Dr. Michael P. Gillespie

SensationSensation is the conscious or subconscious

awareness of changes in the internal or external environment.

Destination of sensory nerve impulses-Spinal cord – reflexes.Lower brain stem – heart rate, breathing rate.Cerebral cortex – we become aware of sensory

stimuli.Perception is the conscious awareness and

interpretation of sensations (primarily occurs in the cerebral cortex).

Sensory ModalitiesEach unique type of sensation is called a

sensory modality.Touch, pain, vision, hearing, etc.

A given sensory neuron carries information for only one sensory modality.

Two classes of sensory modalities:General senses.Special senses.

General SensesGeneral senses refer to both somatic and

visceral senses.Somatic senses include tactile sensations (i.e.

touch, pressure, vibration, itch, tickle), thermal sensations (warm and cold), pain sensations, and proprioceptive sensations. Proprioceptive sensations monitor static positions and movements.

Visceral senses provide information about the organs.

Special senses include the sensory modalities of smell, taste, vision, hearing, and equilibrium or balance.

Process of SensationThe process of sensation begins in a sensory

receptor, which can be either a specialized cell or the dendrites of a sensory neuron.

Each sensory receptor responds to a different stimulus.

The receptor exhibits selectivity.

Sensory Receptor Types

Four Events in Sensation1. Stimulation of the sensory receptor.

Stimulation must occur within the receptive field.

2. Transduction of the stimulus.The receptor transduces (converts) energy in a

stimulus into a graded potential.

Four Events in Sensation3. Generation of nerve impulses.

When a graded potential reaches threshold, it triggers one or more impulses.

Sensory neurons that conduct from PNS to CNS are referred to as first order neurons.

4. Integration of sensory input.Part of the CNS receives and integrates the

sensory nerve impulses.

Types of Sensory ReceptorsSensory receptors can be classified according

to several structural and functional characteristics.1. Microscopic appearance.

Type of potential produced Generator potentials and receptor potentials.

2. Location of receptors and the origin of the stimuli that activate them.

3. According to the type of stimulus they detect.

Microscopic Structural CharacteristicsFree nerve endings of first-order sensory neurons.

Bare dendrites.Pain, thermal, tickle, itch, and some touch

sensations.Encapsulated nerve-endings of first-order sensory

neurons.Dendrites are enclosed in a connective tissue

capsule.Somatic and visceral sensations such as pressure,

vibrations, and some touch sensations.i.e. pacinian corpuscles.

Microscopic Structural CharacteristicsSeparate cells that synapse with first-order

sensory neurons.i.e. hair cells for hearing and equilibrium,

gustatory receptor cells in taste buds, photoreceptors in the retina of the eye, etc.

Types of Graded PotentialsSensory receptors produce two kinds of graded

potentials in response to a stimulus.Generator potentials

Occur in dendrites of free nerve endings, encapsulated nerve endings, and the receptive part of olfactory receptors.

When a generator potential is large enough to reach threshold, it generates an action potential in a first-order neuron.

Receptor potentials Occur in sensory receptors that are separate cells. Receptor potentials trigger release of a

neurotransmitter through exocytosis of synaptic vesicles.

Location of Receptors / Origin of StimuliExteroreceptors

Located at or near the external surface of the body.Sensitive to stimuli outside the body.Monitor the external environment.Hearing, vision, smell, taste, touch, pressure,

vibration, temperature, and pain.Interoreceptors

Located in blood vessels, visceral organs, muscles, and the nervous system.

Monitor the internal environment.Usually not consciously perceived; however, strong

stimuli may be felt as pain and pressure.

Location of Receptors / Origin of StimuliMechanoreceptors

Located in muscles, tendons, joints, and the inner ear.

Provide information about body position, muscle length and tension, and the position and movement of your joints.

There really is no such thing as a proprioceptor. Receptors such as mechanoreceptors participate in proprioceptive pathways. The term proprioceptor is vague and not appropriate; however, its use is ubiquitous in the literature.

Type of Stimulus DetectedMost stimuli are in the following forms:

Mechanical energy – i.e. sound waves or pressure changes.

Electromagnetic energy – i.e. light or heat.Chemical energy – i.e. a molecule of glucose.

Type of Stimulus DetectedMechanoreceptors

Sensitive to mechanical stimuli such as the deformation, stretching, or bending of cells.

Provide sensations of touch, pressure, vibration, proprioception, hearing, and equilibrium.

ThermoreceptorsRespond to changes in temperature.

NociceptorsRespond to painful stimuli from physical or

chemical tissue damage.

Type of Stimulus DetectedPhotoreceptors

Detect light that strikes the retina of the eye.Chemoreceptors

Detect chemicals in the mouth (taste), nose (smell), and body fluids.

OsmoreceptorsDetect the osmotic pressure of body fluids.

