Synapses and Sensory receptors

Synapses and Sensory Receptors

1. Synapses

-> SEQ, HD synaptic transmission

2. Memory and Learning 

-> CC STM, LTM

-> SEQ HD, LTP

3. Sensory Processes 

-> SEQ, HD sensory pathway

4. Human Hearing 

-> SEQ, HD audio reception

I. Synapses 

1. introduction

• A junction between the synaptic terminal in another cell 

2. Types of Synapses 

• Electrical - current flows directly from cell to cell

  • Less common 

• Chemical synapses 

  • Involves neurotransmitters

  • Most synapses 

  • Action potential travels to presynaptic cell and jumps through the synaptic cleft, received by postsynaptic cell (detects neurotransmitters and responds in some way) 

Presynaptic Neuron 

• Synthesize neurotransmitters which are made on the cell body 

• Packaged into vesicles and transported down the whole length of the axon 

• Neurotransmitters stores in synaptic vesicles

• At the end of the axon there is a new set of voltage gated channels

  • Voltage gated calcium channel 

  • Depolarization of synaptic terminal triggers the inflow of calcium 

3. Neurotransmitters

• Diffuse across synaptic cleft and bind/activate specific post-synaptic receptors 

4. Post-synaptoc Potentials 

• Change in membrane potential in postsynaptoc cells 

• Triggered by ligand gated ion channels (ligand = something that binds, neurotransmitter is the ligans)

• Neurotransmitter binds to receptor which causes the receptor to change shape and turn into an open ion channel 

  • Something diffuses through ion channel and membrane potential changes 

• Types 

  • Excitatory postsynaptic potential: depolarizes 

  • Inhibitory postsynaptic potential : hyperpolarizes 

• Summation of postsynaptic potentials 

  • Often 100s of terminals to dendrite, cell body 

  • Some excitatory, some inhibitory

  • Postsynaptic potential due to interactions and distance 

• Temporal summation 

  • 2 EPSPs at 1 synapse 

  • Rapid succession

  • 2nd arrives before MP resets 

    • Stronger depolarization, overlapped in terms of their signal 

• Spatial summation

  • 2 EPSPs nearly simultaneously 

  • Different synapses 

  • Same postsynaptic neuron 

  • Stronger depolarization

IPSI and EPSP interactions 

• Axon hillock - neurons integration system 

• MP at axon hillock - summed effect of all EPSPS and IPSPs

• AP results if sum reaches threshold  

II. Memory Learning 

1. Nervous system development 

• Changes to synapses underlie learning and memory 

• Nervous system macro organization is established during embryonic development regulated by gene expression and signal transduction

• Brain develops and gets remodeled after birth 

2. Neuronal Plasticity 

• Response to activity 

• Remodeling via competition among neurons for growth supporting factors 

  • Produced in limited quantities, active neurons get more, inactive get fewer 

• The more you use a synapse, the more efficient and stronger it's going to be 

• Connections can be modified, forming associations helps learning

3. Memory and Learning 

• Short term memory 

  • Involves cerebral cortex

  • Accessed by temporary link through hippocampus 

  • Essential for acquiring memories but not maintaining 

• Sensory memory - seconds to milliseconds

• Long term memory 

  • Info stored in cerebral cortex 

  • Temporary links replaced by connections with cerebral cortex, unlimited capacity

4. Long Term Potentiation 

• Lasting increase in strength of synaptic transmission 

• Fundamental process of memory storage and learning

• 2 conditions required 

  • Depolarization from another synapse at the same time (spatial summations

  • High frequency serious of APs (rej

In postSynaptic Neuron 

• 2 types of ligand gated ion channels 

• Before LTP - one in membrane, other stored 

  • Requires simultaneous depolarization from other course to reach threshold potential 

• After LTP - both in membrane 

  • Stronger depolarization

  • More likely AP 

III. Sensory Processes

1. Sensory Pathways 

• Need to have some kind of stimulus -> integration (CNS making sense of stimuli) -> motor output/response 

• Follows same components as NS 

• 4 basic components 

  • Sensory reception 

    • Sensory receptor detects change

    • Often sense organ: receptors + associated cells 

  • Sensory transduction 

    • Energy of stimulus converted to receptor potential 

    • Unstimulated receptor

      • Resting potential

    • Stimulated receptor

      • Depolarized -> triggers action potential 

4 Basic Components of Sensory Reception 

• Sensory reception - sensory receptor detects change 

  • Often sense organ - receptors + associated cells

• Sensory transduction - Energy of stimulus converted to receptor potential 

• Transmission - get the action potential to the brain 

  • Large receptor potential -> more frequent action potential

  • Gentle pressure - low frequency 

  • More pressure - high frequency 

• Perception - brain processes info 

  • Exists only in your brain 

2. Types of Sensory Receptors

• Characterized by type of stimulus transduced 

  • Chemoreceptors 

  • Photoreceptors

  • Mechanoreceptors 

  • Thermoreceptors 

IV. Human Ear 

1. Functions 

• Has 2 jobs

  • Hearing 

  • Balance 

    • Both via mechanoreceptors

    • Detect mechanical forms of energy pressure, stretch, vibration 

    • Activated by change of shape - push or pull

    • Changed linked to ion channels 

      • bend/stretch -> change membrane permeability

• Focus on hearing 

  • Sound = pressure waves in a fluid that you can detect/perceive as hearing 

  • Hearing = ability to sense those pressure waves and perceive them as sound

2. Anatomy 

• Outer ear 

  • Pinna - collects sound waves 

  • Auditory canal - channels waves to tympanic membrane 

  • Tympanic membrane (eardrum) - thin membrane that vibrates and transmits to middle ear 

• Middle ear

  • Air filled cavity containing 3 bones called ossicles 

    • Bones vibrate and have 2 jobs

      • Transmit the vibrations from the ear drum, through the bones, into the inner ear 

      • Amplify sound 

• Inner Ear

  • Main structure - cochlea 

    • Spiral tube, fluid filled, where sound waves are detected 

    • Line with cells called hair cells - mechanoreceptors with cilia 

      • Vibrations changing orientation of the cilia = mechanoreceptor 

3. Sensory Reception 

• Converting sound to hearing 

• Hair cells stimulated -> depolarization -> action potential