Synaptic Transmission and Nervous System Organization

Parts of a Neuron

• Cell Body: The main part of the neuron.
• Dendrites: Structures that collect electrical signals.
• Axon: Passes electrical signals to other cells; can be very long (over a meter).
• Arrangement of these parts can vary depending on the nerve cell type (Form = Function).

Information Flow in Nerve Cells

• Information flows from dendrites to the axon.
• Dendrites: Collect electrical signals.
• Cell Body: Integrates incoming signals and generates an outgoing signal to the axon.
• Axon: Passes electrical signals to dendrites of another cell or to an effector cell.
• Neurons form networks for information flow, communicating through synapses.

Neuron Membrane Potential

• Nerve cells are inherently charged; this charge is measured in millivolts (mV).
• This charge results from ions within and around nerve cells.
• Resting Potential: The inherent charge of a nerve cell.
• Typical resting potential is around -65 mV.

Resting Potential Maintenance

• K+ Leak Channel:
  • Potassium ions (K+) leak along their concentration gradient, from inside to outside the cell.
• Na+/K+ Pump:
  • Ensures the highest concentration of potassium (K+) is inside the cell.

Resting Potential to Action Potential

• Depolarization: Membrane becomes more positive (less polarized).
• Repolarization: Rapid return to a negative charge.
• Hyperpolarization: Repolarization overshoots, becoming slightly more positive to reach the resting potential.

Voltage Gated Channels

• Besides the Na+/K+ Pump, ions like Na+, K+, Ca2+, and Cl- can move across the membrane through voltage-gated channels.
• These channels open when the membrane potential changes.

Action Potential - Detailed Explanation

• The movement of Na+ into the cell makes the interior membrane more positive, leading to depolarization.
• Key membrane proteins:
  • K+ Channels
  • Na+ Voltage Gated Channels

Action Potential Phases

• Resting Potential
• Depolarization Phase
• Repolarization Phase
• Hyperpolarization Phase

Action Potentials Travel

• Action potential spreads as a wave of depolarization.
• Na+ enters the axon, causing charge to spread.
• This depolarization opens downstream channels, allowing more Na+ to enter.

Saltatory Conduction

• Action potentials "jump" down myelinated axons, which is much faster than in unmyelinated axons.

Saltatory Conduction - Detailed Explanation

• Schwann cells prevent Na+ from leaking.
• Charge triggers action potential at the Node of Ranvier.
• This process repeats downstream.

Electrochemical Message Passing Between Neurons

• Gap Junctions
  • Allow direct electrical signals to pass between neurons.
  • Connexin proteins form channels between cells.

Neurotransmitters

• Action Potential triggers release of neurotransmitter.
• Process:
  • Action potential arrives at the presynaptic membrane.
  • Voltage-gated Ca^{2+} channels open.
  • Synaptic vesicles release neurotransmitter into the synaptic cleft.
  • Neurotransmitter triggers a change in the postsynaptic cell potential.
  • Neurotransmitter is then broken down or released.

Synaptic Transmission

• Neurotransmitters can cause excitatory or inhibitory responses in a postsynaptic neuron.
• Inhibitory response makes the membrane potential less likely to fire.

Neurotransmitters Categories

Excitatory neurotransmitters: Make action potentials more likely in postsynaptic cells.
Inhibitory neurotransmitters: Make them less likely.
Modulatory neurotransmitters: Modify the response at other synapses.
Drugs: That prevent reuptake of neurotransmitters increase their activity.

• Acetylcholine:
  • Site of Action: Neuromuscular junction, some CNS pathways.
  • Action: Excitatory (inhibitory in some parasympathetic neurons).
    • Drugs That Interfere: Botulism toxin blocks release, Black widow spider venom increases, then eliminates release, a-bungarotoxin (in some snake venoms) binds to receptor
• Monoamines
  • Norepinephrine:
    • Site of Action: Sympathetic neurons, some CNS pathways
    • Action: Excitatory or inhibitory
  • Dopamine:
    • Site of Action: Many CNS pathways
    • Action: Primarily excitatory
  • Serotonin:
    • Site of Action: Many CNS pathways
    • Action: Inhibitory or modulatory
    • Drugs That Interfere: Ritalin (used for attention deficit hyperactivity disorder) increases release, Some antidepressants prevent reuptake, Cocaine prevents reuptake Amphetamines prevent reuptake, MDMA (ecstasy) causes increased release
  • Amino Acids
    • Glutamate:
      • Site of Action: Many CNS pathways
      • Action: Excitatory
      • Drugs That Interfere: PCP (angel dust) blocks receptor
    • Gamma-aminobutyric acid (GABA)
      • Site of Action: Some CNS pathways
      • Action: Inhibitory
      • Drugs That Interfere: Ethanol mimics response to GABA
    • Peptides
      • Endorphins, enkephalins, substance P
        • Site of Action: Used in sensory pathways (pain)
        • Action: Inhibitory
        • Drugs That Interfere:

Summation

• Neurons receive signals from many other neurons.
• Summation determines whether an action potential will be generated.

