Synaptic Transmission - Types: chemical and electrical synapses. Neurotransmitters. CNS Details - Brain regions: cerebrum, cerebellum, brainstem. Spinal cord anatomy and functions. Peripheral Nervous System - Somatic and autonomic nervous systems. Cranial and spinal nerves. Special Senses - Vision: eye anatomy, photoreceptors. Hearing: ear anatomy, sound transduction. Taste and smell: receptors and pathways. 1st chapter

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Chapter 1: The Nervous System — Organization, Synaptic Transmission & Special Senses

Part 1 — Synaptic Transmission

The Synapse

The synapse has two sides: presynaptic and postsynaptic, indicating the usual direction of information flow. The presynaptic side is generally an axon terminal; the postsynaptic side is usually a dendrite or soma of another neuron. The narrow space between them is the synaptic cleft.

"At most synapses, information in the form of electrical impulses traveling down the axon is converted by the presynaptic terminal into a chemical signal that crosses the synaptic cleft. On the postsynaptic membrane, this chemical signal is converted again into an electrical one." — Neuroscience: Exploring the Brain, 5th Ed.

A. Chemical Synapses

Chemical synapses transmit signals in one direction only (presynaptic → postsynaptic). This is the predominant synapse type in the CNS.

Mechanism of transmission:

An action potential arrives at the presynaptic terminal
Voltage-gated Ca²⁺ channels open → Ca²⁺ influx
Ca²⁺ triggers fusion of synaptic vesicles with the presynaptic membrane
Neurotransmitter is released into the synaptic cleft (200–300 Å wide)
Neurotransmitter binds to postsynaptic receptors — either ionotropic (direct ion channel opening) or metabotropic (G-protein/second messenger cascade)
Result: change in postsynaptic membrane potential, biochemical cascades, or gene expression

Chemical synapse diagram — Ca²⁺-triggered vesicle release, ionotropic and metabotropic receptors

Fig. Chemical synapse: action potential → Ca²⁺ influx → neurotransmitter release → ionotropic or metabotropic receptor activation (Guyton & Hall Medical Physiology)

Postsynaptic receptor types:

Receptor Type	Mechanism	Speed	Example
Ionotropic	Ligand-gated ion channel	Fast (ms)	AMPA, GABA-A, nAChR
Metabotropic	G-protein → 2nd messenger	Slow (seconds)	mGluR, GABA-B, muscarinic

Neurotransmitter inactivation occurs by:

Reuptake into the presynaptic terminal via monoamine transporters (SERT, NET, DAT — SLC6 family, 12-transmembrane proteins)
Enzymatic degradation (e.g., acetylcholinesterase for ACh)
Diffusion away from the cleft

B. Electrical Synapses

Electrical synapse — gap junction channels, bidirectional transmission, intercellular gap 20–40 Å

Fig. Electrical synapse: connexin gap junction channels allow bidirectional ionic current flow (Guyton & Hall Medical Physiology)

Electrical synapses use gap junctions (connexin proteins) that physically connect the cytoplasm of two neurons. The intercellular gap is only 20–40 Å.

Feature	Chemical	Electrical
Direction	One-way	Bidirectional
Delay	~0.5 ms	Near-instantaneous
Mechanism	Neurotransmitter	Gap junction current
Modifiability	High (plasticity)	Low
Location	Widespread in CNS	Hypothalamic hormone neurons, retina, cardiac muscle

Functional role of electrical synapses: coordinate synchronous firing of large neuronal groups, detect coincident subthreshold depolarizations, enable simultaneous hormone secretion in hypothalamic neurons.

C. Neurotransmitters

Major neurotransmitter classes:

Class	Examples	Actions
Amino acids	Glutamate (excitatory), GABA, Glycine (inhibitory)	Primary fast signaling
Monoamines	Dopamine, Norepinephrine, Serotonin	Modulation, mood, reward
Acetylcholine	ACh	NMJ, autonomic, cortical arousal
Neuropeptides	Substance P, Enkephalins, NPY	Pain, satiety, slow modulation
Purines	ATP, Adenosine	Neuromodulation
Gases	NO, CO	Retrograde signaling

G-protein second messenger pathway (metabotropic): neurotransmitter binds receptor → G-protein α subunit activates → (1) ion channel opening, (2) adenylyl cyclase → cAMP, (3) intracellular enzyme activation, (4) gene transcription. The G protein is inactivated when GTP bound to α is hydrolyzed to GDP.

