Topic 22: COMMUNICATION BETWEEN CELLS - NEURONS & NERVES (lecture 34) OBJECTIVES: 1. Be able to describe the relative concentrations of Na +, K+, Cl- and organic anions between the inside and the outside of neuronal membranes. 2. What is the membrane potential and how is it maintained? 3. Be able to describe an action potential and the role of voltage gated ion channels in this process. 4. Know how an action potential is propagated in unmyelinated and myelinated axons. 5. Understand how acetyl choline is released and its impact on the post-synaptic cell. 6. Understand the electrical effects of an excitatory vs. an inhibitory neurotransmitter. Communication by hormone is slow due to the fact that the hormone must be transported, it must interact with a receptor on the target cell and then the biological effect takes place. Most complex organisms have nervous systems which transport information very rapidly. Neuron- the fundamental cellular component of the nervous system (fig. 48.2); each neuron consists of dendrite, soma (cell body), axon hillock, long axon, terminal branches and axon termini. Neurons function by carrying waves of electrical excitation; this excitation is typically unidirectional; soma synaptic termini. (note: neurons are usually bundled together to form nerves) There are a number of types of neurons/nerves 1. sensory neurons- carry information away from receptors (receptors = structures that detect changes in the internal/external environment; ca., light, sound, heat, blood pressure) 2. motor neurons- carry excitation to skeletal muscle 3. interneurons- connect neurons to each other 4. autonomic neurons- are involved in control of the functioning of internal organs (two kinds- sympathetic & parasympathetic) Fig. 48.3- gives an example of a neural pathway indicating importance of speed of communication. Voltage- electrical term which defines the extent to which there is a difference in charge between two places; actually referred to as potential difference. In the cases of cells there is a surplus of negative charges on the inside of the cell relative to the outside. This potential difference is measured as a very small voltage (see fig. 48.6) and is known as the membrane potential ( membrane potential = the voltage measured across the plasma membrane of a cell). 1 Virtually every cell has a small membrane potential. However, excitable cells like neurons and muscle cells (fibers) have a relatively large membrane potential and this membrane potential changes during excitation. What makes neurons negatively charged on the inside? Fig. 48.7. There is an asymmetry of ion distribution between the inside and the outside of the cellmore K+ in than out; more Na+ out than in; more Cl- out than in; inside there are also negatively charged (anions) organic molecules which are not present on the outside. 1. membrane is more permeable to K+ than Na+ 2. K+ flows out down its concentration gradient 3. As it flows out, the inside becomes negatively charged because of anions left behind 4. The Na+-K+ ATPase (pump) maintains this ion asymmetry by pumping K+ back in and Na+ out Neurons are excitable cells because the permeability of their membranes to inorganic ions can change and these changes may profoundly impact the membrane potential. Membranes contain protein/protein-complexes known as ion channels that allow inorganic ions to pass through the membrane. There are three (3) basic kinds: 1. passive channel- always open to ion movement 2. electrically-gated - permeability is controlled by the membrane potential of the cell 3. chemically-gated - permeability is controlled by small molecular weight signaling molecules known as neurotransmitters Most ion channels are highly selective for the ion that each transports; thus, there are potassium, sodium, chloride, calcium etc channels. Also, for each ion there may be passive, voltage-gated & chemically-gated channels ( ca., voltage-gated K+channel) Excitation (fig. 48.8)- a neuron receives some kind of stimulus (chemical, electrical, mechanical) usually in the dendritic region or soma. This causes the membrane potential to become less negative (called a depolarization). If this depolarization reaches a certain critical level called threshold, rapid changes take place in the membrane known as an action potential. Action potential- a transient reversal of the membrane potential (inside becomes more positive than outside) that is transferred down the length of the neuron. Fig. 48.9- resting state; voltage-gated ion channels closed 1. rising (depolarizing) phase- once threshold has been reached, voltage-gated Na+ channels open, Na+ flows in and cell depolarizes. 2 2. repolarizing phase- voltage-gated Na+ channels close, voltage-gated K+ channels open, K+ flows out and cell becomes more negative (membrane potential moves towards original state) 3. undershoot- voltage-gated K+ channels still open so membrane potential is more negative than normal (it is hyperpolarized) 4. duration of action potential for neurons is very short (3-10 msec, 3-10 x 10-3 sec!) Propagated action potential- (fig. 48.10); action potential literally travels from one region of the membrane to adjacent regions. In vertebrates most axons are covered with lipid insulation (myelin) with gaps of exposed membrane (fig. 48.11). Action potentials only take place in the region of exposed membrane. This conduction of action potentials is known as saltatory conduction. The velocity of conduction of of action potentials varies with the diameter of the axon and whether it is myelinated or not. Conduction velocity (m/sec) in selected neurons : Squid giant axon – 25 Large motor axon to leg muscle in a mammal- 120 Synaptic transmission. Once the action potential reaches the synaptic terminal, adjacent cells (other neurons, muscle cells, endocrine cells etc) are impacted by the process of synaptic transmission (synapse = the junction between a neuron and another cell); two fundamental kinds of synaptic transmission: 1. electrical – neuron (pre-synaptic cell) is in direct contact with the post-synaptic cell; depolarization of action potential literally spreads to the post-synaptic cell; not very common 2. chemical- the neuron releases a neurotransmitter which diffuses across the space between the two cells (synaptic cleft) and causes some change in the behavior of the post-synaptic cell. Fig. 48.12 Table 48.1- neurotransmitters (NT)- acetyl choline, biogenic amines, amino acids and neuropeptides Excitatory NT’s produce depolarizations of the post-synaptic membrane (move membrane potential closer to threshold) while inhibitory NT’s typically produce hyperpolarizations of the post-synaptic membrane (move membrane potential further away from threshold). 3 Chemical synapses are targets of a variety of drugs, poisons and pharmacological agents. 4