The Somatic Nervous System
The somatic nervous system is the part of you that deals with the outside world. It carries sensation in, and it sends voluntary commands to your skeletal muscles out. Voluntary means the movements you choose to make. This chapter follows that two-way traffic from start to finish. We begin where every feeling starts: at the receptors in your skin. Then we move up to the special senses. These are the eye and its visual pathway, the ear that lets you hear and keeps you balanced, and the chemical senses of taste and smell. Last, we close the loop with the fastest circuit in the body, the reflex arc. Here a signal comes in and a muscle answers before your brain has even been told. By the end, you will be able to trace a signal from the world all the way to a response.
By the end of this chapter you can
- Tell general senses from special senses. Sort sensory receptors by what they detect (mechano-, thermo-, noci-, chemo-, and photoreceptors).
- Name the skin receptors for light touch, deep pressure, temperature, and pain. Explain sensory adaptation.
- Name the three tunics of the eye. Trace the path of light to the retina, and explain refraction and accommodation.
- Follow the visual pathway from the light-sensing cells, through the optic chiasm, to the visual cortex. Predict what vision is lost when a given spot is damaged.
- Describe the outer, middle, and inner ear. Explain how the organ of Corti turns a sound wave into a nerve signal.
- Explain how taste and smell detect chemicals, and why smell is so closely tied to memory and feelings.
- List the five parts of a reflex arc. Compare a one-synapse stretch reflex with a many-synapse withdrawal reflex.
Section 1How sensation begins: the receptors
Two families of senses
Everything you sense starts with a sensory receptor. This is a special nerve ending that turns some kind of energy into an electrical signal. The nervous system can then read that signal. This change is called transduction, and it is the first step in every sense.
We sort the senses into two families. The general senses are touch, pressure, vibration, temperature, pain, and your sense of where your body parts are (proprioception). Their receptors are spread all over the body, in the skin, muscles, joints, and organs. The special senses are vision, hearing, balance, taste, and smell. They sit in complex organs in the head, and they fill the rest of this chapter.
Receptors are named for what they detect
There are dozens of receptor types. The easiest way to sort them is by the kind of signal each one responds to. Learn these five groups, and you can place almost any receptor in the body:
| Receptor type | Responds to | Example |
|---|---|---|
| Mechanoreceptor | Being pushed or bent out of shape: touch, pressure, stretch, vibration, sound | Tactile corpuscles; hair cells of the ear |
| Thermoreceptor | Temperature and changes in it | Warm and cold free nerve endings in skin |
| Nociceptor | Tissue damage; these are the pain receptors | Free nerve endings that signal cuts, burns, and swelling |
| Chemoreceptor | Chemicals in liquid or in the air | Taste and smell receptors; sensors for O₂ and CO₂ in the blood |
| Photoreceptor | Light | Rods and cones of the retina |
The touch receptors of the skin
Your skin alone holds many kinds of nerve endings. Each one is tuned to a slightly different feeling. Free nerve endings are the simplest. They are bare fibers that report pain, temperature, and rough touch. Tactile (Merkel) discs sit just under the surface and pick up fine texture and edges. Tactile (Meissner’s) corpuscles are the egg-shaped parts in the figure above. They sit in small bumps of the dermis in hairless skin, like the fingertips and lips. They sense light touch and gentle, slow (low-frequency) vibration.
Deeper down are the large lamellated (Pacinian) corpuscles. They are built in layers, like an onion. They respond to deep pressure and fast, buzzing (high-frequency) vibration. Bulbous (Ruffini) corpuscles sense skin stretch and steady pressure.
How many receptors you have in one spot decides how sensitive it is. Your fingertips are crowded with receptors. Each one covers a tiny patch of skin, called its receptive field. So you can feel two close points as two separate points. That is why you read Braille with a fingertip, not an elbow.
Another key trait is adaptation. Many receptors are phasic, or fast-adapting. They fire hard when a touch starts or stops. But they go quiet if the touch holds steady. That is why you stop noticing your shirt seconds after you put it on.
Find and name each of the skin’s receptors below.
- Transduction turns a stimulus into a nerve signal. General senses are found all over the body. Special senses live in organs in the head.
- Receptors are grouped by what they sense: mechano-, thermo-, noci-, chemo-, and photoreceptors.
