Somatosensory Tracts and Maps NBIO 401 – Wednesday October 2, 2013

(a) Introduction

Objectives: - Be able to describe the pathways for each of the three types of somatosensory information, from primary afferents to the pathway destination in the cerebral cortex or cerebellum. You should also be able to explain where the pathway starts, where each cell body is, where each axon synapses, and where axons cross the midline. - Be able to describe the somatotopic organization of somatosensory information in 1) the dorsal columns, 2) the spinothalamic tract, 3) the VP thalamus, and 4) the somatosensory cortex. - Know and be able to explain 1) how descending and local inputs to the pain pathway suppress pain transmission, 2) how the responses of Clarke’s column neurons are different from those of individual receptor responses, and 3) the two states of thalamic neurons relaying somatosensory information to the cortex.

Objectives: - Be able to describe the pathways for each of the three types of somatosensory information, from primary afferents to the pathway destination in the cerebral cortex or cerebellum. You should also be able to explain where the pathway starts, where each cell body is, where each axon synapses, and where axons cross the midline. - Be able to describe the somatotopic organization of somatosensory information in 1) the dorsal columns, 2) the spinothalamic tract, 3) the VP thalamus, and 4) the somatosensory cortex. - Know and be able to explain 1) how descending and local inputs to the pain pathway suppress pain transmission, 2) how the responses of Clarke’s column neurons are different from those of individual receptor responses, and 3) the two states of thalamic neurons relaying somatosensory information to the cortex.

Objectives: - Be able to describe the pathways for each of the three types of somatosensory information, from primary afferents to the pathway destination in the cerebral cortex or cerebellum. You should also be able to explain where the pathway starts, where each cell body is, where each axon synapses, and where axons cross the midline. - Be able to describe the somatotopic organization of somatosensory information in 1) the dorsal columns, 2) the spinothalamic tract, 3) the VP thalamus, and 4) the somatosensory cortex. - Know and be able to explain 1) how descending and local inputs to the pain pathway suppress pain transmission, 2) how the responses of Clarke’s column neurons are different from those of individual receptor responses, and 3) the two states of thalamic neurons relaying somatosensory information to the cortex.

Objectives: - Be able to describe the pathways for each of the three types of somatosensory information, from primary afferents to the pathway destination in the cerebral cortex or cerebellum. You should also be able to explain where the pathway starts, where each cell body is, where each axon synapses, and where axons cross the midline. - Be able to describe the somatotopic organization of somatosensory information in 1) the dorsal columns, 2) the spinothalamic tract, 3) the VP thalamus, and 4) the somatosensory cortex. - Know and be able to explain 1) how descending and local inputs to the pain pathway suppress pain transmission, 2) how the responses of Clarke’s column neurons are different from those of individual receptor responses, and 3) the two states of thalamic neurons relaying somatosensory information to the cortex.

1) pain, temp 2) fine touch, 2-point discrimination 5) hair bend 4) skin stretch, proprioception 3) fine touch, 2-point discrimination 6) pressure, vibration Somatosensory Receptors in the Skin

Somatosensory Receptors in Muscles & Joints Golgi Tendon Organ (muscle tension) Muscle Spindle (muscle stretch) Joint Receptors (angle, tension)

Muscle & joint proprioceptors Skin receptors Pain & temperature Pain, temperature, & itch

conscious touch – your perception of surface texture and shape conscious proprioception – your perception of the posture and position of your body parts relative to each other pain – aversive sensation temperature – your perception of how hot or cold a touched object is unconscious proprioception – information about the posture and position of your body parts relative to each other of which you are not aware of; used to guide and improve movements

conscious touch – your perception of surface texture and shape conscious proprioception – your perception of the posture and position of your body parts relative to each other pain – aversive sensation temperature – your perception of how hot or cold a touched object is unconscious proprioception – information about the posture and position of your body parts relative to each other of which you are not aware of; used to guide and improve movements

conscious touch – your perception of surface texture and shape conscious proprioception – your perception of the posture and position of your body parts relative to each other pain – aversive sensation temperature – your perception of how hot or cold a touched object is unconscious proprioception – information about the posture and position of your body parts relative to each other of which you are not aware of; used to guide and improve movements

