Sensory, Motor & Integration Systems

Chapter 15

Sensation & Perception

• Sensation is the detection of stimulus of internal or external receptors. It can be either conscious or subconcious

• Perception is the awareness and conscious interpretation of sensations. It is how the brain makes sense of or assigns meaning to the sensation.

Modalities• A modality is a unique type or classification of

stimulus.

• We often divide them between general and special senses.

• General Senses include:– Somatic

• Touch

• Temperature

• Pain

• Pressure

• Proprioception

Modalities (cont’d)• Visceral Senses

– Sense internal conditions, e.g.:• pH• Osmolarity• O2 and CO2 levels

Special Senses:• Vision• Hearing• Equilibrium• Taste• Olfaction

Sensation

• Sensory receptors are “tuned” or selective to specific types of stimulus

• They are specific for a particular region of the body or receptive field

• For a stimulus to be detected it must be transduced

Transduction• Transduction is the conversion of a stimulus into

an electrical event or potential

• A potential is a change in the membrane’s electrical condition

• There are graded potentials which are localized, variable in amplitude and fade with distance

• They can “sum” (or result in summation)

• If there is sufficient stimulus (reaching threshold, then an action potential may be generated

• Sensory neurons carrying impulses to the PNS are called first order neurons

Sensory Receptors

• Sensory receptors may be classified by– Anatomical type

– Modality

– Location

Fig.15.01

Anatomical classesBased on microscopic features

Figure 15–2

Receptive Field

• Area is monitored by a single receptor cell• The larger the receptive field, the more difficult it

is to localize a stimulus

Classification by location

• Exteroceptors: sense stimuli from outside the body (includes cutaneous receptors and most special senses except equilibrium)

• Interoceptors: sense stimuli from within (chemoreceptors, visceral stretch and pressure and pain)

• Proprioceptors: deal with muscle & joint position and equilibrium sense

Tonic Receptors• Are always active

Phasic Receptors• Are normally inactive

• Become active for a short time whenever a change occurs

• Provide information about the intensity and rate of change of a stimulus

Sensory adaptation

• Generator or receptor potential amplitudes will decline over time if a stimulus remains constant or below threshold for a given length of time

• Some adapt rapidly, some slowly

Rapidly vs. Slowly

• Rapid– Pressure

– Touch

– Smell

• Slow– Proprioceptors

– pH & osmoreceptors

– Pain (really doesn’t adapt much)

Any receptor can act as a pain receptor if the stimulus is of adequate amplitude!

Phasic adaptation

• Response characteristic of phasic receptors

Tonic Adaptation

• Show little peripheral adaptation

• Called slow-adapting receptors

• Remind you of an injury long after the initial damage has occurred

General Sensesby location

• Exteroceptors - Provide information about the external environment

• Proprioceptors - Report the positions of skeletal muscles and joints

• Interoceptors - Monitor visceral organs and functions

Classification Systemby stimulus

• Divides the general sensory receptors into 4 types by the nature of the stimulus that excites them:– nociceptors (pain)– thermoreceptors (temperature)– mechanoreceptors (physical distortion)– chemoreceptors (chemical concentration)

Pain Receptors

• Also called nociceptors• Are common in the:

– superficial portions of the skin

– joint capsules– within the periostea of bones– around the walls of blood

vessels

• Free nerve endings with large receptive fields

Figure 15–2

Type A and Type C Fibers(Type B fibers are found in the ANS)

• Carry painful sensations

Myelinated Type A Fibers

• Carry sensations of fast pain, or prickling pain, such as that caused by an injection or a deep cut

• Sensations reach the CNS quickly and often trigger somatic reflexes

• Relayed to the primary sensory cortex and receive conscious attention

Type C Fibers

• Carry sensations of slow pain, or burning and aching pain

• Cause a generalized activation of the reticular formation and thalamus

• You become aware of the pain but only have a general idea of the area affected

Thermoreceptors

• Also called temperature receptors

• Are free nerve endings located in:– the dermis– skeletal muscles– the liver– the hypothalamus

Thermoreceptors

• Also called temperature receptors

• Are free nerve endings located in:– the dermis– skeletal muscles– the liver– the hypothalamus

Mechanoreceptors

• Sensitive to stimuli that distort their cell membranes

• Contain mechanically regulated ion channels whose gates open or close in response to:– stretching– compression– twisting– or other distortions of the membrane

3 Classes of Mechanoreceptors

• Tactile receptors: – provide the sensations of touch, pressure, and

vibration:• touch sensations provide information about shape or

texture

• pressure sensations indicate degree of mechanical distortion

• vibration sensations indicate pulsing or oscillating pressure

3 Classes of Mechanoreceptors

• Baroreceptors: – detect pressure changes in the walls of blood

vessels and in portions of the digestive, reproductive, and urinary tracts

• Proprioceptors: – monitor the positions of joints and muscles – the most structurally and functionally complex

of general sensory receptors

Proprioceptors (not shown – Joint kinesthetic

receptors.)

