Nervous System 3

synapse

• junction between synaptic terminal and another cell

  • pass signal to another cell

electrical synpase

• current flows directly from cell to cell - less common

• although transmitting from axon

• PM’s hold each other

chemical synpase

• involves signaling via neurotransmitters - more common

• transmit signal using neurotransmitters

• involve

  • presynaptic cell

    • axon terminal sending signal

  • synaptic cleft

    • short distance

    • fast

  • postsynaptic cell

    • cell that responds to signal

presynaptic neuron

• synthesizes neurotransmitters in cell body

  • synthesized in cell body

• stored in synaptic vesicles → transported down axon terminal

• AP triggers voltage-gated Ca²⁺ channels

• Ca²⁺ enters cell → triggers exocytosis of neurotransmitters

  • rushes into synaptic terminal → causes exocytosis of neurotransmitter

neurotransmitter signaling

• diffuse across synaptic cleft

• bind to and activate specific postsynaptic receptors

• leads to postsynaptic potential

postsynaptic potential

• change in MP of postsynaptic cells

• triggered by ligand-gated ion channels - open when bound to neurotransmitters

  • when nothing is bound to the ligand-gated ion channels, they are close

  • when bound to it = channel lets ions into it

types of post-synaptic potentials

• excitatory postsynaptic potential (EPSP): depolarizes

• inhibitory postsynaptic potential (IPSP): hyperpolarizes

• interactions with each other:

  • axon hillock — neuron integration system

  • MP at axon hillock is summed effect of all EPSPs and IPSPs

  • AP results if sum reaches threshold

summation of postsynaptic potentials

• often 100s of terminals to dendrites, cell body

• mix of excitatory and inhibitory signals

  • process of determining signals → net effect

temporal summation

• 2 or more EPSPS (depolarizes) at 1 synapse in rapid succession

• second signal arrives before MP resets → stronger depolarization

  • E₂ arrives before E₁ fades → creates a signal that passes threshold and create action potential

Spatial summation

• 2 or more signals nearly simultaneously

• different synapses on same postsynaptic cell

• add signals together — can be synergistic or antagonistic

  • pulling in opposite direction

  • become excretory, others inhibitory

neuronal plasticity

• response to activity

• remodeling via competition among neurons for growth-supporting factors

• highly active synapses increased, unused eliminated

• use it or lose it

LTP steps (the what)

• repeated depolarization of 2 or more synapses at same time (spatial summation)

  • triggers development of LTP — strengthens synapses that are active together

    • need both synapses to fire at the same time

• high-frequency series of APs at strengthened synapse (temporal summation)

  • uses LTP once established — facilitates recall

• note:

  • Pre-LTP = more synapses

    • summation

  • post-LTP = less synapses

    • temporal

process of establishing LTP

• pre-LTP — just one type of receptor in postsynaptic membrane

  • weak postsynaptic potential, no AP in postsynaptic cell

• repeated use with other synapses (spatial summation)

  • second type of receptor added to membrane (LTP established)

  • stronger postsynaptic potential possible

• High- frequency stimulation of synapse with LTP (temporal summation_

  • summed EPSPs trigger AP without “help” from other synapses

sensory pathway

• sensory reception: sensory receptor detects stimulus

  • often a sense organ - receptor + associated cells

• sensory transduction: E of stimulus converted to receptor potential

  • unstimulated receptor → resting potential

  • stimulated receptor → depolarized → triggers AP

• transmission: sensory information travels as AP from receptor to brain

  • large receptor potential → more frequent APs → stronger sensation

• perception: brain processes information, interprets as sensation

  • exists only in brain

  • e.g. a tree makes sound only when there is someone around to perceive the sound

• when the strength of the stimulus increases, so does the receptor potential

• summary

  • stimulus → sensory receptor → sensory transduction → transmission → perception

sensory reception

• sensory receptor detects stimulus

  • often a sense organ - receptor + associated cells

• 1st step in sensory pathway

sensory transduction

• E of stimulus converted to receptor potential

  • unstimulated receptor → resting potential

  • stimulated receptor → depolarized → triggers AP

• 2nd step in sensory pathway

transmission

• sensory information travels as AP from receptor to brain

  • large receptor potential → more frequent APs → stronger sensation

• 3rd step in sensory pathway

perception

• brain processes information, interprets as sensation

  • exists only in brain

  • e.g. a tree makes sound only when there is someone around to perceive the sound

• 4th step in sensory pathway

auditory pathway

• reception: vibrations depolarize hair cells in cochlea (wave back and forth)

• transduction: causes AP in auditory nerve

• transmission: auditory nerve caries AP to brain

• perception: brain interprets AP as sound

reception

• causes AP in auditory nerve

• 1st step in auditory pathway

transduction

• causes AP in auditory nerve

• 2nd step in auditory pathway

transmission

• auditory nerve caries AP to brain

• 3rd step in auditory pathway

perception

• brain interprets AP as sound

• 4th step in auditory pathway