conscious touch – your perception of surface texture and shape conscious proprioception – your perception of the posture and position of your body parts relative to each other pain – aversive sensation temperature – your perception of how hot or cold a touched object is unconscious proprioception – information about the posture and position of your body parts relative to each other of which you are not aware of; used to guide and improve movements

conscious touch – your perception of surface texture and shape conscious proprioception – your perception of the posture and position of your body parts relative to each other pain – aversive sensation temperature – your perception of how hot or cold a touched object is unconscious proprioception – information about the posture and position of your body parts relative to each other of which you are not aware of; used to guide and improve movements

conscious touch – your perception of surface texture and shape conscious proprioception – your perception of the posture and position of your body parts relative to each other pain – aversive sensation temperature – your perception of how hot or cold a touched object is unconscious proprioception – information about the posture and position of your body parts relative to each other of which you are not aware of; used to guide and improve movements

conscious touch – your perception of surface texture and shape conscious proprioception – your perception of the posture and position of your body parts relative to each other pain – aversive sensation temperature – your perception of how hot or cold a touched object is unconscious proprioception – information about the posture and position of your body parts relative to each other of which you are not aware of; used to guide and improve movements

conscious touch – your perception of surface texture and shape conscious proprioception – your perception of the posture and position of your body parts relative to each other pain – aversive sensation temperature – your perception of how hot or cold a touched object is unconscious proprioception – information about the posture and position of your body parts relative to each other of which you are not aware of; used to guide and improve movements

conscious touch – your perception of surface texture and shape conscious proprioception – your perception of the posture and position of your body parts relative to each other pain – aversive sensation temperature – your perception of how hot or cold a touched object is unconscious proprioception – information about the posture and position of your body parts relative to each other of which you are not aware of; used to guide and improve movements

(b) Conscious Touch & Proprioception -Lower Body -Upper body

(receptor) axon - separate cell (receives input from receptor; cell body is in dorsal root ganglion) Receptors are before the 1 st neuron in the pathway. They provide input to the 1 st neuron. For example, here a Merkel receptor provides input to the axon of the first neuron in the conscious touch and proprioception pathway. The cell body of this neuron is in the DRG.

1st Neuron

1st Neuron – Axons ascend in big fiber tracts called dorsal columns. Axons array in a somatotopy in the dorsal columns with lower body medial and upper body lateral.

2nd Neuron – Receives input from axons of 1 st neuron. Medial, lower body, axons terminate on the medial gracile nucleus. Lateral, upper body axons terminate on the more lateral cuneate nucleus. Axons of gracile and cuneate neurons cross the midline and form the large medial lemniscus. There is a somatotopy in the medial lemniscus. Upper body is represented dorsally and lower body is represented ventrally. Each medial lemniscus represents the contralateral side of the body. axon of 1 st neuron carrying info from lower body terminating on 2 nd neuron axon of 1 st neuron carrying info from upper body terminating on 2 nd neuron

2 nd Neuron to 3 rd Neuron – The 3 rd neuron is in the ventral posteriolateral (VPL) nucleus of the thalamus. Lateral VPL neurons receive input from the gracile nucleus (lower body, contralateral side). Medial VPL neurons receive input from the cuneate nucleus (upper body-contralateral side). axons of 2nd neurons (from gracile and cuneate neurons) 3 rd neuron

3rd Neuron – Lateral VPL neurons (lower body-contralateral side) send their axons to terminate in the medial part of somatosensory cortex. Medial VPL neurons (upper body-contralateral side) send their axons to terminate in the neighboring and more lateral part of the somatosensory cortex.

Lower Body 3rd Neuron – The orderly organization of the entire pathway, from the periphery to the somatosensory cortex, creates a somatotopy in the somatosensory cortex.

Lower Body 3rd Neuron – Within the medial area representing lower body (contralateral side) the genitals are represented most ventrally.

Lower Body 3rd Neuron – Within the medial area representing lower body (contralateral side) the genitals are represented most ventrally. The foot is represented just dorsal to that,

Lower Body 3rd Neuron – Within the medial area representing lower body (contralateral side) the genitals are represented most ventrally. The foot is represented just dorsal to that, and the leg just dorsally to that.

