Sensory Receptors Quiz

Nervous System Physiology Study Notes

• Nervous system physiology is the study of how the nervous system functions.
• Today's topic is sensory physiology, sensory receptors, and receptor potentials.
• Learning objectives include identifying receptor types, describing receptor potentials, classifying fiber types, and explaining receptor adaptation.

Sensory Receptors

• Sensory receptors are specialized cells or structures that detect stimuli from the internal or external environment.
• Receptors are classified into various types based on the type of stimulus they detect.
• Mechanoreceptors detect pressure, touch, and vibration, including tactile sensations like touch, pressure, vibration, itch, position (proprioception), equilibrium.
• Thermoreceptors detect temperature (cold and warmth).
• Nociceptors detect pain.
• Electromagnetic receptors detect light (e.g., in the retina).
• Chemoreceptors detect chemicals (e.g., taste, smell, changes in blood chemistry).
• Osmoreceptors detect changes in osmolarity.
• Sensory receptors are either specialized cells or free nerve endings.

Sensory Units and Receptive Fields

• A sensory unit consists of a single sensory axon and all its peripheral branches.
• Each sensory unit has a receptive field, the area of skin that, when stimulated, generates a signal in that specific sensory unit.
• The size of the receptive field varies, with smaller fields in areas with high sensory acuity (e.g., fingertips) and larger fields in areas with lower acuity (e.g., back).

Types of Sensory Receptors

• Each receptor is highly sensitive to one type of stimulus.
• Mechanoreceptors include: skin tactile sensibilities (epidermis and dermis), free nerve endings, expanded tip endings, Merkel's discs, spray endings, Ruffini's endings, encapsulated endings.
• Thermoreceptors include cold and warmth receptors.
• Nociceptors are free nerve endings.
• Electromagnetic receptors include rods and cones (vision).
• Chemoreceptors include taste buds, olfactory epithelium, arterial oxygen receptors, and receptors in aortic and carotid bodies.

Modulation of Sensation

• Each of the principle types of sensation (e.g., pain, touch) is considered a modality.
• Different nerve fibers transmit different modalities of sensation.
• The labeled line principle states that each nerve tract terminates at a specific point in the central nervous system (CNS).
• The modality of sensation determined by the specific nerve fiber that is stimulated and the location in the CNS where it synapses.
• If a pain fiber is stimulated, a person perceives pain, even if the physical stimulus is not pain itself (e.g., electricity, excessive heat, crushing the fiber).

Modality of Sensation: Phantom Limb

• Phantom limb is the sensation that an amputated body part is still attached.
• 70% of people experience phantom limb sensations.
• Missing limb frequently feels as if it is in a distorted and painful position.
• Phantom limb phenomena may be caused by irritation, inflammation of severed nerve endings.

Modality of Sensation: Transduction

Each receptor is sensitive to a specific form of energy.

• Receptors can also be stimulated by nonspecific stimuli of extremely large intensity or amplitude.
• A stimulus creates a receptor potential (a change in membrane potential).
• The amplitude and duration of the receptor potential depend on the stimulus amplitude and duration.

Modality of Sensation: Transduction of Sensory Stimuli

• Receptor potentials are graded potentials that depend on the intensity of the stimulus.
• Action potentials are all-or-none signals that transmit information along a neuron.
• The frequency of action potentials depends on the amplitude of the receptor potential.
• A receptor potential that exceeds a certain threshold triggers action potentials in the nerve fiber.

Adaptation of Receptors

• Rapidly adapting receptors (phasic): responds with a progressively slower rate if a continuous sensory stimulus is applied (e.g., Meissner corpuscle, Pacinian corpuscle, hair end-organ).
• Slowly adapting receptors (tonic): never adapt completely (e.g., baroreceptors, chemoreceptors, muscle spindle, Golgi tendon organ; Merkel cell end-organ, Ruffini corpuscle, joint receptors, pain receptors).

Encoding Stimulus Intensity

• The CNS perceives stimulus intensity by changing the frequency of impulses induced by the receptor.
• The amplitude of receptor potential increases as stimulus intensity increases.
• This results in an increase in the frequency of action potentials in the fibers.

Classification of Fibers

• Nerve fiber conduction velocity depends on fiber diameter.
• Fiber types include Aα, Aβ, Aγ, Aδ, and C.
• Different fiber types carry different types of sensory information.

Tactile Sensations

• Touch, pressure, and vibration occur through receptors.
• Touch is the stimulation of tactile receptors.
• Pressure results from deformation of deeper tissues.
• Vibration results from rapidly repetitive sensory signals.
• Tactile receptors include Meissner's corpuscles, Pacinian corpuscles, Ruffini's corpuscles, Merkel's discs, and free nerve endings.
• Hair end-organs adapt readily.

Receptors for Position Senses (Proprioception)

• Three receptors are involved in position sense: mechanoreceptors in joints (e.g., Ruffini endings), mechanoreceptors in the skin, and muscle spindle receptors.

