stimuli from the PNS to the CNS

Question 1

Description of Neurons

Answer 1

• specialized cells that send & receive signals to/from other cells.
• Communication b/t neurons occurs at synapses.
• Consist of
  
  • Cell body-contains the nucleus and other essential organelles.
  • Dendrites-processes that receive inputs to the neuron.
  • Axon-transmit signal from cell body to the neuron’s target(s).
    
    • often insulated w/ myelin sheath to speed the rate of AP

Question 2

Glial cells of CNS:

Answer 2

• Provide structural & metabolic support to neurons during development and in the mature brain.
• Oligodendrocytes - form the myelin sheath around CNS axons.
• Astrocytes: provide scaffolds for growing axons and migrating neurons during development
  
  • maintain appropriate extracellular ion concentration
  • contribute to the formation of the blood-brain barrier.
• Microglia - act as macrophages or scavengers in the CNS.
• Ependymal cells - line the fluid-filled cavities (ventricles) of the brain and spinal cord.

Question 3

Glial cells in the PNS

Answer 3

• Provide structural & metabolic support to neurons during development and in adult
• Schwann cells: myelinate axons of peripheral nerves.
  
  • Role in regeneration following injury to a peripheral nerve axon.
• Satellite cells-surround/support nerve cells in peripheral ganglia

Question 4

Information Processing in the CNS

Answer 4

• Most CNS neurons act as miniature computational units
  
  • integrate inputs from multiple sources.
• Balance of excitatory & inhibitory inputs
  
  • determines whether the neuron generates an AP
• many presynaptic inputs to postsynaptic cell required to reach threshold & fire an AP
• Components: Many CNS neurons to One CNS neuron
• Synaptic Input: Excitatory and inhibitory inputs
• Transmitters: Various chemical transmitters interacting with a variety of receptor types
• Electrical Activity: Many action potentials firing synchronously –> An AP in the target neuron

Question 5

Information Processing in the PNS

Answer 5

• postsynaptic skeletal muscle fiber at the NMJ excited by a single presynaptic alpha motor neuron
• Components: One alpha motor neuron to one muscle fiber
• Synaptic Input: Excitatory inputs only
• Transmitters: One chemical transmitter and One receptor type
• Electrical Activity: Action potential in a motor neuron–> Action potential in muscle fiber

Question 6

Define Synaptic Potential

Answer 6

graded, monophasic changes in postsynaptic membrane potential that are generated at a synapse

Question 7

Structural Features of the Synapse

Answer 7

• Synapses are intercellular junctions that are specialized for the transmission of nerve impulses.
• Presynaptic neuron (usually axon terminal) releases a chemical substance into the synaptic cleft.
• Chemical substance interacts w/ receptors in membrane of postsynaptic neuron (dendrites or soma)
• Fewer synapses also occur between axons and other axons.

Question 8

Physiological Properties of Synaptic Potentials

Answer 8

• Release of an excitatory NT depolarizes the postsynaptic membrane.
  
  • This is called an excitatory postsynaptic potential (EPSP).
• Release of an inhibitory neurotransmitter hyperpolarizes the postsynaptic membrane.
  
  • This is called an inhibitory postsynaptic potential (IPSP).
• Size of graded synaptic potential related to amt of transmitter released & density of receptors on postsynaptic membrane.
• Amplitude of graded synaptic potentials can vary at each synapse and over time at a given synapse.
• Graded synaptic potentials spread passively thru membrane of the postsynaptic dendrite & soma
• Graded synaptic potentials decay with time and distance.
• EPSPs at a single synapse are generally subthreshold and will not generate an AP

Question 9

difference between excitatory and inhibitory postsynaptic potentials.

Answer 9

• Release of an excitatory NT depolarizes the postsynaptic membrane.
  
  • This is called an excitatory postsynaptic potential (EPSP).
• Release of an inhibitory neurotransmitter hyperpolarizes the postsynaptic membrane.
  
  • This is called an inhibitory postsynaptic potential (IPSP).

Question 10

Types of Integration of Synaptic Input

Answer 10

• AP occurs when multiple subthreshold EPSPs sum to bring the membrane potential to threshold.
• Temporal summation: Consecutive EPSPs at the SAME site add to depolarize toward threshold.
• Spatial Summation: Simultaneous EPSPs at DIFFERENT synapses on the same neuron sum to depolarize the membrane toward threshold.

Question 11

Principal Site of Synaptic Integration

Answer 11

• The axon hillock of a neuron is the usual site of integration of graded postsynaptic potentials.
• When the sum of graded potentials in a postsynaptic cell is large enough the neuron will generate an AP
• High density of voltage-gated Na+ channels at the axon hillock make it the zone for initiating the AP

Question 12

Synaptic Connections: Divergence

Answer 12

-One presynaptic neuron synapses on multiple postsynaptic neurons.
-Ex: The sensory signal produced by touching a hot stove
diverges to retract the affected limb & inform higher CNS centers that you touched a hot stove.

Question 13

Synaptic Connection: Convergence

Answer 13

• multiple axon terminals synapse on the same postsynaptic cell.
• Converging inputs on the postsynaptic cell can be inhibitory and/or excitatory signals.
• The postsynaptic cell integrates the converging inputs chiefly by spatial summation.

