Source: Lecture slides, "A Brain-Mind Odyssey" | Textbook: Luo, Principles of Neurobiology
Tags: neural circuits, brain anatomy, CNS, PNS, cerebral cortex, neurons, action potentials, sensory systems, visual pathway, somatosensory system, Brodmann areas, white matter, grey matter, stretch reflex, neuroscience
The nervous system is organised at multiple scales: individual neurons, local microcircuits, and large-scale brain systems. Neural circuits transmit and process information using action potentials (spikes), and different brain regions are specialised for different functions. A brain becomes necessary when an organism needs fast, flexible, and adaptive responses at a spatial scale beyond what single-cell diffusion can handle.
Neural circuit
A network of interconnected neurons that processes information and produces a functional output (sensation, movement, cognition).
Action potential (spike)
An all-or-none electrical signal that travels along an axon. Spikes encode and transmit information between neurons.
Central nervous system (CNS)
The brain and spinal cord. The retina is also technically part of the CNS.
Peripheral nervous system (PNS)
All nerves and neurons outside the CNS.
White matter
Bundles of long-range, myelinated axons that connect distant brain regions.
Grey matter
Brain tissue composed of cell bodies, dendrites, and short-range axons. Found in the cerebral cortex and subcortical nuclei.
Synapse
A junction between two neurons where information is transmitted, either chemically (via neurotransmitter) or electrically (via gap junctions/connexins).
EPSP (excitatory postsynaptic potential)
A graded depolarisation in the postsynaptic neuron caused by synaptic input. EPSPs summate, and if they reach spike threshold, the neuron fires an action potential.
Feedforward system
A neural pathway in which information flows in one direction, from sensory input through relay stations to cortex (e.g. touch receptor to dorsal column nuclei to thalamus to somatosensory cortex).
Microcircuit
A local network of neurons within a single brain area that performs a specific computation on its inputs.
Brodmann areas
A map of cerebral cortex regions defined by K. Brodmann (1909) based on cytoarchitecture (the cellular organisation of each area). Different Brodmann areas correspond to different functions.
Single-celled organisms manage behaviour through diffusion-based signalling. One organism, one function, one cell-scale process.
A brain becomes necessary when:
The organism operates at a spatial scale too large for diffusion
Fast, flexible, and adaptive reactions and movements are required
The organism needs to remember past experiences and modify behaviour accordingly (learning)
Qualia and perception require integration of experience
René Descartes recognised that the body could produce simple reflexes mechanically (he described what we now call "pain reflexes"), but a brain was needed for more complex, experience-dependent behaviour. His concept of "animal spirits" maps loosely onto what we now understand as action potentials travelling along defined neural pathways.
The course frames everything along a hierarchy:
Neurons (single cells that fire action potentials)
Circuits (local networks of neurons)
Large-scale brain systems (multiple interconnected regions)
Behaviour and cognition (the output)
The central questions linking these levels: how do circuits process information, mediate sensation, generate movement, store memories, drive innate behaviours, and mediate emotion, attention, and cognitive function?
Sagittal view (midline):
Cerebral cortex (outermost layer, folded surface)
Corpus callosum (connects left and right hemispheres)
Thalamus (sensory relay station)
Midbrain
Pons
Cerebellum (motor coordination)
Medulla (autonomic functions)
Spinal cord
Brain stem comprises midbrain, pons, and medulla.
Spinal cord regions (top to bottom): cervical, thoracic, lumbar, sacral.
The cortex surface has sulci (grooves) and gyri (ridges). Major landmarks include the central sulcus and the Sylvian fissure.
Four lobes:
Frontal lobe (anterior to central sulcus)
Parietal lobe (posterior to central sulcus)
Temporal lobe (below Sylvian fissure)
Occipital lobe (posterior)
Key functional areas (small sample):
M1 (primary motor cortex, precentral gyrus)
S1 (primary somatosensory cortex, postcentral gyrus)
S2 (secondary somatosensory)
V1, V2, V4 (visual cortex areas, occipital lobe)
MT, MST, LIP (motion and spatial processing)
A1 (primary auditory cortex)
IT (inferotemporal cortex, object recognition, includes "face patches")
dlPFC (dorsolateral prefrontal cortex)
FEF (frontal eye fields)
SMA, PMd, PMv (premotor and supplementary motor areas)
EC (entorhinal cortex)
Piriform cortex (olfaction)
In a coronal brain section you can see:
Grey matter on the outside (cortex) and in deep structures (thalamus, striatum, hippocampus)
White matter in between, forming the connections
Lateral ventricles (fluid-filled spaces)
Hippocampus (medial temporal lobe, important for long-term potentiation, LTP, and memory)
Two example sensory systems illustrate how circuits connect regions:
Somatosensory (touch) pathway:
Skin receptor → dorsal root ganglion → spinal cord → dorsal column nuclei (synapse) → thalamus VPL (synapse) → primary somatosensory cortex (S1)
Visual pathway:
Retina → LGN of thalamus (synapse) → visual cortex (V1)
Both are feedforward systems with synaptic relays at each stage. Information is encoded by spiking at each stage.
