Neuron Biology Notes

Neuron Biology

Summary

  • Review of neuron types and electrochemical potentials.
  • Characterization of impulse transmission.
  • How receptor and neuron types, muscle type, and stimulus strength affect impulse transmission.

Learning Outcomes

  • Describe neuron types.
  • Describe resting membrane, action, and receptor potentials.
  • Explain differences between electrical potentials in somatic receptors, axons, and muscles.
  • Explain synaptic transmission and mediation via neurotransmitters and receptors.
  • Describe adaptation in different receptor types.
  • Describe how stimulus strength affects neural transmission.
  • Describe the effect of presynaptic inhibition and facilitation.
  • Describe impulse transmission in neuronal pathways and oscillating circuits.
  • Describe how impulses activate/inhibit muscles.

Neurons

  • Basic functional units of the nervous system.
  • Multipolar neuron structure:
    • Cell body (soma).
    • Short, branched dendrites.
    • Long, single axon.

Structural Classification of Neurons

  • Anaxonic Neurons
    • Small with more than two processes.
    • Axons indistinguishable from dendrites.
    • Found in the brain and sense organs.
  • Bipolar Neurons
    • Small with two processes separated by the cell body (one dendrite and one axon).
    • Found in special sensory organs (sight, smell, hearing).
  • Unipolar Neurons
    • Also called pseudounipolar neurons.
    • Have very long axons/process.
    • Fused dendrites and axon.
    • Cell body situated to one side.
    • Sensory neurons of PNS.
  • Multipolar Neurons
    • Have very long axon.
    • Multiple dendrites, one axon.
    • Common in the CNS.
    • Include all skeletal muscle motor neurons.

Functional Classifications of Neurons

  1. Sensory neurons
    • Afferent neurons of PNS.
    • Conduct impulses to the spinal cord or brain.
  2. Motor neurons
    • Efferent neurons of PNS.
    • Conduct impulses away from the spinal cord or brain toward muscles or glandular tissue.
  3. Interneurons
    • Association neurons

Sensory Neurons

  • Functions:
    • Monitor internal environment (visceral sensory neurons).
    • Monitor effects of the external environment (somatic sensory neurons).
  • Structures:
    • Unipolar.
    • Cell bodies grouped in sensory ganglia.
    • Processes (afferent fibers) extend from sensory receptors to CNS.

Three Types of Sensory Receptors

  1. Interoceptors
    • Monitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive).
    • Internal senses (taste, deep pressure, pain).
  2. Exteroceptors
    • External senses (touch, temperature, pressure).
    • Distance senses (sight, smell, hearing).
  3. Proprioceptors
    • Monitor position and movement (skeletal muscles and joints).

Motor Neurons

  • Functions:
    • Carry instructions from CNS to peripheral effectors via efferent fibers (axons).
  • Two major efferent systems:
    • Somatic nervous system (SNS): Includes all somatic motor neurons that innervate skeletal muscles.
    • Autonomic (visceral) nervous system (ANS): Visceral motor neurons innervate all other peripheral effectors (smooth muscle, cardiac muscle, glands, adipose tissue).
  • Two groups of motor efferent axons:
    • Preganglionic fibers.
    • Postganglionic fibers.

Interneurons

  • Located in the brain, spinal cord, and autonomic ganglia (between sensory and motor neurons).
  • Responsible for:
    • Distribution of sensory information.
    • Coordination of motor activity.
  • Involved in higher functions (memory, planning, learning).

Reflex Arc

  • A signal conduction route to and from the CNS.
  • Three-neuron arc: afferent neurons, interneurons, and efferent neurons.
    • Afferent neurons: conduct impulses to the CNS from the receptors.
    • Interneurons: involved in processing the sensory information.
    • Efferent neurons: conduct impulses from the CNS to effectors (muscle or glandular tissue).
  • Two-neuron arc: simplest form, consists of afferent and efferent neurons.

Ion Movements and Electrical Signals

  • All plasma (cell) membranes produce electrical signals by ion movements.
  • Transmembrane potential is particularly important to neurons.

Five Main Membrane Processes in Neural Activities

  1. Resting potential:
    • The transmembrane potential of resting cell.
  2. Graded potential:
    • Temporary, localized change in resting potential.
    • Caused by stimulus.
  3. Action potential:
    • Is an electrical impulse.
    • Produced by graded potential.
    • Propagates along the surface of axon to synapse.
  4. Synaptic activity:
    • Releases neurotransmitters at presynaptic membrane.
    • Produces graded potentials in postsynaptic membrane.
  5. Information processing:
    • Response (integration of stimuli) of postsynaptic cell.

