Lecture 5: Receptors Perception: Exteroception (vision, hearing, smell, touch) Interoception (internal objects, organs) Proprioception (position of body parts)

Proprioceptor Neuron The body of a sensory neuron is located in a ganglion near the spinal cord. One branch of its T-shaped axon goes to the peripheral sensory ending, and another branch goes through the dorsal roots into the spinal cord. Ganglion T-shaped axon Sensory ending Neuron body Spinal cord

Proprioceptor Neuron Body, long T-shaped axon, and sensory ending Body is in spinal ganglia Sensory endings generate APs in response to a specific stimulus (deformation, light, etc.) Antidromic conduction No dendrites, no synapses on the body

Muscle Spindle Ia II Extrafusal fiber BFdyn BFst CF A muscle spindle is oriented parallel to extrafusal muscle fibers. It is covered with a capsule and contains two types of intrafusal muscle fibers: bag fibers (BF) and chain fibers (CF). Two types of sensory endings can be found in muscle spindles: primary (Ia) and secondary (II). Primary endings are typically seen in virtually all intrafusal fibers. Secondary endings are seen in CF and in static BF, but not in dynamic BF.

Spindle Response to Stretch (Primary Ending) Length Time Action potentials Typical responses of a primary spindle ending to an externally imposed muscle stretch at different velocities. The response increases with muscle length and with the velocity of the stretch.

Spindle Response to Stretch (Secondary Ending) Length Time Action potentials A typical response of a secondary spindle ending to an externally imposed muscle stretch and shortening. The response increases with muscle length and does not depend on velocity.

Gamma Motoneurons There are two types of small motoneurons (g-motoneurons) innervating intrafusal fibers of muscle spindles. BFdyn BFst CF g dyn -motoneuron st -motoneurons Dynamic g-motoneurons innervate dynamic bag fibers and change the sensitivity of primary endings. Static g-motoneurons innervate static bag and chain fibers. They change the sensitivity of primary and secondary endings.

Gamma Motoneurons: Effects on Spindle Reaction to Stretch Length Time g-dynamic stimulation The effects of an activity of dynamic g-motoneurons on a response of a primary spindle ending to muscle stretch and shortening. In the lower graph, a g-dynamic stimulation was applied during the same changes in muscle length.

Muscle Spindle Two types of endings: Primary (Ia afferents): sensitive to length and velocity of muscle fibers Secondary (II afferents): sensitive only to length of muscle fibers Gamma-motoneurons: A system to modify sensitivity of the spindle endings Gamma-MNs innervate intrafusal muscle fibers

Golgi Tendon Organs Ganglion Golgi tendon organ Ib afferent Muscle fibers Tendon Golgi tendon organs are located in series with extrafusal muscle fibers at their junction with the tendon. They are innervated with fast-conducting Ib axons of sensory neurons in spinal ganglia.

Response to Muscle Force Golgi Tendon Organs: Response to Muscle Force A response of a Golgi tendon organ to muscle force. Note that it is similar to the response of secondary spindle endings to muscle length. Muscle force Time Action potentials

Articular Receptors Most articular receptors fire in rather narrow ranges of joint angle, mostly close to the anatomical limits. An increase in muscle force leads to an increase in joint capsule tension, and articular receptors increase their response (bold lines). Angle Frequency of firing Anatomical limits

GTO and Articular Receptors GTO: a passive sensory ending sensitive only to tendon force Articular receptors: Sensitive to joint angle close to the anatomical limits of joint rotation Sensitive to joint capsule tension

Cutaneous and Subcutaneous Receptors Skin surface Meissner corpuscles Merkel disks Ruffini endings Pacinian corpuscles Major types of cutaneous and subcutaneous mechanoreceptors in the glabrous skin of the hand.

Cutaneous and Subcutaneous Receptors Name What They Measure Features Merkel disks Vertical pressure Several are innervated by one axon Meissner Quickly changing Each is innervated by corpuscles pressure ≥ 2 axons Ruffini Deformation of Slowly adapting endings large skin areas Pacinian Rapidly changing Huge (1–5 mm) corpuscles mechanical deformation (such as vibration)

of Proprioceptive Neurons Central Axons of Proprioceptive Neurons MN INs Ia Ia, II Ib Spindle Articular receptor Golgi organ Skin receptors To the brain Afferent nerves from peripheral receptors go into the spinal cord through the dorsal roots. There they make synapses on interneurons and motoneurons (only primary spindle endings) and send signals to the brain. Note that the same interneurons may receive signals from afferents of different modalities.