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Neural Integration I: Sensory Pathways and the Somatic Nervous System. Martini Chapter 15 March 2008. General Senses. Sensation of temperature, pain, touch, pressure, vibration, and proprioception Sensation : Information provided by sensory receptors to the CNS.

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Neural integration i sensory pathways and the somatic nervous system

Neural Integration I: Sensory Pathways and the Somatic Nervous System

Martini Chapter 15

March 2008

General senses
General Senses

  • Sensation of temperature, pain, touch, pressure, vibration, and proprioception

    Sensation: Information provided by sensory receptors to the CNS.

    Perception: The conscious awareness of sensation.

Special senses
Special Senses

  • More complex than general senses, having sensory organs that protect receptors.

    • olfaction (smell)

    • vision

    • gustation (taste)

    • equilibrium (balance)

    • hearing

Free nerve endings
Free Nerve Endings

  • Simplest type of Sensory receptor

  • Dendrites of Sensory Neurons detect stimuli

  • No accessory structures, so they detect numerous types of stimuli (low receptor specificity)

Sensory receptors1
Sensory Receptors

  • Detects stimuli and translates into action potential that can be conveyed to the CNS in a process called transduction.

    • Can humans transduce all stimuli?


  • receptor potential

    • transduction begins when a stimulus changes the transmembrane potential of a sensory neuron

      • graded effect: stronger stimulus = larger receptor potential.

  • generator potential

    • when the receptor potential exceeds the threshold and an action potential is fired

Tonic and phasic receptors
Tonic and Phasic Receptors

  • Tonic Receptors

    • always active, indicative of background level of stimulation

    • change in frequency of action potentials indicates increase or decrease of stimulation

  • Phasic Receptors

    • normally inactive

    • activated for short period by a stimulus

    • provide information about the intensity and rate of change of stimuli

Specificity of sensory receptors
Specificity of Sensory Receptors

  • Most receptors detect a specific type of stimulus

  • due to structure of receptor and presence of other accessory cells surrounding receptor

Receptive field
Receptive Field

  • The area is monitored by a single receptor cell

  • The larger the receptive field, the more difficult it is to localize a stimulus

Labeled lines
Labeled Lines

  • The link between the peripheral receptor and the cortical neuron.

  • Where the labeled line arrives in the cortex determines what you perceive

  • If the labeled line is activated at a point beyond the receptor it can cause perceptions in the absence of sensation:

    • seeing halos when you rub your eyes is an example of how our perception of reality is sometimes misleading.

Sensory adaptation
Sensory Adaptation

  • Reduction in sensitivity of a receptor in the presence of a constant stimulus

    • Peripheral Adaptation:

      • level of receptor activity decreases

    • Central Adaptation:

      • signal decreases along sensory pathway after receptor

      • usually inhibition of nuclei along pathway by other neurons

Sensory adaptation1
Sensory Adaptation

  • fast-adapting receptors

    • phasic receptors

    • high level of peripheral adaptation

    • example is thermoreceptors

      • you are not constantly aware of temperature, only change in temperature

General senses1

Classification by location


touch, pressure, pain, special senses


chemical changes, stretching of tissues, temperature


Location of self

only in skeletal muscle, tendons, joints, ligaments, & CT coverings of bones & muscles

Classification by sensation






deformed by force

touch, pressure (BP), vibration, stretch, itch


chemicals in solution

Smell, taste, blood chemistry

General Senses


  • Free nerve endings with large receptive fields

  • May be sensitive to:

    • extremes of temperature

    • mechanical damage

    • dissolved chemicals, such as chemicals released by injured cells

  • Very strong stimuli may activate all 3 receptor types.


  • Are common in the:

    • superficial portions of the skin

    • joint capsules

    • within the periostea of bones

    • around the walls of blood vessels

  • slow-adapting tonic receptors

  • Two types of Axons carry painful sensations:

    • Type A fibers

    • Type C fibers

  • Carried on pathway to thalamus, reticular formation, primary sensory cortex

Fast pain
Fast Pain

  • carried by large myelinated Type A fibers

  • reach CNS quickly and trigger fast reflex

  • specific activation of primary sensory cortex

  • easy to localize where pain occurred

Slow pain
Slow Pain

  • carried by small unmyelinated Type C fibers

  • reaches CNS slowly

  • generalized activation of the thalamus

  • difficult to localize area of pain

The perception of pain
The Perception of Pain

  • chronic pain can lead to facilitation of pain pathways making lesser stimuli seem more painful.

  • damage to the sensory neuron can lead to the perception of pain in the absence of a painful stimulus

Neurotransmitters and pain
Neurotransmitters and Pain

  • Sensory Neurons

    • glutamate and Substance P

      • excitatory

      • facilitate neurons along pain pathway

Neurotransmitters and pain1
Neurotransmitters and Pain

  • Natural painkillers

    • endorphins and enkephalins (in small red neuron)

    • inhibit the perception of pain by the CNS by inhibiting the release of Substance P from sensory neurons

Keep in mind:

The perception of pain may be gone, but the stimulus remains.


