Neural Control and Coordination — NEET notes

Neuron Structure & Types — The cell body, its wiring, and why myelin is nature's insulation tape

The neuron is the structural and functional unit of the nervous system. It has three parts: dendrites (receive), cell body/soma (integrate), and axon (transmit). Myelin sheath (Schwann cells in PNS, oligodendrocytes in CNS) insulates axons for faster conduction. Nodes of Ranvier allow saltatory conduction. Neurons are classified as sensory (afferent), motor (efferent), or interneurons (association). Structurally: unipolar, bipolar, or multipolar.

Neuron Anatomy

A neuron has three main structural regions: dendrites (short, branched, receive input), cell body or soma (contains nucleus and Nissl granules), and axon (single, long, transmits impulse away from soma). The axon hillock is where action potentials are initiated.

The neuron (nerve cell) is the structural and functional unit of the nervous system. It is specialized for receiving stimuli, integrating information, and transmitting electrical impulses. A single motor neuron from the spinal cord to your toe can be over 1 meter long — making it one of the longest cells in the human body.

• Dendrites — short, highly branched extensions from the cell body. They RECEIVE signals from other neurons/receptors and carry impulses TOWARD the cell body
• Cell body (soma/cyton) — contains the nucleus, Nissl granules (dense RER + ribosomes for protein synthesis), mitochondria, and Golgi apparatus. The metabolic command center
• Axon — single, long process that carries impulses AWAY from the cell body. Ends in branched axon terminals (synaptic knobs/boutons) that form synapses
• Axon hillock — cone-shaped junction between soma and axon. This is the trigger zone where action potentials are initiated
• Nissl granules — found in cell body and dendrites, but ABSENT from the axon and axon hillock. They stain with basic dyes (used to identify neurons histologically)

Myelin Sheath & Nodes of Ranvier

The myelin sheath is a lipid-rich insulating layer around axons formed by Schwann cells (PNS) or oligodendrocytes (CNS). Gaps in the myelin are called Nodes of Ranvier where ion exchange occurs. Myelinated fibers conduct faster via saltatory conduction.

Not all axons are bare. Many are wrapped in a fatty myelin sheath — a multilayered lipid membrane that acts like insulation on an electrical wire. This insulation dramatically increases the speed of nerve impulse transmission.

• Myelin sheath — formed by Schwann cells in PNS, oligodendrocytes in CNS. Lipid-rich, white in color (gives 'white matter' its name)
• One Schwann cell wraps around ONE segment (internode) of ONE axon. One oligodendrocyte can myelinate segments of MULTIPLE axons
• Nodes of Ranvier — regular gaps (1-2 μm) between adjacent Schwann cells where the axon membrane is exposed to extracellular fluid
• Saltatory conduction — impulse 'jumps' from node to node (Latin: saltare = to jump). Much faster than continuous conduction
• Myelinated fibers — white matter of brain/spinal cord, cranial nerves, spinal nerves. Speed: 12-120 m/s
• Unmyelinated fibers — grey matter, autonomic nerves, pain fibers. Speed: 0.5-2 m/s

Types of Neurons

Functional classification: sensory (afferent) neurons carry impulses from receptors to CNS; motor (efferent) neurons carry impulses from CNS to effectors; interneurons link the two within CNS. Structural classification: unipolar, bipolar, multipolar based on number of processes.

The ~86 billion neurons in your body aren't all the same. They are classified by function (what direction they carry signals) and by structure (how many processes extend from the cell body). NEET focuses heavily on the functional classification.

• Unipolar neurons — one process from cell body (rare in adults; found in embryonic tissue and some sensory ganglia of invertebrates)
• Bipolar neurons — one dendrite + one axon. Found in retina of eye, olfactory epithelium of nose, cochlea of ear
• Multipolar neurons — many dendrites + one axon. MOST COMMON type. All motor neurons and most interneurons are multipolar
• Pseudo-unipolar neurons — appear unipolar but actually have one process that splits into two. Found in dorsal root ganglia (sensory neurons)

Quick recall

Where are bipolar neurons found in the human body?

Three places: (1) Retina of the eye, (2) Olfactory epithelium of the nose, (3) Cochlea of the ear. All are sensory locations — bipolar neurons are specialized for receiving specific sensory input.

Nerve Impulse — How a -70mV resting neuron fires a +30mV spike and resets in milliseconds

Nerve impulse is an electrochemical wave that travels along the neuron membrane. At rest, neurons are polarized at -70mV (resting potential) due to Na⁺/K⁺ pump and selective permeability. A stimulus opens Na⁺ channels → depolarization (reaches +30mV = action potential) → K⁺ channels open → repolarization → brief hyperpolarization → resting potential restored. Follows all-or-none principle. In myelinated fibers, impulse jumps node to node (saltatory conduction).

