Neurons, Action Potentials, Synapses, and Cerebral Cortex – Study Notes
Neuron Structure and Signaling
Neurons receive excitatory and inhibitory signals via dendrites, which are passed to the soma.
If excitatory input outweighs inhibitory, the neuron generates an action potential.
The action potential (electrical impulse) travels unidirectionally from the soma down the axon to axon terminals.
Myelin sheath insulates the axon, speeding up conduction.
At axon terminals, the electrical signal converts to a chemical signal, releasing neurotransmitters into the synapse (the gap between neurons).
The axon hillock is where the action potential is initiated.
Neurons function as digital units (on/off); brain function emerges from patterns of activity across many neurons.
Action Potential: Properties and Phases
An action potential is a brief electrical impulse (t \in [0, 5] \text{ ms}).
Membrane Potentials:
Resting: V_{rest} \approx -70 \text{ mV}
Threshold: V_{th} \approx -45 \text{ mV}
Peak: V_{max} \approx +30 \text{ mV}
Phases: Depolarization (rise to positive), Repolarization (return to negative), Hyperpolarization (brief dip below rest).
All-or-None Property: Reaching V_{th} triggers a full action potential; insufficient stimulation produces none. Amplitude is constant, intensity is encoded by firing rate.
Refractory Period: During hyperpolarization (t_{ref} \approx 5 \text{ ms}), another action potential cannot be fired.
Frequency Coding: Stronger stimuli increase firing rate, not action potential amplitude.
Signal Transmission at the Synapse
Action potentials trigger neurotransmitter release into the synapse.
Transmission is chemical, not electrical.
The type of neurotransmitter determines the postsynaptic response (excitatory or inhibitory).
Brain Anatomy: Gross Structure
The human brain weighs about 3 \text{ pounds} and contains about 10^{11} neurons.
Gray Matter: Neuronal cell bodies on the cortex surface.
White Matter: Myelinated axons beneath gray matter.
Cerebral Cortex: Highly folded sheet (gyri/sulci) divided into two hemispheres by the longitudinal fissure.
Lobes (Left Hemisphere View):
Frontal Lobe: Motor control, higher cognitive functions.
Parietal Lobe: Bodily sensations, sensory integration.
Occipital Lobe: Visual processing.
Temporal Lobe: Hearing, language processing.
Key Sulci: Lateral sulcus (separates temporal from frontal/parietal), Central sulcus (separates frontal from parietal).
Directional Terminology: Dorsal (top), Ventral (bottom), Anterior (front), Posterior (back), Medial (middle), Lateral (outside).
Internal Brain Structures
Corpus Callosum: Connects the two hemispheres.
Thalamus: Major sensory relay station to the cortex (except smell).
Hypothalamus: Regulates body temperature, thirst, hunger; associated with pituitary gland.
Brainstem (Pons, Medulla Oblongata): Connects to spinal cord.
Cerebellum: Coordinates muscle movements.
Functional Anatomy of the Cerebral Cortex
Frontal Lobe
Primary Motor Area: Voluntary movement.
Broca's Area (Left Hemisphere): Speech production.
Prefrontal Area: Decision-making, planning.
Parietal Lobe
Primary Somatosensory Area: Initial processing of bodily sensations.
Primary Taste Area: Processing taste.
Temporal Lobe
Primary Auditory Area: Initial processing of sound.
Wernicke's Area (Left Hemisphere): Language comprehension.
Occipital Lobe
Primary Visual Area: Initial processing of visual information.
Summary
Neurons are digital units whose collective activity forms perception. Brain processing is based on patterns across neural networks.
Thalamus is a central sensory relay hub.
Hypothalamus and pituitary regulate basic drives/homeostasis.
Cerebellum coordinates movements.
Key Numerical/Time References: V{rest} \approx -70 \text{ mV}, V{th} \approx -45 \text{ mV}, V{max} \approx +30 \text{ mV}, Action potential time: t \in [0, 5] \text{ ms}, Refractory period: t{ref} \approx 5 \text{ ms}.