Chapter 12- 2 Neural tissue

Signal transduction Neurons use gated channels to transmit signals. Activation of these channels cause local changes in the membrane potential.

Types of potential changes Graded (local) potentials all cell membranes can generate act locally strength is related to the strength of the signal get weaker the further they get from the original signal Can be excitatory or inhibitory Action potentials only on excitable membranes are self propagating exhibit “all or none” behavior

Graded potential A change in potential that decreases with distance and varies in strength based on the strength of the stimulus Result in a localized can be depolarization or hyperpolarization Depolarization - a shift in the transmembrane potential toward 0 (becomes less negative) Hyperpolarization - a shift in the transmembrane potential away from 0 (becomes more negative) Repolarization - after a cell depolarizes a shift back towards its typical resting potential

Depolarization and Hyperpolarization opening Na+ channels would depolarize the cell opening of K+ channels would repolarize or hyperpolarize the cell opening of Cl- channels would hyperpolarize the cell Figure 11.9

Depolarization waves Figure 11.10

Local Potentials Graded Magnitude varies from small to large depending on stimulus strength or frequency Can summate or add onto each other Spread (are conducted) over the plasma membrane in a decremental fashion: rapidly decrease in magnitude as they spread over the surface of the plasma membrane. Can cause generation of action potentials Seeley, Stephens and Tate

Action Potential Appears when region of excitable membrane depolarizes to threshold. In neurons, it is a property of the axon and normally begins at the axon hillock. Steps involved 1. Sodium channel activation and depolarization of the membrane 2. Sodium channel inactivation 3. Potassium channel activation and membrane repolarization 4. Return to normal permeability

Generation of an Action Potential Figure 12.16.1

Steps of an action potential

Generation of an Action Potential

Characteristics of action potentials Generation of action potential follows all-or-none principle Refractory period lasts from time action potential begins until normal resting potential returns. Absolute refractory period - the neuron cannot re-fire. Na+ Channels are either open or inactivated Relative refractory period - when Na+ channels closed and can become reactivated. K+ channels are still open so a new stimulus must cause more Na+ entry than K+ exit.

Refractory periods

Characteristics of action potentials Continuous propagation spread of action potential across entire membrane in series of small steps Saltatory propagation action potential spreads from node to node, skipping internodal membrane

Continuous Propagation Propagation along an Unmyelinated Axon

Saltatory Propagation Propagation along a Myelinated Axon

Synapse Site of intercellular communication Neurotransmitters released from synaptic knob of presynaptic neuron to a postsynaptic neuron or other effector. Neuromuscular junction Neuroglandular junction Presynaptic membrane Postsynaptic membrane Synaptic cleft

Electrical synapses Rare Pre- and postsynaptic cells are bound by interlocking membrane proteins that allow ions to pass between them (Gap junctions and connexons)

Chemical synapses Common Release a chemical neurotransmitter between the pre- synaptic and post-synaptic membranes Excitatory neurotransmitters cause depolarization and promote action potential generation Inhibitory neurotransmitters cause hyperpolarization and suppress action potentials. Depends on the receptors and ion channels present on the post-synaptic membrane

Structure of a Typical Synapse

Neurotransmitter Functions Direct effects (ion channels) Figure 11.22a

Neurotransmitter Functions Indirect effects via G proteins

Some neurotransmitters Cholinergic synapses very common release Acetylcholine (ACh) Are always excitatory, work by opening Na+ channels Adrenergic synapses release norepinephrine (NE), depolarizing effects, released in the sympathetic NS Work via G proteins and cAMP

Some neurotransmitters Other important neurotransmitters Dopamine - released in the CNS and in some peripheral nerves. Can be excitatory or inhibitory, causes artery vasodialation, alters mood Serotonin - released in the CNS, low levels lead to depression GABA (gamma aminobutyric acid) (an amino acid) in CNS, is generally in inhibitory neurons, opens Cl- channels

Cholinergic transmission

Cholinergic synapses Synaptic delay occurs as calcium influx and neurotransmitter release take appreciable amounts of time ACh broken down Choline reabsorbed by presynaptic neurons and recycled Synaptic fatigue occurs when stores of ACh are exhausted

Removal of neurotransmitter Seeley, Stephens and Tate

Neuromodulators Are neurotransmitters or other substances that generally influence pre-synaptic or post-synaptic cells response to other neurotransmitter They can be peptides (opiods), gases (CO2, NO), lipids (arachadonic acid), purines (eg ATP, adenosine) They can alter membrane potential or cell function directly (alter ion channels) or indirect effect (activate second messengers) Can exert influence via lipid-soluble gases