SENSORY AND MOTOR SYSTEMS: REFLEXES

D. C. Mikulecky

Professor of Physiology

ADEQUATE STIMULUS

• A SPECIFIC STIMULUS UNIQUE TO A SENSORY RECEPTOR

• FOR EXAMPLE, PHOTORECEPTORS IN THE EYE RESPOND ONLY TO ELECTROMAGNETIC RADIATION

TYPES OF ENERGY TRANSDUCED

RECEPTOR TYPE ENERGY HANDLED

MECHANORECEPTORSTOUCH,PRESSURE,PAIN

MECHANICAL: PRESSURE ORPHYSICAL DISPLACEMENT

THERMORECEPTORS HEAT OR COLD

CHEMORECEPTORS:TASTE AND SMELL

CHEMICAL

PHOTORECEPTORS VISIBLE LIGHT

NOCIOCEPTORS:PAIN

TISSUE DAMAGE

GRADED VS ALL OR NONE

• A RECEPTOR’S RESPONSE TO A STIMULUS IS GRADED

• IF THRESHOLD IS EXCEEDED, THE ACTION POTENTIAL RESULTING IS ALL OR NONE

SENSORY TRANSDUCTION

ADEQUATE STIMULUS

MEMBRANECONDUCTANCE

CHANGE

GENERATORPOTENTIAL

ACTIONPOTENTIAL

EXAMPLE: THE PACINIAN CORPUSCLE

• PRESSURE RECEPTOR

• PRESSURE DISTORTS THE CAPSULE, STRETCHING THE CELL MEMBRANE

• SODIUM CHANNELS OPEN, DEPOLARIZING THE MEMBRANE

• IF BEYOND THRESHOLD, ACTION POTENTIAL RESULTS

HOW IS THE INTENSITY OF THE STIMULUS ENCODED?

• FREQUENCY CODING

• POPULATION CODING

FREQUENCY CODINGLIGHT PRESSURE

LOW FREQUENCY

MORE PRESSURE

HIGHERFREQUENCY

POPULATION CODING

LIGHT PRESSURE

MORE PRESSURE

NEOCORTEX

THALAMUS

LIMBICCORTEX

BRAINSTEM

SPINALCORD

NOSE

EYE

TONGUE

EAR

SKIN

SMELL

VISION

TASTE

AUDITION

SOMATIC

THE LABELED LINE CODE OF STIMULUS QUALITY

LOCALIZATION, DISTRIBUTION, AND ACUITY

• CODING OF LOCATION DEPENDS ON RECEPTOR LOCATION

• AREA COVERED BY RECEPTORS IN A SENSORY UNIT IS A RECEPTIVE FIELD

• ACUITY DEPENDS ON THE DENSITY OF RECEPTORS

GENERAL FEATURES OF THE MOTOR SYSTEM

• HIERARCHICAL ORGANIZATION

• VS

• PARALLEL ORGANIZATION

HIERARCHICAL ORGANIZATION

MOTOR CORTEX

CEREBELLUM AND BASAL GANGLIA

BRAIN STEM

SPINAL CORD

SKELETAL MUSCLE

PARALLEL ORGANIZATIONMOTOR CORTEX

SKELETAL MUSCLE

BRAIN STEM

SPINAL CORD

THE MOTOR UNIT

• A SINGLE MOTOR NEURON AND ALL THE MUSCLES IT INNERVATES

• THE FUNCTIONAL UNIT OF THE MOTOR SYSTEM

• CORRESPONDS TO THE SENSORY UNIT/RECEPTIVE FIELD RELATIONSHIP

• SIZE OF MOTOR UNIT DETERMINES FINENESS OF MOTOR RESPOSE

THE MOTOR RESPONSE SYSTEM: SPINAL LEVEL

• THE ORGANIZATION OF MOTOR NERVES IN THE SPINAL COLUMN

• THE ROLE OF INTERNEURONS

• THE ROLE OF LOCOMOTOR GENERATORS

THE ORGANIZATION OF MOTOR NERVES IN THE SPINAL COLUMN

• DORSAL AREA OF VENTRAL HORN: FLEXOR MOVEMENTS

• VENTRAL AREA OF VENTRAL HORN: EXTENSOR MOVEMENTS

• DORSOLATERAL AREA OF VENTRAL HORN: INNERVATE EXTREMITIES

• VENTROMEDIAL REGION: AXIAL MUSCLES TO MAINTAIN POSTURE

LOCATION OF CELL BODIES FOR EXTENSORS AND FLEXORS

FLEXORS

EXTENSORS

THE ROLE OF INTERNEURONS

• LOCATED IN THE INTER MEDIATE ZONE OF THE SPINAL CORD

• LATERAL SIDE NEURONS SYNAPSE IPSILATERALLY (TO DISTAL LIMB MUSCLES)

