Quick reference · read before Ch. 1

Basic Concepts & Terms of the Nervous System

The organizing ideas and vocabulary every neuroanatomy chapter assumes you already know — how the system is divided, what a nucleus/tract/ganglion actually means, the directional language used in the brainstem vs the cord, and the generic rules (three-neuron chains, decussation) that repeat in every pathway you'll meet later.

🗂️ How the Nervous System Is Divided

The same "system" gets sliced up several different ways depending on whether you're talking about location, function, or voluntary control — these three divisions overlap, they don't replace each other.

CNS (central nervous system)
brain + spinal cord
PNS (peripheral nervous system)
everything outside the brain/cord — nerves, ganglia, plexuses
somatic nervous system
voluntary control of skeletal muscle; conscious sensation
autonomic nervous system (ANS)
involuntary control of viscera, glands, smooth/cardiac muscle
sympathetic division
"fight or flight" branch of the ANS
parasympathetic division
"rest and digest" branch of the ANS
enteric nervous system
semi-independent neural network of the gut wall
afferent
carrying information toward the CNS (sensory, "arrives")
efferent
carrying information away from the CNS (motor, "exits")
visceral
relating to internal organs
somatic (sensory/motor)
relating to body wall/skeletal muscle rather than organs
general vs special
"general" = widespread modality (touch, pain); "special" = restricted to a sense organ (vision, hearing)

🧱 Gray Matter, White Matter & the Words Built From Them

Same two tissue types, different names depending on shape and whether you're in the CNS or PNS.

gray matter
neuron cell bodies + dendrites + unmyelinated processes
white matter
myelinated axons — appears pale because of myelin lipid
nucleus (CNS)
a cluster of neuron cell bodies inside the CNS
ganglion (PNS)
a cluster of neuron cell bodies outside the CNS
cortex
a sheet/layer of gray matter on the surface (cerebral, cerebellar cortex)
tract / fasciculus
a bundle of axons inside the CNS sharing origin, course, and destination
nerve
a bundle of axons outside the CNS (PNS equivalent of a tract)
lemniscus
a ribbon-like sensory tract (e.g., medial lemniscus)
peduncle
a large stalk-like bundle connecting two regions (e.g., cerebellar peduncles)
commissure
fibers crossing the midline connecting corresponding areas of the two sides (e.g., corpus callosum)
decussation
a crossing point of fibers/tracts from one side to the other
column / funiculus
a longitudinal region of white matter in the spinal cord (dorsal, lateral, ventral)
horn (spinal cord)
a projection of gray matter in cord cross-section (dorsal, ventral, lateral horn)
plexus
a network of interweaving nerves (or vessels)
ramus
a branch of a nerve
rootlet / root
the fine filaments (rootlets) that converge into a spinal nerve root (dorsal = sensory, ventral = motor)

🧭 Directional & Positional Language in Neuroanatomy

Because the neuraxis bends at the cephalic flexure, "anterior/posterior" flips meaning between the cord/brainstem and the cerebrum — rostral/caudal keeps the same meaning everywhere along the axis.

rostral
toward the front of the neuraxis (toward the nose), regardless of body posture
caudal
toward the tail end of the neuraxis
dorsal
toward the back — in the cord/brainstem, dorsal = sensory side
ventral
toward the belly — in the cord/brainstem, ventral = motor side
ipsilateral
same side of the body
contralateral
opposite side of the body
bilateral
both sides
medial
toward the midline
lateral
away from the midline
superior / inferior
above / below — used loosely with rostral/caudal in the forebrain
proximal / distal (in a pathway)
closer to / farther from the cell body or CNS
longitudinal axis / neuraxis
the long axis running from spinal cord through brainstem to cerebrum

🛡️ Meninges & Coverings

meninges
the three connective tissue layers wrapping the CNS
dura mater
outermost, tough fibrous layer
arachnoid mater
middle, web-like avascular layer
pia mater
innermost layer, closely applied to the CNS surface, highly vascular
epidural space
potential space external to the dura (true space in the spinal canal)
subdural space
potential space between dura and arachnoid
subarachnoid space
true space between arachnoid and pia, contains CSF
arachnoid granulations
projections of arachnoid into dural venous sinuses that resorb CSF
falx cerebri / tentorium cerebelli
dural folds separating the cerebral hemispheres / the cerebrum from cerebellum
epineurium / perineurium / endoneurium
connective tissue layers wrapping a peripheral nerve, its fascicles, and individual axons respectively

