Physiology Chapter on Sensory Receptors

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Questions and Answers

Which type of sensory receptors primarily detect touch, pressure, and vibration?

Thermoreceptors

Chemoreceptors

Mechanoreceptors (correct)

Photoreceptors

What type of sensation is associated with the body's internal organs?

Visceral sensations (correct)

Proprioceptive sensations

Deep sensations

Exteroreceptive sensations

Which statement about proprioceptive sensations is incorrect?

They can detect pressure sensations from the feet.

They are influenced by external stimuli. (correct)

They provide position sensations.

They include tendon and muscle sensations.

What constitutes a sensory unit in the context of sensory receptors?

A single sensory axon and all of its peripheral branches Signup and view all the answers

Which of the following is classified as a special sense?

Smell Signup and view all the answers

Which of the following is NOT a type of somatic sensation?

Vision Signup and view all the answers

Deep sensations include which of the following?

Deep pressure and pain Signup and view all the answers

What type of receptor is responsible for sensing pain?

Nociceptors Signup and view all the answers

Which of the following correctly describes sensory receptors?

They can be specialized cells or free nerve endings. Signup and view all the answers

Which classification includes sensations that arise from the body's surface?

Exteroreceptive sensations Signup and view all the answers

What primarily determines the modality of sensation registered by a sensory receptor?

The specific energy form each receptor is sensitive to Signup and view all the answers

Which phenomenon describes the sensation of an amputated limb still being present?

Phantom limb sensation Signup and view all the answers

What mechanism causes phantom pain in an amputated limb?

Irritation of severed nerve endings Signup and view all the answers

How does the brain identify that a certain type of impulse signifies a specific sensation?

By the specific pathway the sensory impulses take Signup and view all the answers

What might occur when extremely high intensity or amplitude stimuli are applied to sensory receptors?

Perception of sensations not normally felt by that receptor Signup and view all the answers

What is the primary role of sensory receptors in an organism?

To inform about changes in the environment Signup and view all the answers

Why do different nerve fibers transmit different modalities of sensation?

Because they are influenced by the type of sensory receptor Signup and view all the answers

What is meant by the term 'labeled line principle' in the context of sensory modalities?

Different sensory modalities use different types of receptors that connect to specific brain areas Signup and view all the answers

What changes can lead to the misinterpretation of sensations in the brain, particularly in relation to amputated limbs?

Neuromas sending incorrect signals Signup and view all the answers

Which sensory pathway principle suggests that sensory impulses are projected to the brain based on their locality?

Law of projection Signup and view all the answers

What initiates the receptor potential in sensory receptors?

Mechanical deformation of the receptor membrane Signup and view all the answers

What are graded potentials in sensory transduction?

Potentials that are dependent on stimulus intensity Signup and view all the answers

Which of the following best describes transduction in sensory receptors?

The conversion of physical stimuli into electrical signals Signup and view all the answers

At what maximum amplitude does the receptor potential usually reach?

~100mV Signup and view all the answers

What determines the frequency of action potentials in sensory neurons?

The increase in receptor potential Signup and view all the answers

What happens when the receptor potential exceeds the threshold in a sensory neuron?

Action potentials are generated Signup and view all the answers

How do temperature changes affect sensory receptors?

They influence the permeability of the membrane Signup and view all the answers

In the pacinian corpuscle, what is the result of compression on the outer layers?

Deformation of the central fiber Signup and view all the answers

What distinguishes receptor potentials from action potentials?

Receptor potentials are graded and not all-or-none Signup and view all the answers

What occurs during the genesis of action potential phase in sensory receptors?

Transduction and action potential generation can occur separately Signup and view all the answers

Which type of receptor does not adapt completely to a continuous stimulus?

Chemoreceptors Signup and view all the answers

What is the mechanism of adaptation in rods and cones?

Change in light-sensitive chemicals Signup and view all the answers

How do rapidly adapting receptors primarily detect stimuli?

By responding to changes in stimulus strength Signup and view all the answers

Which of the following statements about conduction velocity is true?