Resting Potential

When not transmitting signals, a neuron maintains a potential difference of about -70mV across its membrane (inside negative, outside positive). This is maintained by the Na⁺/K⁺ ATPase pump (3 Na⁺ out, 2 K⁺ in) and selective permeability of the membrane.

A resting neuron is like a loaded gun — it's not firing, but it's ready. The inside of the neuron is negatively charged relative to the outside. This voltage difference across the membrane is the resting membrane potential, approximately -70 mV. The neuron is said to be polarized.

• Resting potential ≈ -70 mV (inside of neuron is negative relative to outside)
• Na⁺/K⁺ ATPase pump — actively transports 3 Na⁺ OUT and 2 K⁺ IN per ATP hydrolyzed. Creates ionic gradient
• At rest: high Na⁺ outside, high K⁺ inside the neuron
• Membrane is selectively permeable — more permeable to K⁺ than Na⁺ at rest (K⁺ leak channels are open)
• K⁺ leaks out along its concentration gradient → leaves behind negative charges → inside becomes more negative
• This combination of pump activity + K⁺ leak maintains the -70mV polarized state

Action Potential

When a stimulus reaches threshold, voltage-gated Na⁺ channels open → Na⁺ rushes in → membrane depolarizes from -70mV to +30mV (action potential). Then Na⁺ channels inactivate, K⁺ channels open → K⁺ rushes out → repolarization. A brief hyperpolarization follows before returning to resting potential.

When a stimulus is strong enough to reach the threshold potential (~-55 mV), an explosive chain reaction begins. Voltage-gated Na⁺ channels fly open, Na⁺ floods in, and the membrane potential rockets from -70 mV to about +30 mV. This rapid reversal of polarity is the action potential — the nerve impulse.

• Step 1 — Stimulus: A stimulus depolarizes the membrane slightly. If it reaches threshold (~-55 mV), an action potential is triggered
• Step 2 — Depolarization: Voltage-gated Na⁺ channels open → Na⁺ rushes INTO the cell → membrane potential shoots from -70 mV to +30 mV (inside becomes positive)
• Step 3 — Peak (+30 mV): Na⁺ channels INACTIVATE (automatically close after ~1 ms). Na⁺ entry stops
• Step 4 — Repolarization: Voltage-gated K⁺ channels open (with slight delay) → K⁺ rushes OUT → membrane potential drops back toward -70 mV
• Step 5 — Hyperpolarization: K⁺ channels close slowly → membrane briefly overshoots to about -80 mV (undershoot/hyperpolarization) → Na⁺/K⁺ pump restores original -70 mV

All-or-None Principle & Saltatory Conduction

Action potentials follow the all-or-none law: if threshold is reached, the full action potential fires at maximum amplitude regardless of stimulus strength. Stronger stimuli increase frequency, not amplitude. In myelinated neurons, impulses jump between Nodes of Ranvier (saltatory conduction) for faster transmission.

A nerve impulse doesn't come in sizes. It's either full strength or nothing — like a gun being fired. You can't fire a bullet gently. This is the all-or-none principle: once threshold is reached, the action potential fires at its maximum amplitude (+30 mV) regardless of whether the stimulus was barely above threshold or massively strong.

• All-or-none law: Sub-threshold stimulus → NO action potential. Threshold or above → FULL action potential (+30 mV)
• Stimulus strength does NOT change the amplitude of the action potential
• Stronger stimuli increase the frequency of action potentials (more spikes per second), not the size of each spike
• This is how the nervous system codes intensity: a gentle touch = few action potentials per second; a hard slap = many per second

Conduction speed depends heavily on whether the axon is myelinated. In unmyelinated fibers, the impulse crawls continuously along the membrane. In myelinated fibers, it leaps from one Node of Ranvier to the next — saltatory conduction (Latin: saltare = to jump).

• Continuous conduction — in unmyelinated fibers. Ion exchange occurs at every point along the axon. Slow (0.5-2 m/s)
• Saltatory conduction — in myelinated fibers. Ion exchange only at Nodes of Ranvier (myelin prevents ion flow elsewhere). Fast (up to 120 m/s)
• Saltatory conduction is faster AND more energy-efficient (fewer ions move, less pumping needed to restore gradients)
• Speed also increases with axon diameter — thicker axons conduct faster (less internal resistance)

Quick recall

A neuron receives two stimuli: one at 60 mV above threshold and one at 5 mV above threshold. How do the resulting action potentials differ?