- Skin touch receptors range from free nerve endings and Merkel discs to Meissner’s (light touch), Pacinian (deep pressure and vibration), and Ruffini (stretch) corpuscles.
- Phasic receptors adapt to a steady stimulus. Nociceptors (pain) hardly adapt at all.
Section 2The eye: catching the light
Three layers wrapped around a ball of fluid
The eyeball is built from three coats, one inside the other. Each coat is called a tunic. The tough outer coat is the fibrous tunic. Most of it is the white sclera, which gives the eye its shape. In front, it becomes the clear, curved cornea. The cornea is the eye’s main window, and it bends light more than any other part.
Under that is the vascular tunic, which carries the blood supply. The dark choroid feeds the retina and soaks up stray light. The ciliary body makes fluid and adjusts the lens. The colored iris opens and closes the pupil to control how much light gets in. The inner coat is the neural tunic, or retina. This is where light finally becomes a nerve signal.
| Tunic | Structures | Job |
|---|---|---|
| Fibrous (outer) | Sclera; cornea | Protects the eye and gives it shape; the cornea does most of the focusing |
| Vascular (middle) | Choroid; ciliary body; iris | Brings blood, makes aqueous humor, and controls the lens and pupil |
| Neural (inner) | Retina (rods, cones, and relay nerve cells) | Turns light into nerve signals (action potentials) |
The path of a light ray
Follow one ray of light from the world to the retina. It passes through the cornea → aqueous humor → pupil → lens → vitreous humor → retina. Two fluids fill the eye and keep it firm. The watery aqueous humor fills the space in front of the lens. It feeds the cornea and lens, which have no blood vessels. It drains out all the time through the scleral venous sinus (canal of Schlemm).
Behind the lens, a jelly called the vitreous humor fills the large back space of the eye. Refraction means the bending of light. The cornea does most of it. The lens makes the fine changes.
Accommodation and the sharpest spot
To focus on something close, the eye uses accommodation. The ciliary muscle tightens. This loosens the small ligaments that hold the lens. The stretchy lens then bulges into a rounder, stronger shape. For things far away, the ciliary muscle relaxes. The ligaments pull tight, and the lens flattens.
The image lands on the retina. The retina has two kinds of light receptors. Rods work very well in dim light, but they see no color. Cones need bright light, and they give us color and detail. A small pit called the fovea centralis is packed almost only with cones. It gives the sharpest, most detailed color vision. Where the optic nerve leaves the eye, there are no light receptors at all. This spot is the optic disc, your natural blind spot.
Explore every part of the eyeball in the slice below.
- The eye has three tunics: fibrous (sclera and cornea), vascular (choroid, ciliary body, iris), and neural (retina).
- Light travels cornea → aqueous humor → pupil → lens → vitreous → retina. The cornea bends light the most, and the lens fine-tunes it.
- Accommodation: the ciliary muscle tightens → the lens gets rounder for close-up vision.
- The fovea (all cones) gives the sharpest vision. The optic disc is the blind spot. Rods work in dim light, and cones give color.
Section 3The visual pathway: from retina to cortex
The route to the back of the brain
Once the retina makes a signal, the signal must reach the part of the brain that can make sense of it. That part is the visual cortex in the occipital lobe, at the very back of the head. The trip goes like this: ganglion cell axons of the retina → optic nerve (CN II) → optic chiasm → optic tract → lateral geniculate nucleus of the thalamus → optic radiations → primary visual cortex.
The most important stop is the optic chiasm. This is an X-shaped crossing just under the brain. Here, fibers from the nasal half of each retina (the half nearer the nose) cross to the other side. Fibers from the temporal half (the half nearer the temple) stay on the same side.
Why each brain half sees the opposite world
This partial crossing has a neat result. The left half of what you see is handled by the right side of the brain, and the right half by the left side. That may sound backwards. But remember that the lens flips every image. So the left half of the world lands on the nasal retina of the left eye. It also lands on the temporal retina of the right eye. Both of those fiber groups end up in the right brain.
This crossing is not an accident. It lets the brain blend the slightly different views of your two eyes into one 3-D picture. From the cortex, the signal splits again, as the figure above shows. An upper, or dorsal “where/how” stream, goes toward the parietal lobe. A lower, or ventral “what” stream, goes toward the temporal lobe.