Transmission of Tactile Signals

• Rapidly conducting sensory signals determine precise localization and intensity gradations (e.g., Meissner's corpuscles, Pacinian corpuscles, Ruffini's corpuscles)
• Slowly conducting sensory signals determine poorly localized touch & tickle and crude pressure (e.g., free nerve endings).

Dorsal Root, Ventral Root

• Afferent fibers, carrying sensory signals, enter the spinal cord through dorsal roots.
• Sensory afferent fibers are the projections of dorsal root ganglia (DRG) cells.
• Ventral root consists of thick alpha motor axons and medium-sized gamma motor axons, and autonomic preganglionic axons.

Sensory Pathways

• Sensory signals enter the spinal cord through the dorsal roots and are conducted along sensory pathways, including the dorsal column-medial lemniscal system and the anterolateral (spinothalamic) system.
• The dorsal column-medial lemniscal system transmits fine touch, pressure, vibration, and proprioception.
• The anterolateral (spinothalamic) system transmits pain, temperature, crude touch, and pressure.

Function of the thalamus in somatic sensation

• The thalamus acts as a primary relay station for sensory information conducted to the cerebral cortex.
• Lesions to the thalamus can lead to a loss of sensory perception.
• The thalamus has a slight ability to discriminate between tactile sensations, pain, and temperature sensations.

Cortical Control of Sensory Sensitivity-Corticofugal signals

• The cerebral cortex transmits signals to lower sensory relay stations.
• These signals help control the intensity of sensory input.

Dermatomes

• Each spinal nerve innervates a segmental area of the skin.
• Dermatomes are useful for determining the level of spinal cord damage or injury.

Neural Lesions

• Ventral root lesions often result in muscle weakness and hypotonia.
• Dorsal root lesions often result in numbness/loss of sensation.
• Peripheral nerve lesions affect specific sensory and motor functions of a specific region of the body. Often resulting in loss of sensation and muscle weakness.

Pain

• Pain receptors are widespread in superficial layers of the skin, periosteum, arterial walls, joint surfaces, dura, and tentorium.
• Nociceptors are free nerve endings that respond to several types of stimuli.
• These are thermal stimuli, mechanical stimuli, and chemical stimuli.
• Chemicals that stimulate pain include bradykinin, serotonin, histamine, proteolytic enzymes, K+, acids, and acetylcholine.
• Tissue ischemia, or lack of blood flow, can lead to pain by releasing acidic or toxic products
• Muscle spasm can also lead to pain by compressing blood vessels and causing ischemia.
• Pain signals travel through two pathways in the anterolateral system (neospinothalamic and paleospinothalamic pathways).
• Fast pain signals are transmitted by large-diameter Aδ fibers.
• Slow pain signals are transmitted by small-diameter C fibers.
• The neospinothalamic tract transmits fast pain signals to the ventrobasal thalamus.
• The paleospinothalamic tract transmits slow pain signals to the reticular formation to other brain stem areas.
• Cortical control of pain modulation occurs via the periaqueductal gray matter.
• The stimulation of this area reduces pain perception
• Opiate receptors in the CNS provide pain suppression

Referred Pain

• The pain that one feels in one location that is remote from the site of the tissue damage.
• Referred pain arises due to visceral organ pain being sensed in a different location via sensory signals travelling along shared neural pathways (e.g., pain originating in the heart felt in the left arm).

Headache

• Brain tissue is insensitive to pain, headaches usually arise from irritation of meninges, stretching of venous sinuses, or damage to the tentorium.
• Intracranial headaches can arise from conditions like meningitis, low cerebrospinal fluid pressure, alcohol use, constipation, and migraine.
• Migraines have both environmental and genetic causes with various triggers-odors, loud noise, bright light, and fatigue among others.
• Extracranial headaches are caused by muscular contractions of the head and neck, irritation of structures within the nose and eyes, such as exposure to UV light

Thermal Sensations

• Thermal sensations are sensed by free nerve endings of the specialized thermal receptors
• Different receptors respond to different temperatures, but all receptors respond to noxious temperatures (very hot or very cold)
• The receptors responsible for cold include TRPM8 and TRPA1.
• The receptors responsible for heat include TRPV1, TRPV3, and TRPV4.

Motor Functions of the Spinal Cord, Cord Reflexes

• Afferent neurons enter the CNS through spinal or cranial nerves.
• Spinal cord has different sensory and motor areas.
• Efferent fibers leave through the ventral roots of the spinal cord.
• Cord reflexes are involuntary, instantaneous actions that occur at the spinal cord level to respond to stimuli.