Question 14

Synaptic Connections: Axoaxonic Synapses

Answer 14

• Type 1: Presynaptic axon terminal synapses on the initial segment (unmyelinated) of the postsynaptic axon
• Type 2: Presynaptic axon terminal synapses on the axon terminal of a 2nd neuron that is presynaptic to the soma or dendrite of a 3rd neuron
• The presynaptic axon terminal modulates the entry of Ca2+ ions into the postsynaptic axon terminal
• Modulation of Ca2+ entry regulates the amount of NT released by the postsynaptic terminal

Question 15

Presynaptic Facilitation:

Answer 15

• NT released at axoaxonic synapse increases entry of Ca2+ into axon terminal of postsynaptic neuron
• This increases the amt of transmitter released from postsynaptic neuron
  
  • generates a larger PSP in dendrites or soma of the 3rd neuron

Question 16

Presynaptic Inhibition

Answer 16

• NT released at axoaxonic synapse decreases entry of Ca2+ into axon terminal of postsynaptic neuron
• Decreases the amt of NT released from the postsynaptic neuron
  
  • generates a smaller PSP in dendrites or soma of the 3rd neuron

Question 17

Feed forward Processes

Answer 17

• Information flows unidirectionally through a series of neurons.
• Excitation: excitatory neurons project to other excitatory neurons, which, in turn, excite other neurons
  
  • Neuron A excites Neuron B which excites Neuron C
• Inhibition: excitatory neurons activate inhibitory neurons, which then inhibit adjacent populations of excitatory neurons
  
  • Neuron A excites Neuron B which inhibits Neuron C

Question 18

Disinhibition

Answer 18

• Four neurons connected in series:
  
  • Neurons 1 and 4 are excitatory
  • Neurons 2 and 3 are inhibitory.
• Excitation of Neuron 2 by 1 inhibits Neuron 3.
• Decreased firing by Neuron 3 releases Neuron 4 from inhibition
• Result of disinhibition is to increase the activity level of Neuron 4.
• Comparable to a “double negative,”
• inhibition (by Neuron 2) of an inhibitory interneuron (Neuron 3) results in facilitation of activity of its target (Neuron 4).

Question 19

Retrograde Reaction Following Neuron Damage

Answer 19

• Morphological reaction of proximal axon, cell body and dendrites of a damaged nerve
• Reaction occurs in the direction opposite of conduction
• The primary retrograde reaction occurs at the neuronal cell body.
• swelling of the cell body and nucleus
• displacement of nucleus from center of cell to eccentric location.
• dispersion of Nissl substance into homogeneous particles of decreased basophilia (chromatolysis)
• Ribosome-studded endoplasmic reticulum are dispersed and replaced with polyribosomes.

Question 20

Anterograde Reaction Following Neuron Damage

Answer 20

• Morphological reaction of the axon, its myelin sheath and axon terminals distal to the injury site
• Reaction occurs in the direction of conduction
• Also termed Wallerian degeneration
• Involves degeneration & clearance of the axon, myelin sheath & axon terminals distal to the injury site
  
  • allows for potential regeneration of the injured axon.
• Schwann cells near the injured nerve dedifferentiate & divide.
• Schwann cells & macrophages phagocytose the degenerative debris in PNS injury
  
  • In CNS this is done by microglia, astrocytes and macrophages.

Question 21

PNS Nerve Regeneration

Answer 21

• able to repair, including restoring functionally useful connections.
• After injury, tips of proximal stumps swell & experience some retrograde degeneration
  
  • once the debris is cleared, it begins to sprout axons
  • presence of growth cones can be detected
• Growth cones sprout initially at the nearest Node of Ranvier of the proximal segment
  
  • must grow across injury & into Schwann cell guidance tunnels
• Growing axons contact Schwann cells forming columns
  
  • second wave of Schwann cell proliferation occurs.
• Schwann cells from 2nd proliferation form guidance tunnels along the former course of the axon.
• Elongating axons innervate target tissue, myelinate and functional recovery occurs.
• Axon growth rates can reach 2 mm/day in small nerves & 5 mm/day in large nerves.
• The growth rate is determined by the slow anterograde transport rate (1 -6 mm/day).

Question 22

Factors That Influence PNS regeneration and reinnervation

Answer 22

Type of Nerve Injury

• crush: more regeneration b/c endoneurial sheath remain intact.
• transection: continuity of axoplasm lost
  
  • regeneration more difficult because of misalignment of axons
  • if suture ends of nerve together, chance of recovery increases

Site of Injury:
-The closer to target site the nerve is damaged the greater the likelihood for regeneration.

Age
-younger = regenerative activity is greater

Question 23

CNS Neuron Regeneration Capability

Answer 23

• axonal regeneration is typically abortive.
• axons initially sprout from the proximal axon
  
  • regrowth is limited by several factors:
• Loss of molecules that promote axonal growth
  
  • laminin and fibronectin persist in PNS but are absent in CNS
• Expression of molecules that inhibit axonal growth.
  
  • Inhibitory glycoproteins in myelin processes of oligodendroglia.
  • Not in Schwann cells.
• Oligodendroglia do not form “guidance tunnels”.
• Glial cells release cytokines that decrease axonal growth.
• Development of glial scar at injury site impedes growth of axons due to proteoglycan production that inhibits sprouting.