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