They DON'T differ in amplitude — both produce the same +30 mV action potential (all-or-none). But the stronger stimulus may cause a higher FREQUENCY of firing (more action potentials per second).

Synapse — The tiny gap where electricity becomes chemistry and back again

A synapse is the functional junction between two neurons (or a neuron and effector). In chemical synapses, the action potential triggers Ca²⁺-dependent release of neurotransmitters (acetylcholine, dopamine, serotonin, GABA) from synaptic vesicles into the 20 nm cleft. These bind post-synaptic receptors to generate excitatory or inhibitory responses. Electrical synapses use gap junctions for faster but unmodulated transmission.

Chemical Synapse

The most common synapse type. Pre-synaptic terminal releases neurotransmitters into the synaptic cleft, which bind receptors on the post-synaptic membrane. The process is Ca²⁺-dependent and ensures unidirectional signal transmission.

Neurons don't physically touch each other. There's a tiny gap — about 20 nm wide — called the synaptic cleft between the axon terminal of one neuron and the dendrite (or cell body) of the next. This entire junction is called a synapse, and it's where the electrical signal is converted to a chemical signal and back.

• Pre-synaptic terminal — the axon terminal of the transmitting neuron. Contains mitochondria (for ATP) and synaptic vesicles filled with neurotransmitter molecules
• Synaptic cleft — the ~20 nm gap between pre-synaptic and post-synaptic membranes
• Post-synaptic membrane — the membrane of the receiving neuron's dendrite/cell body. Contains specific receptor proteins for neurotransmitters

How chemical transmission works (step by step):

• Step 1: Action potential arrives at the axon terminal (pre-synaptic terminal)
• Step 2: Voltage-gated Ca²⁺ channels open → Ca²⁺ ions flood INTO the pre-synaptic terminal
• Step 3: Ca²⁺ causes synaptic vesicles to fuse with the pre-synaptic membrane (exocytosis) → neurotransmitter molecules are released into the synaptic cleft
• Step 4: Neurotransmitters diffuse across the cleft and bind to specific receptors on the post-synaptic membrane
• Step 5: Binding opens ion channels in the post-synaptic membrane → generates a new electrical signal (either excitatory or inhibitory)
• Step 6: Neurotransmitter is rapidly removed — by enzymatic breakdown (e.g., acetylcholinesterase breaks ACh), reuptake into pre-synaptic terminal, or diffusion away

Neurotransmitters

Neurotransmitters are chemicals stored in synaptic vesicles and released into the synaptic cleft. Major ones include acetylcholine (neuromuscular junction), noradrenaline (sympathetic), dopamine (reward/pleasure), serotonin (mood), GABA (main inhibitory), and glutamate (main excitatory).

Different synapses use different neurotransmitters. Each one has specific receptors and specific effects. Some excite the post-synaptic neuron (make it more likely to fire), others inhibit it (make it less likely to fire). The balance between excitation and inhibition is what makes the nervous system functional.

• Excitatory neurotransmitters (ACh, glutamate, noradrenaline) → depolarize post-synaptic membrane → increase probability of firing
• Inhibitory neurotransmitters (GABA, glycine) → hyperpolarize post-synaptic membrane → decrease probability of firing
• The SAME neurotransmitter can be excitatory or inhibitory depending on the receptor type (e.g., ACh excites skeletal muscle but inhibits/slows the heart)
• After release, neurotransmitters must be quickly removed to prevent continuous stimulation

Electrical Synapse & Excitatory vs Inhibitory

Electrical synapses use gap junctions for direct ion flow between neurons — faster but less modifiable than chemical synapses. At chemical synapses, signals can be excitatory (EPSP, depolarizing) or inhibitory (IPSP, hyperpolarizing). The post-synaptic neuron sums all inputs to decide whether to fire.

Not all synapses use neurotransmitters. Electrical synapses have gap junctions — protein channels that directly connect the cytoplasm of two neurons. Ions flow directly from one cell to the next, making transmission almost instantaneous.