It varies between all nerve fiber types. Signup and view all the answers

What increases when the intensity of a stimulus increases?

Frequency of action potentials increases Signup and view all the answers

What is the conduction velocity of Aδ fiber when a stimulus is applied to the fingertip?

25 m/sec Signup and view all the answers

What factor does NOT influence the strength of stimulus detection by receptors?

Duration of the stimulus Signup and view all the answers

Which of the following is an example of a slowly adapting receptor?

Golgi tendon organ Signup and view all the answers

What is the approximate conduction speed of C fibers?

0.5 m/sec Signup and view all the answers

Which receptor type is responsible for quickly transmitting information about sudden pressure?

Pacinian corpuscle Signup and view all the answers

Study Notes

Nervous System Physiology

The study covers nervous system physiology, sensory receptors, and receptor potentials.
Learning objectives include listing receptor types, describing receptor potential, classifying fiber types, classifying somatic sensation, describing receptor functions, and explaining receptor adaptation.

Sensory Receptors

Sensory receptors are categorized into special senses (vision, hearing, smell, taste, equilibrium) and somatic senses.
Somatic sensations are further classified as mechanoreceptive (tactile &positional sensations), pain/nociception, & others.

Mechanoreceptive Somatic Senses

Tactile sensations include touch, pressure, vibration, and itch/pricking. They occur via various receptors such as Meissner corpuscles, Pacinian corpuscles, Ruffini's corpuscles, Merkel's disks, and free nerve endings.
Position (proprioception) sensations involve sensing the position of the body.
Equilibrium sensations involve maintaining balance.

Other Classifications of Somatic Sensations

Exteroreceptive sensations arise from the body surface.
Proprioceptive sensations relate to the body's physical state (position & tendon/muscle sensations).
Pressure sensations are from the bottom of the feet.
Visceral sensations originate from visceral organs.
Deep sensations arise from deep tissues (e.g., fasciae, muscles, & bone). Deep pressure, pain, and vibration are types of deep sensations.

Types of Sensory Receptors

Mechanoreceptors are responsible for tactile and positional sensations (e.g., touch, pressure, vibration.)
Thermoreceptors detect temperature (e.g., cold, warmth).
Nociceptors detect pain (e.g., pain from injury).
Electromagnetic receptors detect light (e.g., rod and cone cells in the eyes).
Chemoreceptors detect chemicals (e.g., taste buds, olfactory epithelium, receptors in the aortic and carotid bodies).

Sensory Receptors: Classification

Cellular receptors are on the surface of the cellular membrane or inside the cytoplasm.
Specialized receptor cells are in specific parts of the body (e.g., eyes, ears, nose, tongue). These structures have chemoreceptors, osmoreceptors, thermoreceptors, baroreceptors, proprioceptors, and other mechanoreceptors.

Sensory Units and Receptive Fields

A sensory unit consists of a single sensory axon and all of its peripheral branches.
Each has a receptive field, the area where the stimulus evokes action potentials in the sensory neuron.

Modality of sensation

The labeled line principle states that each specific type of sensation has its own dedicated nerve fibers throughout the central nervous system. A designated spot in the central nervous system receives impulses from the corresponding nerve fibers, which determines the perception of the stimulus (e.g., pain, heat, and touch).
A phantom limb is the sensation an amputated body part still exists.

Transduction of Sensory Stimuli

Receptor potentials are graded potentials that vary in amplitude depending on stimulus intensity and duration.
Receptor potentials lead to action potentials that are transmitted along the nerve fibers.

Adaptation of Receptors

Rapidly adapting (phasic) receptors respond rapidly but with a progressively slower rate when a continuous stimulus is applied (e.g., Meissner corpuscles, Pacinian corpuscles, hair end-organs).
Slowly adapting (tonic) receptors never adapt completely (e.g., baroreceptors, muscle spindles, Golgi tendon organs, Merkel cell end-organs, Ruffini corpuscles).