Membrane Potentials

  • All living cells maintain a difference in the concentration of ions across their membranes.
  • Membrane potential:
    • Slight excess of positively charged ions on the outside of the membrane.
    • Slight deficiency of positively charged ions on the inside of the membrane.
  • Difference in electrical charge is called potential because it is a type of stored energy.

Polarized Membrane

  • A membrane that exhibits a membrane potential.
  • Magnitude of potential difference is measured in volts (V) or millivolts (mV).
  • The sign indicates the charge on the inside surface.

Transmembrane Potential

  1. ECF and cytosol differ greatly in ionic composition (Na+, K+).
  2. Cells have selectively permeable membranes.
  3. Membrane permeability varies by ion.

Passive Forces Acting Across the Plasma Membrane

  • Chemical gradients: Concentration gradients of ions (Na+, K+).
  • Electrical gradients: Separate charges of positive and negative ions, resulting in a potential difference.
  • Equilibrium potential:
    • The transmembrane potential at which there is no net movement of a particular ion across the cell membrane.
    • Example: K+=–90mVK^+ = –90 mV, Na+=+66mVNa^+ = +66 mV

Resting Membrane Potential

  • Maintained by a nonconducting neuron's plasma membrane; typically −70mV-70 mV.
  • Selective permeability maintains a slight excess of positive ions on the outer surface.

Sodium-Potassium Pump

  • Active transport mechanism that transports sodium (Na+) and potassium (K+) ions in opposite directions.
  • Ejects 3 Na+ ions for every 2 K+ ions.
  • Stabilizes the resting potential when the ratio of Na+ entry to K+ loss is 3:2.

Local Potentials

  • Slight shift away from the resting membrane potential.
    • Excitation: Stimulus opens additional Na+ channels, leading to depolarization.
    • Inhibition: Stimulus opens additional K+ channels, leading to hyperpolarization.
  • Graded potentials: Magnitude of deviation is proportional to the stimulus magnitude.

Action Potential

  • The membrane potential of a neuron conducting an impulse.
  • Mechanism:
    • Adequate stimulus opens stimulus-gated Na+ channels.
    • Na+ diffuses into the cell, causing local depolarization.
    • Threshold potential is reached; voltage-gated Na+ channels open.
    • More Na+ enters, causing further depolarization.
    • Membrane begins to repolarize as K+ channels open, allowing K+ to diffuse outward.
    • Brief period of hyperpolarization occurs.
    • The Na-K pumps restore resting membrane potential.

Refractory Period

  • Period where a neuron cannot produce another action potential.
    1. Absolute refractory period
      • Lasting approximately 0.5 ms
      • Membrane resists restimulation
      • Will not respond to a stimulus, no matter how strong.
    2. Relative refractory period
      • Time when the membrane is repolarized and restoring the resting membrane potential
      • The few milliseconds after the absolute refractory period
      • Will respond only to a very strong stimulus

Synapses

  • Where nerve signals are transmitted from one neuron to another.

  • Types:

    • Neuromuscular junction: Synapse between neuron and muscle.
    • Neuroglandular junction: Synapse between neuron and gland.

Synaptic Transmission

  • Electrical synapses: Action potential continues along postsynaptic membrane via gap junctions.
  • Chemical synapses: Presynaptic cells release neurotransmitters across a gap to the postsynaptic cell, inducing an action potential.

Neuronal Pools

  • Functional groups of interconnected interneurons.
  • Limited input sources and output destinations.
  • May stimulate or depress parts of the brain or spinal cord.

Five Patterns of Neural Circuits in Neuronal Pools

  1. Divergence: Spreads stimulation to many neurons or neuronal pools in the CNS.
  2. Convergence: Brings input from many sources to a single neuron.
  3. Serial processing: Moves information in a single line.
  4. Parallel processing: Moves the same information along several paths simultaneously.
  5. Reverberation: Positive feedback mechanism that functions until inhibited.

Convergence

  • A mechanism for providing input to a single neuron from multiple sources.

Sensory Receptors

  • Receptor potential: The potential that develops when an adequate stimulus acts on a receptor (graded response).
  • When a threshold is reached, an action potential in the sensory neuron's axon is triggered.
  • Impulses travel over sensory pathways to the brain and spinal cord, where they are interpreted or initiate a reflex.
  • Adaptation: Receptor potential decreases over time in response to a continuous stimulus, leading to decreased impulse conduction and sensation intensity.

Responses of Sensory Receptors

  • Interaction of stimulus with sensory receptor produces a local potential.
    • Primary: Have axons that conduct action potential in response to receptor potential
    • Secondary: Have no axons and receptor potentials produced do not result in action potentials but cause release of neurotransmitters
  • Accommodation or adaptation: Decreased sensitivity to a continued stimulus