At chemical synapses, the post-synaptic response depends on which neurotransmitter binds and which receptor is activated:

• EPSP (Excitatory Post-Synaptic Potential) — neurotransmitter causes depolarization of post-synaptic membrane (e.g., Na⁺ channels open). Pushes neuron TOWARD threshold → promotes firing
• IPSP (Inhibitory Post-Synaptic Potential) — neurotransmitter causes hyperpolarization (e.g., Cl⁻ channels open or K⁺ channels open). Pushes neuron AWAY from threshold → prevents firing
• Summation — a single EPSP is usually too small to reach threshold. The neuron adds up (sums) multiple inputs: temporal summation (rapid-fire from one synapse) and spatial summation (inputs from many synapses at once)
• Decision at the axon hillock: if total EPSPs - total IPSPs ≥ threshold → fire. Otherwise → no action potential

Central Nervous System — Brain + spinal cord: the command center that runs the whole show

The CNS consists of the brain (protected by cranium and meninges) and the spinal cord (in the vertebral column). The brain has three major parts: forebrain (cerebrum, thalamus, hypothalamus), midbrain (corpora quadrigemina, cerebral peduncles), and hindbrain (cerebellum, pons, medulla oblongata). The spinal cord handles reflexes and relays signals. Grey matter (cell bodies) and white matter (myelinated axons) are organized differently in brain vs spinal cord.

Brain Overview & Protection

The human brain weighs ~1.4 kg and is protected by the cranium (skull), three meninges (dura mater, arachnoid mater, pia mater), and cerebrospinal fluid (CSF). It has three major divisions: forebrain, midbrain, and hindbrain.

The human brain is the most complex organ in the known universe. Weighing about 1.4 kg (~2% of body weight but consuming ~20% of the body's oxygen), it is protected by the bony cranium (skull) and three membranous coverings called meninges.

• Three meninges (outer to inner): Dura mater (tough, outermost) → Arachnoid mater (web-like, middle) → Pia mater (delicate, innermost, directly covers brain surface)
• Mnemonic for order: DAP — Dura, Arachnoid, Pia (outer to inner)
• Cerebrospinal fluid (CSF) — fills the space between arachnoid and pia mater (subarachnoid space) and the ventricles of the brain. Acts as a shock absorber and provides nutrients
• CSF is produced by choroid plexus in the ventricles
• Brain has 4 ventricles — 2 lateral ventricles (in cerebral hemispheres), 3rd ventricle (in diencephalon), 4th ventricle (between cerebellum and pons/medulla)

The brain is divided into three major parts:

• Forebrain (Prosencephalon): Cerebrum + Diencephalon (thalamus + hypothalamus) — the largest part, handles thinking, sensory processing, hormonal control
• Midbrain (Mesencephalon): Connects forebrain to hindbrain. Contains corpora quadrigemina (visual and auditory reflexes) and cerebral peduncles
• Hindbrain (Rhombencephalon): Cerebellum + Pons + Medulla oblongata — controls balance, breathing, heart rate, and relays signals

Cerebrum

The cerebrum is the largest part of the brain (~2/3 of total mass), divided into left and right hemispheres connected by the corpus callosum. The surface (cortex) has gyri (ridges) and sulci (grooves). Four lobes: frontal (motor, reasoning), parietal (sensory), temporal (hearing, memory), occipital (vision). The cerebral cortex contains sensory, motor, and association areas.

The cerebrum is the largest part of the brain, making up about 2/3 of the total brain mass. It is divided into left and right cerebral hemispheres connected by a thick bundle of nerve fibers called the corpus callosum. The surface (cerebral cortex) is highly folded — ridges called gyri (singular: gyrus) and grooves called sulci (singular: sulcus) increase the surface area enormously.

• Cerebral cortex — the outer grey matter (2-4 mm thick, contains ~15-20 billion neurons). This is where all conscious thinking happens
• Inner white matter — myelinated axons connecting different cortical areas and hemispheres
• Corpus callosum — largest commissure (white matter) connecting left and right hemispheres. Allows communication between them

• Association areas — large cortical regions that integrate information from multiple sensory/motor areas. Responsible for complex functions: thinking, planning, learning, memory, personality
• Motor area — in the pre-central gyrus (frontal lobe). Controls voluntary movements on the OPPOSITE side of the body
• Sensory area — in the post-central gyrus (parietal lobe). Receives sensory input from the OPPOSITE side
• Left hemisphere — dominant for language, logic, math in most people. Right hemisphere — dominant for spatial ability, creativity, facial recognition

Thalamus, Hypothalamus, Midbrain & Hindbrain

Thalamus relays all sensory information (except smell) to the cerebrum. Hypothalamus controls body temperature, hunger, thirst, circadian rhythm, and links the nervous and endocrine systems via the pituitary gland. Cerebellum coordinates voluntary movements and maintains posture. Pons relays signals between cerebrum and cerebellum. Medulla oblongata controls vital involuntary functions (breathing, heart rate, blood pressure, swallowing, vomiting).

Below the cerebrum, a series of vital structures handle everything from relaying sensory data to keeping you alive during sleep. Let's go through them top to bottom.