🌱 Development Vocabulary

Just the naming logic — full developmental detail lives in the Autonomic & Development chapter.

neural tube
the CNS precursor, formed by neurulation from the neural plate
neural crest
cells at the neural tube margins → PNS ganglia, Schwann cells, and more
neuropore
the open ends of the neural tube before closure (rostral/anterior and caudal/posterior)
primary vesicles
prosencephalon, mesencephalon, rhombencephalon — the first three brain swellings
secondary vesicles
telencephalon + diencephalon (from prosencephalon); metencephalon + myelencephalon (from rhombencephalon)
telencephalon
→ cerebral hemispheres, basal ganglia
diencephalon
→ thalamus, hypothalamus, epithalamus
mesencephalon
→ midbrain (stays undivided)
metencephalon
→ pons + cerebellum
myelencephalon
→ medulla oblongata
alar plate / basal plate
dorsal (sensory) / ventral (motor) halves of the developing neural tube
sulcus limitans
groove separating alar (sensory) from basal (motor) plate

🔁 Generic Rules Every Pathway Follows

Most ascending sensory and descending motor pathways you'll meet later are variations on this same skeleton.

1st-order neuron ➜ synapse ➜ 2nd-order neuron (usually decussates) ➜ synapse (often thalamus) ➜ 3rd-order neuron → cortex
three-neuron chain
the generic template for a sensory pathway: 1st order (periphery→CNS), 2nd order (decussates, →thalamus), 3rd order (thalamus→cortex)
two-neuron chain
generic template for a motor pathway: upper motor neuron (cortex/brainstem→cord) and lower motor neuron (cord→muscle)
upper motor neuron (UMN)
neuron with cell body in cortex or brainstem, synapses onto the LMN
lower motor neuron (LMN)
neuron with cell body in the anterior horn or cranial nerve nucleus, directly innervates muscle
first-order neuron
sensory neuron with cell body in a peripheral (dorsal root/cranial) ganglion
second-order neuron
relay neuron, cell body in cord/brainstem, usually the point of decussation
third-order neuron
final relay, cell body in thalamus, projects to cortex
homunculus
the distorted map of the body represented on motor/sensory cortex, sized by function not size
somatotopy
orderly point-to-point spatial mapping of the body onto a neural structure
reflex arc
the shortest possible circuit: receptor → afferent → (interneuron) → efferent → effector
monosynaptic reflex
reflex with a single synapse between afferent and efferent neuron (e.g., knee jerk)
dermatome
skin area supplied by a single spinal nerve's sensory fibers
myotome
muscle group supplied by a single spinal nerve's motor fibers

⚡ Cellular & Functional Basics

Fuller detail on neurons, glia, and myelin lives in the Neurons & Myelination chapter — these are just the words used everywhere else.

neuron
the excitable, signal-conducting cell of the nervous system
glia (neuroglia)
non-neuronal supporting cells (astrocytes, oligodendrocytes, microglia, ependyma, Schwann cells)
synapse
the junction where one neuron communicates with the next
neurotransmitter
a chemical messenger released at a synapse
myelin
the lipid-rich insulating sheath that speeds conduction
action potential
the all-or-none electrical impulse conducted along an axon
receptor (sensory)
a specialized structure that transduces a stimulus into a neural signal
effector
the muscle or gland acted on at the end of a motor pathway
excitatory / inhibitory
whether a signal makes the next neuron more or less likely to fire
circuit / network
a set of interconnected neurons performing a shared function

🩺 Clinical Shorthand You'll See Constantly

lesion
any area of damage/dysfunction in the nervous system
paresis
partial weakness
plegia / paralysis
complete loss of motor function
hemi- / para- / quadri-/tetra-
prefixes for one side / both legs / all four limbs affected
ataxia
loss of coordinated, smooth movement
hypotonia / hypertonia
decreased / increased resting muscle tone
hyperreflexia / hyporeflexia
exaggerated / diminished deep tendon reflexes
Babinski sign
extensor plantar response — suggests an upper motor neuron lesion
nystagmus
involuntary rhythmic oscillation of the eyes
dysdiadochokinesia
impaired rapid alternating movements — classic cerebellar sign
anesthesia / hypoesthesia
absent / reduced sensation
paresthesia
abnormal spontaneous sensation (tingling, "pins and needles")
localizing sign
a clinical finding that points to the anatomical site of a lesion
StepWise USMLE Notes · Basic Concepts & Terms of the Nervous System