• MIDLINE NEURONS SYNAPSE BOTH SIDES (MUSCLES FOR POSTURE) ALPHA MOTOR NEURONS:EXCITE SYNERGISTIC AND INHIBIT ANTAGONISTIC

THE ROLE OF LOCOMOTOR GENERATORS

• RHYTHMIC ACTION OF ALTERANATING FLEXION AND EXTENSION INVOVED IN WALKING MOVEMENTS (CHICKEN WITH ITS HEAD CUT OFF)

• SPINAL TRANSECTED ANIMALS CAN WALK ON TREADMILL

• UNDER CONTROL OF LOCOMOTOR COMMAND CENTER IN THE BRAIN STEM

SENSORY FIBERS IN THE MUSCLE

• THE MUSCLE SPINDLE AS LENGTH DETECTOR

• GOGLI TENDON ORGANS: TENSION RECEPTORS

THE MUSCLE SPINDLE AS LENGTH

DETECTOR(SENSORY FIBERS) • TYPE Ia NERVE FIBERS: TRANSMIT

INFORMATION ABOUT LENGTH AND VELOCITY TO THE CNS

• TYPE II NERVE FIBERS:TRANSMIT INFORMATION ABOUT MUSCLE LENGTH TO CNS

•

TWO TYPES OF INTRAFUSAL FIBERS

NUCLEARCHAIN

FIBER

NUCLEARBAGFIBER

TYPE IISENSORYFIBER

TYPE IASENSORYFIBER

TWO TYPES OF MOTOR NEURON

• ALPHA MOTOR NEURON: INNERVATES EXTRAFUSAL FIBER

• GAMMA MOTOR NEURON: INNERVATES INTRAFUSAL FIBERS

STRETCHING AN INTRAFUSAL FIBER SENDS SIGNALS TO CNS

THE ROLE OF GAMMA MOTOR NEURONS IN REGULATING MUSCLE SPINDLE RESPONSE

RECORD

RECORD

STIMULATE

Afferent Activity

Muscle Force

THE ROLE OF GAMMA MOTOR NEURONS IN REGULATING MUSCLE SPINDLE RESPONSE

RECORD

RECORD

STIMULATE

Afferent Activity

Muscle Force

STIMULATE

GOGLI TENDON ORGANS: TENSION RECEPTORS

• IN SERIES WITH EXTRAFUSAL FIBERS

• TRANSMITS INFORMATION ABOUT FORCE OR TENSION TO CNS

• FREQUENCCY CODING

SPINAL REFLEXES

• THE STRETCH REFLEX

• THE INVERSE MYOTACTIC REFLEX

• THE FLEXOR WITHDRAWAL REFLEX

THE STRETCH REFLEX (KNEE JERK REFLEX)

• TAP PATELLAR TENDON BELOW KNEE

• SENSORY SIGNALS FROM MUSCLE SPINDLE INDICATE FLEXION OF LEG

• FLEXOR IS INHIBITED

• EXTENSOR ACTIVATED

• LEG EXTENDS

KNEE JERK REFLEX

SPINALCORD

SENSORY NEURON FROMMUSCLE SPINDLE DETECTSSTRETCH

FLEXOR INHIBITED

EXTENSORACTIVATED

INVERSE MYOTACTIC REFLEX

ARM EXTENDS

WEIGHTRELEASED

BICEPS & SYNERGYSTIC MUSCLERELAXEDEXTENSOR CONTRACTED

THE FLEXOR WITHDRAWAL REFLEX

• PAINFUL STIMULUS DETECTED• IPSILATERAL EXTENSORS INHIBITED• IPSILATERAL EXTENSORS EXCITED• LIMB IS WITHDRAWN• IF INVOLVING ONE FOOT WHILE

STANDING, CONTRALATERAL SIDE EXTENSORS ACTIVATED AND FLEXORS INHIBITED TO SUPPORT GREATER WEIGHT

FLEXOR WITHDRAWAL REFLEX

STEPON TACK

SENSORYSIGNAL TO SPINE

VIAINTERNEURONS

IPSILATERAL EXTENSORS INHIBITEDIPSILATERAL FLEXORS CONTRACT-LEG LIFTSCONTRALATERAL EXTENSORS CONTRACTCONTRALATERAL FLEXORS INHIBITED- MAINTAIN BALANCE AND SUPPORT WEIGHT