Chapter 16

Sensory, Motor & Integrative Systems

 

Lecture Outline

INTRODUCTION

•      The components of the brain interact to receive sensory input, integrate and store the information, and transmit motor responses.

•      To accomplish the primary functions of the nervous system there are neural pathways to transmit impulses from receptors to the circuitry of the brain, which manipulates the circuitry to form directives that are transmitted via neural pathways to effectors as a response.

Chapter 16
Sensory, Motor & Integrative Systems

•      Levels and components of sensation

•      Pathways for sensations from body to brain

•      Pathways for motor signals from brain to body

•      Integration Process

–   wakefulness and sleep

–   learning and memory

SENSATION

•      Sensation is a conscious or unconscious awareness of external or internal stimuli.

Is Sensation Different from Perception?         

•       Perception is the conscious awareness & interpretation of a sensation.

–    precisely localization & identification

–    memories of our perceptions are stored in the cortex

•       Sensation is any stimuli the body is aware of

–    Chemoreceptors, thermoreceptors, nociceptors, baroreceptors

–    What are we not aware of?

•    X-rays, ultra high frequency sound waves, UV light

–    We have no sensory receptors for those stimuli

Sensory Modalities

•      Sensory Modality is the property by which one sensation is distinguished from another.

•      Different types of sensations

–   touch, pain, temperature, vibration, hearing, vision

–   Generally, each type of sensory neuron can respond to only one type of stimulus.

•      Two classes of sensory modalities

–   general senses

–   special senses

Sensory Modalities

•      The classes of sensory modalities are general senses and special senses.

–   The general senses include both somatic and visceral senses, which provide information about conditions within internal organs.

–   The special senses include the modalities of smell, taste, vision, hearing, and equilibrium.

Process of Sensation

•      Sensory receptors demonstrate selectivity

–   respond to only one type of stimuli

•      Events occurring within a sensation

–   stimulation of the receptor

–   transduction (conversion) of stimulus into a graded potential

•   vary in amplitude and are not propagated

–   generation of impulses when graded potential reaches threshold

–   integration of sensory input by the CNS

Sensory Receptors

•      Receptor Structure may be simple or complex

–   General Sensory Receptors (Somatic Receptors)

•   no structural specializations in free nerve endings that provide us with pain, tickle, itch, temperatures

•   some structural specializations in receptors for touch, pressure & vibration

–   Special Sensory Receptors (Special Sense Receptors)

•   very complex structures---vision, hearing, taste, & smell

Alternate Classifications of Sensory Receptors

•      Structural classification

•      Type of response to a stimulus

•      Location of receptors & origin of stimuli

•      Type of stimuli they detect

Structural Classification of Receptors

•      Free nerve endings

–   bare dendrites

–   pain, temperature, tickle, itch & light touch

•      Encapsulated nerve endings

–   dendrites enclosed in connective tissue capsule

–   pressure, vibration & deep touch

•      Separate sensory cells

–   specialized cells that respond to stimuli

–   vision, taste, hearing, balance

 

Structural Classification

•       Compare free nerve ending, encapsulated nerve ending and sensory receptor cell

Classification by Stimuli Detected

•      Mechanoreceptors

–   detect pressure or stretch

–   touch, pressure, vibration, hearing, proprioception, equilibrium & blood pressure

•      Thermoreceptors detect temperature

•      Nociceptors detect damage to tissues

•      Photoreceptors detect light

•      Chemoreceptors detect molecules

–   taste, smell & changes in body fluid chemistry

Classification by Response to Stimuli

•      Generator potential

–   free nerve endings, encapsulated nerve endings & olfactory receptors produce generator potentials

–   when large enough, it generates a nerve impulse in a first-order neuron

•      Receptor potential

–   vision, hearing, equilibrium and taste receptors produce receptor potentials

–   receptor cells release neurotransmitter molecules on first-order neurons producing postsynaptic potentials

–   PSP may trigger a nerve impulse

•      Amplitude of potentials vary with stimulus intensity

 

 

 

Classification by Location

•      Exteroceptors

–   near surface of body

–   receive external stimuli

–   hearing, vision, smell, taste, touch, pressure, pain, vibration & temperature

•      Interoceptors

–   monitors internal environment (BV or viscera)

–   not conscious except for pain or pressure

•      Proprioceptors

–   muscle, tendon, joint & internal ear

–   senses body position & movement

Adaptation in Sensory Receptors

•      Most sensory receptors exhibit adaptation – the tendency for the generator or receptor potential to decrease in amplitude during a maintained constant stimulus.