Lesions map to the pathway
A lesion is a damaged spot. Because the pathway is so orderly, damage at each point causes a loss of vision you can predict. This shows up on almost every nerve exam:
| Lesion site | Visual field result |
|---|---|
| Optic nerve (one side) | Total blindness in that one eye |
| Optic chiasm (center) | Loss of both outer (temporal) halves of vision, called bitemporal hemianopia |
| Optic tract or beyond (one side) | Loss of the same half of vision on the opposite side in both eyes, called homonymous hemianopia |
Step through the whole trip, from light receptor to cortex, below.
- The pathway: retina → optic nerve → optic chiasm → optic tract → thalamus (LGN) → visual cortex in the occipital lobe.
- At the chiasm, nasal fibers cross. So each side of the brain handles the opposite half of what you see.
- The cortex feeds a dorsal “where” stream (parietal) and a ventral “what” stream (temporal).
- Where the damage is predicts what vision is lost. Damage to the chiasm gives bitemporal hemianopia.
Section 4The ear: hearing and balance
Three parts passing a wave inward
The ear does two jobs at once. It hears, and it keeps you balanced. It is built in three parts. The outer ear is the flap you can see (the auricle) and the ear canal (external acoustic meatus). It funnels sound to the tympanic membrane, or eardrum, and makes it vibrate.
The middle ear is filled with air. It holds the three smallest bones in the body, called the ossicles. They are the malleus, incus, and stapes. They act like levers. They pass the vibration across and press it onto the oval window. The auditory (Eustachian) tube links the middle ear to the throat to even out pressure. That is the tube that “pops” on a plane.
The inner ear is filled with fluid and buried in the temporal bone. It holds the spiral cochlea for hearing. It also holds the vestibule and semicircular canals for balance.
| Region | Key structures | Job |
|---|---|---|
| Outer ear | Auricle, ear canal, tympanic membrane | Gathers sound and makes the eardrum vibrate |
| Middle ear | Malleus, incus, stapes; auditory tube | Makes the vibration stronger and passes it to the oval window |
| Inner ear | Cochlea; vestibule & semicircular canals | Turns vibration into hearing; senses head position and motion |
How the cochlea turns motion into sound
The coiled cochlea holds three fluid-filled tubes. When the stapes rocks the oval window, it sends a pressure wave through a fluid called perilymph. That wave makes the basilar membrane shiver. Sitting on the basilar membrane is the organ of Corti (the spiral organ). It is dotted with hair cells. Each hair cell has tiny hairs called stereocilia. They brush against the tectorial membrane, which hangs over them.
When the hairs bend, small gates (ion channels) pop open. The hair cell becomes charged (depolarizes) and fires the cochlear nerve, which is part of cranial nerve VIII. The round window bulges out to let off the pressure, so the wave can keep moving.
Pitch is coded by place. At the base of the cochlea, the basilar membrane is narrow and stiff. There it responds to high-frequency (high-pitched) sounds. At the tip, or apex, it is wide and floppy. There it responds to low-frequency (low-pitched) sounds. So the brain knows what note it hears by which hair cells fire. It is a lovely case of the body’s shape doing the math.
Next door, the vestibule senses gravity and moving in a straight line. The semicircular canals sense spinning and turning. Together they give you your sense of balance, through the same eighth nerve.
Send a sound wave inward through all three parts of the ear below.
- The ear has three parts: the outer ear gathers sound, the middle ear’s ossicles make it stronger, and the inner ear holds the cochlea and balance organs.
- The organ of Corti sits on the basilar membrane. Its hair cells turn vibration into a cochlear nerve (CN VIII) signal.
- Pitch is mapped by place on the basilar membrane: the base for high tones, the apex for low tones.
- The vestibule and semicircular canals handle balance. Hair-cell (sensorineural) loss is permanent.
Section 5Taste and smell: the chemical senses
Tasting the dissolved, smelling the airborne
Taste and smell both use chemoreceptors. They detect molecules, not energy. The only difference is where the molecules are. Taste (gustation) reads chemicals dissolved in your spit (saliva). Smell (olfaction) reads chemicals floating in the air.