• Thalamus — the 'relay station.' ALL sensory input (except smell) passes through the thalamus before reaching the cerebral cortex. It filters and prioritizes information
• Hypothalamus — the 'thermostat and hormone boss.' Controls: body temperature, hunger center, thirst center, satiety center, circadian rhythm (sleep-wake cycle), emotional behavior (part of limbic system)
• Hypothalamus is the link between nervous and endocrine systems — it controls the pituitary gland (master gland) via releasing and inhibiting hormones
• Limbic system — includes hypothalamus, amygdala (fear, aggression), hippocampus (memory formation). Controls emotional responses and memory

• Midbrain — contains: corpora quadrigemina (4 small lobes: superior colliculi for visual reflexes, inferior colliculi for auditory reflexes) and cerebral peduncles (motor pathways from cerebrum to spinal cord)
• Pons — acts as a bridge between cerebrum and cerebellum. Contains pneumotaxic center (regulates breathing rhythm by limiting inspiration)
• Cerebellum — the 'little brain' behind the pons. Controls: coordination of voluntary movements, maintenance of posture and balance, fine-tuning of motor activities. Damage → ataxia (uncoordinated movements)
• Medulla oblongata — the 'vital functions center.' Controls: breathing (respiratory center), heart rate (cardiac center), blood pressure (vasomotor center), swallowing, coughing, sneezing, vomiting. Damage = death

Spinal Cord & Reflex Arc

The spinal cord extends from the medulla oblongata to the lumbar region (L1-L2), protected by the vertebral column. It has an inner H-shaped grey matter (cell bodies) and outer white matter (myelinated tracts). Functions: conducts impulses between brain and body, and controls spinal reflexes. A reflex arc is the neural pathway for a reflex action.

The spinal cord is a cylindrical structure about 45 cm long extending from the foramen magnum (base of skull) to the level of L1-L2 vertebra (lower back). Below this, only spinal nerves continue as the cauda equina (horse's tail). The spinal cord is protected by the vertebral column and the three meninges (same as the brain).

• Grey matter (inner, H-shaped): Contains cell bodies of neurons. Has dorsal horns (receive sensory input), ventral horns (contain motor neurons), and lateral horns (in thoracic region, contain autonomic neurons)
• White matter (outer): Contains myelinated ascending (sensory) and descending (motor) nerve tracts
• NOTE: This is OPPOSITE to the brain where grey matter is outer (cortex) and white matter is inner
• Central canal — runs through the center of the spinal cord, filled with CSF, continuous with brain ventricles
• 31 pairs of spinal nerves arise from the spinal cord (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal)

The Reflex Arc — the fastest response system in your body:

• A reflex is an involuntary, rapid, automatic response to a stimulus. It does NOT require conscious thinking
• Reflex arc pathway: Receptor (sense organ) → Sensory (afferent) neuron → Integration center (spinal cord/brain) → Motor (efferent) neuron → Effector (muscle/gland)
• Spinal reflexes — processed in the spinal cord WITHOUT brain involvement. Example: withdrawal reflex (pulling hand away from a hot surface), knee-jerk reflex
• Cranial reflexes — processed in the brain stem. Example: pupillary light reflex, salivation reflex
• Monosynaptic reflex — only one synapse (sensory → motor directly). Example: knee-jerk (patellar) reflex
• Polysynaptic reflex — involves interneurons between sensory and motor neurons. Example: withdrawal reflex (touching hot object)

Quick recall

In the knee-jerk reflex, why is there no interneuron?

It's a monosynaptic (stretch) reflex — the sensory neuron from the muscle spindle synapses DIRECTLY with the motor neuron in the spinal cord. Only one synapse involved. This makes it the fastest possible reflex (~25 ms).

Peripheral Nervous System — 12 cranial + 31 spinal nerves, and the somatic vs autonomic split

The PNS includes all nerves outside the brain and spinal cord: 12 pairs of cranial nerves (from brain) and 31 pairs of spinal nerves (from spinal cord). Functionally divided into somatic nervous system (voluntary, skeletal muscle control) and autonomic nervous system (involuntary, smooth muscle/glands). The ANS is further split into sympathetic (fight-or-flight, thoracolumbar) and parasympathetic (rest-and-digest, craniosacral) divisions that act antagonistically.

Cranial & Spinal Nerves

Cranial nerves (12 pairs) arise from the brain and innervate mostly head and neck structures (exception: vagus nerve innervates thoracic and abdominal organs). Spinal nerves (31 pairs) arise from the spinal cord and are all mixed (both sensory and motor). They innervate the trunk and limbs.

The peripheral nervous system is everything OUTSIDE the brain and spinal cord — all the nerves that connect the CNS to the rest of the body. It's divided into cranial nerves (12 pairs, from the brain) and spinal nerves (31 pairs, from the spinal cord).

