Module 11: Neuron Anatomy & Physiology

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What are the 2 basic types of cells nervous tissue contains?

neurons and neuroglia

What are neurons?

Individual nerves cells that conduct signals in the NS.

How many neurons are in the human brain?

100 billion

What do nerve cell serve?

sensorimotor activities and higher mental functions (e.g., memory, thought, language, reasoning and calculation.)

What are neuroglia cells?

supporting and insulating cells. also interstitial cells

What is the function of neuroglia cells?

they hold neurons in place.

Where are neurons not encapsulated/insulated from each other?

at the synapses

By providing insulation, glial cells prevent _____

abberant spread of signals

_____ cells far outnumber neurons by __-__ times.

Glial cells far outnumber neurons by 5-50 times.

What makes up half of the CNS volume?

Glial cells

What are equivalent to glial cells in the PNS?

Schwann Cells

What forms myelin in the PNS?

Schwann cells

What forms the axon covering in the PNS?

Schwann cells

What is neurolemma known as?

Schwann cells

What forms myelin in the CNS?

Oligodendroglia

When does myelination begin?

prenatal

When are pathways in the CNS fully myelinated?

2 years of age

What are the 3 types of glia in the CNS?

Oligodendroglia
Astroglia
Microglia

What are Oligodendroglia?

Equivalent to Schwann Cell in the PNS
lay down the myelin sheath in the CNS

What has sheet like processes that extend out among neurons?

astroglia

What occupies spaces between neural processes in the gray matter?

astroglia

What insulates the synapse?

astroglia

Why are astroglia believed to play a role in transport of substances between capillaries and neurons and in the blood brain barrier?

Because they have end-feet that contact capillaries

What are the largest and most numerous of the glial cells?

astroglia

What are the cells that remove cell debris generated in normal cell death and in degenerating brain tissue/cell death due to brain injury?

microglia

What cells look like Halloween ghosts?

microglia

Where are microglia found?

in choroid plexus and other structures related to the ventricles

What is composed of glial cells that line the ventricles and choroid plexus?

ependyma

What functions as phagocytes (macrophages)?

microglia

When do microglia remove cell debris?

in normal death or in cell death due to injury

What is a soma?

cell body

What is the metabolic center of the neuron?

soma

What is known as the receiving tree?

dendrites

What are the neoplasms?

Tumors

What is glioma?

tumor involving glia

What is glioblastoma?

most frequent type of tumor in adults

What is are three types of glia tumors?

glioma, glioblastoma, astrocytoma

What do dendrites receive?

integrative synaptic input from other neurons

Together with the ___ ___, dendrites constitute the ______ segment of the neuron

Together with the cell body, dendrites constitute the receptive segment of the neuron.

What unit is specialized as a receptor and integrator of info received from other neurons?

dendrite-cell body

What is a single process?

axon

What conveys coded info from the dendrite cell body unit to synaptic junctions?

conductile segment

What is called initial segment where action potential is initiated?

axon hillcock

Each main axon or its collaterals, terminates by branching into several terminal filaments called _______

teledendria

Where does each telodendron end?

at the terminal bouton

What is part of the synaptic junction?

terminal bouton

What constitutes the transmissive segment of the neuron?

boutons

What does a bouton contain?

presynaptic vessicles with neurotransmitter

What determines if a neuron will have an excitatory or inhibitory effect on the neuron with which it synapses?

neurotransmitter

What is another name for pre-synaptic terminal?

axon terminal

What is another name for the synaptic knob?

bouton

Axon length can vary from ___ ____ (in the brain) to as long as __ ______ (in the spinal cord)

Axon length can vary from a few micrometers (in the brain) to as long as 2 meters (in the spinal cord)

What are the 3 types of neurons?

Multipolar
Bipolar
Unipolar

What has many dendrites and one axon?

multipolar neurons

What is the most typical neuron?

multipolar

Which neuron is found in the brain?

multipolar

What are bipolar neurons?

have 2 processes extending from each pole of the body

In bipolar neurons, what is the peripheral process?

dendrite

In bipolar neurons, what is the axon?

central process

What is an example of a bipolar neuron?

rods and cones of eye, visual receptor cells

What neuron is T-shaped?

unipolar

What has one process extending from the body and divides into central and peripheral processes?

unipolar neurons

What kind of neurons are the cells in the dorsal/sensory roots of the spinal cord?

unipolar or pseudopolar

What is the effector segment?

synapse

What is the synapse?

site of contact with one neuron with another

What is neurosomatic?

neuron to cell body (soma) or dendrite of a neuron

What is neuroaxonal?

neuron to axon of a second neuron

What is nerve to muscle fiber synapse?

neuromuscular

What is the synaptic cleft?

space between the
bouton of one neuron and the cell body, dendrite or axon or another neuron
or the space between the bouton and the cell membrane of a muscle fiber neuron to muscle synapse

What is the cell membrane of the bouton?

pre-synaptic membrane

What is the cell membrane of the other cell body, dendrite, axon, or muscle fiber?

post-synaptic membrane

How many micrometers is a myelinated neuron?

greater than 2 micrometers in diameter

How many micrometers is an unmylinated neuron?

less than 2 micrometers

What is the range of conduction speed for myelinated neurons?

2 - 100 meters/sec.

