Ch 6: Neuronal Signaling and the Structure of the Nervous System

• axon - (nerve fiber) extends from body and carries output to target cells
  
• the region that arises from the cell body is the initial segment (or axon hillock)
  
• axon branches – collaterals
  

• releases neurotransmitters from the axon
  
• chemical messengers diffuse across an extracellular gap to the cell opposite the terminal
  
• some neurons release messengers from bulging areas along the axon known as varicosities

• form sections of myelin sheaths along axons in PNS
  
• gaps in myelin called nodes of Ranvier where the plasma membrane is exposed to extracellular fluid
  

• movement of organelles and materials b/w cell body and axon
  
• depends on rails of microtubule and specialized motor proteins called kinesins and dyneins
  
• anterograde – from cell body to axon terminals
  
• retrograde – from terminals to cell body
  

• afferent – convey info from tissues and organs to the CNS
  
• efferent – convey info away from the CNS to effector cells (muscle, gland, etc)
  
• interneurons – connect neurons w/in the CNS

• per 1 afferent, 10 efferent & 200,000 interneurons
  
• sensory receptors
  
• at the ends afferent neurons
• respond to physical and chemical changes in the environment

• groups of afferent and efferent neurons together with connective tissue and blood vessels in PNS
  
• CNS – afferent neurons have cell bodies outside of CNS whereas efferent neurons have cell bodies within CNS
  

• junction b/w 2 neurons
  
• communicate by neurotransmitters
  
• understand differences in the term ‘receptor’
  
• presynaptic neuron – conducts a signal -> synapse (terminal area)
  
• postsynaptic neuron – conducts a signal <- synapse (cell body/dendrite area)
  

stimulates formation of tight juncts b/w cells in capillaries

forms blood-brain barrier to prevent toxins from entering brain

provides glucose & removes NH3

• potential across plasma membranes
  
• extracellular voltage = 0 V
• ions collect against the surfaces of the plasma membrane
• most of the fluids (intra/extracellular) are neutral

• diff in [specific ion] in intra & extracellular fluid
  
• diff in membrane permeabilities to different ions, reflecting the # of open channels
  
• equilibrium potent – membrane potent where 2 fluxes become = in magnitude but opp in direction – 0 net flux
  

• equilibrium potential for any ion species (p 144)
  
• GHK equation – calculate the potential of a membrane (Vm)
  
• leak K+ channels cause a concentration gradient
  
• electrogenic pump – moves net charge across the membrane directly causing the potential
  

• depolarized – membrane is < - than resting level
  
• overshoot – reversal of memb potent polarity – becomes + relative to the outside
  
• repolarizing – memb potent that was depolarized returns toward RMP
  
• hyperpolarized – potent is more - than resting level

• changes in potential that are confined to a small region of the membrane
  
• signal over short distances
  
• produced when a specific change in the environment acts on a specialized region of the membrane
  

• charge flows b/w the place of origin and the potential and adjacent regions of the plasma membrane that are at resting potential
  
• local current is decremental – flow of charge decreases as the distance from the point of origin increases
  

• summation – addition stimuli occur before the graded potential dies, they are added to the depolarization from the first stimulus
  
• all cells are capable of producing graded potentials
  
• depolarize or hyperpolarize
  

• large alterations in the membrane potential
  
• rapid, high frequencies
  
• excitable membranes – membranes capable of producing APs (neuron and muscle cells, etc)
• excitability - ability to generate
• only excitable membranes can produce APs

• ligand- & mechanically-gated - initial stimulus of an AP
  
• VG channels allow membrane 2 undergo APs
  
• Na+ channels have inactivation gates – limits Na+ flux by blocking channel shortly after depolarizing opens it
  
• Na+ channels opne/close faster than K+
  

• absolute refractory period – when Na+ channels are open or have proceeded to the inactivated state due to an AP
  
• relative refractory period – with a great stimulus, can last longer & occur during afterhyperpolarization (as channels are closing)
  

• depolar. of adj areas along a neuron to cause more APs
• not conducted decrementally like GPs
  
• refractory - only move in 1 direction
  
• not refractory - move in either direction
  
• salutatory conduction – APs seem to leap from 1 section to the next b/c myelin
  

• receptor potential – initial depolarization achieved by a GP in afferent neurons
  
• generated in sensory receptors at peripheral ends
  
• synaptic potential – depolar is due to GP or synaptic input  or a spontaneous change in potential (pacemaker potential)

• membrane potent
  
• depolarized at an excitatory syn
• hyperpolarized at an inhibitory syn
• convergence – 1000s of synapses from diff presyn cells can affect a single postsyn cell
  
• divergence – a single postsyn cell can affect many other postsyn cells
  

kinds of chemical synapses have different effects on the postsynaptic cell

·      if [Ca2+] is high in the terminal, more ions will be released than usual

• no nerves in CNS
  
• a group of axons is called a pathway or tract
• one that links R and L halves of the CNS is a commissure
• long pathways carry info b/w brain and SC or two parts of the brain

• smooth and cardiac muscle
  
• has 2-neuron chain b/w CNS and effector organ
• can be excitatory or inhibitory
• enteric NS  - network of nerves in gastrointestinal tract
  
• includes sensory neurons and interneurons
  

allows substances to enter the brain quickly