• 12 pairs of cranial nerves — arise from the brain (most from the brain stem). Named by Roman numerals I through XII
• Can be sensory only (I Olfactory, II Optic, VIII Vestibulocochlear), motor only (III Oculomotor, IV Trochlear, VI Abducens, XI Accessory, XII Hypoglossal), or mixed (V Trigeminal, VII Facial, IX Glossopharyngeal, X Vagus)
• Vagus nerve (X) — the longest cranial nerve. Only cranial nerve to extend beyond head/neck into thorax and abdomen. Innervates heart, lungs, digestive tract (parasympathetic)
• 31 pairs of spinal nerves — ALL are mixed (carry both sensory and motor fibers): 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal
• Each spinal nerve has a dorsal root (sensory fibers, has a dorsal root ganglion with sensory cell bodies) and a ventral root (motor fibers)

Somatic vs Autonomic

The somatic nervous system (SNS) controls voluntary skeletal muscles with a single motor neuron from CNS to muscle. The autonomic nervous system (ANS) controls involuntary functions (heart, glands, smooth muscle) using a two-neuron chain (pre-ganglionic and post-ganglionic). The ANS has sympathetic and parasympathetic divisions.

The PNS is functionally divided into two systems based on whether you have conscious control or not. The somatic nervous system handles things you choose to do (walking, waving). The autonomic nervous system handles things that happen automatically (heartbeat, digestion, pupil size).

Sympathetic vs Parasympathetic

Sympathetic division (thoracolumbar origin) prepares the body for emergency: increases heart rate, dilates pupils, inhibits digestion, releases adrenaline. Parasympathetic division (craniosacral origin) promotes relaxation: decreases heart rate, constricts pupils, stimulates digestion. They act antagonistically on most organs to maintain homeostasis.

The autonomic nervous system has two divisions that work like opposing pedals in a car. The sympathetic division is the accelerator (revs you up for emergencies). The parasympathetic division is the brake (calms you down for rest and digestion). Most organs receive BOTH — and the balance between them determines the organ's state.

• Both divisions use ACh as the pre-ganglionic neurotransmitter — this is a common trick question
• Post-ganglionic difference: Sympathetic uses noradrenaline (adrenergic), Parasympathetic uses ACh (cholinergic)
• Dual innervation — most organs receive both sympathetic and parasympathetic fibers. Exception: adrenal medulla (only sympathetic), sweat glands (only sympathetic), blood vessels of skeletal muscles (only sympathetic)
• The vagus nerve (X cranial nerve) carries ~75% of all parasympathetic fibers

Sensory Reception: Eye — How your 576-megapixel biological camera converts light into nerve impulses

The eye is the organ of vision. Light enters through cornea → aqueous humor → pupil (controlled by iris) → lens (accommodation by ciliary muscles) → vitreous humor → retina. The retina contains photoreceptors: rods (dim light, black-and-white, rhodopsin) and cones (bright light, color vision, iodopsin — 3 types for red, green, blue). Image formed is real, inverted, and diminished. Fovea centralis has only cones (sharpest vision). Blind spot (optic disc) has no photoreceptors. Defects: myopia (nearsightedness, concave lens), hypermetropia (farsightedness, convex lens), presbyopia, astigmatism, cataract, glaucoma.

Structure of the Eye

The eyeball has three layers: sclera/cornea (outer fibrous coat), choroid/ciliary body/iris (middle vascular coat), and retina (inner nervous coat). The lens divides the eye into an anterior segment (aqueous humor) and a posterior segment (vitreous humor). The iris controls light entry through the pupil.

The human eye is a nearly spherical organ, about 2.5 cm in diameter, housed in the bony orbit of the skull. It has three concentric layers (tunics), each with a specific function.

• Outer layer (Fibrous tunic): Sclera (white, tough, protective coat covering most of the eye) + Cornea (transparent anterior part, does most of the light refraction — ~2/3 of total focusing power)
• Middle layer (Vascular tunic/Uvea): Choroid (highly vascular, provides nutrition, contains melanin to prevent internal light reflection) → Ciliary body (contains ciliary muscles that change lens shape for focusing) → Iris (colored diaphragm, controls pupil size)
• Inner layer (Nervous tunic): Retina — contains photoreceptor cells (rods and cones), bipolar neurons, and ganglion cells. The actual 'film' of the eye where image is formed