The primate visual system involves roughly 20 cortical areas, each specialised for different subtasks.
Information flows from LGN (with magnocellular and parvocellular divisions) to V1 (primary visual cortex), then branches:
V1 → V2 → V4 → TEO → TE: colour and shape processing (ventral/"what" stream, temporal lobe)
V1 → V2 → V3/MT → parietal areas (7a, 7b, LIP, VIP): movement and spatial position (dorsal/"where" stream, parietal lobe)
V1 is primary visual cortex. V2 is secondary visual cortex. V1 contains subdivisions (4B, interblob, blob) that receive different LGN inputs.
The knee-jerk (stretch) reflex is a two-neuron, one-synapse circuit:
A tap on the patellar tendon stretches the quadriceps muscle
Sensory neuron (muscle spindle) detects the stretch, fires action potentials
Sensory neuron synapses directly onto a motor neuron in the spinal cord (using glutamate)
Motor neuron fires, releasing ACh at the neuromuscular junction (NMJ)
Quadriceps contracts
Signal processing at each stage follows the same pattern: input → integration → conduction → output (transmitter release or muscle contraction).
Sensory neuron: graded receptor potential → action potential → action potential conduction → glutamate release
Motor neuron: graded synaptic potential → action potential → conduction → ACh release at NMJ
Muscle: graded synaptic potential → action potential → conduction → contraction
Spikes convey information. They are the universal currency of neural communication.
Sagittal brain diagram with major structures labelled (cortex, corpus callosum, thalamus, midbrain, pons, cerebellum, medulla, spinal cord)
Coronal section showing grey matter, white matter, ventricles, hippocampus, thalamus, striatum
Cortical lobe map with central sulcus and Sylvian fissure
Functional area map (M1, S1, V1, A1, IT, etc.)
Somatosensory and visual pathway diagrams (feedforward, with synapse points)
Visual system hierarchy (LGN → V1 → V2 → branching dorsal/ventral streams)
Stretch reflex circuit (sensory neuron → motor neuron → muscle)
⚠️ Know the distinction between CNS and PNS (and that the retina counts as CNS).
⚠️ Be able to label major brain structures on both sagittal and coronal sections.
⚠️ Understand the difference between white matter (myelinated axon bundles) and grey matter (cell bodies and dendrites).
⚠️ The stretch reflex is the simplest possible circuit (two neurons, one synapse). It illustrates the input → integration → conduction → output processing chain that repeats at every stage.
⚠️ Glutamate is the transmitter at the sensory-to-motor synapse in the stretch reflex. ACh is the transmitter at the neuromuscular junction.
⚠️ The visual system hierarchy (LGN → V1 → V2 → branching streams) is a key example of how many specialised areas work together. Roughly 20 cortical areas are involved in primate vision.
⚠️ Brodmann areas are defined by cytoarchitecture, not by function, though they often correspond to functional boundaries.
Q: What are the two divisions of the nervous system, and what does each include?
A: The central nervous system (CNS) includes the brain, spinal cord, and retina. The peripheral nervous system (PNS) includes all nerves and neurons outside the CNS.
Q: Why do large, complex organisms need a brain rather than relying on diffusion-based signalling?
A: Diffusion is too slow and spatially limited for organisms that operate at a large scale. A brain provides fast signalling along defined pathways, enabling flexible and adaptive responses, memory, and experience-dependent behaviour modification.
Q: In the stretch reflex, what neurotransmitter does the sensory neuron release onto the motor neuron, and what does the motor neuron release at the muscle?
A: The sensory neuron releases glutamate. The motor neuron releases acetylcholine (ACh) at the neuromuscular junction.
Q: What is the difference between white matter and grey matter?
A: White matter consists of bundles of long-range, myelinated axons connecting distant brain regions. Grey matter contains neuronal cell bodies, dendrites, and short-range axons.
Q: Trace the somatosensory (touch) pathway from skin to cortex, naming each relay point.
A: Skin receptor → dorsal root ganglion → spinal cord → dorsal column nuclei (synapse) → thalamus VPL (synapse) → primary somatosensory cortex (S1).
Q: In the visual system hierarchy, what are the two major processing streams and what does each handle?
A: The ventral stream (V1 → V2 → V4 → IT/TE, running into the temporal lobe) processes colour and shape ("what"). The dorsal stream (V1 → V2 → MT → parietal areas, running into the parietal lobe) processes movement and spatial position ("where").
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