Chapter 3 Synapses

1. Chapter 3Synapses

2. The Concept of the Synapse • Neurons communicate by transmitting chemicals at junctions called “synapses” • In 1906, Charles Scott Sherrington coined the term synapse to describe the specialized gap that existed between neurons. • Sherrington conducted his research investigating how neurons communicate with each other by studying reflexes (automatic muscular responses to stimuli).

4. The Concept of the Synapse • Sherrington observed three important points about reflexes: • Reflexes are slower than conduction along an axon. • Several weak stimuli presented at slightly different times or slightly different locations produces a stronger reflex than a single stimulus does. • As one set of muscles relaxes, another set becomes excited.

5. The Concept of the Synapse • Sherrington observed a difference in the speed of conduction in a reflex arc from previously measured action potentials. • He believed the difference must be accounted for by the time it took for communication between neurons. • Evidence validated the idea of the synapse.

7. The Concept of the Synapse • Sherrington observed that repeated stimuli over a short period of time produced a stronger response. • Led to the idea of temporal summation or that repeated stimuli can have a cumulative effect and can produce a nerve impulse when a single stimuli is too weak.

8. The Concept of the Synapse • Sherrington also noticed that several small stimuli on a similar location produced a reflex when a single stimuli did not. • This led to the idea of spatial summation or that synaptic input from several locations can have a cumulative effect and trigger a nerve impulse.

11. The Concept of the Synapse • Presynaptic neuron – the neuron that delivers the synaptic transmission • Postsynaptic neuron – the neuron that receives the message • Excitatory postsynaptic potential (EPSP) is a graded potential that decays over time and space. • The cumulative effect of EPSPs are the basis for temporal and spatial summation.

12. The Concept of the Synapse • Sherrington also noticed that during the reflex that occurred, the foot of a dog that was pinched retracted while the other three feet were extended. • He suggested that an interneuron in the spinal cord sent an excitatory message to the flexor muscles of one leg and an inhibitory message was sent to the other three legs.

14. The Concept of the Synapse • This led to the idea of inhibitory postsynaptic potential or the temporary hyperpolarization of a membrane. • An ISPS occurs when synaptic input selectively opens the gates for positively charged potassium ions to leave the cell or negatively charged chloride ions to enter the cells. • Serves as an active “brake”, that suppresses excitation.

16. The Concept of the Synapse • Neurons can have thousands of synapses. • Both temporal and spatial summation can occur within a neuron. • The likelihood of an action potential depends upon the ratio of IPSPs to EPSPs at a given moment.

18. The Concept of the Synapse • The spontaneous firing rate refers to the periodic production of action potentials despite synaptic input. • EPSPs increase the number of action potentials above the spontaneous firing rate. • IPSPs decrease the number of action potentials below the spontaneous firing rate.

19. Chemical Events at the Synapse • German physiologist Otto Loewi was the first to convincingly demonstrate that communication across the synapse occurs via chemical means. • Neurotransmitters are chemicals that travel across the synapse and allow communication between neurons.

21. Chemical Events at the Synapse • The major sequence of events allowing communication between neurons across the synapse: • The neuron synthesizes chemicals that serve as neurotransmitters. • Neurons store neurotransmitters in axon terminals or transport them there. • An action potential triggers the release of neurotransmitters into the synaptic cleft.

22. Chemical Events at the Synapse (cont.) • The neurotransmitters travel across the cleft and attach to receptors on the postsynaptic neuron. • The neurotransmitters separate from the receptors. • The neurotransmitters are taken back into the presynaptic neuron, diffuse away, or are inactivated by chemicals. • The postsynaptic cell may send negative feedback to slow the release of further neurotransmitters.

24. Chemical Events at the Synapse • Major categories of neurotransmitters include the following: • Amino acids. • Neuropeptides. • Acetylcholine. • Monoamines. • Purines. • Gases.