Taste happens in taste buds. Most sit on the bumps of the tongue, called papillae. There are mushroom-shaped fungiform papillae, ridged foliate papillae, and large circumvallate papillae at the back. The pointed filiform papillae have no taste buds. They only grip food and sense texture. Each bud is a cluster of gustatory cells, or taste cells. Their taste hairs poke up through a taste pore to sample the fluid. Basal cells sit behind them. They keep replacing worn-out taste cells every week or two.
| Basic taste | Signals | Trigger |
|---|---|---|
| Sweet | Sugars, energy | Glucose and other carbohydrates |
| Sour | Acids, unripe or spoiled food | Hydrogen ions (H⁺) |
| Salty | Minerals and body salts (electrolytes) | Sodium ions (Na⁺) |
| Bitter | Possible poisons | Alkaloids and many poisons |
| Umami | Protein, a savory taste | Glutamate (as in broth or aged cheese) |
Smell: a neuron with its feet in the air
High in the roof of the nose sits the olfactory epithelium. Its receptor cells are true nerve cells (neurons). They have tiny hairs (cilia) bathed in mucus. There are only about 400 kinds of smell receptors. Yet the nose can tell apart thousands of odors. That is because each smell turns on its own mix of receptors. It is like a chemical fingerprint that the brain learns to read.
The fibers of these nerve cells thread up through tiny holes in the cribriform plate of the ethmoid bone. They reach the olfactory bulb. Here is what makes smell strange and special. Unlike every other sense, smell signals go straight to the smell and limbic areas of the brain, without stopping first in the thalamus. And unlike most nerve cells, smell neurons grow back all through life.
Switch between the two chemical senses and explore their receptors below.
- Both senses use chemoreceptors. Taste reads molecules dissolved in liquid. Smell reads molecules in the air.
- Taste buds on the papillae hold gustatory cells. They detect five basic tastes: sweet, sour, salty, bitter, umami.
- Smell neurons reach the brain through the cribriform plate. They are among the few nerve cells that regenerate, or grow back.
- Only smell skips the thalamus. It links straight to the limbic system, which ties it to memory and feelings.
Section 6The reflex arc: sensation answered
The fastest circuit in the body
We began this chapter with feelings coming in. We end it with a response going out. A reflex is a fast, automatic reaction to a stimulus that you can predict. Its key feature is that it does not wait for you to think. The signal runs a short, built-in loop called the reflex arc. It often turns around in the spinal cord. It fires a muscle before your brain even knows what happened. That speed keeps you safe. You jerk your hand off a hot stove long before you feel the burn.
The five parts of every reflex arc
Some reflexes are simple and some are complex. But every reflex arc has the same five parts in the same order. Learn them once, and you can draw any reflex in the body:
| Step | Component | What it does |
|---|---|---|
| 1 | Receptor | Senses the stimulus (for example, a muscle spindle senses stretch) |
| 2 | Sensory (afferent) neuron | Carries the signal into the brain or spinal cord (the CNS) |
| 3 | Integration center | One or more synapses (nerve-to-nerve links) in the spinal cord or brainstem |
| 4 | Motor (efferent) neuron | Carries the order back out |
| 5 | Effector | The muscle or gland that responds |
Knee-jerk simple, withdrawal complex
The classic stretch (patellar) reflex is the knee-jerk a doctor tests with a small hammer. It is the simplest kind. It is a monosynaptic arc, with just one synapse between the sensory and motor nerve cells. Here is how it works. Tapping the tendon stretches the thigh muscle. The muscle spindle, a stretch sensor in the muscle, fires. The sensory nerve cell links straight to a motor nerve cell in the cord. The quadriceps muscle tightens, and the leg kicks out.
At the same moment, an interneuron (a go-between nerve cell) blocks the opposite muscle, the hamstring. So the hamstring relaxes and gets out of the way. This trick is called reciprocal inhibition, and you can see it in the figure above. Other reflexes are polysynaptic. They pass through one or more interneurons. One example is the withdrawal reflex that yanks your foot off a tack. It is paired with a crossed-extensor reflex. That reflex stiffens your other leg so you do not fall.
Set off the knee-jerk and follow the signal through all five parts of the arc.
- A reflex is a fast response you do not choose. It turns around in the cord or brainstem, not the thinking brain.
- Every arc has five parts: receptor → sensory neuron → integration center → motor neuron → effector.
- The stretch reflex is monosynaptic (one synapse). Withdrawal reflexes are polysynaptic and add interneurons.
