Cell Structure in the Nervous System

Cells & the Brain

• All info comes from sensory receptors

• This information is transformed in the brain into perceptions & commands for movement.

• The complexity of the brain results from the number, not the variety, of cells

• Brain contains ~ 100 billion nerve cells

Types of Cells• 2 classes of cells in nervous system

– Neurons or nerve cells– Glial cells (glia)

• Neurons not initially recognized as single cells because they differ from other cells:– much larger (axon can be 0.1 mm to 2 m)– different in shape– don't touch each other directly– Golgi stain

• Glia surround neurons – from the Greek for ‘glue’

Glial Cells• There are about 1 trillion glial cells in the brain

– 10 - 50 x more than there are neurons

• Glial cells have perform important functions including:– surround & support neurons– separate & insulate groups of neurons– some produce myelin– some are scavengers– maintain concentration of K+ in extracellular space– some take up & remove chemical transmitters– some guide migration of neurons during

development– some help form an impermeable lining in capillaries

& venules, creating the blood-brain barrier

Types of Glial Cells• Astrocytes

– Most numerous in brain– Fill spaces between neurons– Serve to regulate composition of the

extracellular space

• Oligodendroglia– Wrap axons with myelin sheath in brain and

spinal cord (Not all axons are myelinated)– Each oligodendrocyte wraps several axons

• Microglia– Phagocytes - clean debris from dying neurons &

glia

•

More Glial Cells

• Schwann Cells– Wrap axons with myelin sheath outside

the brain and spinal cord– Each Schwann cell wraps only one axon

• Ependymal Cells– Line ventricles– Involved with secretion and absorption

of cerebral spinal fluid– Also play a role in routing embryonic

cells during development 

Neurons• Basic cells of the nervous system• There are about 100 billion

neurons in the brain– more in the spinal cord, peripheral

nervous system and sensory organs • Neurons derive form the neural

tube during development• Neurons signal information

electrically with nerve impulses

The Neuron Doctrine• Golgi stain – A reduced silver method using

silver chromate that stains a few, isolated neurons in their entirety– Invented by Camillo Golgi– Santiago Ramon y Cajal used it to survey the

variety of structures in vertebrate nervous systems

• Led Cajal to propose the neuron doctrine: – nerve cells are the structural and functional basis

of the nervous system and they must communicate with one another

– confirmed by electron microscopy 50 years later – Golgi and Cajal shared a Nobel Prize

The Structure of a Neuron

• 4 morphologically distinct regions:

• Cell Body (soma)• Axon• Dendrites• Presynaptic

Terminals

The Cell Body• Also called soma (somata) - perikaryon • Metabolic center of the cell

– Assembly of new membrane

• Gives rise to dendrites & axon• Contains the nucleus

– Site of DNA in chromosomes

• Contains rough & smooth endoplasmic reticulum– Rough ER: Stacks of flattened membrane

compartments with ribosomes attached– Smooth Endoplasmic Reticulum: connects RER

with Golgi apparatus and serves to further process membrane proteins; serves to sequester and release calcium to control cytoplasmic composition

Other Elements of the Cell Body

• Site of protein synthesis (ribosomes) – Free ribosomes and polyribosomes loose in

cytoplasm• Contains the Golgi apparatus

– Stacks of membrane compartments– Packaging of secretory products

(neurotransmitter peptides)• Mitochondria

– Site of conversion of energy from chemical bonds in food to ATP, the common energy currency of the cell

The Cytoskeleton• Superstructure of the cell – 3 components:• Microtubules

– Largest - 20 nm thick-walled tubes– Spirals of alpha and beta tubulin molecules– Railroad tracks for movement of organelles, etc.

