Neurophysiology – Rapid Review

Nerve Fibers

• Structural types: myelinated vs. non-myelinated
• Distribution: somatic (skeletal mm.) vs. visceral/automatic (organs)
• Origin: cranial vs. spinal
• Function: sensory (afferent) vs. motor (efferent)
• Neurotransmitter: adrenergic (NA) vs. cholinergic (ACh)
• Erlanger-Gasser (diameter & velocity)
– $A\alpha$ (12–24 µm, $70–120\,m/s$)
– $A\beta\;6–12\,µm$
– $A\gamma\;5–6\,µm$
– $A\delta\;2–5\,µm$
– $B\;1–2\,µm$
– $C<1.5\,µm$ (non-myelinated)

Nerve Action Potential & Fiber Properties

• RMP: nerve $-70\,mV$; muscle $-90\,mV$
• All-or-none, refractory period, non-fatiguing
• Local (electrotonic) potentials: graded, non-propagated, obey summation
• Saltatory conduction: Na⁺ entry at Nodes of Ranvier → $\times50$ speed

Synaptic Transmission

• One-way (Bell-Magendie); synaptic delay $0.3–0.5\,ms$
• Excitatory (EPSP, ACh, Glu) vs. inhibitory (IPSP, GABA, Gly)
• Presynaptic inhibition – axo-axonic ↓ Ca²⁺ entry
• Postsynaptic inhibition – IPSP via ↑ Cl⁻ / K⁺ conductance
• Summation: spatial & temporal
• Renshaw negative-feedback; feed-forward & reciprocal inhibition

Neuroglia

• CNS: astrocytes (BBB, K⁺ buffer), microglia (phagocytes), oligodendrocytes (myelin)
• PNS: Schwann cells (myelin, regeneration), satellite cells (support, chemical milieu)

Receptors & Sensory Coding

• Exteroceptors (cutaneous mechano-, chemo-, tele-), interoceptors – viscero, proprio
• Properties: specificity (Müller); adaptation – phasic vs. tonic; Weber-Fechner $R=k\log S$
• Receptor potential: graded, non-propagated, leads to AP at first node

Reflex Arc & Spinal Tracts

• Components: receptor → afferent → center → efferent → effector
• Ascending tracts: ant./lat. spinothalamic (crude touch | pain & temp), dorsal columns (fine touch, proprio, vibration), spinocerebellars (unconscious proprio)
• Descending: pyramidal (lat./ant. corticospinal) – voluntary; extrapyramidal (vestibulo-, rubro-, reticulo-, tectospinal) – tone & posture

Degeneration & Regeneration

• Wallerian (distal), retrograde (cell body), trans-neuronal
• Degrees of injury: neuropraxia → axonotmesis → neurotmesis
• Regrowth via Schwann tubes; $1–4\,mm/day$

Basal Ganglia & Cerebellum

• BG: caudate, putamen, globus pallidus, STN, substantia nigra
– Functions: tone ↓, initiation & scaling of movement, automatic associated acts
– Parkinsonism: ↓ dopamine → rigidity, resting tremor, bradykinesia
• Cerebellum:
– Vestibulo- (flocculonodular) → equilibrium
– Spino- (vermis, paravermis) → tone & posture
– Cortico-/neo- (lat. hemispheres) → planning, coordination, comparator, timing
– Lesion: ataxia, hypotonia, intention tremor, dysdiadochokinesia

Thalamus & Hypothalamus

• Thalamus: relay (VPL/VPM, MGB, LGB), sensory processing, crude perception, motor integration, arousal
• Hypothalamus:
– Endocrine (posterior pituitary ADH/oxytocin; releasing hormones)
– ANS control (symp post-lat, parasymp ant)
– Temperature (preoptic heat loss, posterior heat gain)
– Hunger (LH), satiety (VMH); thirst (LH + osmoreceptors)
– Circadian clock (suprachiasmatic)
– Emotion (reward ↔ punishment zones)

Limbic System & Reticular Formation

• Limbic: hippocampus (explicit memory), amygdala (emotion), cingulate, septal, hypothalamus – motivation, behavior, memory (Papez circuit)
• RF: ARAS – wakefulness; descending facilitatory & inhibitory systems – tone, autonomic control

Muscle Spindle & Tone

• Intrafusal fibers: nuclear bag & chain; innervation – primary $Ia$ annulospiral, secondary $II$ flower-spray; γ-motor (static vs. dynamic)
• Stretch (myotatic) reflex – monosynaptic, maintains posture
• Tone: baseline γ-drive, balanced by supraspinal facilitation vs. inhibition; hypertonia (UMN), hypotonia (LMN or cerebellar)

EEG & Sleep

• Alpha $8–12\,Hz$ eyes closed → blocked by opening
• Beta $>15\,Hz$ alert
• Theta $4–7\,Hz$ children, light sleep
• Delta $<4\,Hz$ deep NREM, pathology when awake
• Sleep cycles: NREM (Stages I–IV, slow waves) ≈$75\%$; REM (paradoxical) ≈$25\%$ – dreams, memory consolidation

Learning, Memory & Conditioning

• Non-associative: habituation (↓ response), sensitization (↑)
• Associative: classical & operant conditioning; positive vs. negative reflexes
• Memory stores: sensory (<$1\,s$), short-term/working (sec–min), long-term (days–life)
• Explicit (declarative) – hippocampus; implicit (procedural) – basal ganglia, cerebellum
• Consolidation via rehearsal; LTP – synaptic facilitation; amnesia – anterograde (hippocampal), retrograde

Speech

• Wernicke (area 22) – comprehension
• Broca (areas 44/45) – motor programming
• Arcuate fasciculus links them
• Motor cortex executes via cranial nerves; lesions → aphasias (Broca – non-fluent, Wernicke – fluent)

Autonomic Nervous System

• Sympathetic (thoracolumbar; short pre-, long post-ganglionic, NA):
– ↑ HR & BP, bronchodilation, pupil dilatation, sweat ↑, GI ↓, bladder relaxation
• Parasympathetic (craniosacral; long pre-, short post-, ACh):
– Opposite effects: ↓ HR, GI ↑, bronchoconstrict, pupil constrict, bladder voiding