• Pupil — the opening in the center of the iris. Dilates in dim light (sympathetic), constricts in bright light (parasympathetic)
• Lens — transparent, biconvex, elastic structure behind the iris. Held in position by suspensory ligaments (zonules) attached to the ciliary body
• Aqueous humor — watery fluid in the anterior chamber (between cornea and lens). Maintains intraocular pressure, provides nutrition to cornea and lens
• Vitreous humor — gel-like substance filling the posterior chamber (behind the lens). Maintains eye shape, holds retina in place
• Conjunctiva — thin transparent mucous membrane covering the front of the sclera and inner surface of eyelids. Keeps the eye moist

Retina, Rods & Cones

The retina contains two types of photoreceptors: rods (~120 million, for dim light/scotopic vision, contain rhodopsin, absent in fovea) and cones (~6 million, for bright light/photopic vision, contain iodopsin/photopsins, concentrated in fovea). The fovea centralis is the point of sharpest vision. The blind spot (optic disc) is where the optic nerve leaves — no photoreceptors here.

The retina is the neural layer of the eye — it converts light into electrical signals. It contains photoreceptor cells arranged in a specific pattern, along with bipolar neurons and ganglion cells that process and relay signals to the brain via the optic nerve.

• Fovea centralis (yellow spot/macula) — small pit in the center of the retina. Contains ONLY cones, no rods. Gives the sharpest, most detailed color vision. When you focus on something, its image falls on the fovea
• Blind spot (optic disc) — where the optic nerve exits the eye and blood vessels enter. NO photoreceptors here → no image formation. You don't notice it because your brain fills in the gap
• Rhodopsin — visual pigment in rods = opsin protein + retinal (derivative of Vitamin A). Light breaks rhodopsin → generates nerve impulse. Dark adaptation = regeneration of rhodopsin
• Night blindness (nyctalopia) — caused by Vitamin A deficiency → insufficient rhodopsin production → rods can't function properly in dim light

Image Formation, Accommodation & Eye Defects

The image formed on the retina is real, inverted, and diminished — the brain interprets it as erect. Accommodation is the ability of the lens to change shape (curvature) to focus on near or distant objects via ciliary muscle contraction/relaxation. Common defects: myopia (corrected by concave lens), hypermetropia (convex lens), presbyopia (loss of accommodation with age), astigmatism (irregular cornea/lens), cataract (lens opacity), glaucoma (increased intraocular pressure).

Light from an object is refracted by the cornea and lens to form an image on the retina. This image is real, inverted, and diminished. The visual cortex in the occipital lobe of the brain processes this inverted image and interprets it as an erect image — so you see the world right-side-up.

• Accommodation — the process by which the eye changes the focal length of the lens to focus on objects at different distances
• Near vision: Ciliary muscles CONTRACT → suspensory ligaments become SLACK → lens becomes MORE convex (rounded) → increased refractive power → near objects focused on retina
• Far vision: Ciliary muscles RELAX → suspensory ligaments become TAUT → lens becomes FLATTER (less convex) → decreased refractive power → distant objects focused on retina
• Near point (least distance of distinct vision): ~25 cm for a normal young adult. Decreases with age
• Far point: At infinity for a normal eye (can focus on objects infinitely far away)

Sensory Reception: Ear — Hearing from air vibrations, balance from fluid-filled canals

The ear has three parts: external ear (pinna + ear canal + tympanic membrane), middle ear (3 ossicles: malleus, incus, stapes — amplify sound 20x; connected to pharynx via Eustachian tube), and inner ear (bony labyrinth containing cochlea for hearing and vestibular apparatus for balance). In the cochlea, sound vibrations are transduced by hair cells of the organ of Corti on the basilar membrane. The vestibular apparatus (utricle, saccule, semicircular canals) detects head position and rotational movement.

External & Middle Ear

The external ear (pinna, external auditory canal, tympanic membrane) collects and channels sound waves to the eardrum. The middle ear contains three tiny ossicles (malleus, incus, stapes) that amplify sound vibrations ~20 times and transmit them to the oval window of the inner ear. The Eustachian tube equalizes pressure.

The ear does double duty — it's the organ for both hearing and balance (equilibrium). Sound processing starts in the external ear, gets amplified in the middle ear, and converted to nerve impulses in the inner ear.

• Pinna (auricle) — the visible part of the ear. Made of elastic cartilage covered by skin. Collects sound waves and directs them into the ear canal
• External auditory canal (meatus) — tube from pinna to eardrum (~2.5 cm long). Lined with ceruminous glands that secrete cerumen (ear wax) — traps dust and repels insects
• Tympanic membrane (eardrum) — thin, semi-transparent membrane at the end of the ear canal. Vibrates when sound waves hit it. Separates external ear from middle ear

The middle ear is an air-filled cavity in the temporal bone. It contains the three smallest bones in the human body — the ear ossicles.

