Topic 2 - Unit 2- Fundamentals of sensory physiology

Neuron structure and function

The neuron on the right consists of a cell body, dendrites, and an axon, or nerve fiber. The neuron on the left that receives stimuli from the environment has a receptor in place of the cell body.

Neuron Components: Cell body (keeps cell alive), dendrites (receive signals from other neurons), and axon/nerve fiber (conducts electrical signals). Sensory receptors are specialized neurons that respond to environmental stimuli.

Neural Processing: Neurons are interconnected in complex networks—signals don't travel in straight lines but through interconnected pathways where they meet and are affected by other signals. This neural processing creates neurons that respond to specific features like slanted lines, faces, movement direction, or tastes.

How Electrical Signals in Neurons Are Studied:

Electrical signals are recorded from the axons (or nerve fibers) of neurons using small electrodes to pick up the signals.

The same neuron is stimulated at different intensities

Researchers record its electrical activity using electrodes

Rate of firing: 

Meaning

• Stronger stimulation → more action potentials per second

• Weak stimulation → fewer action potentials

Example:

• Soft touch → slow firing

• Strong pressure → rapid firing

Regularity of firing: 

Meaning

• Stronger stimuli produce:

  • More consistent

  • More regular firing patterns

Weak stimuli:

• Produce irregular or sporadic firing

Action potentials

1. Resting State (–70 mV)

What this means

• The inside of the neuron is more negative than the outside.

• This difference in electrical charge is called the resting membrane potential.

• –70 mV is normal for most neurons.

Why this happens

• Different ions (Na⁺, K⁺) are unevenly distributed

• The membrane is more permeable to K⁺

• The sodium–potassium pump maintains this imbalance

 Measuring the Charge (Electrode)

Meaning

• An electrode is placed near or inside the neuron.

• It measures the voltage difference between inside and outside.

• At rest, it reads –70 mV.

3. Rising Phase (Depolarization)

What’s happening

• A stimulus triggers the neuron

• Voltage-gated sodium (Na⁺) channels open

• Sodium rushes into the neuron

Result

• Inside becomes less negative, then positive

• This is called depolarization

• Appears as the rising phase on the graph

4. Falling Phase (Repolarization)

What’s happening

• Sodium channels close

• Potassium (K⁺) channels open

• Potassium flows out of the neuron

Result

• The inside becomes negative again

• This is called repolarization

• Appears as the falling phase of the action potential

5. Return to Resting State: 

What happens next

• The neuron may briefly become more negative than –70 mV (hyperpolarization)

• The sodium–potassium pump restores normal ion distribution

• The neuron is ready for another signal

Synaptic transmission

Action potentials can't jump the synapse (gap between neurons). Instead:

1. Action potential triggers release of neurotransmitters from synaptic vesicles

2. Neurotransmitters flow across synapse

3. Bind to matching receptor sites on receiving neuron (like key in lock)

4. Cause voltage change in receiving neuron

Two Response Types

Excitatory: Causes depolarization (more positive inside), increases firing likelihood

Inhibitory: Causes hyperpolarization (more negative inside), decreases firing likelihood

Multiple inputs combine to determine if receiving neuron fires. Both excitation and inhibition are essential for neural processing.

Sensory coding: how neurons represent information

Specificity Coding

One neuron represents one specific stimulus/concept (e.g., "grandmother cell" fires only for your grandmother). Quiroga et al. found neurons responding specifically to Steve Carell or Halle Berry from multiple views and representations. However, limited recording time means these neurons might respond to other stimuli if tested further—grandmother cells likely don't exist.

Sparse Coding

A small group of neurons with overlapping responses represents each stimulus through their firing pattern. Most neurons remain silent. Evidence supports this for visual objects, auditory tones, and odors.

Population Coding

Large groups of neurons create unique patterns for each stimulus. Allows representation of huge numbers of stimuli through different patterns. Evidence exists across all senses.

Brain representation and modularity

Historical Context

Phrenology (Gall & Spurzheim, 18th century) wrongly claimed skull bumps revealed mental faculties but correctly proposed different brain areas serve different functions.

Modularity Evidence

Neuropsychology: Broca's area (frontal lobe) = speech production; Wernicke's area (temporal lobe) = speech comprehension

fMRI studies: Superior temporal sulcus (STS) = "voice area" responds specifically to vocal sounds vs. environmental sounds

fMRI Method

Measures blood flow changes in brain voxels (2-3 mm cubes containing many neurons). Active areas consume more oxygen, changing hemoglobin's magnetic properties. Colors indicate activation levels during tasks vs. baseline.

Distributed representation

Perceptions involve networks of brain areas working together, not just isolated modules.

Examples:

• Pain: Activates multiple areas for sensory (location, intensity), emotional (unpleasantness), and motor (reflexive) components

Object perception: Houses, faces, chairs show maximum activity in separate areas BUT also activate widespread cortical regions

Connectivity between brain areas

Structural Connectivity

Physical fiber pathways connecting brain regions (the "road map")

Functional Connectivity

Neural activity patterns flowing through networks during specific functions (the "traffic patterns")