BIO 201 Exam 3 (All Notes) Fall 2025 Russell

The PNS is composed of what?

nervous system structures outside the brain and spinal cord; nerves, ganglia, sensory receptors and efferent nerve endings

nerve

a cordlike organ composed of numerous nerve fibers (axons) bound together by connective tissue

A nerve may contain how many nerve fibers?

anywhere from a few to hundreds of thousands

Spinal nerve axons are grouped within what?

connective tissue sheaths

fiber

a single axon within an endoneurium

fascicle

a bundle of fibers within a perineurium

nerve

a bundle of fascicles within an epineurium

Numerous blood vessels are within the covering of what PNS structure?

nerves

Peripheral nerves, either cranial or spinal are classified according what?

the direction they transmit impulses

Mixed nerves

contain both sensory and motor fibers

Most nerves are what kind?

mixed nerves

Sensory (afferent) nerves

only carry impulses toward the CNS

Motor (efferent) nerves

only carry impulses away from the CNS

Ganglia

collections of neuron cell bodies associated with nerves in the PNS

Ganglia associated with afferent nerve fibers are what?

cell bodies of sensory neurons

Ganglia associated with efferent nerve fibers are what?

mostly cell bodies of autonomic motor neurons

If damage to a PNS nerve fiber occurs, what happens?

If the cell body is intact, the axon will regenerate

Damaged CNS nerve fibers almost never do what?

regenerate

Why do damaged CNS nerve fibers almost nerve regenerate?

oligodendrocytes have growth-inhibiting proteins that do not support regrowth of axons

What is the key component of PNS axons ability to regenerate?

Schwann cell participation

Sensory receptors

specialized cells or multicellular structures that collect information from the environment. They stimulate neurons to send impulses along sensory fibers to the brain

Sensory receptors can be classified by what?

modality, origin or distribution

thermoreceptors

respond to heat and cold

photoreceptors

respond to light

nociceptors

pain receptors that respond to tissue injury or potentially damaging situations

chemoreceptors

respond to chemicals, including odors, tastes, and body fluid composition

mechanoreceptors (baroreceptors)

respond to physical deformation caused by vibration, touch, pressure, stretch, or tension, such as sound waves, touch, gravity, and movements

Exteroceptors

sense stimuli external to the body

Interoceptors

detect stimuli in the internal organs

Proprioceptors

sense the position and movements of the body or its parts

General (somatic) senses

employ receptors widely distributed in the skin, muscles, tendons, joint capsules, and viscera

Special senses

limited to the head and employ complex sense organs

Receptors for the general senses have what structural characteristics?

they're structurally simple and consist of one or a few sensory nerve fibers and usually a sparse amount of connective tissue

On a microscopic level, sensory receptors are what?

free-nerve endings, cilia, encapsulated nerve endings at the dendrites of a-order sensory neurons, or separate cells that synapse with first order sensory neurons

Encapsulated nerve endings

free nerve fibers wrapped in glial cells or connective tissue; most are mechanoreceptors for touch, pressure, and stretch

Tactile sensations are

touch, pressure, and vibration plus itch and tickle

Crude touch

the ability to perceive that something has simply touched the skin

fine touch

provides specific information about a touch sensation such as location, shape, size, and texture of the source of stimulation

Pressure sensations generally result from stimulation of

tactile receptors in deeper tissues and are longer lasting and have less variation in intensity than touch sensations

Pressure is a sustained sensation that is felt of over a (larger/smaller) area than touch.

larger

Vibration sensations result from

rapidly repetitive sensory signals from tactile receptors

The receptors for vibration sensations are

corpuscles of touch and lamellated corpuscles

Touch corpuscles detect what?

low-frequency vibrations

Lamellated corpuscles detect what?

high-frequency vibrations

Free nerve endings have what structure?

they are bare dendrites that lack structural specialization and are the simplest receptors

Free nerve endings are common in what type of tissue?

epithelial tissue

Free nerve endings respond to what types of stimuli?

pain, temperature, tickle, itch and some touch

What is the only sensation that we cannot elicit ourselves?

tickle

Merkel's (tactile) discs

tonic receptors for light touch and also textures, edges, and shapes

Merkel's (tactile) discs are _______ adapting touch receptors.

slowly

Merkel's (tactile) discs have what structure?

saucer-shaped, flattened free nerve endings

Merkel's discs are found on what parts of the body?

fingertips, hands, lips, and external genitalia

Hair root plexuses are rapidly adapting touch receptors for what function?

so that we are not constantly stimulated by clothing, but are very sensitive to light touch

Hair root plexus have what structure?

free nerve endings wrapped around hair follicles

Hair root plexuses detect what?

movements on the skin surface that disturb hairs - fine touch

Meissner's corpuscles consist of what?

two or three nerve fibers meandering upward through a mass of flattened Schwann cells

Meissner's corpuscle's

phasic receptors for light touch and texture; allow us to distinguish between two points on the skin, kisses

Meissner's corpuscle are ______ adapting receptors.

rapidly

Meissner's corpuscles are found where?

