Download Ch 2 lec 2

Chapter 2
Structure and functions of cells of the nervous system
Review

Basic Genetics


Genes
Chromosomes

Made up of 4 nucleotide bases





Adenine-thymine, guanine-cytosine
Replication
Duplication errors
Sex Chromosomes and Sex-linked Traits
Structural and Operator genes
Review
Cells of the Nervous
System
•
–
Neurons
–
Basic structure
Cells of the Nervous System

Neurons




Multipolar
Unipolar
Bipolar
Glial cells


Various types
Provide a wide variety of supportive functions
Cells of the Nervous System

Types of Neurons

Multipolar Neuron – neuron with one axon and many
dendrites attached to its soma; most common type in CNS.
Figure 2.1
Cells of the Nervous System
Types of Neurons

Bipolar Neuron – neuron with one
axon and one dendrite attached it its
soma.


sensory systems (vision and audition)
Unipolar Neuron – neuron with one
axon attached to its soma; the axon
divides, with one branch receiving
sensory information and the other
sending the information into the central
nervous system.

somatosensory system (touch, pain, etc)
Figure 2.2
Copyright © 2006 by Allyn and Bacon
Figure 2.5 The Principal Internal
Structures of a Multipolar Neuron
Inside the Cell Body
From DNA (nucleus) to protein synthesis (cytoplasm)
•
Transcriptional and translational processes take place in the cell body
Genetic Code and
Genetic Expression

Mechanism of gene expression
1. Strand of DNA unravels
2. Messenger RNA (mRNA)
synthesized from DNA
3. mRNA leaves nucleus and
attaches to ribosome in the
cell’s cytoplasm
4. Ribosome synthesizes protein
according to 3-base sequences
(codons) of mRNA
Cells of the Nervous System

Internal Structure

Figure 2.20
Membrane – a structure consisting principally of lipid
molecules that defines the outer boundaries of a cell and also
constitutes many of the cells organelles, such as the Golgi
apparatus
Cells of the Nervous System

Internal Structure


Cytoplasm – the viscous, semi-liquid substance contained in
the interior of the cell; contains organelles
Mitochondria – an organelle that is responsible for
extracting energy from nutrients; ATP (adenosine triphosphate.
Figure 2.5
Cells of the Nervous System

Internal Structure

Endoplasmic Reticulum – parallel layers of membrane in the cytoplasm;
stores and transports chemicals through the cell; 2 types


Rough ER – contains ribosomes; produces proteins secreted by the cell
Smooth ER – site of synthesis of lipids; provides channels for the segregation of
molecules involved in various cellular processes
Cells of the Nervous System

Internal Structure

Golgi Apparatus – special form of smooth ER; some complex
molecules are assembled here; also acts as a packaging plant, where
products of a secretory cell are wrapped


Exocytosis – the secretion of a substance by a cell through means
of vesicles; the process by which neurotransmitters are secreted
Lysosomes – an organelle containing enzymes that break down
waste products; produced by Golgi apparatus.
Cells of the Nervous System

Internal Structure



Cytoskeleton – formed of microtubules and other protein fibers giving the
cell its shape.
Microtubule – a long strand of bundles of 13 protein filaments arranged
around a hollow core; part of the cytoskeleton and involved in transporting
substances from place to place within the cell.
Axoplasmic Transport – active process by which substances are
propelled along microtubules; 2 types


Anterograde axoplasmic transport – movement from the soma to the terminal
buttons; accomplished by kinesin and ATP; fast (500 mm/day)
Retrograde axoplasmic transport – movement from the terminal buttons to the
cell body; accomplished by dynein; about ½ as fast as antergrade transport
Cells of the Nervous System

Supporting Cells

Glia (glial cells) - Supporting cells that “glue” the nervous
system together; 3 most important types are:



Astrocytes
Oligodendrocytes
Microglia
Glial Cells

Astrocytes – largest glia, many functions

Myelin producers



Oligodendrocytes (CNS)
Schwann cells (PNS)
Microglia – involved in response to injury or disease
Astrocytes


Provide support to neurons
Clean up debris




phagocytosis.
Provide nutrients and other
substances
Regulate chemical composition of
the extracellular fluid
Some of astrocyte’s processes are
wrapped around blood vessels;
other processes are wrapped
around parts of neuron
 Astrocytes
receive glucose from
capillaries and break it down to
lactate
 Lactate released into extracellular
fluid and then taken up by neurons
Astrocytes and the
Blood-Brain-Barrier
 ‘Selectively permeable’

Some substance can pass
through the BBB
 BBB is not uniform

Area postrema (medulla)
Compromised
Normal
Figure 2.12
Glial Cells
Oligodendrocytes


Myelinate axons in the CNS
Support axons and produce the myelin sheath
A
sheath that surrounds axons and insulates them,
preventing messages from spreading between adjacent
axons
 The sheath is not continuous (the bare portions are
called nodes of Ranvier)
 A given oligodendrocyte produces up to 50 segments
of myelin
Oligodendrocyte
Figure 2.10
Glial Cells:
Oligodendrocytes
 Myelin
 80% lipid
 20% protein
 Nodes of Ranvier
 1-2 μm
Figure 2.10
Glial Cells: Schwann Cells
 Peripheral cells
 Located in the
PNS
 Can aid in the
removal of dead or
dying neurons
 Can then guide
axonal
sprouting
 CNS: axonal sprouts
are hindered by glial
scars (gliosis)
Figure 2.11
Glial Cells:
Microglia

