Neuroscience Lecture Notes: Somatotopy, Neurons, Thresholds, and Recording Methods

Somatotopic Maps, Penfield, and Sherrington

• Not physically connected, but still able to communicate.
• Sherrington acted as the mental supervisor for a little of Penfield, Miles of Penfield.
• Penfield is known for mapping parts of the body on the cortex (somatotopic map) and the homunculus stems from that work.
• The idea of a somatotopic map and the homunculus comes from Penfield's work; Horace Barlow refined ideas about perception depending on the combination of specialized neurons signaling different stimulus attributes.
• There are different touch neurons signaling different touch sensations; the brain interprets touch via the combination of inputs from these specialized neurons.

Brain areas and somatosensory representation

• The frontal lobe does not have a huge primary sensory role.
• The parietal lobe, especially the postcentral gyrus, contains the primary somatosensory cortex where the body map is located.
• The temporal lobe is involved in related processing; it sits near the parietal region.

Tools, figures, and biophysics foundations (Helmholtz, Cajal, etc.)

• Hermann von Helmholtz contributed to biophysics: neurons obey the rules of physics and chemistry; signaling involves electrochemical processes.
• Helmholtz described devices for examining the eye/retina (ophthalmoscope) and emphasized the physical basis of neural signaling.
• A modern alternative to the ophthalmoscope is a fundus camera that photographs the retina for examination.
• Helmholtz helped popularize biophysics: neurons are not magical; they operate with physical and chemical principles.
• Cajal is famous for drawings of neurons and for contributions to understanding synapses; he was awarded the Nobel Prize for medicine for those contributions. Today, photos of neurons look very different, but the basic neuron and synapse concepts remain.

Neurons: structure, membranes, and action potentials

• Neurons are a sequence of parts: cell body, axon, axon terminals.
• The cell membrane (bilayer) allows ions to pass through ion channels.
• Resting membrane potential is typically around $V_m \approx -70 \,\text{mV}$, due to charge separation across the membrane and selective ion distribution (more potassium inside, more sodium outside).
• Electrical potentials are the voltage difference across the membrane: the inside relative to the outside.
• An action potential is initiated when the membrane depolarizes toward a threshold, caused by a large influx of sodium ions.
• During an action potential, the membrane potential rises toward approximately $+30 \,\text{mV}$ to $+40 \,\text{mV}$.
• Sodium channels open, allowing Na⁺ to flow into the cell, causing depolarization.
• After a brief period, sodium channels close and voltage-gated potassium channels open, leading to repolarization.
• Repolarization brings the membrane potential back toward the resting level; the membrane may hyperpolarize below the resting potential before stabilizing.
• Potassium tends to leave the cell due to the higher internal concentration, contributing to repolarization.
• The extra positive charge exiting during repolarization causes the drop in membrane potential; potassium channels eventually close, leaving leaky potassium channels to set the resting state again.
• The Na⁺/K⁺ pump helps reestablish typical intracellular and extracellular ion concentrations after activity.
• As Na⁺ channels open, permeability to Na⁺ increases massively; the membrane potential moves toward the Na⁺ Nernst potential, which is positive, before channels close and K⁺ channels drive repolarization.
• The precise Nernst potentials (e.g., for Na⁺ and K⁺) are not the focus of this class, but the concept is acknowledged.
• In the context of the neuron, the concentrations near the membrane change a lot during signaling, even if the overall concentrations in the cell do not.
• Action potentials are all-or-none events; the brain must distinguish occasional random firing from rapid, stimulus-driven firing, using perceptual thresholds.