Somatic SensationsSomatic sensations arise from stimuli of

sensory receptors in the skin or subcutaneous layer; in mucous membranes of the mouth, vagina, and anus; in muscles, tendons, and joints; and in the inner ear.

Somatic sensory receptors are distributed unevenly.Highest density – tip of the tongue, lips,

fingertips.Cutaneous sensations are those arising from

stimulating the surface of the skin.

Four Modalities of Somatic SensationTactileThermalPainProprioceptive

Tactile SensationsThe tactile sensations include touch,

pressure, vibration, itch, and tickle.Tactile receptors in the skin or subcutaneous

layer include Meissner corpuscles, hair root plexuses, Merkel discs, Ruffini corpuscles, pacinian corpuscles, and free nerve endings.

Structure and Location of Sensory Receptors

TouchSensations of touch arise from stimulation of

receptors in the skin and subcutaneous layer.Rapidly adapting touch receptors:

Meissner corpuscles Corpuscles of touch. Located in the dermal papillae of hairless skin. Egg shaped mass of dendrites enclosed by a

capsule.Hair root plexuses

Free nerve endings wrapped around hair follicles.

TouchSlowly adapting touch receptors:

Merkel discs (tactile discs or type I cutaneous mechanoreceptors. Saucer shaped, flattened free nerve endings that

make contact with Merkel cells. Plentiful in the fingertips, hands, lips, and external

genitaliaRuffini corpuscles (type II cutaneous

mechanoreceptors). Elongated, encapsulated receptors located deep in

the dermis, and in ligaments and tendons. Present in the hands and soles. Sensitive to stretching of digits and limbs.

PressurePressure is a sustained sensation that is felt over

a larger area than touch.It occurs with deformation of deeper tissues.Meissner corpuscles, Merkel discs, and pacinian

corpuscles contribute to pressure sensation.Pacinian corpuscles (lamellated corpuscles) are

large oval structures composed of a multi-layered connective tissue capsule enclosing a dendrite.Located in the dermis and subcutaneous layer; in

submucosal tissues; around joints, tendons, and muscles; in the periosteum; and in the mammary glands, external genitalia, and certain viscera, such as the pancreas and urinary bladder.

VibrationVibration sensation results from rapidly

repetitive sensory signals from tactile receptors.

Meissner corpuscles and pacinian corpuscles detect vibration.

Meissner – lower-frequency vibrations.Pacinian – higher-frequency vibrations.

ItchItch results from stimulation of free nerve

endings by certain chemicals, such as bradykinin, often due to a local inflammatory response.

TickleFree nerve endings are thought to mediate

the tickle sensation.

Thermal SensationsThermoreceptors are free nerve endings.The thermal sensations of coldness and

warmth are detected by different receptors.Temperatures below 10⁰ and above 48⁰C

primary stimulate pain receptors.

Thermal SensationsCold receptors:

Located in the stratum basale of the dermis. Attached to medium-diameter type A myelinated

fibers.Temperatures between 10⁰ and 40⁰C activate them.

Warm receptors:Located in the dermis.Not as abundant as cold receptors.Attached to small-diamtere unmyelinated C fibers.Temperatures between 32⁰ and 48⁰C activate them.

Phantom Limb SensationPatients who have had a limb amputated may

still experience sensations such as itching, tingling, or pain as if the limb were still there.

This is called phantom limb sensation.Possible causes:

Impulses from the proximal portions of sensory neurons that previously carried impulses from the limb.

Neurons in the brain that previously received input from the missing limb are still active, giving false sensory perceptions.

Phantom Limb SensationTreatments such as acupuncture, electrical

nerve stimulation, and biofeedback can be helpful in treating phantom limb pain.

Pain SensationsPain serves a protective function by signaling

the presence of noxious, tissue-damaging conditions.

The subjective description and indication of the location of pain may help identify the underlying disease.

The receptors for pain are called nociceptors (noci = harmful).

They are free nerve endings found in every tissue of the body except the brain.

Pain SensationsIntense thermal, mechanical, or chemical

stimuli can activate nociceptors.Tissue irritation or injury releases chemicals

such as prostaglandins, kinins, and potassium ions that stimulate nociceptors.

Pain SensationsPain can persist long after the pain-producing

stimulus is removed because the pain mediating chemicals linger.

Conditions that elicit pain include excessive distention (stretching) of a structure, prolonged muscular contractions, muscle spasms, or ischemia.

Types of PainTypes of pain based upon speed of impulses:

Fast pain Medium-diameter, myelinated A fibers. Occurs within 0.1 seconds after a stimulus is

applied. Referred to as acute, sharp, or pricking pain. Needle puncture or knife cut to the skin. Not felt in deeper tissues.

