🧠 Lecture 12 – Nervous System Organization

neural circuit

a group of interconnected neurons that process specific information and produce specific outputs

excitatory connection

a synapse that increases the likelihood of the postsynaptic neuron firing an action potential (produces EPSPs)

inhibitory connection

a synapse that decreases the likelihood of the postsynaptic neuron firing an action potential

convergence

when multiple presynaptic neurons form synapses on a single postsynaptic neuron, allowing integration of many inputs

divergence

when one presynaptic neuron sends signals to multiple postsynaptic neurons, spreading information to several targets

recurrence (feedback inhibition)

when a postsynaptic neuron sends a divergent pathway that synapses back on its presynaptic partner; also called reverberation

information flow through a simple circuit

sensory neuron → interneuron (integration) → motor neuron → effector (e.g. muscle)

example

spinal reflec arcs (like the knee-jerk reflex) illustrates convergence, divergence, and inhibitory control in neural circuits

skeleton

the skull and vertebral column protect the brain and spinal cord from mechanical injury

meninges

three connective tissue layers (dura mater, arachnoid mater, pia mater) that enclose and protect the CNS

cerebrospinal fluid (CSF)

a clear fluid produced by ependymal cells in the ventricles that cushions the brain and spinal cord, and cleanses wastes through glymphatic flow

ventricles

fluid-filled cavities in the brain where CSF is produced and circulates into the subarachnoid space

blood-brain barrier (BBB)

a highly selective barrier formed by tight junctions in CNS capillary endothelium; regulates the exchange of molecules between blood and neural tissue

transport across BBB

only lipophilic molecules or hydrophilic molecules with specific transport proteins can cross

astrocytes at BBB

help regulate local blood flow and maintain homeostasis of the CNS environment

primary cortical area

a brain region that serves as the main site of sensory input or motor output for a specific modality

primary motor cortex (M1)

located in the frontal lobe; responsible for initiating voluntary movements

primary somatosensory cortex (S1)

located in the parietal lobe; responsible for conscious perception of touch, pressure, pain and proprioception

primary visual cortex (V1)

located in the occipital love; processes visual input from the eyes (organized retinotopically)

primary auditory cortex (A1)

located in the temporal love; processes sound information

topographic mapping

the organization of neurons such as that adjacent neurons process adjacent regions of the body or sensory space (e.g. somatopy in M1/S1, retinotopy in V1)

crossed projections (decussation

axons from one side of the brain project to or receive input from the opposite side of the body

lateralization

the specialization of certain functions to one hemisphere of the brain (e.g. language centers on the left)

ascending pathways

carry sensory info from the body to the brain via relay synapses (e.g. spinal cord → thalamus → cortex)

descending pathways

carry moto commands from the brain to the body (e.g. cortex → spinal cord → muscles)

lesion effects

• damage to S1 → sensory deficits (e.g. numbness)

• damage to M1 → motor deficits (e.g. weakness or paralysis)

• because of decussation, damage to one hemisphere causes symptoms on the opposite side of the body

example

• stroke in the right motor cortex → left-sided paralysis

• stroke in the left somatosensory cortex → right-sided numbness

EEG

measures electrical fields generated by groups of neurons active in synchrony, detected by scalp electrodes

EEG signal strength

strongest when many neurons in a region are active simultanesly (synchronous activity)

EEG use cases

detects sleep stages, seizures, and levels of consciousness (fast sampling rate: 10-20 kHz)

fMRI (functional magnetic resonance imaging

measures changes in blood flow in different brain regions as an indirect indicator of neural activity

fMRI signal basis

increased local blood flow increases the oxygenated blood signal (“lights up” on the scan)

fMRI use cases

identifies active brain regions during tasks or in disease states (slow sampling rate: ~6 seconds)

key difference

EEG = direct electrical activity (fast, poor spatial resolution)

fMRI = indirect metabolic activity (slow, high spacial resolution