Synapses lect 18

Lecture 18 - 4/2 and 4/9:

1. Synapses

   1. Introduction

      1. Presynaptic Cell - signal comes from this cell

         1. Synthesizes & stores neurotransmitter

      2. Chemicals cross the Synaptic cleft = gap (~20nm)

      4. Postsynaptic cell

         1. Starts action potential in this cell

   2. Types

      1. The presynaptic neuron

         1. AP depolarizes membrane

         2. Voltage gated Ca2+ open

         3. Vesicles fuse with membrane & release neurotransmitter

         4. Neurotransmitter diffuses across cleft

      2. On postsynaptic membrane

         1. 4. Neurotransmitter binds to ligand-gated ion channels

            1. → and they open

            2. *the neurotransmitter does not enter the cell

               1. A postsynaptic potential is generated

               2. Can stimulate or inhibit

   3. Neurotransmitters

      1. Small chemicals signal across a synapse

      2. Active specific receptors

      3. Ex: glutamate

4. Postsynaptic potentials

   1. Excitatory Postsynaptic Potential (EPSP): Depolarizes

   2. Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes

   4. Axon hillock = integrates EPSPs - IPSPs

   5. If sum of EPSPs minus IPSPs greater than threshold at axon hillock

      1. If membrane reaches threshold → AP!

   6. Excitatory Postsynaptic Potential (EPSP)

      1. If ligand-gated ion channels permeable to Na+/K+

      2. Postsynaptic cell membrane depolarizes

         1. Postsynaptic cell more likely to generate an AP at Axon hillock

   7. Inhibitory Postsynaptic Potential (IPSP)

      1. If ligand-gated ion channels permeable to only K+ or Cl-

      2. Postsynaptic cell membrane hyperpolarizes

2. Memory and Learning

   1. Nervous System Development

      1. Pre-birth (embryonic development)

         1. (review development)

      2. Structure of nervous system →

         1. Gene expression

         2. Signal Transduction 

      3. Brain develops & remodels neurons

   2. Neuronal Plasticity

      1. After birth brain has Neuronal Plasticity

      3. Reinforced through experiences

         1. Use it or lose it

   3. Memory & Learning

      1. Short Term Memory

         1. Held for short time

         2. Accessed by temporary links in hippocampus

      2. Long Term Memory

         1. Permanent connections in cerebral cortex

         2. Stored memories

         3. FYI: SLEEP consolidates memories

   4. Long-term Potentiation (LTP)

      1. Strengthening synapses transmission long 

      3. Preconditions for LTP

         1. In presynaptic neuron:

            1. Two conditions:

               1. High frequency of AP’s

               2. Depolarization from a second stimulus

                  1. Strengthens synapse

         2. In postsynaptic neuron:

            1. 2 types of ligand-gated channels

            2. Before LTP - 

               1. One stored - AMPA

               2. One in membrane - NMDA

      4. Before LTP

         1. Glutamate

         2. → NMDA receptors

         3. But…blocked by Mg2+

      5. Initiating LTP

         1. Depolarization!

         2. → NMDA releases Mg2+

         3. Some Na+ & Ca2+ flow in

         4. AMPA receptors upregulate

3. Sensory Processes

   1. Sensory Pathways

      1. Sensory Reception

         1. Detect Stimulus

         2. Sense Organs

      2. Sensory Transduction

         1. Converts stimulus to receptor potential

            1. Receptor potential = change in membrane potential

      3. Transmission

         1. Sensory receptor

         2. Depolarizes → AP triggered

         3. Higher receptor potential = more frequent AP’s

      4. Perception Brain processes information

   2. Types of Sensory Receptors

      1. Receptor examples:

         1. Chemo - taste and smell

         2. Mechano - touch, hearing

         3. Photo - light, seeing

4. Human Ears

   1. Functions

      1. Hearing = sense changes in external vibrations

      2. Equilibrium = balance controlled by sensing movement of fluid in ears

      3. Both processes use mechanoreceptors

      4. The parts of the ear change shape when they receive vibrations and transmit sound

      5. Sound = waves of air or water pressure

      6. Hearing = ability to sense changes in pressure

   2. Anatomy

      1. Outer Ear

         1. Pinna: collects sound waves

         2. Auditory Canal: channels waves to tympanic membranes

         3. Tympanic Membrane (ear drum): Thin membrane - vibrates → transmits to middle ear

            1. Separates outer ear from middle

      2. Middle Ear: air-filled cavity

         1. Ossicles: 3 small bones - Malleus → Incus → Stapes

            1. Amplify sound, transmit to oval window

               1. → Transmit to inner ear

      3. Inner Ear:

         1. Cochlea: Spiral tube, where sound is detected

         2. 3 fluid filled chambers (fluid = perilymph)

         3. Contains hair cells - 

            1. Mechanoreceptors with cilia

         4. Vibrate against membrane

   3. Sensory Reception & Processing

      1. Converting sound to hearing

      2. Hair cells stimulated → Depolarization → AP

      3. Axons bundled into auditory nerve