• Part Fundamentals of Physiology• Part II Food, Energy, and Temperature• Part III Integrating systems• Part IV Movement and Muscle• Part V Oxygen, Carbon dioxide, and Internal

Transport• Part VI Water, Salts and Excretion

Part III Integrating System• Chp 11 Neurons• Chp 12 Synapses• Chp 13 Sensory processes• Chp 14 Nervous system organization and

biological clocks• Chp 15 Endocrine and Neuroendocrine

Physiology• Chp 16 Reproduction• Chp 17 Integrating Systems at Work: Animal

Navigation

Chp 12Synapse

Integrating System

http://trc.ucdavis.edu/biosci10v/bis10v/week10/08nervevolution.html

Vertebrate nervous system

Synapses: electrical or chemical

Electrical synapses

• Signals are transmitted instantly

• Can be transmitted both ways

• Found in simpler animals: squid, crayfish

Chemical synapse: characteristics• Synapses have a discontinuity• Complex series of events• Post-synaptic response can be

modulated• Can be excitatory or

inhibitory• Can amplify signal• Only one way• Are plastic (can be modified)• Various processes:

– Ionotropic– metabotropic

Synaptic excitability• If Na+ channels open: membrane potential less negative excitation EPSP• If K+ channels open: membrane potential more negative inhibition IPSP• Signals can add up in time (temporal summation)• Or in space (spatial summation)• Synapses are present on the dendrites (axodendritic) or on the soma

(axosomatic)

Synaptic events• AP reached axon terminal• Voltage gated Ca++ channels open Ca++

rush in• They activate enzymes which promote

vesicle fusion and opening at the cell membrane neurotransmitter empties into the synapse

• The neurotransmitter binds to the receptors located on the post synaptic neuron

• The channels open (most often Na+) triggering a less negative voltage

• In case of a neuromuscular junction, the muscle fiber depolarizes

• Neurotransmitter, still present in the synapse, must be degrades quick if the synapse s to be responsive neurotransmitter is degraded or taken back (reuptake)

AP - EPSP - IPSP

• The neurotransmitter is broken down and recycled

• The pre-synaptic neuron releases vesicles full of neurotransmitter

Neurotransmitters

• Small-molecule neurotransmitters– Cholinergic– Noradrenergic

• Neuroactive peptides

• Dale’s law: a differentiated neuron releases only one kind of neurotransmitter

Neurotransmitter’s characteristics• 1- present at the pre-synaptic

terminal (along with the synthetic machinery)

• 2- released in the synapse upon pre-synaptic stimulation

• 3- if added, it mimics the pre-synaptic stimulation

• 4- a mechanism for removal should exists

• 5- effects of some drugs should mimic the potential neurotransmitter

Vertebrate neurotransmitters• Most CNS synapses use amino acid neurotransmitters

– Glutamate for EPSPs– Glycine, GABA for IPSPs

• Biogenic amine (Ach, Ne, Dopamine, serotonin) are present in few neurons but these neurons project widely

• Peptides are released with other neurotransmitters and modulate the signal

• Different post-synaptic receptors (for the same neurotransmitter) will induce different effects

• Peptide neurotransmitters are synthesized in the body of the neuron, not the axon terminal – can be depleted

• Ach, GABA, glutamate, dopamine, serotonin also found in invertebrates

Types of receptors

• Ionotropic– Fast – channel– Ach nicotinic (ex:

neuromuscular synapse, ray electric organ

• Metabotropic

Receptors

Ionotropic receptors

Metabotropic receptors

• Act via cAMP and protein kinase

• Can act through IP3 and DAG, calmodulin

• Modulate post-synaptic permeability and pre-synaptic inhibition

Synaptic plasticity• Change in synaptic strength

over time• Learning and memory

• Synaptic facilitation: increase in amplitude of postsynaptic potential in response to successive pre-synaptic impulses

• Synaptic antifacilitation or depression: opposite

• Especially present in hippocampus and cerebral cortex

Aplysia • Habituation: decrease in

intensity of a reflex response to a stimulus when the stimulus is presented repeatedly

• Sensitization: prolonged enhancement of a reflex response to a stimulus which results from the presentation of a second stimulus that is novel or noxious

Non-associative conditioning

• Habituation: after many stimulation, there is less neurotransmitter released by the presynaptic neuron decreased EPSP on the postsynaptic neuron– The smaller signals are due to

inhibition of calcium channels less calcium released

• Sensitization due to a shock on the neuron from the head increased EPSPs– More calcium released in

facilitation

Classical conditioning• A conditioning signal triggers

a response after a series of stimulus (ex: Pavlov’s dog and the bell)

• Learning areas in vertebrates: hippocampus and cerebral cortex:

• Long term potentiation LTP: long lasting enhancement of synaptic transmission following intense stimulation

LPT• Receptors: NMDA AMPA

receptors

• NMDAs, activated by glutamate, work only if the cell is depolarized

• In resting state, it is blocked by Mg++

• The EPSP depends on activation of the AMPAs

• During depolarization, the Mg++ is released, NMDAs are unblocked

• Ca++ enters and activate various kinase

Memory

• The LTP leads to increase numbers of AMPA receptors on the membrane increased response

• Similarly, low Ca++ leads to removal of AMPA receptors Long Term Depression

• These changes induces protein synthesis from gene transcription

• Also, effects at the level of the synapses themselves with changes in the dendrites