Nervous System
• Brain, spinal cord, efferent and afferent neurons
• Pattern of information flow:
Receptor Afferent path Integration Efferent Path Effect
Central Nervous
System (CNS)
• Main cell types are neurons and glial cells

Typical Arrangement of
Neural Connections
• Neurons communicate via electrical signaling
• They are excitable
• Structurally the soma (cell body) has an extensive ER and prominent nucleoli
• Long appendages or processes:
• Dendrites (receive info)
• Axons (deliver info); some are covered by myelin
A collection of axons is called a NERVE

Types of glial cells:
CNS = oligodendrocytes, astrocytes, microglia, ependymal cells
PNS = Schwann cells, satellite cells

Myelin acts as an insulator and inhibits ion movement in the axonal membrane that it surrounds.

Neurons as Excitable Tissue
• Excited by altering the resting membrane potential (-90 mV)
• Depolarize
• Hyperpolarize
• Most changes in membrane potential occur through the opening or closing of certain ion channels (they are voltage-gated or gated via second messenger system activation).
Ion
Na+
Cl-
Ca2+
K+
Proteins (-)
Intracellular
(mM)
2
10
10 -8
150
65
Extracellular
(mM)
140
105
2.5
5
2

• Gates can be chemically opened by neurotransmitters
• Gates can be opened via signal transduction mechanisms linked to neurotransmitter binding to receptor
• Gates can be opened by stretch, pressure, etc.

Stimulus = anything that can cause the opening or closing of gated channels in a neuronal membrane
What happens to the resting membrane potential of the membrane adjacent to the site of Na+ entry?
How about here?

The axon hillock (trigger zone) is sensitive to changes in ion concentration and is the site at which an action potential is initiated.
An action potential is a self-propagating depolarization of the axonal membrane that initiates at the hillock and runs to the axon terminus without diminishing in strength .
All action potentials are the same “size”.
What determines whether an action potential will occur or not?

If the graded potential doesn’t change the resting membrane potential enough, the signal from the stimulus will die out and the neuron will not respond with an action potential.
The amount of change in membrane potential necessary to generate an action potential is called a threshold stimulus .

Action Potential = depolarization along the axon
1
2
3

If the trigger area of the axon reaches threshold, the influx of Na+ and the generation of the action potential will be repeated over and over again in one direction, at each segment of membrane, down the axon.

What will happen at this area of membrane?
What will happen at this area of membrane?

One portion of the membrane has just been depolarized and is relatively insensitive to changes in cation concentration. It is said to be refractory to stimulus. Downstream membrane is at resting potential, and can be influenced by cation influx.

Saltatory conduction in myelinated neurons

Action potentials cause the release of neurotransmitter from the presynaptic axon terminus

What will happen to the resting membrane potential if the activation gate is opened?
How could a cell open this activation gate?

Strength of stimulus determines neuronal response

mV
EPSP time mV time
IPSP time

Neurotransmitter activity is stopped by: diffusion away from the synapse, transport into cells
(glial or back into presynaptic neuron), or degradation by specific enzymes.

What is the response in the post-synaptic neuron?

What will determine whether this postsynaptic neuron will respond?

A B Red neuron is releasing serotonin which causes an
IPSP. The neuron is firing at
70 APs/sec
Neuron A is releasing dopamine, causing and EPSP.
The neuron is firing at 40
APs/sec
Neuron B is releasing acetylcholine to create an
EPSP. It is firing at 20
APs/sec.
What will the outcome be in the postsynaptic cell?

Transmitter Molecule
Acetylcholine
Serotonin
5-Hydroxytryptamine (5-HT)
GABA
Glutamate
Aspartate
Glycine
Histamine
Epinephrine
Derived From
Choline
Tryptophan
Glutamate
Histidine
Tyrosine
Site of Synthesis
CNS, parasympathetic nerves
CNS, chromaffin cells of the gut, enteric cells
CNS
CNS
CNS spinal cord hypothalamus adrenal medulla, some CNS cells
Norpinephrine
Tyrosine CNS, sympathetic nerves
Dopamine
Adenosine
Tyrosine
ATP
CNS
CNS, periperal nerves

Neurotrans.
Types of receptors
Mode of action Result in postsynaptic cell
Opens ion channels EPSP Acetylcholine
Serotonin
GABA
Nicotinic
Muscarinic
Two main classes; multiple subclasses
GABA-A
GABA-B
Norepinephrine

Receptor
G-protein coupled receptors; both AC and IP3/DAG
Depends on receptor type
Receptor Cl- channel
G-linked K+ channel IPSP in all cases
G-protein linked to cAMP
IPSP
Dopamine b receptor
G-protein linked to cAMP
EPSP
D1, D2, D3,
D4, and D5
G-protein linked to cAMP, direct channel opening, cAMP to
K+ channel opening
EPSP and IPSP
Target
CNS neurons; skeletal muscle
Platelet aggregation, smooth muscle contraction, satiety, vomiting
Throughout CNS and in retina
Relaxes smooth muscles of gut, bronchial tree, and vessels to skel. muscle
Increases rate and strength of cardiac contraction; excites smooth muscle in vessels
D1-3 are located in the striatum of the
CNS, and the basal ganglia
D3-5 play a role in mood, psychosis and neuroprotection