Dr. Michael P. Gillespie
Sensation
 Sensation 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 Modalities
 Each 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 Senses
 General 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 Sensation
 The 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 Sensation
 1. 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 Sensation
 3. 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 Receptors
 Sensory 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 Characteristics
 Free 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 Characteristics
 Separate 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 Potentials
 Sensory 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 Stimuli
 Exteroreceptors
 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 Stimuli
 Mechanoreceptors
 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 Detected
 Most 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 Detected
 Mechanoreceptors
 Sensitive to mechanical stimuli such as the deformation,
stretching, or bending of cells.
 Provide sensations of touch, pressure, vibration,
proprioception, hearing, and equilibrium.
 Thermoreceptors
 Respond to changes in temperature.
 Nociceptors
 Respond to painful stimuli from physical or chemical
tissue damage.
Type of Stimulus Detected
 Photoreceptors
 Detect light that strikes the retina of the eye.
 Chemoreceptors
 Detect chemicals in the mouth (taste), nose (smell), and
body fluids.
 Osmoreceptors
 Detect the osmotic pressure of body fluids.
Somatic Sensations
 Somatic 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 Sensation
 Tactile
 Thermal
 Pain
 Proprioceptive
Tactile Sensations
 The 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
Touch
 Sensations 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.
Touch
 Slowly 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 genitalia
 Ruffini 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.
Pressure
 Pressure 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.
Vibration
 Vibration 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.
Itch
 Itch results from stimulation of free nerve endings by
certain chemicals, such as bradykinin, often due to a
local inflammatory response.
Tickle
 Free nerve endings are thought to mediate the tickle
sensation.
Thermal Sensations
 Thermoreceptors 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 Sensations
 Cold 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 Sensation
 Patients 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 Sensation
 Treatments such as acupuncture, electrical nerve
stimulation, and biofeedback can be helpful in
treating phantom limb pain.
Pain Sensations
 Pain 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 Sensations
 Intense 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 Sensations
 Pain 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 Pain
 Types 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 Pain
 Slow 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 Pain
 Types 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 Pain
 Fast pain
 Very precisely localized to the stimulated area.
 i.e. pin prick
 Somatic slow pain
 Well localized, but more diffuse
Localization of Pain
 Visceral slow pain
 Some is localized to the area of pain
 Much 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
Analgesia
 Analgesia (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 firstorder 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 Sensations
 Proprioceptive 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 Sensations
 Proprioceptors 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.
Mechanoreceptors
 Three types:
 Muscle spindles

Located within skeletal muscles
 Tendon organs

Located within tendons
 Joint kinesthetic receptors

Located within synovial joint capsules
Muscle Spindles
 Muscle 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 Spindles
 They 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 Organs
 Tendon 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 Receptors
 Several 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 Pathways
 Somatic 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 neurons
 First-order neurons
 Second-order neurons
 Third-order neurons
First-order Neurons
 Conduct 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 Neurons
 Conduct 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 Neurons
 Conduct impulses from the thalamus to the primary
somatosensory cortex on the same side.
Relay Stations
 Regions 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 Pathways
 Somatic 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 Pathway
 This 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 Pathway
 First 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 Pathway
 The 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 Cortex
 This 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 Cortex
 The 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 Cortex
 Figure 16.7
Mapping the Primary
Somatosensory Area
Somato-Sensory and SomatoMotor Maps in Cerebral Cortex
Sensory Homunculus
Somatic Sensory Pathways to
the Cerebellum
 The 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 Pathways
 Lower motor neurons
 Have cell bodies in the brain stem and spinal cord.
 Innervate skeletal muscles
 Referred to as the final common pathway because only
LMNs provide output from the CNS directly to skeletal
muscle fibers
 Upper motor neurons
 Carry signals form the cerebral cortex to LMNs.
 Execution of voluntary movements.
 Maintain balance and coordination.
Direct Motor Pathways
 Lateral corticospinal tract
 Anterior cotricospinal tract
 Corticobulbar tract
Indirect Motor Pathways
 Rubrospinal
 Tectospinal
 Vestibulospinal
 Medial 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 Tract
 Connects 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.
Rubrospinal
 Controls large muscle movement such as the arms and
legs.
 Some fine motor control.
 Facilitates flexion and inhibits extension in the upper
extremities.
Tectospinal
 Coordinates head and eye movements.
 Mediates reflex postural movements in response to
visual and auditory stimuli.
Vestibulospinal
 The 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.
Reticulospinal
 The 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