Neurotransmitter transporters belong to the SLC6 gene family:

SERT (serotonin transporter) — substrate: serotonin; false substrate: MDMA (Ecstasy)
NET (norepinephrine transporter) — false substrate: amphetamine, dopamine
DAT (dopamine transporter) — target of cocaine

(Stahl's Essential Psychopharmacology)

Part 2 — Central Nervous System

A. Brain Regions

1. Cerebrum

The cerebral hemispheres consist of:

Cerebral cortex — four lobes: frontal, parietal, temporal, occipital; separated by sulci
White matter (subcortical)
Deep nuclei: basal ganglia, hippocampus, amygdala

Cortical functional areas:

Primary motor cortex — upper motor neurons projecting to spinal cord
Primary somatosensory cortex — receives peripheral sensory input
Primary visual cortex (occipital), Primary auditory cortex (temporal)
Association areas — integrate diverse information; the limbic association area handles motivation, memory, and emotion

Basal ganglia (caudate, putamen, globus pallidus): receive input from all cortical lobes; project via thalamus to frontal cortex to assist movement regulation.

Hippocampus: memory consolidation (part of limbic system).

Amygdala: emotion processing; communicates with autonomic nervous system via the hypothalamus.

Thalamus: relay station — processes nearly all sensory information going to the cortex and all motor information from cortex to brainstem/spinal cord.

Hypothalamus: regulates body temperature, food intake, water balance; endocrine control of pituitary (releases releasing/inhibiting hormones into hypophysial portal blood); contains posterior pituitary neurons secreting ADH and oxytocin.

(Costanzo Physiology, 7th Ed.)

2. Cerebellum

"The cerebellum helps control the rate, range, force, and direction of movements (collectively known as synergy). Damage to the cerebellum results in lack of coordination."

The cerebellum lies in the posterior fossa, attached to the brainstem by three cerebellar peduncles (afferent + efferent fibers).

Three main divisions:

Division	Primary Input	Function
Vestibulocerebellum	Vestibular organs	Balance, eye movements
Spinocerebellum	Spinal cord	Synergy (coordination) of movement
Pontocerebellum	Cerebral cortex (via pons)	Planning and initiation of movements

Cerebellar cortex layers (from inside out):

Cerebellar cortex layers: granular, Purkinje cell, and molecular layers with climbing and mossy fiber inputs

Fig. Cerebellar cortex structure — granular layer (granule cells, Golgi II), Purkinje cell layer (inhibitory output), molecular layer (parallel fibers, basket/stellate cells). Mossy fibers and climbing fibers provide the two excitatory inputs (Costanzo Physiology, 7th Ed.)

Granular layer (innermost): granule cells + Golgi II cells; mossy fiber input
Purkinje cell layer (middle): output always inhibitory (GABAergic)
Molecular layer (outermost): parallel fibers (axons of granule cells), basket cells, stellate cells

Two input systems:

Climbing fibers (from inferior olive) → directly on Purkinje cells; generate complex spikes; role in motor learning
Mossy fibers (majority of input) → via granule cells → parallel fibers → Purkinje cells

3. Brainstem

The brainstem consists of the midbrain, pons, and medulla oblongata. Key functions:

Houses nuclei for cranial nerves III–XII
Contains reticular formation (arousal, consciousness, cardiorespiratory control)
Mediates relay of information between cerebral cortex and spinal cord
Controls vital functions: respiration, heart rate, blood pressure, swallowing, vomiting

Midbrain: superior and inferior colliculi (visual and auditory reflexes); red nucleus, substantia nigra (motor control).

Pons: pontine nuclei relay cortical signals to cerebellum; pneumotaxic center for breathing; cranial nerve nuclei V, VI, VII, VIII.

Medulla oblongata: cardiovascular and respiratory centers; inferior olive (input to cerebellum); cranial nerve nuclei IX, X, XI, XII; pyramidal (corticospinal) decussation.

