RAVEN- Vertebrates

• Nervous System


  Allows animals to respond to environmental stimuli
• Nervous System


  • Consists of sensory receptors, motor effectors, and the nervous system that links the two
  • Consists of neurons and supporting cells
• Central Nervous System (CNS)


  Brain and spinal cord
• Types of Neurons


  • Sensory neurons (afferent neurons)
  • Motor neurons (efferent neurons)
  • Interneurons (association neurons)
• Sensory neurons


  Carry impulses to central nervous system (CNS)
• Motor neurons


  Carry impulses from CNS to effectors (muscles and glands)
• Interneurons


  Provide more complex reflexes and associative functions (learning and memory)
• Peripheral Nervous System (PNS)


  Sensory and motor neurons
• Components of the PNS

  • Somatic NS (stimulates skeletal muscles)
  • Autonomic NS (stimulates smooth and cardiac muscles, as well as glands)
  • Sympathetic and parasympathetic NS (counterbalance each other)
• Neuron


  • Cell body
  • Dendrites (short, cytoplasmic extensions that receive stimuli)
  • Axon (single, long extension that conducts impulses away from cell body)
• Neuroglia


  • Support neurons both structurally and functionally
  • Schwann cells and oligodendrocytes produce myelin sheaths surrounding axons
  • In the CNS, myelinated axons form white matter, dendrites/cell bodies form gray matter
  • In the PNS, myelinated axons are bundled to form nerves
• Resting potential


  Potential difference that exists across every cell's plasma membrane when the neuron is not being stimulated, ranging from -40 to -90 millivolts (mV) with an average of -70 mV
• Sodium-potassium pump


  Brings two K+ into cell for every three Na+ it pumps out
• Ion leakage channels


  Allow more K+ to diffuse out than Na+ to diffuse in
• Equilibrium potential


  Balance between diffusional force and electrical force
• Graded potentials


  Small consistent changes in membrane potential due to activation of gated ion channels
• Action potentials


  Transient disruptions triggered by a threshold change in potential, the actual signals that move along an axon
• Chemically-gated or ligand-gated channels


  Ligands are chemical signals like hormones or neurotransmitters that induce opening and cause changes in cell membrane permeability
• Depolarization


  Makes the membrane potential more positive
• Hyperpolarization


  Makes the membrane potential more negative
• Summation


  The ability of graded potentials to combine
• Voltage-gated Na+ channels


  Have activation gate and inactivation gate, transient influx of Na+ causes the membrane to depolarize
• Voltage-gated K+ channels


  Have a single activation gate that is closed in the resting state, K+ channel opens slowly and efflux of K+ repolarizes the membrane
• Phases of an action potential

  • Rising, falling, and undershoot
• Nerve impulse propagation


  Each action potential, in its rising phase, reflects a reversal in membrane polarity, positive charges due to influx of Na+ can depolarize the adjacent region to threshold, and so the next region produces its own action potential
• Ways to increase velocity of conduction

  • Axon has a large diameter (less resistance to current flow, found primarily in invertebrates)
  • Axon is myelinated (action potential is only produced at the nodes of Ranvier, impulse jumps from node to node, saltatory conduction)
• Synapses


  Specialized intercellular junctions with the other neurons, with muscle cells, or with gland cells
• Types of synapses


  • Electrical synapses (involve direct cytoplasmic connections between the two cells formed by gap junctions)
  • Chemical synapses (have a synaptic cleft between the two cells, end of presynaptic cell contains synaptic vesicles packed with neurotransmitters)
• Chemical synapses


  1. Action potential triggers influx of Ca2+

  2. Synaptic vesicles fuse with cell membrane

  3. Neurotransmitter is released by exocytosis

  4. Diffuses to other side of cleft and binds to chemical- or ligand-gated receptor proteins

  5. Produces graded potentials in the postsynaptic membrane

  6. Neurotransmitter action is terminated by enzymatic digestion or cellular uptake
• Acetylcholine (ACh)


  Crosses the synapse between a motor neuron and a muscle fiber, binds to receptor in the postsynaptic membrane, causes ligand-gated ion channels to open, produces an excitatory postsynaptic potential (EPSP) that stimulates muscle contraction, acetylcholinesterase (AChE) degrades ACh causing muscle relaxation
• Inhibitory neurotransmitters


  • Glycine
  • GABA (γ-aminobutyric acid)
• Biogenic amine neurotransmitters


  • Epinephrine (adrenaline)
  • Norepinephrine
  • Dopamine
  • Serotonin
• Neuropeptide neurotransmitters


  • Substance P
  • Enkephalins
  • Endorphins
  • Nitric oxide (NO)
• Synaptic integration


  Integration of EPSPs (depolarization) and IPSPs (hyperpolarization) occurs on the neuronal cell body, small EPSPs add together to bring the membrane potential closer to the threshold, IPSPs subtract from the depolarizing effect of EPSPs to deter the membrane potential from reaching threshold
• Spatial summation


  Many different dendrites produce EPSPs
• Temporal summation


  One dendrite produces repeated EPSPs
• Habituation


  Prolonged exposure to a stimulus may cause cells to lose the ability to respond to it, cell decreases the number of receptors because there is an abundance of neurotransmitters
• Cocaine


  Affects neurons in the brain's "pleasure pathways" (limbic system), binds dopamine transporters and prevents the reuptake of dopamine, dopamine survives longer in the synapse and fires pleasure pathways more and more
• Nicotine


  Binds directly to a specific receptor on postsynaptic neurons of the brain, brain adjusts to prolonged exposure by making fewer receptors to which nicotine binds and altering the pattern of activation of the nicotine receptors
• Subdivisions of the Central Nervous System

  • Spinal cord (spinal reflexes, relays sensory and motor information)
  • Hindbrain (medulla oblongata, pons, cerebellum)
  • Midbrain
  • Forebrain (diencephalon, telencephalon)