•      Receptors may be rapidly or slowly adapting.

Adaptation of Sensory Receptors

•      Change in sensitivity to long-lasting stimuli

–   decrease in responsiveness of a receptor

•   bad smells disappear

•   very hot water starts to feel only warm

–   potential amplitudes decrease during a maintained, constant stimulus

•      Variability in tendency to adapt:

–   Rapidly adapting receptors (smell, pressure, touch)

•   specialized for detecting changes

–   Slowly adapting receptors (pain, body position)

•   nerve impulses continue as long as the stimulus persists – Pain is not easily ignored.

SOMATIC SENSATIONS

•      Receptors for somatic sensation are summarized in Table 16.2)

Tactile Sensations

•      Tactile sensations are touch, pressure, and vibration plus itch and tickle.

•      receptors include (Figure 16.2)

–   corpuscles of touch (Meissner’s corpuscles),

–   hair root plexuses,

–   type I (Merkel’s discs)

–   type II cutaneous (Ruffini’s corpuscles) 

–   mechanoreceptors,

–   lamellated (Pacinian) corpuscles,

–   free nerve endings

Touch

•      Crude touch refers to the ability to perceive that something has simply touched the skin

•      Discriminative touch (fine touch) provides specific information about a touch sensation such as location, shape, size, and texture of the source of stimulation.

 

•      Receptors for touch include corpuscles of touch (Meissner’s corpuscles) and hair root plexuses; these are rapidly adapting receptors.

•      Type I cutaneous mechanoreceptors (tactile or Merkel discs) and type II cutaneous mechanoreceptors (end organs of Ruffini) are slowly adapting receptors for touch (Figure 16.2).

Pressure and Vibration

•      Pressure is a sustained sensation that is felt over a larger area than touch.

–   Pressure sensations generally result from stimulation of tactile receptors in deeper tissues and are longer lasting and have less variation in intensity than touch sensations

–   Receptors for pressure are type II cutaneous mechanoreceptors and lamellated (Pacinian) corpuscles.

•   Like corpuscles of touch (Meissner’s corpuscles), lamellated corpuscles adapt rapidly.

•      Vibration sensations result from rapidly repetitive sensory signals from tactile receptors

–   receptors for vibration sensations are corpuscles of touch and lamellated corpuscles, which detect low-frequency and high-frequency vibrations, respectively.

Itch and Tickle

•      Itch and tickle receptors are free nerve endings.

–   Tickle is the only sensation that you may not elicit on yourself.

Meissner’s Corpuscle

•       Dendrites enclosed in CT in dermal papillae of hairless skin

•       Discriminative touch & vibration-- rapidly adapting  

•       Generate impulses mainly at onset of a touch

Hair Root Plexus

Merkel’s Disc

•       Flattened dendrites touching cells of stratum basale

•       Used in discriminative touch (25% of receptors in hands)

 

Ruffini Corpuscle

•       Found deep in dermis of skin

•       Detect heavy touch, continuous touch, & pressure

Pacinian Corpuscle

•       Onion-like connective tissue capsule enclosing a dendrite

•       Found in subcutaneous tissues & certain viscera 

•       Sensations of pressure or high-frequency vibration

Somatic Tactile Sensations - Summary

•      Touch

–   crude touch is ability to perceive something has touched the skin

–   discriminative touch provides location and texture of source

•      Pressure is sustained sensation over a large area

•      Vibration is rapidly repetitive sensory signals

•      Itching is chemical stimulation of free nerve endings

•      Tickle is stimulation of free nerve endings only by someone else

Thermal Sensations

•      Free nerve endings with 1mm diameter receptive fields on the skin surface

–   Cold receptors in the stratum basale respond to temperatures between 50-105 degrees F

–   Warm receptors in the dermis respond to temperatures between 90-118 degrees F

•      Both adapt rapidly at first, but continue to generate impulses at a low frequency

•      Pain is produced below 50 and over 118
 degrees F.

Pain Sensations

•      Pain receptors (nociceptors) are free endings that are located in nearly every body tissue

–   Free nerve endings found in every tissue of body except the brain

–   adaptation is slight if it occurs at all.