Fine Touch and Pressure Receptors

• Are extremely sensitive• Have a relatively narrow receptive field• Provide detailed information about a source of

stimulation, including:– its exact location– shape– size– texture– movement

Crude Touch and Pressure Receptors

• Have relatively large receptive fields

• Provide poor localization

• Give little information about the stimulus

Tactile ReceptorsFigure 15–3

Baroreceptors

• Monitor change in pressure

• Consist of free nerve endings that branch within elastic tissues in wall of distensible organ (such as a blood vessel)

• Respond immediately to a change in pressure, but adapt rapidly

Chemoreceptors

• Respond only to water-soluble and lipid-soluble substances dissolved in surrounding fluid

• Receptors exhibit peripheral adaptation over period of seconds, central adaptation may also occur

Organization of the Primary Motor and Somatosensory cortices

• The Primary Motor Cortex is located on the precentral gyrus of the cerebral cortex.

• The Primary Somatosensory cortex is located on the postcentral gyrus

• The same areas of the body are represented in both hemispheres but they are connected contralaterally

Somatic Sensory Pathways

The pathways to the sensory areas of the

cerebral cortex can be organized according to

the following hierarchy

• First-order neurons: somatic receptors to spinal cord or brain stem

• Second-order neurons: brain stem or spinal cord to thalamus (decussation occurs here)

• Third-order neurons: thalamus to cortex

3 Major Somatic Sensory Pathways

Figure 15–4

Fig. 15.05

The homunculus

Posterior Column Pathway

• Carries sensations of highly localized (“fine”) touch, pressure, vibration, and proprioception

Figure 15–5a

Figure 15–5a, bSensory Homunculus

Figure 15–5c

Sensory Homunculus

Sensations Bound

for Cerebral Cortex

• Ascend within the anterior or lateral spinothalamic tracts:– the anterior

spinothalamic tracts carry crude touch and pressure sensations

Figure 15–5b

Sensations Bound

for Cerebral Cortex

• The lateral spinothalamic tracts carry pain and temperature sensations

Figure 15–5c

Feeling Pain

• An individual can feel pain in uninjured part of body when pain actually originates at another location

Strong Visceral Pain

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

• Activity in interneurons leads to stimulation of primary sensory cortex, so an individual feels pain in specific part of body surface: – also called referred pain

Referred Pain• The pain of a heart attack is frequently felt in the left arm• The pain of appendicitis is generally felt first in the area

around the navel and then in the right lower quadrant

Figure 15–6

The Spinocerebellar Pathway

• Cerebellum receives proprioceptive information about position of skeletal muscles, tendons, and joints

Figure 15–7

Spinocerebellar Tracts

• Axons of these second-order neurons ascend in 1 of the spinocerebellar tracts:– the posterior spinocerebellar tracts:

• contain axons that do not cross over to the opposite side of the spinal cord:

– axons reach cerebellar cortex via inferior cerebellar peduncle of that side

Spinocerebellar Tracts– the anterior spinocerebellar tracts:

• dominated by axons that have crossed over to opposite side of spinal cord

• contain significant number of uncrossed axons as well:

– sensations reach the cerebellar cortex via superior cerebellar peduncle

– many axons that cross over and ascend to cerebellum then cross over again within cerebellum, synapsing on same side as original stimulus

Spinocerebellar TractsTable 15–1

Main Aspects of Sensory Perception

• Perceptual detection – detecting that a stimulus has occurred and requires summation

• Magnitude estimation – how much of a stimulus is acting

• Spatial discrimination – identifying the site or pattern of the stimulus

Main Aspects of Sensory Perception

• Feature abstraction – used to identify a substance that has specific texture or shape

• Quality discrimination – the ability to identify submodalities of a sensation (e.g., sweet or sour tastes)

• Pattern recognition – ability to recognize patterns in stimuli (e.g., melody, familiar face)

