Nervous System Part

Home , Excitatory synapse, Graded potential, Postsynaptic potential

SYNAPSES AND NEURAL INTEGRATION The Synapse

 Syn = “to clasp or join”  Junction between  Adjacent neurons  Neuron and an effector cell  Mediates information transfer  Electrical  Chemical

The Synapse

 Presynaptic neuron  Conducts impulses toward the synapse  Postsynaptic neuron  Transmits impulses away from the synapse

Axodendritic synapses Dendrites Axosomatic synapses Cell body Axoaxonic synapses

(a) Axon

Axosomatic synapses

Cell body (soma) of postsynaptic neuron (b)

 Electrical Synapses  Less common than chemical synapses  Neurons are electrically coupled (joined by gap junctions)  Communication is very rapid  May be unidirectional or bidirectional  Important in  Embryonic tissue  Some brain regions  Synchronizing groups of neurons

The Synapse

 Chemical Synapses  Release of neurotransmitter from presynaptic neuron  Unidirectional  Here’s how it works…

Signal transmission at the chemical synapse

 Binding of NT to postsynaptic membrane  Excitatory Postsynaptic Potentials (EPSP)  Inhibitory Postsynaptic Potentials (IPSP)

The Synapse

 Binding of NT to postsynaptic membrane  Excitatory Postsynaptic Potentials (EPSP)  Depolarizes postsynaptic cell membrane  Excitatory neurotransmitters  Helps trigger AP

An EPSP is a local depolarization of the postsynaptic membrane that brings the neuron closer to AP threshold. Neurotransmitter binding opens chemically gated Threshold ion channels, allowing the simultaneous pas- sage of Na+ and K+.

Membrane potential Membrane(mV) potential Stimulus

Time (ms) (a) Excitatory postsynaptic potential (EPSP)

 Binding of NT to postsynaptic membrane  Inhibitory Postsynaptic Potentials (IPSP)  Hyperpolarizes postsynaptic cell membrane  Inhibitory neurotransmitters  Decreases chance of AP

An IPSP is a local hyperpolarization of the postsynaptic membrane and drives the neuron away from AP threshold. Neurotransmitter binding opens K+ or Cl– channels. Threshold

Membrane potential Membrane(mV) potential Stimulus

Time (ms) (b) Inhibitory postsynaptic potential (IPSP)

 Excitatory  Acetylcholine  Norepinephrine  Glutamate  Inhibitory  Serotonin  GABA  Glycine Thought Question

Strychnine is a pesticide that is used against small vertebrates. This chemical is an antagonist to glycine, what symptoms might an animal or human experience if they ingest this substance? The Synapse

 Inactivation of NT’s 1. Diffusion 2. Reuptake 3. Degradation (enzymatic inactivation)  Examples  Cholinesterase & ACh  Monoamine Oxidase & Norepinephrine Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron

Presynaptic Postsynaptic neuron neuron

1 Action potential arrives at axon terminal. 2 Voltage-gated Ca2+ channels open and Ca2+ Mitochondrion enters the axon terminal. Ca2+ Ca2+ Ca2+ Ca2+ 3 Ca2+ entry causes Synaptic neurotransmitter- cleft containing synaptic Axon terminal Synaptic vesicles to release their vesicles contents by exocytosis. 4 Neurotransmitter diffuses across the synaptic cleft and binds to specific Postsynaptic receptors on the neuron postsynaptic membrane.

Ion movement Enzymatic Graded potential degradation Reuptake

Diffusion away from synapse

5 Binding of neurotransmitter opens ion channels, resulting in graded potentials. 6 Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse.

 Pharmacology  Anticholinesterase neurotoxins  Causes excitotoxicity (overstimulation)  Example: Organophosphates, Nerve Gas  Local Anesthetics  Block Na+ channels  Inhibitory  Example: Lidocaine Question From Monday

 Neuroplasticity  Idea that the brain is not hardwired. Following injury or changes in use, the brain is able to changes its neural circuits. Neurons can adapt and change their functions dependent on need.  Synaptic plasticity  Changes in the use of a neuronal connection can lead to changes at the synapse.  Increased/decreased amount of calcium released at the axon terminal (this changes the amount of NT released)  Altering the number of receptors on the postsynaptic cell membrane (happens by triggering a second messenger system which affects gene transcription for the receptors)

Neural Integration

 Summation  A single EPSP cannot induce an action potential  EPSP’s can summate to reach threshold  IPSP’s can also summate with EPSP’s  Cancel each other out E1 E1

Threshold of axon of postsynaptic neuron Resting potential

E1 E1 E1 E1 Time Time (a) No summation: (b) Temporal summation: 2 stimuli separated in time 2 excitatory stimuli close cause EPSPs that do not in time cause EPSPs add together. that add together.

Excitatory synapse 1 (E1)

Excitatory synapse 2 (E2)

Inhibitory synapse (I1)

Figure 11.19a, b E1 E1

E2 I1

E1 + E2 I1 E1 + I1

Time Time (c) Spatial summation: (d) Spatial summation of 2 simultaneous stimuli at EPSPs and IPSPs: different locations cause Changes in membane EPSPs that add together. potential can cancel each other out.

Figure 11.19c, d Summation

 Types  Temporal summation  One or more presynaptic neurons transmit impulses in rapid-fire order  Spatial summation  Postsynaptic neuron is stimulated by a large number of terminals at the same time Neuronal Circuits

 Features  Allow for a wide variety of neuronal interaction  May consist of thousands of neurons  Often include excitatory and inhibitory neurons

Presynaptic (input) fiber

Facilitated zone Discharge zone Facilitated zone

 Diverging circuit  One incoming fiber stimulates an ever-increasing number of fibers  May affect a single pathway or several  Common in both sensory and motor systems Figure 11.22a Figure 11.22b Neuronal Circuits

 Converging circuit  Opposite of diverging circuits  Results in either strong stimulation or inhibition  Also common in sensory and motor systems Figure 11.22c, d Reciprocal Inhibition

 Found in Chapter 13  We’ll come back to this when we cover reflexes

Questions?

 Don’t forget homework for lab, due at the beginning…  Lab Exercise 17 pg.’s 265 (all) and 265 #9 & 10 only (top)  PreLab 1 Activity

 Complete Nerves 9  Neuron Worksheet #1-11