Chapter 15

Somatic

What are sensory pathways?

Neuron chains that carry sensory info from receptors to the CNS.

What are sensory receptors?

Special cells or neuron endings that detect specific conditions in the body or environment.

Afferent Pathways

• Bring body (somatic) and organ (visceral) info to the CNS

• Body info → goes to the thinking part (cerebral cortex)

• Organ info → goes to the automatic control center (brainstem)

Efferent Pathways

• Carry signals out of the CNS

• Control skeletal muscles (movement)

• Brain sends motor commands → muscles move

General senses

temperature, pain, touch, pressure, vibration, proprioception (body position)

Special senses

olfaction (smell), gustation (taste), vision (sight), equilibrium (balance), hearing

Receptor Specificity

• Each receptor senses only one type of thing (like touch, light, or sound)

Receptive Field

• Area one receptor watches over

• Bigger field = harder to know the exact spot of the stimulus

Sensory Unit

• One sensory nerve + all its receptors = one sensation

Receptive Field (Sensory Unit)

• Area on the body that one sensory unit covers

Convergence

• Many sensors send signals to the same brain cell

• Makes it harder to tell two close points apart

Receptor Potentials

Receptors turn a stimulus into an electrical signal

Depolarize = closer to firing (threshold)

Hyperpolarize = farther from firing

Stronger stimulus = bigger signal

Sensory Adaptation

• Receptors stop responding when a stimulus stays constant

• Example: you stop noticing your clothes on your skin

Phasic Receptors

• React fast at first

• Then quickly stop responding if stimulus continues

Clothes on your skin, the chair you’re sitting on, or putting on a watch. You feel it at first, but then the feeling fades away

Tonic Receptors

• Keep firing as long as the stimulus is there

• Example: pain → doesn’t “fade out” like phasic receptors

Pain from a cut, the ache in your muscles after exercise, or holding your arm stretched. The feeling stays as long as the stimulus is there.

Exteroceptors

• Sense the outside world (touch, temperature, smell, vision, hearing)

👉 Example: feeling the wind on your skin.

Proprioceptors

Sense body position and muscle movement
👉 Example: Knowing where your arm is even with your eyes closed

Interoceptors

Sense what’s happening inside your body (organs)
👉 Example: Feeling hungry or needing to pee

Nociceptors

Detect Pain

Thermoreceptors

Detect Temperature

phasic

Mechanoreceptors

Sense physical forces like touch, pressure, stretch, or vibration
👉 Example: Feeling someone poke your arm

• Work by ion channels opening/closing when the membrane is bent or stretched

Chemoreceptors

Sense chemicals in the body or air
👉 Example: Smelling food or detecting oxygen levels in blood

Tactile Receptors (Touch Receptors)

Detect touch, texture, pressure, and vibration

Fine touch receptors

• Very sensitive

• Small area covered → can tell exact spot & details

Crude touch receptors

• Not very sensitive

• Large area covered → can’t pinpoint well

👉 Example: Fingertips (fine) vs. back (crude)

Free Nerve Endings

• Sense touch & pressure

• Always active (tonic)

• Small area covered

Root Hair Plexus

• Sense hair movement

• Fast adapting (phasic)

• Made of free nerve endings

👉 Example: Feeling a bug land on your arm hair

Tactile Discs (Merkel Discs)

• Sense fine touch & light pressure

• Always active (tonic)

Bulbous (Ruffini) Corpuscles

• Sense deep pressure & skin stretch

• Always active (tonic)

• Found deep in the skin (dermis)

👉 Example: Feeling your skin stretch when you grab something tightly

Lamellar (Pacinian) Corpuscles

• Sense deep pressure & vibration

• Fast adapting (phasic) → react quickly, then stop

• Built like an onion (layers around a nerve ending)

👉 Example: Feeling your phone buzz in your pocket

Tactile (Meissner) Corpuscles

• Sense fine touch, light pressure & low-frequency vibration

• Fast adapting (phasic) → react quickly, then fade

👉 Example: Feeling the texture of fabric with your fingertips

Baroreceptors

• Sense pressure changes in blood vessels & organs (like stomach, lungs, bladder)

• Free nerve endings inside stretchy organ walls

👉 Example: Blood pressure sensors in arteries

Proprioceptors (Body Position Sensors)

Tell the brain where your joints & muscles are

Muscle spindles

→ sense muscle stretch, trigger reflex to resist overstretching

Reflex when doctor taps your knee

Golgi tendon organs

→ sense tension at muscle-tendon joint, prevent too much force

Prevents lifting something too heavy

Joint receptors

• → sense pressure & movement in joints

Feeling your knee move when you bend it

Chemoreceptors

• Sense chemicals dissolved in body fluids

• Monitor pH, CO₂, and O₂ in the blood

• Carotid bodies → in carotid arteries (neck)

• Aortic bodies → in aortic arch (near heart)

• Fast adapting (phasic)

Sensory Homunculus

• Brain map for feeling (touch)

• Each body part has a spot in the brain

• Bigger brain area = more nerves there (not bigger body part)

👉 Example: Hands & lips take up a lot of brain space because they’re super sensitive.

Motor Homunculus

• Brain map for movement control

• Each body part has its own spot in the brain

• Bigger brain area = more fine control
  👉 Example: Hands, face, and tongue take up lots of space because they need precise movements