10-20% of glial cells are microglia

Cells originate in the periphery

Phagocytosis- breakdown dying
neurons, protect from invading
microorganisms

Primarily gray matter

Hippocampus, olfactory
telencephalon, basal ganglia,
substantia nigra
Phagocytosis
6 month
24 month
Lucin and Wyss-Coray (2009)


Reactive microglia present in aging rats
Stress also shown to activate microglia

Cagnin et al. (2001)
The Lancet
 [11C]-PK11195:


Peripheral BZP binding
site present on
activated microglia
AD:

entorhinal,
temporoparietal, and
cingulate cortex
Lipid bilayer

Selectively permeable
to very few ions
Proteins embedded
in the bilayer

Channel proteins


Selective for ion type
Receptor proteins
 Signalling
The Cell Membrane
devices
Neuronal Charge: Simple Design

Measuring
membrane voltage

Requires:


ONE recording
electrode inside the
cell (intracellular)
ONE recording
electrode outside the
cell (extracellular)
Figure 2.15
The Ionic Basis of the
Resting Membrane Potential

Membrane potential: The
voltage across the neuronal membrane
at any given time.

Resting Membrane Potential:
The voltage when a neuron is at rest
(without synaptic input)

At rest (RMP)
 -65

- -70 mV
During an action potential
 -65
to +30 mV
Resting Membrane Potentials

The RMP is entirely
dependent upon


The types of ions
Where they are found
(distribution across the
membrane)
65 mV

It is because these ions are
unequally distributed across
the membrane, that the
inside of the cell sits more
negative in reference to the
external environment.
IONS OF INTEREST
substance
symbol
-anions
A–
potassium
K+
sodium
Na+
chloride
Cl–
IONS Concentrations at Rest
Uneven distribution of ions across the
membrane
Ions of Interest: Resting Membrane
Potential
Figure 2.18
Membrane Potentials: The Pressures
Figure 2.18

Membrane (lipid bilayer) is only selectively
permeable to K+, Na+, Cl- (not permeable to A-)
Membrane Potentials: The Pressures
Figure 2.20


Two passive processes- Require NO energy
One active process- Energy consuming
The Movement of Ions:
Passive Processes
1) Diffusion
 Dissolved ions distribute evenly

Ions flow down concentration
gradient

Diffusion of ions:


Channels permeable to specific ions
Concentration gradient across the
membrane
The Movement of Ions:
Passive Processes
2) Electrical (Electrostatic) Processes

Opposite charges
attract

Like charges repel
Cation
Attract
Repel
Anion
The Movement of Ions: Active Processes

Sodium-Potassium
Transporter (also known
as the Na+/K+ pump or
Na+/K+-ATPase)

Active mechanism in the
membrane that extrudes 3
Na+ out and transports 2
K+ in.
Figure 2.20
Channel Proteins (summarized)
How Ions are Transferred Across the Membrane
1. Active
1. Na+/K+Pump
2. LEAK
3. Needs voltage to open
(passive diffusion)
2. Non-Gated
(always open)
3. VoltageGated
(open or closed)
An Action
Potential
Action potentials require a
threshold level of
depolarization to occur
+
+
+
4
Figure 2.17
Action Potential Summary
An Action Potential


Temporal and
sequential
importance of ion
transfer across the
membrane.
Dependent on
voltage-gated
(dependent)
channels
Figure 2.21
Summary: Things to think about

Membrane potentials




Lipid bilayer
Ion types (cations and anions contributing)
Distribution of ions across the membrane
Membrane proteins


Channels
Pumps/transporters:


Passive vs active movement of ions
Action potentials


Threshold
Temporal explanation of ion movement across the
membrane.
Communication Within a Neuron

Conduction of the Action Potential

All-or-None Law – Principle that once the action potential begins, it
proceeds without decrement to the terminal buttons.
Figure 2.23
Communication Within a Neuron

Conduction of the Action Potential

Rate Law – principle that variations in the intensity of a
stimulus or other information being transmitted in an
axon are represented by variations in the rate at which
that axon fires.
Figure 2.24
Communication Within a Neuron
Rate Law
 A single action potential is not the basic element of
information
 Variable information is represented by an axon’s rate of
firing


A high rate of firing causes a strong muscular contraction
Strong stimulus (bright light) casus a high rate of firing in
axons of the eyes
Communication Within a Neuron

Cable Properties – passive conduction of electrical
current, in a decremental fashion, down an axon.
Figure 2.25
Communication Within a
Neuron

Saltatory Conduction – conduction of
action potentials by myelinated axons. The
action potential appears to jump from
one node of Ranvier to the next.
No flow of Na+
Figure 2.26
Factors Influencing Conduction Velocity

Saltatory conduction

High density of Na+ V-D at
Nodes of Ranvier
2 advantages of Saltatory
Conduction
 Economical


Much less Na+ enters cell (only at
nodes of Ranvier) mush less has
to be pumped out.
Speed

Conduction of APs is faster in
myelinated axons because the
transmission between the nodes
is very fast.
Communication Within a Neuron

Multiple sclerosis


Autoimmune degradation of myelin in PNS
Without myelin the spread of + charge is diminished