Encoding Stimulus Intensity

Intensity of a stimulus is encoded by the frequency of action potentials in the sensory fibers. A stronger stimulus will result in a higher frequency of action potentials A stronger stimulus results in a higher frequency of action potentials.

Classification of Nerve Fibers.

Nerve fibers are classified based on their diameter and speed of conduction. Larger-diameter fibers have faster conduction velocities.

Tactile Sensations: Touch, Pressure, & Vibration

Different types of tactile receptors detect touch, pressure, and vibration by responding to different stimuli. Each receptor is tuned to respond best to a particular type of stimulation (e.g., Meissner's corpuscles for low frequencies, Pacinian corpuscles for high frequencies).
Free nerve endings respond to temperature, pressure, stretch, and pain, and they are widespread in the skin and other tissues.

Receptors for Position Senses

Transmission of Tactile Signals.

Rapidly conducting sensory fibers determine precise localization and fine gradations of intensity.
Slowly conducting sensory signals determine poorly localized touch, tickle, and crude pressure.
Detection of vibration uses Pacinian corpuscles (30-800Hz) and Meissner's corpuscles (3-80Hz).
Tickle & itch sensations use rapidly adapting free nerve endings (small type C unmyelinated fibers).

Dorsal & Ventral Roots

Sensory fibers in the dorsal root carry sensory information into the spinal cord.
Motor fibers in the ventral root carry motor information from the spinal cord to muscles and glands.

Sensory Pathways for Transmitting Signals into CNS

Sensory signals enter the spinal cord through the dorsal roots of the spinal nerves. Dorsal column
medial lemniscal system
Anterolateral system (spinothalamic tract)

Dorsal Column-Medial Lemniscal System

Transmits fine touch, including tactile discriminative sense, which concerns the precise localization of stimuli and gradations of intensities (e.g., touch, pressure, and vibrations).

Motor Cortex

The motor cortex (neocortex) plans & executes complex skilled movements
Motor cortex is divided into three regions • Primary motor cortex (Brodmann's area 4) • Premotor area (Brodmann's area 6) • Supplementary motor area (Brodmann's area 6)

Motor Association Cortex (Premotor Area + Supplementary Motor Area)

It plans complex movements, receives input from the posterior parietal cortex to assess the spatial location of body parts & the environment, and integrates these inputs with intended movements from the premotor & supplementary motor areas.

Specialized Areas of Motor Control (Broca, Wernicke areas).

Broca's area (speech function) is located in the categorical hemisphere (left in most people) of the dominant side.
Wernicke's area (language comprehension) is associated with Broca's area in language processing.

Voluntary Eye Movement

The premotor area above Broca’s area directs voluntary eye movements toward specific objects Area is responsible for skilled hand movements

Motor Signals carried to spinal cord: two ways

Directly from cortex to spinal cord (corticospinal/pyramidal tract)
Indirectly through basal ganglia, cerebellum, and other brain stem nuclei (extrapyramidal tracts)

Extrapyramidal System

Consists of basal ganglia, subthalamic nuclei, red nucleus & reticular formation—controls balance, posture, and involuntary movements. It acts like a braking system preventing unwanted movements (i.e., involuntary movements). Some types are vestibulospinal, tectospinal, and rubrospinal.

Corticospinal (pyramidal) Tract:

The pyramidal tract consists of the majority of the descending motor fibers from the motor cortex
The nerve fibers pass directly from the motor cortex to the spinal cord.
A minority of nerve fibers synapse first in the brain stem. (cortico-rubro-spinal pathway)

Role of the Brain Stem in Controlling Motor Functions

The brain stem plays important roles in controlling: Respirations, Cardiovascular System, Gastrointestinal tracts
Other voluntary movements
Eye movements
Motor and sensory functions for the head and face

