Receptor Afferent path Integration Efferent Path Effect 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 is 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). Ion Intracellular (mM) Extracellular (mM) Na+ Cl- Ca2+ K+ Proteins (-)

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

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 neuron membrane What happens to the resting membrane potential of the membrane adjacent to the site of Na+ entry? How about here?

What determines whether an action potential will occur or not? 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. 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 3 2

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?

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

Strength of stimulus determines neuronal response

EPSP mV time time mV time IPSP

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?

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

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