Types of PainSlow pain

Small-diameter, unmyelinated C fibers. Begins a second or more after the stimulus is

applied. Increases in intensity over several seconds or

minutes. Referred to as chronic, burning, or throbbing pain. Can occur in skin, deeper tissues, or internal

organs.

Types of PainTypes of pain based upon location of pain

receptors:Superficial somatic pain – stimulation of

receptors in the skin.Deep somatic pain - stimulation of receptors in

skeletal muscles, joints, tendons, and fascia.Visceral pain – stimulation of receptors in

visceral organs.

Localization of PainFast pain

Very precisely localized to the stimulated area.i.e. pin prick

Somatic slow painWell localized, but more diffuse

Localization of PainVisceral slow pain

Some is localized to the area of painMuch is referred to the skin that overlies the

organ or to a surface area far from the stimulated organ.

Know as referred pain.In general, the visceral organ and the area to

which the pain is referred are served by the same segment of the spinal cord.

Distribution of Referred Pain

AnalgesiaAnalgesia (an = without, algesia = pain) is pain

relief.Types of analgesia:

Analgesic drugs such as aspirin and ibuprofen block the formation of prostaglandins, which stimulate nociceptors.

Local anesthetics such as novacaine block the conduction of nerve impulses along the axons of first-order pain neurons.

Morphine and other opiate drugs alter the quality of pain perception in the brain. Pain is still sensed, but no longer experienced as so noxious.

Proprioceptive SensationsProprioceptive sensations allow us to know

where our head and limbs are located and how they are moving even if we are not looking at them.

Kinesthesia (kin = motion, esthesia = perception) is the perception of body movements.

Proprioceptive sensations arise in receptors termed proprioceptors.

Proprioceptive SensationsProprioceptors are embedded in muscles and

tendons. These tell us the degree to which the muscle is contracted, the amount of tension on tendons, and the position of joints.

Hair receptors in the inner ear monitor the orientation of the head relative to the ground and the head position during movements.The provide information for maintaining balance

and equilibrium.Proprioceptors also allow for weight

discrimination.

MechanoreceptorsThree types:

Muscle spindles Located within skeletal muscles

Tendon organs Located within tendons

Joint kinesthetic receptors Located within synovial joint capsules

Muscle SpindlesMuscle spindles are located in skeletal

muscles.They consist of several slowly adapting sensory

nerve endings that wrap around 3-10 specialized muscle fibers, called intrafusal muscle fibers.

Muscle spindles monitor changes in the length of skeletal muscles.

The main function of a muscle spindles is to measure muscle length (how much a muscle is being stretched).

Muscle SpindlesThey participate in stretch reflexes.

Activation of the muscle spindle causes contraction of a skeletal muscle, which relieves stretching.

They help maintain the level of muscle tone (the small degree of muscle contraction present while the muscle is at rest).

Tendon OrgansTendon organs are located at the junction of

a tendon and a muscle.They consist of a thin capsule of connective

tissue that encloses a few tendon fascicles.The participate in tendon reflexes to protect

tendons and their associated muscles from damage due to excessive tension.

Tendon reflexes decrease muscle tension by causing muscle relaxation.

Muscle Spindles & Tendon Organs

Joint Kinesthetic ReceptorsSeveral types of joint receptors are present

within or around the articular capsule of synovial joints.Free nerve endings and Ruffini corpuscles

respond to pressure.Pacinian corpuscles respond to acceleration

and deceleration of the joint.Articular ligaments contain receptors similar

tendon organs that adjust reflex inhibition of adjacent muscles.

Somatic Sensory PathwaysSomatic sensory pathways relay information

from the somatic sensory receptors to the primary somatosensory area in the cerebral cortex and to the cerebellum.

Three sets of neuronsFirst-order neuronsSecond-order neuronsThird-order neurons

First-order NeuronsConduct impulses from somatic receptors

into the brain stem or spinal cord.Impulses from the face, mouth, teeth, and eyes

travel along the cranial nerves.Impulses from the neck, trunk, limbs, and

posterior aspect of the head travel along spinal nerves.

Second-order NeuronsConduct impulses from the brain stem or

spinal cord to the thalamus.The axons decussate in the brain stem or

spinal cord before ascending.Consequently, all somatic sensory

information from one side of the body reaches the thalamus on the opposite side.

Third-order NeuronsConduct impulses from the thalamus to the

primary somatosensory cortex on the same side.

Relay StationsRegions within the CNS where neurons

synapse with other neurons that are part of a particular sensory or motor pathway are known as relay stations.

The Thalamus serves as a major relay station.Neural signals are being relayed from one

region of the CNS to another.

Direct Motor Pathways

Somatic Sensory PathwaysSomatic sensory impulses ascend to the

cerebral cortex via three general pathways.Posterior column-medial lemniscus pathway.Anterolateral (spinothalamic) pathways.Trigeminothalamic pathway.