•      Stimulated by excessive distension, muscle spasm, & inadequate blood flow

•      Tissue injury releases chemicals such as K+, kinins or prostaglandins that stimulate nociceptors

Types of Pain

•      Fast pain (acute)

–   occurs rapidly after stimuli (.1 second)

–   sharp pain like needle puncture or cut

–   not felt in deeper tissues

–   larger A nerve fibers

•      Slow pain (chronic)

–   begins more slowly & increases in intensity

–   aching or throbbing pain of toothache

–   in both superficial and deeper tissues

–   smaller C nerve fibers

 

Types of Pain

•      Somatic pain that arises from the stimulation of receptors in the skin is superficial, while somatic pain that arises from skeletal muscle, joints, and tendons is deep.

•      Visceral pain, unlike somatic pain, is usually felt in or just under the skin that overlies the stimulated organ

–   localized damage (cutting) intestines may cause no pain, but diffuse visceral stimulation can be severe

•   distension of a bile duct from a gallstone

•   distension of the ureter from a kidney stone

–   pain may also be felt in a surface area far from the stimulated organ in a phenomenon known as referred pain (Figure 16.3).

Referred Pain

•       Visceral pain that is felt just deep to the skin overlying the stimulated organ or in a surface area far from the organ.

•       Skin area & organ are served by the same segment of the spinal cord.

–    Heart attack is felt in skin along left arm since both are supplied by spinal cord segment T1-T5

Pain Relief

Multiple sites of analgesic action:

•      Aspirin and ibuprofen block formation of prostaglandins that stimulate nociceptors

•      Novocaine blocks conduction of nerve impulses along pain fibers

•      Morphine lessen the perception of pain in the brain.

Proprioceptive Sensations

•      Receptors located in skeletal muscles, in tendons, in and around joints, and in the internal ear convey nerve impulses related to muscle tone, movement of body parts, and body position. This awareness of the activities of muscles, tendons, and joints and of balance or equilibrium is provided by the proprioceptive or kinesthetic sense.

Proprioceptive or Kinesthetic Sense

•      Awareness of body position & movement

–   walk or type without looking

–   estimate weight of objects

•      Proprioceptors adapt only slightly

•      Sensory information is sent to cerebellum & cerebral cortex

–   signals project from muscle, tendon, joint capsules & hair cells in the vestibular apparatus

–   receptors discussed here include muscle spindles, tendon organs (Golgi tendon organs), and joint kinesthetic receptors (Figure 16.4).

 

Muscle Spindles

•       Specialized intrafusal muscle fibers enclosed in a CT capsule and innervated by gamma motor neurons

•       Stretching of the muscle stretches the muscle spindles sending sensory information back to the CNS

•       Spindle sensory fiber monitor changes in muscle length

•       Brain regulates muscle tone by controlling gamma fibers

Golgi Tendon Organs

•       Found at junction of tendon & muscle

•       Consists of an encapsulated bundle of collagen fibers laced with sensory fibers

•       When the tendon is overly stretched, sensory signals head for the CNS & resulting in the muscle’s relaxation

Joint Receptors

•      Ruffini corpuscles

–   found in joint capsule

–   respond to pressure

•      Pacinian corpuscles

–   found in connective tissue around the joint

–   respond to acceleration & deceleration of  joints

SOMATIC SENSORY PATHWAYS

•      Somatic sensory pathways relay information from somatic receptors to the primary somatosensory area in the cerebral cortex.

•      The pathways consist of three neurons

–   first-order,

–   second-order, and

–   third-order

•      Axon collaterals of somatic sensory neurons simultaneously carry signals into the cerebellum and the reticular formation of the brain stem.

Somatic Sensory Pathways

•      First-order neuron conduct impulses to the CNS (brainstem or spinal cord)

–   either spinal or cranial nerves

•      Second-order neurons conducts impulses from brain stem or spinal cord to thalamus

–   cross over to opposite side of body

•      Third-order neuron conducts impulses from thalamus to primary somatosensory cortex (postcentral gyrus of parietal lobe)

Posterior Column-Medial Lemniscus Pathway to the Cortex

•      The nerve impulses for conscious proprioception and most tactile sensations ascend to the cortex along a common pathway formed by three-neuron sets (Figure 16.16a).

