Neurotransmission & Nerve Conduction

• Nerve Conduction

  •  The movement of nerve impulses down neurons. This is sometimes called the propagation of nervous impulses
• Neurotransmission

  • The transmission of nerve impulses across a synapse
• Conduction of nerve impulses

  • Plasma membrane of axon separating negative cytoplasm from positive extracellular fluid. The ions are lined up along the membrane. More positive on the outside than on the inside. This is the same in all living cells. In an axon this potential difference is exploited to send a signal
• The wave of depolarization along the axon

  • In excitable tissues like axons and muscle fibers, a wave of positive potential moves away from the point where the stimulus is initiated. In an axon this wave is initiated at the axon hillock
• Axon hillock: a small area in the cell where the axon meets the soma
• Steps of the wave of depolarization

  1. Stimulus (increase in voltage) triggers an action potential which depolarizes the local membrane
  2. The inside of the membrane becomes briefly positive. This increase in voltage triggers the next area of membrane to become positive because of opening voltage sensitive Na+ channels
  3. The voltage increase is terminated shortly after it begins by closure of the Na+ channels and opening of the K channels such that only a short section of axon is depolarized at any one time but the action potential moves along the axon
• The axons cannot send a second message until they have repolarized. This is because the message is an action potential with a refractory period
• Purpose of the refractory period

  • Assures that transmission is a one way event
  • The voltage-sensitive channels on the axon just respond to voltage changes so the message could go both ways
  • Issues that the Aps are separate events
  • Stimulus strength is coded for be frequency of discharges (APs) not the amplitude since all APs are the same
  • Allows the cell recover to the resting state
  • Important for keeping the neurons from running down due to loss of ions and energy shortages brought about by the use of large quantities of energy used to re-establish the RMP
• Saltatory Conduction Steps

  1. Stimulus (increase in voltage) triggers an AP which depolarizes the local membrane
  2. The inside of the axon membrane becomes briefly positive. This increase in voltage triggers the next areas of exposed membrane to become positive. The Ap thus leaps from one Node of Ranvier to the next
• Nodes of Ranvier: areas of the axon membrane without myelin in a myelinated axon
• Myelin speeds up the process because it jumps from node to node, because only a short section of axon has to be depolarized then the wave of depolarization jumps to the next node
• Thick axons speed this process up too because there is less resistance to current flow with a large membrane
• Myelinated axons can conduct messages at a absolute maximum of about 150m/sec compared to unmyelinated which are never more than 10m/s
• Heavy myelinated bix neurons have less resistance so they can conduct much more quickly
• Dull visceral pain is due to short unmyelinated axons
• Electrical Synapse

  • The electrical signal is simply transmitted across the synapse by inducing ionic movement in the adjacent cell
  • This allows neurotransmission between adjacent muscle cells with no delay
  • Allows cells to stay synchronized in their action
  • Gap junction are often found between smooth muscle cells
• Electrical synapses are found in

  • Heart muscle (myocardium)
  • Uterine muscle (myometrium)
  • Large parts of the brain
• Gap junction: a tiny hole between 2 adjacent cells and allows fro electrical synapses
• Signal Transmission across an electrical synapse

  1. Stimulus (increase in voltage) triggers an AP which depolarizes the local membrane
  2. The inside of the axon membrane becomes briefly positive. This increase in voltage triggers the next area of membrane to become positive
  3. The wave of depolarization meets the axon terminal and the positive ions flow through the ion channels causing the postsynaptic membrane to depolarize. The signal is thus propagated across the synapse
• EPSPs increase in voltage (brings the cell closer to threshold)

  • Often due to Na+ channels opening and Na+ enters the cell
• IPSPs decrease in voltage (brings the cell further from threshold)

  • Often due to K+ channels opening and K+ moves out of the cell
• If enough potentials are added up spatially and temporally then threshold is reached and an action potential in a muscle or in the axon hillock of a neuron is triggered
• Neurotransmitter: a substance that transmits signals across the synapse
• Neurotransmitters are released from axons (the presynaptic membrane) and infrequently from the postsynaptic membrane
• Some neurotransmitters change the membrane voltage as in the generate of EPSPs and IPSPs while other alter cell function in some other way
• Biogenic amines

  • Acetylcholine
  • Catecholamines
• Acetylcholine or ACh is the neurotransmitter at all ganglia of the ANS and neuromuscular junction
• Catecholamines

  • Dopamine
  • Norepinephrine (noradrenaline) - released into the blood from the adrenals and also released at synapses
  • Epinephrine (adrenaline) - sympathetic, postganglionic neurotransmitter
• Serotonin (5-HT)

  • Associated with sleep and dreaming, nausea and vomiting, affective tone (mood)
  • Biogenic amine
• Histamine

  • Particularly important in the CNS 
  • Mostly associated with allergic response in the rest of the body
  • Biogenic amine
• Amino acids

  • Glutamate (excitatory)
  • Aspartate (excitatory)
  • GABA (inhibitory)
  • Glycine (inhibitory)
  • Only found in the CNS
• Peptides

  • Substance P is the pain neurotransmitter
  • Oxytocin stimulates uterine contraction and the expression of milk
  • Enkephalins provide an opiate-like analgesia by inhibiting neurotransmission in ascending pain pathways
• Alpha-methyltyrosine interferes with the enzyme that leads to the synthesis of L-DOPA
• L-DOPA is given to Parkinson’s Disease sufferers in order to enhance dopamine production
• Neurotransmitter Removal

  • Inactivation
  • Reuptake
• Neurotransmitter inactivation

  • Some specific enzyme breaks down the neurotransmitter in the synapse
• Neurotransmitter Removal

  • The dominant removal method is reuptake into presynaptic neuron
  • All catecholamines are taken back up
  • Type of active transport
• Reuptake can be blocked

  • Selective Serotonin Reuptake Inhibitors (SSRIs)
  • Prozac inhibits the reuptake of serotonin from the synapse and can be used as an antidepressant since increased serotonin is associated with increased affective tone
  • Norepinephrine/dopamine reuptake inhibitors (NDRIs)
  • Wellbutrin 
  • Cocaine
  • Acts to block epinephrine reuptake which leads to high synaptic levels of epinephrine and the manic feeling associated with cocaine uses
• Uptake inhibitors should never be combined with MAOi drugs like tranylcypromine (a tricyclic antidepressant)