Somatic Sensory Pathways

Posterior Column-Medial Lemniscus PathwayThis pathways conveys information for touch,

pressure, vibration, and conscious proprioception from the limbs, trunk, neck, and posterior head.Posterior column – in spinal cord.Medial lemniscus – in brain stem.

Posterior Column-Medial Lemniscus PathwayFirst order neurons from the upper limbs, upper

trunk, neck, and posterior head travel in the cuneate fasciculus.

First order neurons from the lower limbs and lower trunk travel along the gracile fasciculus.

The axons synapse with second order neurons in the cuneate and gracile nuclei respectively.

The axons of the second-order neurons decussate in the brain stem and enter the medial lemniscus.

Posterior Column-Medial Lemniscus PathwayThe second-order neurons traveling in the

medial lemniscus synapse with third-order neurons in the thalamus.

Axons from the third order neurons project into the primary somatosensory area of the cortex.

Posterior Column-Medial Lemniscus Pathway

Anterolateral Pathway to the Cortex (Spinothalamic)This pathway conveys information for pain,

temperature, itch, and tickle from the limbs, trunk, neck, and posterior head.

First order neurons connect to a receptor of the limbs, trunk, neck, or posterior head.Cell bodies are located in the dorsal root

ganglion.The first order neurons synapse with second

order neurons in the spinal cord.Cell bodies are located in the posterior gray

horn of the spinal cord.

Anterolateral Pathway to the Cortex (Spinothalamic)The axons of the second order neurons

decussate and move to the brain stem via the spinothalamic tract.

The axons of the second order neurons synapse with third order neurons in the thalamus.

The third-order neurons project to the primary somatosensory area of the cortex on the same side as the thalamus.

Anterolateral Pathway to the Cortex (Spinothalamic)Figure 16.6

Trigeminothalamic Pathway to the CortexThis pathway conveys information for most

somatic sensations from the face, nasal cavity, oral cavity, and teeth.

First-order neurons extend from somatic sensory receptors in the face, nasal cavity, oral cavity, and teeth into the pons via the trigeminal nerve.

They synapse with second order neurons in the pons.

Trigeminothalamic Pathway to the CortexThe second order neurons decussate and

ascend the trigeminothalamic tract to the thalamus.

They synapse with third-order neurons in the thalamus.

Trigeminothalamic Pathway to the CortexFigure 16.7

Mapping the Primary Somatosensory Area

Somato-Sensory and Somato-Motor Maps in Cerebral Cortex

Sensory Homunculus

Somatic Sensory Pathways to the CerebellumThe posterior spinocerebellar and anterior

spinocerebellar tracts convey nerve impulses from proprioceptors to the cerebellum.

This informs the cerebellum of body movements and allows it to coordinate them for smooth, controlled movements.

This helps us to maintain posture and balance.

Somatic Motor PathwaysLower motor neurons

Have cell bodies in the brain stem and spinal cord.

Innervate skeletal musclesReferred to as the final common pathway

because only LMNs provide output from the CNS directly to skeletal muscle fibers

Upper motor neuronsCarry signals form the cerebral cortex to LMNs.Execution of voluntary movements.Maintain balance and coordination.

Direct Motor PathwaysLateral corticospinal tractAnterior cotricospinal tractCorticobulbar tract

Indirect Motor PathwaysRubrospinalTectospinalVestibulospinalMedial and lateral reticulospinal

Lateral Corticospinal Tract (Crossed Pyramidal Tract)The lateral corticospinal tract provides fine

motor control to the limbs and digits.The fibers decussate in the medulla.

Anterior Corticospinal Tract (Direct Pyramidal Tract)The anterior corticospinal tract conducts

voluntary motor impulses from the precentral gyrus to the motor centers of the cord.

Corticobulbar TractConnects the cerebral cortex to the brain

stem.“bulbar” refers to the brainstem.Controls the muscles of the face, head, and

neck.Innervates the cranial motor nuclei.

RubrospinalControls large muscle movement such as the

arms and legs.Some fine motor control.Facilitates flexion and inhibits extension in

the upper extremities.

TectospinalCoordinates head and eye movements.Mediates reflex postural movements in

response to visual and auditory stimuli.

VestibulospinalThe vetsibulospinal tract is a descending

tract that originates from the vestibular nuclei of the medulla.

The vestibulospinal tract facilitates extensor (antigravity) muscle tone.

It assists in maintaining equilibrium.It participates with cranial nerves II, IV, and

VI in controlling eye movements.It helps to control head and neck position.

ReticulospinalThe reticulospinal tract is an extrapyramidal

tract which travels from the reticular formation.

It has integrative functions that help to coordinate automatic movements of locomotion and posture.

Spinal Tracts

Referred Pain Distribution

Stages of Sleep

Reticular Activating System

Input and Output to Cerebellum