L31 - Neurons: Regular Neurons

overview

3 types of neurons:

1. sensory

2. interneuron (regular neurons)

3. motor neuron

Synapses

= how neurons send signals to adjacent cells

Chemical Synapses = export of a chemical (neurotransmitter)

• neuron → neuron or neuron → muscle cell

Components:

1. neurotransmitters = small organic molecules (signal molecules)

ex. acetylcholine (ACh)

2. neurotransmitter receptors = most are ion channels

• transmembrane w multiple alpha-helices

ex. acetylcholine receptor (AChR) - a sodium channel

Release of neurotransmitters (by Presynaptic neurons)

1. resting

• polarized membrane

• vesicles close to plasma memb

  

2. release

• membrane depolarizes

• Ca2+ channels (voltage-gated) open: Ca2+ enter presynaptic neuron → activates SNAREs (indirectly)

• vesicles bind to plasma memb → release neurotransmitters

3. recovery

• Ca2+ channels (voltage-gated) close when membrane repolarizes

• Ca2+ is exported by sodium calcium exchangers (from [low] → [high])

• membrane is recovered by:

  • pinocytosis or kiss-and-run exocytosis

    

→ empty vesicles are sent back to cell body & refilled locally

Reception of neurotransmitters (by Postsynaptic neurons)

1. ion gradients are ready: high Na+ & Cl- outside cell & high K+ inside cell

2. receptors:

• AChR = acetylcholine-gated Na+ chanel (EXCITATORY synapse)

  • Na+ enters cell → cell depolarizes

• GABA receptors = GABA-gated Cl- channel (INHIBITORY synapse)

  • Cl- enters cell → membrane hyperpolarizes

3. recovery

• receptors/channels close when neurotransmitters are removed from synaptic cleft by:

  • diffusion, degradation (by enzymes), or reuptake

ex. ACh is removed by degradation of ACh by AChE (enzyme) on postsynaptic cell

Integration of Signals (in Postynaptic neurons)

• synapses:

  • contact the dendrites & cell body

  • are excitatory: cause depolarization

  • are inhibitory: cause hyperpolarization

  • each active synapse generates a postsynaptic potential (PSP)

Spatial Summation = combines PSPs from diff presynaptic neurons @ SAME time

Temporal Summation = combine PSPs from one presynaptic neuron @ DIFF times

Generation of Output (by Postsynaptic neurons)

PSPs → AP ] via the axon hillock

Relationship b/w combined PSP & APs:

• low combined PSP : NO action potentials

• moderate combined PSP : infrequent action potentials

• high combined PSP : lots of action potentials

→ magnitude of the combined PSP → frequency of action potentials

Examinable Content