What interrupts the myelin sheath?

nodes of ranvier

What is an internode?

distance from one node of ranvier to the next along the course of the axon fiber

The _____ is porportional to the diameter of the fiber.

internode

The thicker the fiber, the longer the _____

internodes

Where are neurotransmitter contained?

in synaptic vessicles of the bouton

When are they released?

at a synapse

What are neurotransmitters?

chemical messengers that transmit signals between neurons and between neurons and muscle

What are alpha motor neurons and all the muscle fibers it innervates?

motor units

What is the nerve to muscle ratio?

describes how many muscle fibers are innervated by a single motor neuron

What are lower motor neurons?

alpha motor neurons

The more precise (fine) a movement, the ____ the ratio

The more precise a movement, the smaller the ratio.
i.e. tongue muscle 1:5

Gross movement has a ____ ratio.

larger ratio
i.e. calf muscle 1:1900

What is the termination of a nerve fiber (and its branches) on a muscle?

neuromuscular (myoneural) junction

What is the synapse of each axon terminal (bouton) of a motor neuron on a voluntary muscle fiber?

motor end-plate

Neuron fluid inside is more _____ than fluid ____ a neuron

Neuron fluid inside is more negative than fluid outside a neuron.

How is electrical potential measured?

voltage across 2 points of the cell membrane

What is the measure across 2 points?

membrane potential

When a cell fires, what happens?

It depolarizes

The membrane potential is always ____

negative in resting cell

What is the membrane potential in neurons?

-70mV

What is the membrane potential in muscle fibers?

around m-90mV

How is the potential (or voltage) measured?

from the cell membrane

How is the membrane potential created?

by differences in ion concentrations across the cell membrane (between the intra vs. extracellular fluid)
cell metabolic activity (diffusion of potassium)
selective transport (gated channels in the cell membrane)

What is there a high concentration of INSIDE the cell?

potassium ions (K+) and anions

What is their a high concentration of OUTSIDE the cell?

sodium ions (NA+) and chloride (C1-)

What is continuously leaking out of cell?

potassium ions (K+)

What is left behind when potassium (K+) leaks out?

anions (large quantities of (-) charged protein molecules that cannot leak out)
which is why the inside of the cell is negative relative to the outside

When a signal is transmitted over a nerve fiber, the membrane potential goes through a series of changes called the ____ ________

Action potential

What does an action potential do to the cell membrane?

any factor that suddenly increase the permeability of the membrane to sodium ions

What are two stimuli that can elicit an action potential?

physical stimulus - sensory nerve ending such as pressure on skin
chemical stimulus - neurotransmitter

How does CNS transmission of information occur?

neurotransmitter

What happens when the action potential permeates?

positively charged sodium ions leak into the cell in great numbers and membrane becomes more positive/less negative by at least 30 mV

Action potential and associated events happen very _____

fast

NA+ leakage lasts for less than a _____ of a second

thousandth

The sodium-potassium pump drives sodium out and _____ ___

potassium in

When is the cell membrane polarized?

at -70mV, in its resting state

What is the initial positive change in the membrane potential?

depolarization (where the normal negatively polarized state no longer exists)

What is the return of membrane potential to its resting state called?

repolarization

What is saltatory conduction?

takes place in myelinated axons. the Action potential jumps from node to node

What occurs immediately after action potential is triggered?

refractory period

How do action potentials code info?

by response frequency

A weak stimulus will evoke only a ____ APs per unit of ___

few APs per unit of time

A strong stimulus will evoke generation of ____ APs per units of time

many APs per unit of time

How long does the absolute refractory period last?

8 milliseconds

During the absolute refractory period, ___ ____ cannot be generated

Action Potential

When the the neuron is hyperpolarized, what is happening to the membrane potential?

it becomes more negative from -70 to -90

What follows the absolute refractory period?

relative refractory period

What is the relative refractory period?

membrane is not fully repolarized but a strong (suprathreshold) stimulus can cause new AP

The _________ period is shorter in larger diameter fibers.

refractory

What is the all-or-none principle?

adequate stimulus causes the entire fiber to fire

What has an all-or-none phenomenon?

Action Potential

The ____ acts as a one way connector

synapse

________ are chemical agents released at the presynaptic terminal.

Neurotransmitters

What influences the postsynaptic membrane

neurotransmitters

What are the steps in the process of synapse and neurotransmitter release?

1. AP arrives at the terminal bouton
2. this causes depolarization of presynaptic membrane
3. Influx of calcium (CA++) from synaptic cleft
activates neurotransmitter release from the synaptic vessicles

What is the main transmitter at neuromuscular junctions within the PNS?

acetycholine

What is graded depolarization?

generator potential

Generator potential relies on _________ of arriving stimuli, which is a local graded response.

summation

What has to happen for the stimulus to to reach the initial segment (axon hillock) and generate a new AP?

Generator potentials need to sum

What are the 2 types of neural summation for GP?

temporal summation
spatial summation

What is temporal summation?

several subthreshold stimuli must sum within a critical time period to initiate new AP

What type of neural summation requires many stimuli to converge on postsynaptic membrane simultaneously to initiate new AP?

spatial summation

Generator potentials code info by response amplitude that is by _____ of the stimulus.

intensity

A weak stimulus evokes a _____ potential

small
small voltage change

A _____ stimulus evokes a large (er) potential

stronger
large voltage change

What is EPSP?

excitation of receptor membrane

What is IPSP?

creates state in which neuron is prevented from firing neurotransmitter substance hyperpolarizes the receptor membrane

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