Chapter 2 Structure and functions of cells of the nervous system.
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Transcript of Chapter 2 Structure and functions of cells of the nervous system.
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 BodyInside the Cell BodyFrom DNA (nucleus) to protein synthesis (cytoplasm)
• Transcriptional and translational processes take place in the cell body
Genetic Code and Genetic Code and Genetic ExpressionGenetic Expression
Mechanism of gene expression1. Strand of DNA unravels2. Messenger RNA (mRNA)
synthesized from DNA3. 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 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
Figure 2.20
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
AstrocytesAstrocytes and the and the Blood-Brain-BarrierBlood-Brain-Barrier
‘Selectively permeable’
Some substance can pass through the BBB BBB is not uniform
Area postrema (medulla)
Figure 2.12 Figure 2.12 Normal
Compromised
Glial CellsOligodendrocytes
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: Glial Cells: OligodendrocytesOligodendrocytes
Figure 2.10 Figure 2.10
MyelinMyelin80% lipid20% protein
Nodes of RanvierNodes of Ranvier1-2 μm
Glial Cells: Glial Cells: Schwann CellsSchwann Cells
Figure 2.11 Figure 2.11
Peripheral cellsLocated in the
PNS
Can aid in the removal of dead or dying neuronsCan then guide
axonal axonal sprouting sprouting
CNS: axonal sprouts are hindered by glial scars (gliosisgliosis)
Glial Cells: Glial Cells: MicrogliaMicroglia
10-20% of glial cells are microglia
Cells originate in the periphery
PhagocytosisPhagocytosis-- breakdown dying neurons, protect from invading microorganisms
Primarily gray matter
Hippocampus, olfactory telencephalon, basal ganglia, substantia nigra
Phagocytosis
Reactive microglia present in aging rats Stress also shown to activate microglia
6 month6 month 24 month24 month
Lucin and Wyss-Coray (2009)
Cagnin et al. (2001)The Lancet
[11[11C]C]--PK11195:PK11195: Peripheral BZP
binding site present on activated microglia
AD:AD: entorhinal,
temporoparietal, and cingulate cortex
The Cell MembraneThe Cell Membrane
Lipid bilayer Selectively
permeable to very few ions
Proteins embedded in the bilayer
Channel proteins Selective for ion
type Receptor
proteins Signalling devices
Neuronal Charge: Simple DesignNeuronal Charge: Simple Design
Measuring membrane voltage
Requires: ONE recording
electrode inside the cell (intracellular)
ONE recording electrode outside the cell (extracellular)
Figure 2.15 Figure 2.15
The Ionic Basis of the Resting Membrane PotentialResting Membrane Potential
Membrane potentialMembrane potential: The voltage across the neuronal membrane at any given time.
Resting Membrane Resting Membrane Potential: 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 PotentialsResting Membrane Potentials
The RMP is entirely dependent upon The types of ions Where they are found
(distribution across the membrane)
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.
65 mV
IONS OF INTERESTIONS 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:Ions of Interest: Resting Membrane Potential
Figure 2.18 Figure 2.18
Membrane Potentials: Membrane Potentials: The Pressures
Figure 2.18 Figure 2.18
Membrane (lipid bilayer) is only selectively permeable to K+, Na+, Cl- (not permeable to A-)
Membrane Potentials: Membrane Potentials: The Pressures
Two passive processes- Two passive processes- Require NO energy
One active process- One active process- Energy consuming
Figure 2.20 Figure 2.20
The Movement of Ions: The Movement of Ions: Passive ProcessesPassive 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: The Movement of Ions: Passive ProcessesPassive Processes
2) Electrical (Electrostatic) ProcessesElectrical (Electrostatic) Processes Opposite charges
attract
Like charges repel Cation Anion
The Movement of Ions: The Movement of Ions: Active ProcessesActive 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)Channel Proteins (summarized) How Ions are Transferred Across the Membrane
1. Na+/K+-Pump
2. Non-Gated
(always open)
3. Voltage-Gated (open or closed)
3. Needs voltage to open (passive diffusion)
2. LEAK1. Active
An Action An Action PotentialPotential
Action potentials require a threshold
level of depolarization to occur
Figure 2.17
++
+
4
Action Potential SummaryAction Potential Summary
An Action PotentialAn 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 aboutSummary: Things to think about
Membrane potentialsMembrane 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 potentialsAction 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 NeuronRate 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.
Figure 2.26
No flow of Na+
Factors Influencing Conduction Factors Influencing Conduction VelocityVelocity
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