•      These neurons are a part of the posterior (dorsal) columns

–   consist of the gracile fasciculus and cuneate fasciculus

•      Impulses conducted along this pathway

–   fine touch,

–   stereognosis,

–   proprioception, and

–   vibratory sensations

Posterior Column-Medial Lemniscus Pathway of CNS

•      Proprioception, vibration, discriminative touch, weight                  discrimination & stereognosis

•      Signals travel up spinal cord in posterior column

•      Fibers cross-over in medulla to become the medial lemniscus pathway ending in thalamus

•      Thalamic fibers reach cortex

Anterolateral Pathways to the Cortex

•      3-neuron pathway

•      The anterolateral or spinothalamic pathways carry mainly pain and temperature impulses (Figure 16.5b).

•      They also relay the sensations of tickle and itch and some tactile impulses.

Spinothalamic Pathway of CNS

•      Lateral spinothalamic tract carries pain & temperature

•      Anterior tract carries tickle, itch, crude touch & pressure

•      First cell body in DRG with synapses in cord

•      2nd cell body in gray matter of cord, sends fibers to other side of cord & up through white matter to synapse in thalamus

•      3rd cell body in thalamus projects to cerebral cortex

Somatosensory Map of Postcentral Gyrus

•      Relative sizes of cortical areas

–   proportional to number of sensory receptors

–   proportional to the sensitivity of each part of the body

•      Can be modified with learning

–   learn to read Braille & will have larger area representing fingertips

Somatic Sensory Pathways to the Cerebellum

•      The posterior spinocerebellar and the anterior spinocerebellar tracts are the major routes whereby proprioceptive impulses reach the cerebellum.

–   impulses conveyed to the cerebellum are critical for posture, balance, and coordination of skilled movements.

•      Table 16.3 summarizes the major sensory tracts in the spinal cord and pathways in the brain.

Sensory Pathways to the Cerebellum

•      Major routes for proprioceptive signals to reach the cerebellum

–   anterior spinocerebellar tract

–   posterior spinocerebellar tract

•      Subconscious information used by cerebellum for adjusting posture, balance & skilled movements

•      Signal travels up to same side inferior cerebellar peduncle

Clinical Application - Syphilis

Syphilis causes a progressive degeneration of the posterior portions of the spinal cord.

–   Sexually transmitted disease caused by bacterium Treponema pallidum.

–   Third clinical stage known as tertiary syphilis

–   Progressive degeneration of posterior portions of spinal cord & neurological loss

•   loss of somatic sensations

•   proprioceptive impulses fail to reach cerebellum

–   People watch their feet while walking, but are still uncoordinated and jerky

 

SOMATIC MOTOR PATHWAYS

•      Lower motor neurons extend from the brain stem or spinal cord to skeletal muscles. 

•      These lower motor neurons are called the final common pathway because many regulatory mechanisms converge on these peripheral neurons.

Somatic Motor Pathways - Overview

•      Control of body movement

–   motor portions of cerebral cortex

•   initiate & control precise movements

–   basal ganglia help establish muscle tone & integrate semivoluntary automatic movements

–   cerebellum helps make movements smooth & helps maintain posture & balance

•      Somatic motor pathways

–   direct pathway from cerebral cortex to spinal cord & out to muscles

–   indirect pathway includes synapses in basal ganglia, thalamus, reticular formation & cerebellum

SOMATIC MOTOR PATHWAYS

•      Four distinct neural circuits (somatic motor pathways) participate in control of movement by providing input to lower motor neurons (Figure 16.7).

–   Local circuit neurons are located close to lower motor neuron cell bodies in the brain stem and spinal cord.

–   Local circuit neurons and lower motor neurons receive input from upper motor neurons.

–   Neurons of the basal ganglia provide input to upper motor neurons.

–   Cerebellar neurons also control activity of upper motor neurons.

SOMATIC MOTOR PATHWAYS

•      Organization of upper motor neuron pathways

–   Direct motor pathways provide input to lower motor neurons via axons that extend directly from the cerebral cortex.

–   Indirect pathways provide input to lower motor neurons from motor centers in the brain stem

 

•      Paralysis:  damage of lower motor neurons produces flaccid paralysis while injury to upper motor neurons causes spastic paralysis.

Primary Motor Cortex

•       The primary motor area is located in the precentral gyrus of the frontal lobe (Figure 16.6b)

–    upper motor neurons initiate voluntary movement

•       The adjacent premotor area and somatosensory area of the postcentral gyrus also contribute axons to descending motor pathways.

•       The cortical area devoted to a muscle is proportional to the number of motor units.

–    More cortical area is needed if number of motor units in a muscle is high

•    vocal cords, tongue, lips, fingers & thumb

Direct motor pathways