B. Spinal Cord Anatomy and Functions

The spinal cord extends from the foramen magnum to approximately L1–L2, where it terminates as the conus medullaris. It has 31 segments: 8 cervical (C1–C8), 12 thoracic (T1–T12), 5 lumbar (L1–L5), 5 sacral (S1–S5), 1 coccygeal.

Spinal cord cross-section — dorsal/lateral/ventral columns, ascending and descending tracts, somatotopic organization

Fig. Spinal cord cross-section showing major ascending (sensory) and descending (motor) tracts and somatotopic organization (Bailey & Love's Surgery, 28th Ed.)

Internal organization:

Gray matter (butterfly-shaped): neuron cell bodies
- Dorsal horn: sensory processing (laminae I–VI)
- Ventral horn: lower motor neurons (laminae VII–IX)
- Intermediate zone (lamina VII): interneurons; T1–L2 = sympathetic preganglionic neurons; S2–S4 = parasympathetic preganglionic neurons
White matter: myelinated axon tracts

Major ascending (sensory) tracts:

Tract	Information	Route
Dorsal columns (fasciculus gracilis + cuneatus)	Fine touch, vibration, proprioception	Ipsilateral → decussates at medulla
Lateral spinothalamic	Pain, temperature	Decussates at cord level
Anterior spinothalamic	Crude touch, pressure	Decussates at cord level
Dorsal spinocerebellar	Proprioception to cerebellum	Ipsilateral

Major descending (motor) tracts:

Tract	Origin	Function
Lateral corticospinal	Primary motor cortex (decussates at medulla)	Voluntary limb movement
Anterior corticospinal	Motor cortex	Axial muscles
Rubrospinal	Red nucleus (midbrain)	Limb flexors
Reticulospinal	Reticular formation	Posture, gait, autonomic

Part 3 — Peripheral Nervous System (PNS)

A. Somatic Nervous System

Carries voluntary motor commands to skeletal muscle (via lower motor neurons from the ventral horn) and sensory information from the periphery (skin, muscle, joints) back to the CNS.

Somatic sensory receptors:

Meissner's corpuscles (light touch), Pacinian corpuscles (vibration/pressure), Merkel discs (sustained touch), Ruffini endings (stretch), free nerve endings (pain/temperature)

Somatic reflex arc: sensory receptor → afferent neuron → spinal cord interneuron → efferent neuron → effector (muscle)

B. Autonomic Nervous System (ANS)

The ANS controls involuntary functions: cardiac muscle, smooth muscle, glands. It always has a two-neuron chain: preganglionic → ganglionic synapse → postganglionic → effector.

Autonomic nervous system — parasympathetic (craniosacral) and sympathetic (thoracolumbar) pathways to all organs

Fig. Autonomic nervous system — parasympathetic (orange) from cranial nerves III/VII/IX/X + sacral S2–S4; sympathetic (green) from T1–L2 thoracolumbar cord (Harrison's Principles of Internal Medicine, 21st Ed.)

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar (T1–L2)	Craniosacral (CN III, VII, IX, X + S2–S4)
Ganglia location	Paravertebral chain (near cord)	Near/in target organ
Preganglionic NT	ACh (nicotinic)	ACh (nicotinic)
Postganglionic NT	Norepinephrine (adrenergic)	ACh (muscarinic)
General effect	"Fight or flight"	"Rest and digest"
Heart	↑ HR, ↑ contractility	↓ HR
Pupil	Dilates (mydriasis)	Constricts (miosis)
Bronchi	Dilates	Constricts
GI	↓ motility	↑ motility
Sweat glands	ACh (exception!)	—

Enteric nervous system: third division of ANS; intrinsic to the GI tract; can function independently of CNS.

C. Cranial Nerves (I–XII)

#	Name	Type	Key Function
I	Olfactory	Sensory	Smell
II	Optic	Sensory	Vision
III	Oculomotor	Motor + Parasympathetic	Eye movement (SR, IR, MR, IO), pupil constriction, lens accommodation
IV	Trochlear	Motor	Superior oblique (eye depresses when adducted)
V	Trigeminal	Mixed	Facial sensation (ophthalmic/maxillary/mandibular); muscles of mastication
VI	Abducens	Motor	Lateral rectus (eye abduction)
VII	Facial	Mixed + Parasympathetic	Muscles of facial expression, taste (anterior 2/3 tongue), lacrimal/salivary glands
VIII	Vestibulocochlear	Sensory	Hearing (cochlear) + balance (vestibular)
IX	Glossopharyngeal	Mixed + Parasympathetic	Taste (posterior 1/3 tongue), pharyngeal sensation, parotid gland, carotid body
X	Vagus	Mixed + Parasympathetic	Visceral sensory/motor, pharynx, larynx, cardiorespiratory, GI
XI	Accessory	Motor	Sternocleidomastoid + trapezius
XII	Hypoglossal	Motor	Tongue movements