Perceptual thresholds, transduction, and sensory testing

• Perceptual threshold: the point at which the brain detects a stimulus; firing rate rises with stimulus intensity from baseline.
• Absolute threshold of hearing (as tested in audiology) measures the softest sounds at different frequencies a person can detect.
• Absolute threshold is often equated with perceptual threshold, though there can be different thresholds depending on context or task.
• Just noticeable difference (JND) refers to the smallest detectable difference between two stimuli, which is a different kind of threshold from the absolute threshold.
• Thresholds can change with context: e.g., after exposure to a loud environment, thresholds become higher (less sensitive to quiet sounds); after moving from bright to dark environments, visual thresholds increase.
• Transduction is the process by which sensory stimuli are converted into neural signals; this will be discussed further for different sensory systems.
• An example given: a sound below the perceptual threshold for some but not others; audiology tests probe the softest sounds detectable.

Recording neural activity: methods, ethics, and interpretation

• Directly recording action potentials from the human brain is invasive and raises ethical concerns; not routinely done.
• In peripheral nerves, needle electrodes can be used to directly record neural activity, but it is uncomfortable and invasive.
• Most brain activity recordings rely on averaging across many trials (event-related potentials, ERPs) to extract a signal from noisy data.
• In some cases, deep brain stimulation (DBS) is used in patients (e.g., Parkinson's disease). DBS involves an implanted electrical stimulator in regions like the basal ganglia and can also allow recording activity from nearby neurons, though not usually at the single-neuron level.
• Spike sorting: when recordings capture multiple neurons, waveform shapes can be used to separate which spikes came from which neuron, enabling analysis of individual neuron activity within a population.
• When people engage in tasks like playing an instrument, parts of the frontal lobe and motor cortex show activation related to planning and executing movements; the brain can appear to rehearse movements while listening to music.
• Mirror neurons: related to imitation and understanding others' actions; the term is touched on but not the central focus of this class.
• The takeaway: invasive recordings are rare and tightly regulated; non-invasive methods are commonly used to study brain activity.

Functional near-infrared spectroscopy (fNIRS) and related techniques

• Functional near-infrared spectroscopy (fNIRS) is a non-invasive method to monitor brain activity.
• It works by shining infrared light into the scalp and measuring how much light is reflected back to assess blood oxygenation changes, similar in principle to a pulse oximeter (which clips onto a finger and uses light to measure blood oxygenation).
• fNIRS provides a way to infer which brain regions are more active during a task by detecting changes in hemoglobin oxygenation.
• It offers a more portable and cost-effective alternative to some imaging modalities, though it is generally less precise spatially than fMRI and has other limitations.
• If a brain region is heavily used, the metabolic and vascular response leads to measurable changes detectable by fNIRS.

Connections and synthesis

• The somatotopic map reflects how the cortex represents the body; Penfield's work showed a map with disproportionate representation of certain body parts (homunculus).
• Perception arises from the integration of signals from specialized neurons (Barlow’s idea of feature-specific inputs combining to produce perception).
• The brain's activity connects structure (neural circuits) with function (sensation, perception, and action). Techniques ranging from single-neuron recordings (invasive) to ERPs (non-invasive averages) to fNIRS (non-invasive hemodynamic-based) provide complementary views.
• Practical considerations include ethics of invasive measurement, the need for averaging in noisy data, and the trade-offs between resolution, invasiveness, and practicality.

Quick reference: key terms and formulas

• Resting potential: $V_m \,\approx\,-70 \,\text{mV}$
• Action potential peak: $V_m \approx +30 \text{ to } +40 \,\text{mV}$
• Nernst potentials (illustrative): $E<em>{Na} \approx +60 \text{ mV}, \quad E</em>{K} \approx -90 \text{ mV}$
• Perceptual threshold: the stimulus level at which detection occurs; can vary with context
• Absolute threshold (as used in audiology): the minimum detectable stimulus intensity at a given frequency
• Event-related potential (ERP): averaged neural response time-locked to a stimulus
• Spike sorting: computational separation of action potentials from multiple neurons recorded together
• Deep brain stimulation (DBS): implanted device delivering electrical stimulation to brain regions (e.g., basal ganglia) for disorders like Parkinson's disease
• fNIRS: functional near-infrared spectroscopy, measures changes in blood oxygenation to infer brain activity