  • free nerve endings

    • dermis

    • skeletal muscles

    • liver

    • hypothalamus

  • fast-adapting phasic receptors

  • cold receptors are 3-4X more numerous than warm receptors

  • carried over same pathways as pain

    • taken to thalamus, reticular formation and primary sensory cortex


  • sensitive to distortions of cell membrane

    • mechanically gated ion channels open

  • 3 classes

    • Tactile receptors

      • touch, pressure, and vibration

    • Baroreceptors

      • pressure changes in the walls of blood vessels and in portions of the digestive, reproductive, and urinary tracts

    • Proprioreceptors

      • positions of joints and muscles

Tactile receptors
Tactile Receptors

  • Range in complexity from free nerve endings to specialized sensory complexes.

    • Fine touch and pressure receptors

      • source of stimulus, exact location, shape, size, texture, and movement

      • small receptive field

    • Crude touch and pressure receptors

      • poor localization (and that’s it really)

      • large receptive field

Tactile receptors1
Tactile Receptors

  • Six types in skin

    • free nerve endings

    • root hair plexus

    • tactile (Merkel’s) discs

    • tactile (Meissner’s) corpuscles

    • lamellated (Pacinian) corpuscles

    • Ruffini corpuscles

Tactile receptors2
Tactile Receptors

  • Free Nerve Endings

    • sensitive to touch and pressure

    • tonic receptors with small receptive fields

    • situated between epidermal cells

    • no difference in structure between these and thermoreceptors or nociceptors

Tactile receptors3
Tactile Receptors

  • Root Hair Plexus

    • movement of hair distorts the dendrites surrounding the hair follicle producing action potentials

    • phasic receptors that adapt quickly to a stable stimulus

    • For this reason you are not aware of your clothing at all times!

Tactile receptors4
Tactile Receptors

  • Tactile (Merkel’s) Discs

    • fine touch and pressure receptors

    • extremely sensitive and tonic receptors

    • close to stratum germinativum

Tactile receptors5
Tactile Receptors

  • Tactile (Meissner’s) Corpuscles

    • fine touch, pressure, and low-frequency vibration

    • dendrites coiled and surrounded by schwann cells and then by fibrous capsule

    • abundant in the eyelids, lips, fingertips, nipples, and genitalia

Tactile receptors6
Tactile Receptors

  • Lamellated (Pacinian) Corpuscles

    • sensitive to deep pressure

    • fast-adapting tonic receptors

    • single dendrite inside collagen fibers and supporting cells

    • found in dermis, mesentery, pancreas, urethra, and bladder

Tactile receptors7
Tactile Receptors

  • Ruffini Corpuscles

    • also sensitive to pressure and distortion of the skin

    • located in the reticular (deep) dermis

    • capsule filled with a network of dendrites and collagen fibers

    • tonic receptors

      • little adaptation

Tickle and itch
Tickle and Itch

  • These sensations are not well understood, but closely related to touch and pain

  • receptors are free nerve endings and are carried over Type C fibers

  • Tickle sensation is usually described as pleasurable.

  • Itch sensations can be more unpleasant than pain.


  • free nerve endings that branch within elastic tissues of extendable organs

    • blood vessels, digestive tract, urinary tract

  • fast-adapting phasic receptors

  • Blood pressure monitored by baroreceptors in:

    • carotid artery

    • aorta


3 major groups (tonic non-adapting receptors)

  • Muscle Spindles

    • monitor skeletal muscle length (stretch reflex)

  • Golgi tendon organs

    • located at the junction between skeletal muscle and its tendon

    • stimulated by tension in tendon

    • monitor external tension developed during muscle contraction

  • Joint capsule receptors

    • free nerve endings detect pressure, tension, and movement at the joint


  • Respond to soluble substances dissolved in surrounding fluid.

  • Receptors are fast-adapting phasic type

  • Located in the:

    • carotid bodies

    • aortic bodies

  • Receptors monitor Ph, carbon dioxide, and oxygen levels in arterial blood

Organization of sensory pathways
Organization of Sensory Pathways

  • First-order neurons

    • cell bodies in DRG or cranial nuclei

    • Conduct impulses from sensory receptors to the spinal cord or brain stem to synapse w/ 2nd order neurons

  • Second-order neurons

    • cell bodies in dorsal horn of spinal cord or medullary nuclei

    • Transmit impulses to the thalamus or cerebellum where they synapse (cross over)

  • Third-order neurons(only from thalamic synapses)

    • cell bodies in thalamus

    • conduct impulses to the somatosensory cortex

Somatic sensory pathways
Somatic Sensory Pathways

  • carry sensory information from the skin and musculature of the body wall, head, neck, and limbs

    • posterior column pathway

    • anterolateral pathway

    • spinocerebellar pathway

The posterior column pathway
The Posterior Column Pathway

  • Carries highly localizes fine touch, pressure, vibration, and proprioception

  • also called dorsal column/medial lemniscus

    First-Order Neuron (DRG, CN)

    2 spinal tracts:

  • fasciculus gracilis

    • lower half of body

    • synapse in nucleus gracilis in medulla

  • fasciculus cuneatus

    • upper half of body

    • synapse in nucleus cuneatus in medulla

The posterior column pathway1
The Posterior Column Pathway

Second-Order Neuron

  • soma in nucleus gracilis in medulla

  • soma in nucleus cuneatus in medulla

  • both ascend and cross over to the opposite side of the brain (decussation) and enter the medial lemniscus tract.

