Nerve Cells and Behavior Notes 5

Nerve Cells and Behavior

Nervous System Overview

  • Adaptive behavior relies on the interplay between the nervous system, the body, and the environment.
  • Sensory receptors detect stimuli from the environment.
  • Sensory input is filtered to prioritize biologically relevant information.
  • The nervous system generates adaptive responses to stimuli.

Nerve Cells

  • Electrically excitable cells; humans have approximately 10^{10} nerve cells.
  • Communication is electrochemical, using less than 10 watts of power.
  • Connections between nerve cells can be excitatory or inhibitory.

Generalized Nerve Cell Structure

  • Cell body (soma): Diameter < 10 μm.
  • Axon: Diameter of 2-3 μm.
  • Dendrites: Diameter ~1 μm.
  • Interconnectivity: Receives inputs from 1,000 to 10,000 other cells.

Examples of Nerve Cell Variation:

  • Motor Neuron from Spinal Cord
  • Mitral Cell from Olfactory Bulb
  • Purkinje Cell
  • Pyramidal Cell from Cortex

Nerve Cell Physiology

  • Plasma membrane (PM) maintains a difference between the inside and outside of the cell.
  • Gradients exist across the PM for Na^+ and K^+.
  • Resting membrane potential: Inside of the cell is negatively charged (~ -70mV) relative to the outside.
  • Depolarization: Changing the gradient can depolarize the membrane, leading to an action potential.
Action Potential (Spike)
  • All-or-nothing event: a wave of depolarization that travels down the axon of a neuron.
  • Speed is crucial.
    • Invertebrates increase speed by increasing axon diameter, which reduces resistance.
    • Vertebrates increase speed by myelinating the axon; this allows the impulse to jump along the axon in a process called "saltation."

Discussion Questions

  1. Diseases involving demyelination: Multiple Sclerosis (MS).
  2. Demyelination and AP transmission: Demyelination would decrease the rate of transmission of action potentials. This can have significant health consequences for the organism.

Synapses

  • Junctions between nerve cells.
    1. Electrical Synapses: direct electrochemical contact between two neurons.
    2. Chemical Synapses: release neurotransmitters to signal other neurons.

Electrical Synapses

  • Less common than chemical synapses.
  • Fast transmission speed: 0.1 milliseconds.
  • Small gap (~ 2nm) bridged by small tubes through which ions flow from one neuron into the next.

Chemical Synapses

  1. Diffusion of neurotransmitter into the synaptic cleft.
  2. Binding of neurotransmitters to receptors on the post-synaptic cell.
Main Categories of Effect of Chemical Synapses
  • Excitatory: Depolarization of the post-synaptic cell.
  • Inhibitory: Hyperpolarization of the post-synaptic cell.

Pathways

  • Afferent: Ascending/toward the central nervous system (CNS), typically sensory.

  • Efferent: Descending/away from the CNS, typically motor.

    • Receptor cell → spinal cord → brainstem → higher brain areas
    • Higher brain regions → brainstem → periphery (effector)

Vertebrate Brain Anatomy

  • Telencephalon: Cerebral hemispheres.
  • Diencephalon: Thalamus & hypothalamus.
  • Mesencephalon: Midbrain.
  • Metencephalon: Pons & cerebellum.
  • Myelencephalon: Medulla.

Sensory Receptors

  • Chemoreceptors: Detect chemicals.
  • Mechanoreceptors: Detect stretch, motion, and pressure.
  • Photoreceptors: Detect light.
  • Electroreceptors: Detect current and electrical fields.
  • Thermoreceptors: Detect temperature changes.

Discussion Questions

  1. Receptors on Tongue: Chemoreceptors (taste).
    Receptors on Skin: Mechanoreceptors (pressure, touch), Thermoreceptors (temperature).
  2. Behavior examples:
    Chemoreceptors: Stimulation leads to salivation, swallowing or rejection of foods.
    Mechanoreceptors: Stimulation leads to withdrawal from painful stimuli.

Receptive Field

  • Region of space where the presence of a stimulus can affect the firing of a neuron.
  • Example: Receptive field in light detection.
    • The receptive field for a ganglion cell includes all photoreceptors synapsing with it.
    • Ganglion cells group together to form the receptive field for the cell in the brain they synapse with.

Response Patterns

  1. Tonic: Continuous activity. Sustained depolarization with continuous sensory nerve firing.
  2. Phasic: Responds to change in stimulus (rapid adaptation). Rapid adaptation is a form of feature extraction.

Neural Encoding

  • How the nervous system encodes information:
    1. Intensity (population) code: Number of cells excited.
    2. Place code: Receptor surface position represents a physical feature in space.
    3. Frequency code: Spike rate is proportional to stimulus strength (refractory period may limit rate).
  • Example: African Clawed Frog (Xenopus laevis).
  • Matched Filter: Another form of feature extraction.

Neural Mechanisms and Behavioral Adaptation

  • Example: Moth's ability to detect echolocating bats (Roeder 1967).

  • A1 and A2 cells in thoracic 'ears'.

    • A1 cell: Sensitive to low-intensity sounds; helps determine the direction and distance of the bat.
    • A2 cells: Fire only when the bat is very close.
      • Originally thought to trigger evasive maneuvers, but new information suggests that evasive maneuvers are caused by combined information from A1 cells.
    • A1 cells fire sooner and more rapidly in the direction of the echolocating bat.
    • When the bat gets close, it switches to very rapid ultrasonic pulses, causing the moth to switch to evasive maneuvers.

  • Both A1 and A2 cells are sensitive to ultrasound (20 – 50 kHz) and are unresponsive to sounds < 18 kHz.

    • A1 = directionality; the moth will turn to equalize inputs from both sides, flying directly away from a distant bat.
    • A2 cells are recruited when the bat is much closer, triggering evasive/erratic flight or free fall. Once the moth is on the ground, it is masked by the ground from echolocation.

Discussion Questions

  • Long-eared bats 'whisper' instead of 'shout', producing quieter sounds when echolocating.
    1. Moth behavior in response to long-eared bats: Moths should exhibit less reliable or delayed responses, especially if the bats remain far away. Since A2 cells only fire when bats are very close, the quieter sounds of long-eared bats may not trigger A2 cell firing at all.
    2. Evolution of Moth Behavior: If long-eared bats are the ONLY predators, moth behavior might evolve towards reduced sensitivity to the typical echolocation frequencies (reduced A1 sensitivity), or increased reliance on other senses, like passive listening for wing sounds. Alternatively, moths could become more sensitive to the quieter echolocation calls of these specialized bats.