Spinal nerves: 31 pairs (8C, 12T, 5L, 5S, 1Co), each with a dorsal root (sensory, dorsal root ganglion cell bodies) and ventral root (motor). Merge to form mixed spinal nerve → anterior and posterior rami → peripheral nerve plexuses (cervical, brachial, lumbar, sacral).

Part 4 — Special Senses

A. Vision — Eye Anatomy & Photoreceptors

Eye cross-section with retina detail showing rods, cones, bipolar cells, amacrine cells, ganglion cells, nerve fiber layer

Fig. Eye anatomy with retina detail — full cellular lamination from RPE and photoreceptors through ganglion cells to nerve fiber layer

Eye anatomy layers (outer → inner):

Sclera: tough outer coat; merges anteriorly with transparent cornea (primary refractive structure)
Uvea: choroid (vascular, nourishes retina) + ciliary body (ciliary muscle adjusts lens curvature for accommodation) + iris (controls pupil diameter)
Lens: transparent, biconvex; changes shape for accommodation; suspended by ciliary zonule fibers
Vitreous humor: posterior chamber; maintains globe shape
Retina: innermost layer; contains photoreceptors

Retinal cell layers (light enters from ganglion cell side):

Retinal pigment epithelium (RPE)
Photoreceptors — rods and cones
Outer plexiform layer (receptor → bipolar synapse)
Bipolar cells, horizontal cells, amacrine cells
Inner plexiform layer
Ganglion cells — axons form the optic nerve (CN II)
Nerve fiber layer

Photoreceptors:

	Rods	Cones
Number	~120 million	~6 million
Location	Peripheral retina	Concentrated at fovea (highest acuity)
Light sensitivity	Scotopic (dim light)	Photopic (bright light, color)
Pigment	Rhodopsin (opsin + retinal)	Three types: L (red), M (green), S (blue)
Acuity	Low	High
Color	No	Yes (trichromatic)

Phototransduction (rods): Light → retinal isomerizes (11-cis → all-trans) → rhodopsin activation → transducin (G-protein) → phosphodiesterase activation → cGMP hydrolysis → Na⁺ channels close → hyperpolarization of photoreceptor → reduced glutamate release → signal propagation to bipolar → ganglion cells → optic nerve → lateral geniculate nucleus (thalamus) → primary visual cortex (V1, occipital lobe).

Visual pathway: optic nerves → optic chiasm (nasal fibers decussate) → optic tracts → lateral geniculate nucleus → optic radiation → visual cortex.

B. Hearing — Ear Anatomy & Sound Transduction

Ear anatomy — outer/middle/inner ear, ossicles, cochlea, organ of Corti, perilymph/endolymph flow

Fig. Ear anatomy showing sound pathway: air → ear canal → eardrum → malleus/incus/stapes → oval window → cochlear fluid waves → organ of Corti → auditory nerve (Bailey & Love's Surgery, 28th Ed.)

Outer ear: auricle + ear canal → funnels sound to tympanic membrane (eardrum).

Middle ear: three ossicles amplify vibrations (~22× mechanical advantage):

Malleus (hammer) → Incus (anvil) → Stapes (stirrup) → oval window
Eustachian tube equalizes pressure with nasopharynx

Inner ear (cochlea):

Bony spiral structure; tonotopic organization (base = high frequency, apex = low frequency)
Three fluid compartments:
- Scala vestibuli (perilymph, high Na⁺) — above
- Scala media / cochlear duct (endolymph, high K⁺) — middle
- Scala tympani (perilymph) — below
Organ of Corti sits on the basilar membrane inside the scala media
- Inner hair cells (IHCs, 1 row): primary sensory cells; relay signals to auditory nerve
- Outer hair cells (OHCs, 3 rows): amplify and tune basilar membrane movement (cochlear amplifier)
- Tectorial membrane sits above stereocilia