  • both synapse in the ventral nuclei of the thalamus

The posterior column pathway2
The Posterior Column Pathway

Third-order Neuron

  • soma in ventral nuclei of the thalamus

  • arriving information is sorted

    • nature of stimulus

    • region of body involved

  • the axon travels to a specific region of the primary sensory cortex

Sensory homunculus
Sensory Homunculus

  • electrical stimulation of human brains was used to create a functional map of the primary sensory cortex

  • the homunculus has a huge face because the face has the most sensory receptors and thus give the most sensory input

The anterolateral pathway
The Anterolateral Pathway

  • conscious, poorly localized sensations of touch, pressure, pain and temperature

  • first-order neurons

    • synapse on second-order neurons in the posterior gray horn of the spinal cord

The anterolateral pathway1
The Anterolateral Pathway

  • second-order neurons (soma in spinal cord)

    • decussate

    • fibers ascend in

      • anterior spinothalamic tract

        • touch and pressure

      • lateral spinothalamic tract

        • pain and temperature

    • synapse on third-order neurons of thalamus

The anterolateral pathway2
The Anterolateral Pathway

  • third-order neurons

    • soma in ventral nuclei of the thalamus

    • arriving information is sorted

      • nature of stimulus

      • region of body involved

    • the axon travels to a specific region of the primary sensory cortex

Phantom limb pain
Phantom Limb Pain

  • An individual can experience painful sensations from an amputated limb, or if they were born without the limb.

  • The sensory neurons are still firing and the signal reaches the primary cortex and thus is perceived as a real stimulus on an absent limb.

Referred pain
Referred Pain

  • Perception of pain that is incorrectly localized.

  • Sensations arriving at segment of spinal cord can stimulate interneurons that are part of anterolateral pathway

    • heart attack

      • referred pain in left arm

    • appendicitis

      • referred pain in the naval and RLQ

The spinocerebellar pathway
The Spinocerebellar Pathway

  • proprioceptive input from muscles, tendons and joints

  • first-order neurons

    • synapse on second-order neurons in dorsal gray horn

The spinocerebellar pathway1
The Spinocerebellar Pathway

  • second-order neurons

    • soma in spinal cord

    • axons ascend in:

      • posterior spinocerebellar tract

        • don’t decussate

      • anterior spinocerebellar tract

        • usually decussate

Visceral sensory pathways
Visceral Sensory Pathways

  • sensation in visceral tissues and organs, primarily in thoracic and abdominopelvic cavities.

  • interoceptors:

    • nociceptors

    • thermoreceptors

    • tactile receptors

    • baroreceptors

    • chemoreceptors

The somatic nervous system
The Somatic Nervous System

  • controls contractions of the skeletal muscles

  • under voluntary control

Motor neurons of the sns
Motor neurons of the SNS

  • SNS pathways have at least 2 motor neurons:

    • Upper motor neuron

      • cell body is in the CNS

      • may facilitate or inhibit LMN

    • Lower motor neuron

      • cell body is in brainstem or spinal cord

      • activation triggers muscle contraction

Pathways of the sns
Pathways of the SNS

  • Corticospinal Pathway

  • Medial Pathway

  • Lateral Pathway

  • Activity in these pathways is monitored and adjusted by the basal nuclei and cerebellum.

Corticospinal pathway
Corticospinal Pathway

  • Sometimes called the pyramidal system

  • Provides voluntary control over skeletal muscles:

    • system begins at pyramidal cells of primary motor cortex

    • axons of these upper motor neurons descend into brain stem and spinal cord

    • synapse on lower motor neurons that control skeletal muscles

Motor homunculus
Motor Homunculus

Sensory Motor

proportions of homunculus relate to number of motor units in region

hand and mouth large because they have many motor units for fine motor control

Medial and lateral pathways
Medial and Lateral Pathways

  • historically called extrapyramidal pathway

  • subconscious motor control

    • Medial Pathway

      • controls gross movements of trunk and proximal limb muscles

    • Lateral Pathway

      • controls distal limb muscles perform precise movments

  • Upper motor neurons in this pathway stimulate the same lower motor neurons as the corticospinal pathway

The role of the basal nuclei and cerebellum
The Role of the Basal Nuclei and Cerebellum

  • help to coordinate conscious and unconscious muscle contractions

    • Basal Nuclei - provide background patterns of movement involved in voluntary motor activities

    • Cerebellum - monitors:

      • proprioceptive (position) sensations

      • visual information from the eyes

      • vestibular (balance) sensations from inner ear as movements are under way