In the dermal papillae, (which are abundant in hairless portions) such as fingertips, hands, eyelids, tip of tongue, lips, nipples, soles, clitoris, and tip of penis

Meissner's corpuscles generate impulses when?

mainly at onset of a touch

Pacinian (lamellated) corpuscles have what structure?

multilayered connective tissue capsule that encloses the dendrite. They are larger receptors, 1-2 mm long, and look like a sliced onion in cross section

Pacinian (lamellated) corpuscles being phasic receptors means what?

they are fast adapting receptors

Pacinian corpuscles are found where?

deeper subcutaneous tissues, tendons and ligaments, periosteum of bone, in joint capsules, mammary glands, external genitalia, pancreas and urinary bladder

Pacinian (lamellated) corpuscles detect what?

heavy pressure, vibrations, and stretching

Ruffini's corpuscles are ______ adapting touch receptors.

slowly

Ruffini's corpuscles are tonic receptors for what?

heavy touch, pressure, skink stretching, and joint movements

What is the structure of Ruffini corpuscles?

elongated, encapsulated receptors

Ruffini's corpuscles are located where?

deep in the dermis and in ligaments and tendons, in the hands, and soles

Receptors located in skeletal muscles, in tendons, in and around joints, and in the internal ear convey nerve impulses related to what?

muscle tone, movement of body parts, and body position

Proprioceptive sensations send information to the spinal cord and CNS about what?

body position and length, and tension of muscles

What are the proprioceptors?

Pacinian corpuscles, muscle spindles, Golgi tendon organs

Pacinian corpuscles detect what sensation in its role as a proprioceptor?

speed of joint movement

Muscle spindles

proprioceptors in skeletal muscles that monitor changes in the length of skeletal muscles and participate in stretch reflexes

A muscle spindle consists of

several slowly adapting sensory nerve endings that wrap around 3 to 10 specialized muscle fibers

Golgi tendon organs

stretch receptors that protect from over stretching

Golgi tendon organs are located where?

at the junction of a tendon and a muscle

Temperature is sensed by what type of receptor?

free nerve endings

Temperature is sensed by ______ adapting receptors.

rapidly

Hot and cold stimuli are responded to by

separate thermoreceptors (there's receptors for hot and receptors for cold)

Warm receptors are sensitive to what temperatures?

above 25 degrees C

Warm receptors are unresponsive to what temperatures?

above 45 degrees C

Cold receptors are sensitive to what temperatures?

between 10 and 20 degrees C

Pain is a vital sensation because

it provides us with information about tissue-damaging stimuli and with signs that may be used for diagnosis of disease and injury

Nociceptors

pain receptors are free nerve endings that are located in nearly every body tissue (except for nervous tissue)

Adaption of nociceptors is

slight if at all

Although no nociceptors occur in the brain, they do occur in the meninges, where they have a role in what?

headaches

Nociceptors are stimulated by what?

intense distortion of the skin, chemical released by damaged tissue, chemical, mechanical or extremes in temperature

Nociceptors respond to what temperatures?

below 10 degrees C and above 45 degrees C

What are the two kinds of pain recognized in the parietal lobe?

somatic and visceral

Visceral pain, unlike somatic pain, is usually felt where?

in or just under the skin that overlies the stimulated organ. It could also be felt on a surface area far from the stimulated organ in a phenomenon known as referred pain

Referred pain

pain felt on a surface area far from the stimulated organ

Projection pathways for pain are

diverse, complex, and the sensation can originate anywhere along any of the routes

Pain signals from the head travel to the brain stem by way of what four cranial nerves?

trigeminal (CN V), facial (CN VII), glossopharyngeal (CN IX), and vagus (CN X)

Pain signals from the head that travel to the brainstem via the trigeminal, facial, glossopharyngeal, and vagus nerves terminate where?

the medulla and second-order neurons arising in the medulla ascend to the thalamus, where they're relayed to the cerebral cortex

Pain signals from the neck down travel by way of what three ascending spinal cord tracts?

spinothalamic, spinorecticular, and gracile fasciculus

Pain signals from the neck down ultimately terminate where?

thalamus, that relays most pain signals through third order neurons to their final destination in the post central gyrus of the cerebrum

referred pain

pain in the viscera that is often mistakenly thought to come from the skin or other superficial sites

Referred pain results from what?

convergence of neural pathways in the CNS

phantom pain

the sensation of pain in a limb that has been amputated

cranial nerves

12 pairs of nerves that attach to the brain and innervate the head and neck