Sound transduction (mechanotransduction):

Stapes vibrates oval window → pressure waves in perilymph
Basilar membrane deflects at position corresponding to sound frequency
Stereocilia of hair cells bend against tectorial membrane → tip links open MET (mechanotransduction) channels
K⁺ (and Ca²⁺) influx from endolymph → hair cell depolarization
Neurotransmitter (glutamate) released at IHC base → activates spiral ganglion neurons (CN VIII)
Signals travel: cochlear nucleus → superior olivary complex → inferior colliculus (midbrain) → medial geniculate nucleus (thalamus) → primary auditory cortex (temporal lobe, Heschl's gyrus)

C. Taste (Gustation) — Receptors & Pathways

Taste receptor cells (TRCs) are clustered in taste buds on papillae of the tongue:

Fungiform papillae — anterior 2/3 tongue
Circumvallate (vallate) papillae — posterior tongue
Foliate papillae — lateral tongue

Five primary taste modalities:

Taste	Mechanism	Key Stimulus
Sweet	GPCR (T1R2+T1R3) → G-protein (gustducin) → cAMP/IP₃	Sugars
Umami	GPCR (T1R1+T1R3)	Glutamate (MSG)
Bitter	GPCR (T2R family)	Alkaloids, toxins
Sour	H⁺ blocks K⁺ channels / OTOP1 channel	Acids
Salty	ENaC Na⁺ channels (epithelial sodium channels)	NaCl

Taste pathway: TRCs → cranial nerves VII (anterior 2/3 tongue), IX (posterior 1/3), X (epiglottis/pharynx) → nucleus tractus solitarius (NTS) in medulla → thalamus (VPM nucleus) → primary gustatory cortex (insular cortex + frontal operculum).

D. Smell (Olfaction) — Receptors & Pathways

Olfactory receptor neurons (ORNs) are located in the olfactory epithelium (roof of nasal cavity, superior turbinate). Humans have ~400 functional olfactory receptor genes (OR genes — largest gene family in mammals).

Mechanism:

Odorant molecules dissolve in mucus → bind olfactory receptors (GPCRs) on cilia of ORNs
OR → Golf (Gαs type) → adenylyl cyclase → ↑ cAMP → opens CNG channels (cyclic nucleotide-gated) → Na⁺/Ca²⁺ influx → depolarization
Ca²⁺ opens Ca²⁺-activated Cl⁻ channels → amplification

Olfactory pathway (only sense that bypasses the thalamus directly):

ORN axons → pass through cribriform plate → olfactory bulb (CN I)
Mitral/tufted cells → olfactory tract → piriform cortex + amygdala (primary olfactory cortex, conscious perception and emotional/memory integration)
Piriform cortex → entorhinal cortex → orbitofrontal cortex (secondary cortex, odor identification/discrimination)
Also → hypothalamus (autonomic + feeding responses)

Key difference from other senses: olfactory signals reach the cortex and limbic system without a mandatory thalamic relay, explaining the powerful link between smell and memory/emotion.

Summary Table — CNS Regions at a Glance

Structure	Key Functions	Damage Effects
Frontal lobe	Motor, executive function, Broca's area	Contralateral weakness, personality change, aphasia
Parietal lobe	Somatosensory, spatial orientation	Contralateral sensory loss, neglect
Temporal lobe	Auditory, memory, Wernicke's area	Hearing deficits, amnesia, receptive aphasia
Occipital lobe	Visual processing	Visual field defects
Cerebellum	Coordination, balance, motor learning	Ataxia, dysmetria, intention tremor
Brainstem	Vital functions, CN nuclei, relay	Cranial nerve palsies, respiratory/cardiovascular failure
Spinal cord	Sensorimotor relay, reflexes, ANS	Paralysis, sensory loss, autonomic dysfunction

Sources: Neuroscience: Exploring the Brain (5th Ed.) · Guyton and Hall Textbook of Medical Physiology · Costanzo Physiology (7th Ed.) · Stahl's Essential Psychopharmacology · Harrison's Principles of Internal Medicine (21st Ed.) · Bailey & Love's Surgery (28th Ed.)

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