- Reciprocal inhibition relaxes the opposite muscle. Testing reflexes checks the whole arc and helps find where damage is.
The chapter in one breath
- The somatic nervous system carries feelings in and sends chosen commands to skeletal muscle out. Every sense begins when a receptor turns a stimulus into a nerve signal.
- Receptors are grouped by what they sense (mechano-, thermo-, noci-, chemo-, photoreceptor). Skin touch endings range from free nerve endings to Meissner’s, Pacinian, and Ruffini corpuscles. Most adapt, but pain does not.
- The eye’s three tunics focus light through the cornea and lens onto the retina. Accommodation rounds the lens for close-up vision. Rods work in dim light, while cones and the fovea give color and detail.
- The visual pathway crosses at the optic chiasm, so each side of the brain sees the opposite half. Where the damage is predicts the vision loss (chiasm → bitemporal hemianopia).
- The ear’s outer, middle, and inner parts pass a wave to the organ of Corti. There, hair cells fire the cochlear nerve. The vestibule and semicircular canals handle balance.
- Taste and smell are chemical senses. Taste buds detect five basic tastes. Smell skips the thalamus and links straight to the limbic system.
- A reflex arc answers a stimulus in five steps (receptor → sensory neuron → integration center → motor neuron → effector). The stretch reflex is the one-synapse model, and testing reflexes helps find damage.
ReferenceWords worth knowing
Here is a quick list of the key words in this chapter. Skim it now, or come back when a word trips you up.
- Accommodation
- How the eye adjusts to see up close. The ciliary muscle tightens, and the lens gets rounder so it bends light more.
- Adaptation (sensory)
- When a receptor fires less because a stimulus stays the same. Phasic (fast-adapting) receptors adapt quickly; tonic ones hardly adapt at all.
- Basilar membrane
- The sheet in the cochlea that holds up the organ of Corti. Different spots on it vibrate for different pitches of sound.
- Chemoreceptor
- A receptor that detects chemicals. Examples are taste and smell receptors and the body’s blood-gas sensors.
- Cochlea
- The coiled, fluid-filled organ of the inner ear. It turns sound waves into nerve signals.
- Cone
- A light receptor in the retina that works in bright light. It gives color and sharp detail, and it is packed into the fovea.
- Fovea centralis
- A tiny pit in the retina packed with cones. It gives the sharpest, most detailed vision.
- General senses
- The senses found all over the body: touch, pressure, vibration, temperature, pain, and proprioception (knowing where your body parts are). Their receptors are in skin, muscle, and organs.
- Gustation
- The sense of taste. Chemoreceptors in the taste buds detect dissolved molecules.
- Mechanoreceptor
- A receptor that responds to being pushed or bent, such as by touch, pressure, stretch, vibration, or sound.
- Nociceptor
- A pain receptor (a free nerve ending). It signals real or possible tissue damage and hardly adapts.
- Olfaction
- The sense of smell. Receptor nerve cells in the roof of the nose detect molecules in the air, and these cells grow back.
- Optic chiasm
- The X-shaped crossing under the brain. Here the nasal fibers of each retina cross to the other side, so each half of what you see goes to the opposite side of the brain.
- Organ of Corti
- The spiral organ that sits on the basilar membrane. Its hair cells turn vibration into a cochlear nerve signal.
- Photoreceptor
- A light-sensing receptor in the retina. The rods and cones are photoreceptors.
- Reflex arc
- The five-part nerve pathway that carries out an automatic reflex: receptor, sensory neuron, integration center, motor neuron, and effector.
- Refraction
- The bending of light as it passes through the cornea and lens, so that it focuses on the retina.
- Retina
- The eye’s inner (neural) tunic. It holds the light receptors and the relay nerve cells that start the visual pathway.
- Rod
- A light receptor in the retina that works very well in dim light but sees no color. Rods are most common in the outer retina.
- Special senses
- The senses housed in complex organs of the head: vision, hearing, balance, taste, and smell.
- Stretch (patellar) reflex
- The knee-jerk reflex. Stretching a muscle makes that same muscle tighten, using just one synapse (monosynaptic).
- Taste bud
- A cluster of taste (gustatory) cells, mostly on the tongue’s papillae. It detects the five basic tastes through a taste pore.