Pain

Pain is a protective mechanism triggered by tissue damage
Nociceptors are free nerve endings that are widespread and respond to various stimuli such as mechanical, thermal, and chemical.
Two pain pathways transmit pain signals to the CNS; fast-sharp and slow-chronic pain
Fast-sharp pain is triggered by mechanical or thermal stimuli.
Slow-chronic pain often is from chemical agents that stimulate and activate the pain fibers in layers II &III of the substantia gelatinosa.
Analgesia system uses descending corticofugal pathways from the brain to help alleviate pain
Referrred pain is when pain is felt at a location other than the damaged tissue

Headache

Brain tissue is insensitive to pain
Headache can be caused by: • Traumatizing, crushing, or stretching blood vessels in meninges • Tugging on venous sinuses • Damaging the tentorium

Types of Intracranial Headache

Meningitis headache
Low cerebrospinal fluid pressure headache
Alcohol headache
Constipation headache
Migraine headache (with prodromal, aura, pain, & postdrome phases)

Thermal Sensations

Receptors for cold (TRPM8) and heat (TRPV1-4) transduce thermal energy into receptor potentials
These receptors are free nerve endings & are responsible for detecting temperature.
There is a difference in how rapidly these receptors adapt depending on the intensity of the stimulus.
Different types of pain fibers detect different temperatures (pain occurs during extreme cold or heat.)

Muscle Sensory Receptors (Muscle Spindles & Golgi Tendon Organs)

Muscle spindles detect muscle length or changes in muscle length.
Golgi tendon organs detect muscle tension or changes in muscle tension.
Both transmit this information to the spinal cord using different types of afferent nerve fibers to control posture and movement.
Proper muscle control requires feedback from these receptors.

Reflexes

Simple reflex is involuntary, unlearned response to a particular stimulus (e.g., pulling hand away from a hot object).
Reflexes arc are complex pathways and include: • Receptor • Afferent pathway • Integrating center (with synapses) • Efferent pathway • Effector

Monosynaptic Reflexes

Contain a single synapse between the afferent and efferent neurons, and they arise from the stretch reflex (e.g., knee-jerk reflex).
If there are one or more interneurons, the reflex is known as polysynaptic, which usually involve more complex pathways (for example, the flexor reflex/withdrawal reflex during painful stimuli).

Muscle Spindle

Consist of a group of a few small intrafusal muscle fibers enclosed in connective tissue
The central portion of intrafusal fibers doesn't contract
The two ends of the fibers do contract when stimulated by gamma motor neurons, which create a reflex to oppose sudden changes in muscle length.
Important in maintaining posture and muscle tone.

Golgi Tendon Organs

Detects muscle tension or changes in muscle tension
Uses type Ib nerve fibers
Prevents excessive muscle tension, and ensures balanced contraction of muscle fibers to prevent injury

Flexor Reflex & Withdrawal Reflex

A painful stimulus causes flexor muscles to contract and withdraw the affected body part from the stimulus.
This occurs at the same time on the contralateral side as an extension movement.
Facilitated by reciprocal inhibition, which inhibits antagonist muscles during flexion.

Role of Motor Cortex & Brain Stem

The motor cortex directly plans & executes voluntary movements.
The brain stem controls postural reflexes & muscle tone; associated with many other specialized motor functions.

Specific Specialized Motor Areas

Broca's area is important for speech.
The area for voluntary eye movement is located above Broca's area.
Detailed mapping of motor areas has been achieved using electrical stimulation of the brain.

The Cerebellum

Vital for rapid muscular activities (e.g., running, typing, etc.), which also require precise movements and timing.
The cerebellum receives its input from the cortex, brain stem, and spinal cord via various pathways.
The cerebellum's output is inhibitory, allowing it to coordinate and refine movements, and to facilitate learned motor skills.
It plays essential roles in balance and posture.

Clinical Abnormalities of the Cerebellum

Dysmetria, ataxia: errors in the intended movements of the limb resulting in overshooting or undershooting the target.
Dysdiadochokinesia: inability to perform rapidly alternating movements
Intention tremor: tremors when attempting to perform a voluntary act
Cerebellar nystagmus: tremor of the eyeballs
Hypotonia from loss of cerebellar facilitation

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