• Three ossicles (in order): Malleus (hammer, attached to eardrum) → Incus (anvil, connects malleus to stapes) → Stapes (stirrup, fits into the oval window of the inner ear)
• Stapes is the smallest bone in the human body
• Function: The ossicles amplify sound vibrations about 20 times (due to lever action and the area difference between eardrum and oval window)
• Eustachian tube (pharyngotympanic tube) — connects the middle ear to the nasopharynx. Equalizes air pressure on both sides of the eardrum. Opens during swallowing and yawning
• Oval window — membrane-covered opening between middle ear and inner ear where the stapes transmits vibrations
• Round window — another membrane-covered opening below the oval window. Acts as a pressure relief valve for the fluid in the inner ear

Inner Ear & Hearing Mechanism

The inner ear (labyrinth) contains the cochlea (coiled tube for hearing) and vestibular apparatus (for balance). The cochlea has three chambers: scala vestibuli, scala media (cochlear duct), and scala tympani. The organ of Corti sits on the basilar membrane in the cochlear duct and contains hair cells — the actual mechanoreceptors for hearing. Hair cells bend against the tectorial membrane → nerve impulse generated → sent via cochlear nerve (part of VIII cranial nerve) to auditory cortex.

The inner ear is a complex fluid-filled system embedded deep in the temporal bone. It's called the labyrinth because of its maze-like structure. It has two parts: the bony labyrinth (outer, filled with perilymph) and the membranous labyrinth (inner, filled with endolymph).

• Cochlea — a coiled, snail-shaped tube (2.5 turns) responsible for HEARING
• Three chambers of the cochlea: Scala vestibuli (upper, perilymph), Scala media/cochlear duct (middle, endolymph), Scala tympani (lower, perilymph)
• Scala vestibuli and scala tympani are connected at the apex (helicotrema) and filled with perilymph (similar to ECF, high Na⁺)
• Scala media (cochlear duct) is filled with endolymph (similar to ICF, high K⁺)
• Reissner's membrane — separates scala vestibuli from scala media
• Basilar membrane — separates scala media from scala tympani. The ORGAN OF CORTI sits on it

How hearing works (step by step):

• Step 1: Sound waves collected by pinna → travel through ear canal → vibrate the tympanic membrane
• Step 2: Vibrations amplified by the 3 ossicles (malleus → incus → stapes) → transmitted to the oval window
• Step 3: Oval window vibrations create pressure waves in the perilymph of scala vestibuli → waves travel through the cochlea
• Step 4: Pressure waves cause the basilar membrane to vibrate → hair cells of the organ of Corti are pushed against the tectorial membrane
• Step 5: Bending of hair cells (stereocilia) → mechanically-gated ion channels open → generates nerve impulse (receptor potential)
• Step 6: Impulse transmitted via cochlear nerve (part of vestibulocochlear nerve, VIII) → auditory cortex in temporal lobe → perception of sound

Balance & Vestibular Apparatus

The vestibular apparatus in the inner ear detects body position and movement. It includes three semicircular canals (oriented in three planes — detect rotational/angular acceleration via cristae in ampullae) and two otolith organs: utricle and saccule (detect linear acceleration and head tilt via maculae containing otoliths/calcium carbonate crystals).

The ear isn't just for hearing — it's also your body's balance center. The vestibular apparatus in the inner ear constantly monitors your head position and movement, sending information to the brain (cerebellum) so you don't fall over.

• Three semicircular canals — oriented in three mutually perpendicular planes (anterior/horizontal/posterior) to detect rotation in ANY direction
• Each canal has a swollen base called an ampulla containing a sensory structure called the crista ampullaris (crista)
• Crista contains hair cells embedded in a gelatinous mass called the cupula. When the head rotates, endolymph lags behind → pushes the cupula → bends hair cells → nerve impulse generated
• Semicircular canals detect angular/rotational acceleration (turning, spinning, nodding)

• Utricle and saccule (otolith organs) — two sac-like structures in the vestibule. Detect static head position (tilt) and linear acceleration (moving in a straight line, elevator movement)
• Each contains a sensory area called a macula with hair cells
• Hair cells of maculae are covered by a gelatinous layer containing tiny calcium carbonate crystals called otoliths (otoconia/ear stones)
• How it works: When the head tilts or moves linearly, gravity pulls the otoliths → gelatinous layer slides → bends hair cells → nerve impulse
• Utricle — macula oriented horizontally, detects horizontal acceleration and head tilt
• Saccule — macula oriented vertically, detects vertical acceleration