• Neurofilaments – 10 nm twisted cables– Tend to be the most static structures of the three

• Microfilaments – 5 nm double helix of actin– Especially common in neurites along with

microtubules– Also found in thick meshes associated with some

regions of the cell membrane

The Neuronal Membrane

• A continuous sheet covering the neuron

• Separates the cytoplasm from the extra-cellular fluid

• Lipid bilayer with many embedded proteins

• The embedded proteins are channels critical to neuronal function

Dendrites• Neurites which receive signals from other

nerve cells (message IN) • Postsynaptic membrane in dendrites have

molecular receptors for neurotransmitters (chemical messengers)

• May have little bulges or pegs called dendritic spines where incoming synapses connect

• Carry impulses from other neurons or receptors toward the cell body

• Most neurons have several dendrites• Have polyribosomes suggesting some local

protein synthesis in dendrites

The Axon• Main conducting unit of the neuron • Each neuron has only one axon• Axon collaterals – many axons do have

branches• Carries impulses away from the cell body to

other neurons or to effectors (message OUT) – Efferent - axon going away from reference neuron– Afferent - axon coming toward reference neuro

• Axon hillock = the conical region at the beginning of an axon where it joins the cell body

• Axon terminal (terminal bouton) – the end of the axon

Visualizing the Neuron

Myelin• Large axons have an insulating sheath -

myelin • Supports, insulates & nourishes the axon

and helps maintain chemical balance. • This sheath is made up of

oligodendrocytes in the CNS & Schwann cells in the PNS.

• Nodes of Ranvier = Gaps in the sheath which allow passage of the electrical signal through ion channels – Saltatory conduction– Transmission very fast.

Visualizing Myelinated Neurons

Presynaptic Terminals• Neurons don't touch; separated by a space

= synaptic cleft (synapse)– Cell transmitting a signal = presynaptic– Cell receiving = postsynaptic; can be another

neuron or an effector such as a muscle or gland

• Signal within cells is electrical, but between cells is chemical (b/c no direct contact)

• Chemical messengers which transmit the signal = neurotransmitters

• Neurotransmitters are stored in vesicles, released from terminals at the end of the axon into the synapse– Terminal buttons– Motor endplates

Anatomy of a Synapse

Diversity of Neurons

• Neurons can be classified in several ways:

• Based on neurites • Based on dendrites • Based on connections/ function• Based on axon length• Based on neurotransmitter

Number of Neurite Processes

Neurons classed in 3 (4) groups by number of processes:

• Unipolar– the simplest neuron– a single primary process with many branches– no dendrites from the cell body– common in invertebrates

• Bipolar– oval shaped soma– 2 processes: dendrite (info in) & axon (info out)– most sensory neurons are bipolar

Number of Processes (Cont.)

• Pseudounipolar– Sensory cells of touch, pressure, pain are special

type of bipolar– 1st develop as bipolar; the 2 processes fuse to

form a single axon– axon splits at the cell body– one goes to spinal cord, other to periphery (skin,

joints, muscles)• Multipolar

– most common type in vertebrates– one axon and one or more dendrite – vary in size and shape

Visualizing Cell Types

Classification Based on Dendrites

• Pyramidal cells/stellate cells• Spiny cells/aspinous cells

Classification Based on Function

• 3 groups:• Sensory (afferent) neurons

– receive stimuli & transmit them to the central nervous system

• Motor neurons – carry impulses away from the central

nervous system to muscles or glands

• Interneurons – link sensory to motor neurons

Visualizing Cell Types - 2

Classification Based on Neurotransmitter

• Cholinergic neurons – – use acetylcholine

• Gabanergic neurons – – use gamma aminobutyric acid

• Dopaminergic neurons – – use dopamine

How Neurons Carry the Message

• Only neurons are involved in transmission of electrical signals.

• Within a nerve cell, message is an electrical signal = action potential– Cascading membrane depolarization creates

the movement of the action potential as a nerve impulse

– rapid, all or none impulses– in myelinated neurons, gaps at regular

intervals allow regeneration of the action potential

• Between nerve cells the message is carried chemically

Principles of Impulse Transmission

• PRINCIPLE #1:• Information conveyed by an action potential is

determined not by the form of the signal, but by the pathway the signal travels.– action potential for sound looks like action potential for odor

• PRINCIPLE #2:• Principle of Dynamic Polarization:

electrical signals flow in a consistent direction• PRINCIPLE #3: • Principle of Connectional Specificity:

Contact is not random - cells communicate with certain targets and not others

Types of Signals

• Each sensory & motor nerve cell generates 4 types of signals:– an input signal– an integration signal (trigger)– a conducting signal– an output signal

Functional Regions

• Almost all neurons have 4 corresponding functional regions:– receptive - local input– integrative - trigger– conductile - signaling– secretory - output