Motor Commands

• Issued by the CNS • Distributed by somatic nervous system

(SNS) and autonomatic nervous system (ANS):– SNS, or the somatic motor system, controls

contractions of skeletal muscles– ANS, or the visceral motor system, controls

visceral effectors, such as smooth muscle, cardiac muscle, and glands

Somatic Motor Pathways

• Always involve at least 2 motor neurons:– upper motor neuron:

• cell body lies in a CNS processing center– lower motor neuron

• cell body lies in a nucleus of the brain stem or spinal cord

Upper Motor Neuron

• Synapses on the lower motor neuron

• Innervates a single motor unit in a skeletal muscle:– activity in upper motor neuron may facilitate or

inhibit lower motor neuron

Lower Motor Neuron

• Triggers a contraction in innervated muscle:– only axon of lower motor neuron extends

outside CNS– destruction of or damage to lower motor neuron

eliminates voluntary and reflex control over innervated motor unit

Figure 15–8

Concious and Subconscious Motor Commands

Concious and Subconscious Motor Commands

• Control skeletal muscles by traveling over 3 integrated motor pathways:– corticospinal pathway– medial pathway– lateral pathway

Corticospinal Pathway

Figure 15–9

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 to synapse on lower motor neurons that control skeletal muscles

3 Pairs of Descending Tracts

• The corticospinal pathway contains 3 pairs of descending tracts:– coricobulbar tracts– lateral corticospinal tracts– anterior corticospinal tracts

Corticobulbar Tracts

• Provide conscious control over skeletal muscles that move the eye, jaw, face, and some muscles of neck and pharynx

• Innervate motor centers of medial and lateral pathways

The Pyramids

• As they descend, corticospinal tracts are visible along the ventral surface of medulla oblongata as pair of thick bands, the pyramids

Crossing Over

• At spinal segment it targets, an axon in anterior corticospinal tract crosses over to opposite side of spinal cord in anterior white commissure before synapsing on lower motor neurons in anterior gray horns

Fig. 15.05

The homunculus

Motor Homunculus

Figure 15–9

Proportions of Motor

Homunculus

Figure 15–5a

Somatic Motor Commands

• Several centers in cerebrum, diencephalons, and brain stem may issue somatic motor commands as result of processing performed at subconscious level

Primary Functions

• These nuclei and tracts are grouped by their primary functions:– components of medial pathway help control

gross movements of trunk and proximal limb muscles

– components of lateral pathway help control distal limb muscles that perform more precise movements

Medial Pathway

• Primarily concerned with control of muscle tone and gross movements of neck, trunk, and proximal limb muscles

Upper Motor Neurons of Medial Pathway

• Are located in:– vestibular nuclei– superior and inferior colliculi– reticular formation

Vestibular Nuclei

• Receive information over the vestibulococlear nerve (VIII) from receptors in inner ear that monitor position and movement of the head:– primary goal is to maintain posture and balance– descending fibers of spinal cord constitute

vestibulospinal tracts

Superior and Inferior Colliculi

• Are located in the roof of the mesencephalon, or the tectum:– colliculi receive visual (superior) and auditory

(inferior) sensations– axons of upper motor neurons in colliculi

descend in tectospinal tracts – these axons cross to opposite side, before

descending to synapse on lower motor neurons in brain stem or spinal cord

Lateral Pathway

• Primarily concerned with control of muscle tone and more precise movements of distal parts of limbs:– axons of upper motor neurons in red nuclei

cross to opposite side of brain and descend into spinal cord in rubrospinal tracts

Lateral PathwayTable 15–2

Basal Nuclei and Cerebellum

• Responsible for coordination and feedback control over muscle contractions, whether contractions are consciously or subconsciously directed

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

Sensory and Motor Pathway Patterns

• All sensory and motor pathways involve a series of synapses, one after the other

• General pattern: – spinal and cranial reflexes provide rapid,

involuntary, preprogrammed responses that preserve homeostasis over short term

Cranial and Spinal Reflexes

• Control the most basic motor activities

Integrative Centers in the Brain

• Perform more elaborate processing

• As we move from medulla oblongata to cerebral cortex, motor patterns become increasingly complex and variable

Primary Motor Cortex

• Most complex and variable motor activities are directed by primary motor cortex of cerebral hemispheres

Reticular Formation

• Loosely organized network of neurons that extends throughout brain stem:– axons of upper motor neurons in reticular

formation descend into reticulospinal tracts without crossing to opposite side

The RAS

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Higher Brain & ANS Function