Monosynaptic Reflexes Stretch Reflex

• A stretch reflex occurs when a muscle is suddenly stretched.
• Muscle spindles and Golgi tendon organs play a role in activating this stretch reflex through their receptors.
• Sensory neurons synapse with motor neurons eliciting a reflex contraction in the stretched muscle. This response works to oppose sudden changes in muscle length.
• The monosynaptic reflex pathway involves a single synapse between afferent and efferent fibers.
• Stretch reflexes are usually elicited by striking a tendon with a reflex hammer
• Alpha and gamma motor neurons participate in the reflex.

Muscle Spindle Receptors & Golgi Tendon Organs

• Muscle spindles are sensitive to muscle length
• Golgi tendon organs are sensitive to tension in a muscle These provide continuous feedback to the spinal cord to maintain muscle coordination during movement and in maintaining posture.

Flexor Reflex & Withdrawal Reflexes

• A painful stimulus triggers a flexor reflex to pull the limb away.
• Both the withdrawal and flexor reflex pathways involve multiple synapses and thus are classified as polysynaptic reflexes.

Reciprocal Inhibition & Reciprocal Innervation

• Reciprocal inhibition limits the extent of the reflex response, which means that stimulation of one muscle leads to the inhibition of its antagonist muscle for smooth movement.

Role of the Motor Cortex & Brain Stem

• The motor cortex generates signals that initiate and control voluntary movements.
• The brain stem plays a significant role in maintaining posture and controlling automatic and habitual movements, as well as reflexive activities

Corticospinal Tract

• The corticospinal tract is the primary pathway for transmitting motor commands from the cortex to the spinal cord.
• Signals travel along the direct pathway to anterior motor neurons or extend to other centers of the CNS (indirect pathway).
• The majority of pyramidal tracts cross in the medulla oblongata descending into the lateral corticospinal tract
• A minor portion does not cross the midline and continues down the ventral (anterior) corticospinal tract
• A 3% of these fibers are large diameter Betz cells(originate from precentral gyri), which control the fine motor actions of the hands and fingers, and the remaining 97% are smaller and conduct background tonic signals to the motor areas.

Extrapyramidal System

• Extrapyramidal pathways include several tracts, the basal ganglia, and other brain stem structures.
• They function in balance, posture and coordination and to control reflexes
• The basal ganglia act as an alternative pathway to the motor cortex for voluntary actions.

Cerebellum

• The cerebellum is involved in the coordination of motor movements and other functions like balance and eye movements and is critical in motor learning.
• The cerebellum integrates sensory information and motor commands.
• It has a layered structure with different functional areas: flocculonodular lobe, vermis, and intermediate and lateral hemispheres.
• Information reaches the cerebellum via several pathways.
• Afferent fibers from the brain, brain stem, and spinal cord provide sensory input, while efferent fibers convey signals back to the cortex, thalamus and brainstem, for both excitatory and inhibitory functions.
• The cerebellum compares the intended and actual movements causing corrections to the motor response
• Dysmetria, dysdiadochokinesia, intention tremor, and cerebellar nystagmus are some of the results of lesions to or loss of function in the cerebellum.

Basal Ganglia

• The basal ganglia are a collection of subcortical nuclei located around the thalamus.
• The basal ganglia function in motor control, learning, and procedural memory
• The basal ganglia exert an inhibitory influence on motor systems, acting as brakes on movements.
• Two major circuits are in basal ganglia: the putamen and caudate circuits.
• Putamen Circuit is involved in subconscious motor action execution.
• Caudate Circuit is involved in instinctive movements requiring cognitive planning

Motor Cortex

• The primary motor cortex is the region directly responsible for generating commands for voluntary movements.
• The motor cortex communicates with the spinal cord to execute movements.
• Premotor and supplementary motor areas play a role in complex movements and planning.
• Some specialized areas control aspects like speech (Broca's area) and eye movements.
• The motor cortex has a somatotopic organization, meaning different parts of the body are represented in specific locations within the motor cortex.

Upper Motor Neuron & Lower Motor Neuron Lesions

• Upper motor neuron (UMN) lesions are characterized by spastic paralysis and hyperactive reflexes.
• Their lesions cause paralysis, muscular atrophy, and hyporeflexia or areflexia, as well as fasciculations.
• Lower motor neuron (LMN) lesions lead to flaccid paralysis and hypoactive reflexes.
• These lesions also result in muscle atrophy and fasciculations.

Vestibular Sensations & Maintenance of Equilibrium

• The vestibular system detects head position and movement.
• The parts of the vestibular system are 2 large chambers (utricle and saccule) & 3 semicircular canals.
• The maculae in the utricle and saccule detect linear acceleration & static equilibrium.
• The crista ampullaris within semicircular canals detect angular/rotational acceleration.
• The vestibular system sends signals through the vestibular nuclei to the cerebellum. Those then transmit to the brainstem’s reticular nuclei and cerebellar nuclei for motor adjustments.

Test your knowledge on sensory receptors and their functions! This quiz covers key concepts such as types of sensations, receptor types, and the origins of visceral sensations. Perfect for students studying biology or psychology.