25. Chemical Events at the Synapse • Neurons synthesize neurotransmitters and other chemicals from substances provided by the diet. • Acetylcholine synthesized from choline found in milk, eggs, and nuts. • Tryptophan serves as a precursor for serotonin. • Catecholimines contain a catechol group and an amine group. (epinephrine, norepinephrine and dopamine)

27. Chemical Events at the Synapse • Smaller neurotransmitters are synthesized in the presynaptic terminal and held there for release. • Example: acetylcholine • Larger neurotransmitters are synthesized in the cell body and transported down the axon. • Example: neuropeptides

29. Neurotransmitters Amino Acids: glutamate, GABA, glycine, aspartate, maybe others A Modified Amino Acid: acetylcholine Monoamines (also modified from amino acids): indoleamines: serotonin catecholamines: dopamine, norepinephrine, epinephrine Peptides (chains of amino acids): endorphins, substance P, neuropeptide Y, many others Purines: ATP, adenosine, maybe others Gases: NO (nitric oxide), maybe others

30. Chemical Events at the Synapse • Vesicles are tiny spherical packets located in the presynaptic terminal where neurotransmitters are held for release. • MAO (monoamine oxidase) is a chemical that breaks down excess levels of some neurotransmitters • Exocytosis refers to the excretion of the neurotransmitter from the presynaptic terminal into the synaptic cleft. • Triggered by an action potential arriving fro the axon.

31. Chemical Events at the Synapse • Transmission across the synaptic cleft by a neurotransmitter takes fewer than .01 microseconds. • Most individual neurons release at least two or more different kinds of neurotransmitters. • Neurons may also respond to more types of neurotransmitters than they release.

33. Chemical Events at the Synapse • Proteins tether neurons together and guide neurotransmitter molecules to receptors. • An ionotropic effect refers to when a neurotransmitter attaches to receptors and immediately opens ion channels. • Most effects occur very quickly and are very short lasting. • Most ionotropic effects rely on glutamate or GABA.

34. Chemical Events at the Synapse • Metabotropic effects occurwhen neurotransmitters attach to a receptor and initiates a sequence of slower and longer lasting metabolic reactions. • Metabotropic events include such behaviors as hunger, fear, thirst, or anger. • When neurotransmitters attach to a metabotropic receptor, it bends the rest of the protein . • Bending allows a portion of the protein inside the neuron to react with other molecules.

36. Chemical Events at the Synapse • The portion inside the neuron activates a G-protein –one that is coupled to guanosine triphosphate (GTP), an energy storing molecule. • G-protein increases the concentration of a “second-messenger”. • The second messenger communicates to areas within the cell. • May open or close ion channels, alter production of activating proteins, or activate chromosomes.

38. Chemical Events at the Synapse • Metabotropic effects utilize a number of different neurotransmitters • Neuropeptides are often called neuromodulators • Release requires repeated stimulation • Released peptides trigger other neurons to release same neuropeptide • Diffuse widely and affect many neurons via metabotropic receptors

39. Chemical Events at the Synapse • A hormone is a chemical secreted by a gland or other cells that is transported to other organs by the blood where it alters activity. • Endocrine glands are responsible for the production of hormones. • Hormones are important for triggering long-lasting changes in multiple parts of the body.

40. Chemical Events at the Synapse • Protein hormones and peptide hormones are composed of chains of amino acids and attach to membrane receptors where they activate second messenger systems. • Hormones secreted by the brain can control the release of other hormones.

42. Chemical Events at the Synapse • The pituitary gland is attached to the hypothalamus and consists of two distinct glands that each release a different set of hormones: • Anterior pituitary- composed of glandular tissue and synthesizes six hormones. • Posterior pituitary- composed of neural tissue and can be considered an extension of the hypothalamus

44. Chemical Events at the Synapse • Neurons in the hypothalamus synthesize the hormones oxytocin and vasopressin which migrate down axons to the posterior pituitary. • Also known as antidiuretic hormones • The hypothalamus secretes releasing hormones. • flow through the blood and stimulate the anterior pituitary to release a number of other hormones.

46. Chemical Events at the Synapse • The hypothalamus maintains a fairly constant circulating level of hormones through a negative-feedback system. • Example : TSH- releasing hormone

48. Chemical Events at the Synapse • Neurotransmitters released into the synapse do not remain and are subject to either inactivation or reuptake. • Reuptake refers to when the presynaptic neuron take sup most of the neurotransmitter molecules intact and reuses the. • Transporters are special membrane proteins that facilitate reuptake. • Example: Serotonin is taken back up into the presynaptic terminal.

49. Chemical Events at the Synapse • Examples of inactivation and reuptake include: • Acetylcholine is broken down by acetylcholinesterase into acetate and choline. • Some serotonin and catecholamine molecules are converted into inactive chemicals: • COMT and MAO are enzymes that convert catecholamine transmitters into inactive chemicals.

50. Chemical Events at the Synapse • Negative feedback in the brain is accomplished in two ways: • Autoreceptors are receptors that detect the amount of transmitter released an inhibit further synthesis and release. • Post synaptic neurons respond to stimulation by releasing chemicals that travel back to the presynaptic terminal where the inhibit further release