Neuronal Anatomy and Communication

Cells of the Nervous System: Neurons

Three types of neurons:Sensory neuronsMotor neurons Interneurons

Neuronal structure

Soma Dendrites Axon Terminal

buttons Synaptic cleft

Neuronal classifications

Bipolar neuron (a)

Unipolar neuron (b)

Multipolar neuron

Internal structure Cell membrane Cytoplasm Mitochondria Nucleus

ChromosomesProteins

Microtubules

Cells of the Nervous System: Glia

Glial cells support neural function

Glial Cells Astrocytes

Arms wrap around blood vessels

and neuronal structures Isolate the synaptic cleftMaintain chemical

composition

of extracellular spaceClean up following cell

death

Glial cells

Oligodendrocytes (CNS) & Schwann cells (PNS)Provide support and insulation

in the form of the myelin sheath

Myelin Nodes of Ranvier

Glial cells

MicrogliaSmallest glial cellsBrain’s immune cells

Blood-Brain Barrier

Composed of tightly-packed cells of the cerebral blood vessels.

Regulates chemicals in the CNS Protects the brain from toxins Semipermeable

The withdrawal reflex: an example of neuronal communication

1. Sensory neuron detects

2. Message is sent

3. Neurotransmitter is released

4. Interneuron

5. Motor neuron sends a message

Communication within a neuron

Based on changes in the membrane potential of the neuron.

Neurons have two basic potentials

Resting membrane potential

The inside of a neuron is negatively charged relative to the outside

Due to concentrations of positively and negatively charged ions in the brainOutside the cell Inside the cell

Resting membrane potential A concentration gradient pulls the sodium,

potassium and chloride ions toward the membrane; electrostatic forces prevent them from crossing it.

The balance between potassium and sodium ions in and out of the neuron is maintained

Membrane potential

The membrane potential can change: Depolarization Hyperpolarization

-80

-70

-60

-50

-40

Depolarization

Hyperpolarization

Resting Membrane Potential

Action potential

A hyperpolarization _________ communication within a cell.

A depolarization _________ the cell, and _________ the chances of communication within the cell.

Threshold of excitation

Action potential

A massive, momentary reversal of the membrane potential.

Carried down the axon from the cell body to the terminal buttons.

Results in the release of a chemical message into the synapse.

Action potential

Chemical messages from other neurons affect the neuron’s charge.

Excitatory Post-Synaptic Potentials (EPSPs) Inhibitory Post-Synaptic Potentials (IPSPs)

When the cell is depolarized to -65mV, an action potential begins.

Steps of the action potential1. Ion channels in the

membrane rapidly open and Na+ enters the cell (-65mV +40mV)

2. As Na+ rushes in, K+ is forced out of the cell.

3. As the action potential peaks, Na+ channels close, and no more Na+ enters the cell.

4. K+ is forced out of the cell, which decreases the charge inside the cell and K+ channels close.

5. K+ ions trapped outside of the cell result in a temporary hyperpolarized membrane potential.

6. Ion channels reset and the Na+/K+ pump returns the ions to the normal gradients.

All-or-None law

An action potential either occurs or it doesn’t.Magnitude is the same.Does not diminish in strength.

Rate law The strength of a response depends on the

firing rate of the cell. More action potentials/second = strong

response, fewer = weak response.

Action potential conduction Action potentials depend on sodium influx

from the extracellular fluid.Nodes of Ranvier.

Saltatory conduction

Communication between neurons Within-neuron communication: electrical

signal Between-neuron communication: chemical

signal Synaptic transmission

Synaptic structure Presynaptic membrane

Terminal buttonVesiclesTransporter molecules

Synaptic cleft Postsynaptic membrane

On the dendrite, soma or axonReceptors

Neurotransmitter binding Binding sites Ligands

Molecule that fits into a specific binding site Endogenous ligands Exogenous ligands

Synaptic firing

1. Initiated by an action potential in the cell

2. Neurotransmitter (NT) binds to the receptor

Prompting specific ion channels to open

Types of receptors Ionotropic receptor

Neurotransmitter

ReceptorG-protein

Enzyme

Ion Channel

GateJoins the

ion channel

Second messenger

Metabotropic receptor

Synaptic firing3. Postsynaptic potentials are produced by the

flow of ions in and out of the cell. Each NT produces a specific postsynaptic

potential Excitatory NTs Inhibitory NTs

Synaptic firing

4. Neural integration is the summation of all postsynaptic potentials.

Determines the response to PSPs.

Synaptic firing Remember – Each neuron has synaptic

connections with hundreds of other neurons, and must summate all incoming PSPs thousands of times each second!

Synaptic firing

5. Removal of NT from the synapse terminates PSPs

Reuptake Enzymatic deactivation

Autoreceptors

Found on the presynaptic membrane

Types of synapses Axodendritic Axosomatic Axoaxonic