Summation of Signals

• Summation: The additive nature of postsynaptic potentials.
  • EPSP (Excitatory Postsynaptic Potential): Depolarization, Na+ inflow; makes action potentials more likely.
  • IPSP (Inhibitory Postsynaptic Potential): Hyperpolarization, K+ outflow or Cl- inflow; makes action potentials less likely.
  • Simultaneous EPSP + IPSP: Simultaneous Na+ inflow plus K+ outflow or Cl- inflow; signals cancel each other out.

Excitation to Action Potential

• Graded potential (hyperpolarization): Stimulus opens K+ channels.
• Graded potential (depolarization): Stronger depolarizing stimulus opens more Na+ channels.
• Action potential: Depolarization reaches threshold potential, triggering an action potential.

Signal Termination

• Nerve impulse leads to neurotransmitter release.
• Neurotransmitter binds to receptors on the postsynaptic membrane, opening ion channels.
• Neurotransmitter is then degraded or reabsorbed.
• When this process is disrupted, drugs like Zoloft (sertraline HCI) can interfere with reuptake.

Neuron Organization

• Neurons are organized into pathways:
  • Sensory neurons
  • Interneurons
  • Motor neurons

Simple Reflex Arc

• Example: Patellar reflex
  • Stretch receptor (muscle spindle) in the quadriceps muscle is stimulated.
  • Sensory neuron carries the signal to the spinal cord.
  • Motor neuron carries the signal back to the quadriceps muscle (effector), causing it to contract.

Nervous Responses Controlled by Negative Feedback

• Components:
  • Sensory receptor (Sensory input)
  • Afferents
  • Integration (Brain and spinal cord, CNS)
  • Efferents
  • Motor output
  • Effector
  • Peripheral nervous system (PNS)

Case Study 1: Pufferfish

• Dr. Marshall Westwood experiences numbness, nausea, and paralysis after eating pufferfish.
• Cause: Tetrodotoxin (TTX).

Tetrodotoxin (TTX)

• Active neurotoxin in pufferfish.
• Produced by symbiotic bacteria within these animals.
• Blocks voltage-gated sodium ion channels.
• This would impact Part B of the action potential graph (depolarization phase).

Pufferfish Poisoning Effects

• Numbness occurs because sensory neurons stop firing, preventing communication with the brain.
• Paralysis occurs because TXX causes motor neurons to stop firing, preventing communication with the muscles.

Case Study 2: Poisonous Birds

• Dr. Westwood experiences numbness after handling a hooded pitohui bird.
• Cause: Homobatrachotoxin.

Homobatrachotoxin

• Active toxin from the hooded pitohui bird.
• Similar to batrachotoxin from poison arrow frogs.
• Acts on voltage-sensitive sodium channels.

Homobatrachotoxin Mechanism

• In experiments, it was found that after depolarizing, the membrane potential remained positive for an extended length of time and the repolarization was often extremely delayed.

Homobatrachotoxin Effects

• The toxin prevents Na+ ion channels from closing.

Organization of the Nervous System

• Central Nervous System (CNS):
  • Brain and spinal cord.
  • Information processing.
• Peripheral Nervous System (PNS):
  • Afferent division transmits sensory information.
  • Efferent division transmits motor information.
    • Somatic nervous system
    • Autonomic nervous system
      • Parasympathetic division
      • Sympathetic division

Autonomic Nervous System

• Parasympathetic:
  • Rest & Digest.
  • Conserve energy.
  • Decreased heart rate, increased digestion.
  • Main control by nerves of the hindbrain and acetylcholine.
• Sympathetic:
  • Fight or Flight.
  • Increased energy consumption.
  • Increased heart rate, decreased digestion.
  • Main control by nerves of the spinal cord (quick response) and norepinephrine.

Parts of the Brain

• Cerebrum:
  • Conscious thought and memory.
• Diencephalon:
  • Information relay and control of homeostasis.
• Brain Stem:
  • Information relay and center of autonomic control of heart, lungs, and digestive system.
• Cerebellum:
  • Complex motor patterns.
  • Contributes to coordination, precision, accurate timing, but does not initiate movement.
  • Regulates fear and pleasure responses.
  • Cognitive functions such as attention and language.

Parts of the Cerebrum

• Frontal Lobe:
  • Cognition, memory, ability to concentrate.
  • Consequence analysis, problem-solving, planning.
  • Personality, emotions.
• Parietal Lobe:
  • Integration of information from the senses.
  • Spatial orientation.
  • Some speech, and pain and touch perception.
• Temporal Lobe:
  • Auditory receptive area.
  • Storage of long-term memory.
  • Visual processing of faces and scenes.
• Occipital Lobe:
  • Visual processing center.
  • Origins of dreams.

Cerebral Lobe Damage

• Anterograde amnesia (inability to transfer new information into long-term store) can be caused by damage to the temporal lobe.
• Epileptic seizures triggered by visual stimuli can be caused by damage to the occipital lobe.

Hemispheres of the Brain

• Right Hemisphere:
  • Controls the left side of the body.
  • Spatial visualization and analysis.
• Left Hemisphere:
  • Controls the right side of the body.
  • Language and math.

Sympathetic Nervous System

• A sudden loud noise triggers the sympathetic nervous system, increasing heart rate and respiratory intake.