Lower Body Upper Body 3rd Neuron – Within the more lateral medial area representing upper body (contralateral side) the

Lower Body Upper Body 3rd Neuron – Within the more lateral medial area representing upper body (contralateral side) the arm is represented lateral to the trunk,

Lower Body Upper Body 3rd Neuron – Within the more lateral area representing upper body (contralateral side) the arm is represented lateral to the trunk, the hand, lateral to the arm,

Lower Body Upper Body 3rd Neuron – Within the more lateral area representing upper body (contralateral side) the arm is represented lateral to the trunk, the hand, lateral to the arm, and the fingers lateral to the hand. (Lateral to that is the representation of the face and mouth, the pathway for which I will describe next.)

1st Neuron

axon of 1 st neuron carrying info from lower body terminating on 2 nd neuron axon of 1 st neuron carrying info from upper body terminating on 2 nd neuron 2nd Neuron

axons of 2nd neurons (from gracile and cuneate neurons) 3 rd neuron 3rd Neuron

Lower Body Upper Body Termination of 3rd Neuron

There are three neurons in this pathway. 1) cell body in dorsal root ganglion (DRG); peripheral process receives input from receptor in skin, muscle, or joint; central process terminates in nucleus in caudal brainstem, i.e., the gracile nucleus or cuneate nucleus 2) cell body in gracile or cuneate nucleus in the caudal brainstem; receives input from the axons of neuron 1; central process terminates in contralateral thalamus Remember: the axons of the 2 nd neuron cross the midline. 3) cell body in the thalamus; receives input from neuron 2; central process terminates in cerebral cortex

(c) Conscious Touch & Proprioception -Face

1 st Neuron - cell body in trigeminal ganglion; peripheral axon receives input from receptor cells in head; central axon terminates in the main sensory trigeminal nucleus. 2 nd Neuron - cell body in the main sensory trigeminal nucleus; receives input from 1 st neuron; central axon crosses the midline and ascends in the medial lemniscus next to the fibers representing upper body.

Lower Body Upper Body 3rd Neuron – The 3 rd neuron is in the ventral-posterior-medial (VPM) nucleus of the thalamus. The axons of VPM neurons terminate lateral to the upper body area of the somatosensory cortex in an area called the face area. Face

Lower Body Upper Body Face 3rd Neuron – The face is represented next to the fingers

Lower Body Upper Body Face 3rd Neuron – The face is represented next to the fingers, the lips lateral to that,

Lower Body Upper Body Face 3rd Neuron – The face is represented next to the fingers, the lips lateral to that, and the tongue lateral to that.

Somatotopy in somatosensory cortex The map of the contralateral body in the somatosensory cortex looks like this. Note that the regions that are very sensitive, the toes, fingers, lips, & tongue have more cortical area devoted to them. The denser peripheral receptors are in an area, the more sensitive it is and the more somatosensory cortex is devoted to it.

(d) Pain & Temperature -Lower Body -Upper body

1st Neuron 2 nd Neuron 1 st neuron

1st Neuron 2 nd Neuron 1 st neuron 2 nd neuron

Lower Body Upper Body

(e) Pain & Temperature -Face

1st Neuron 2 nd Neuron 1 st neuron

1st Neuron 2 nd Neuron 1 st neuron 2 nd neuron

1st Neuron 2 nd Neuron A B

A B

Main sensory trigeminal nucleus Motor Trigeminal nucleus Mesencephalic trigeminal tract Mesencephalic trigeminal nucleus Spinal trigeminal nerve (1° axons on way to terminate in spinal trigeminal nucleus) Main sensory trigeminal nucleus Spinal Trigeminal nucleus Trigeminal nerve Trigeminal nerve Spinal trigeminal nucleus

1st Neuron 2 nd Neuron 1 st neuron 2 nd neuron

Lower Body Upper Body

(f) Unconscious Proprioception -Lower Body

1st Neuron 2 nd Neuron (NO 3 rd neuron)

(still f) Unconscious Proprioception -Upper Body

1st Neuron 2 nd Neuron (NO 3 rd neuron)

(still f) Unconscious Proprioception -Face, Part 1 – Facial Muscles

1st Neuron 2 nd Neuron (NO 3 rd neuron)

(still f) Unconscious Proprioception -Face, Part 2 – Jaw Muscles

1st Neuron (NO 2 nd Neuron) (NO 3 rd Neuron) (weird uh?)

Main sensory trigeminal nucleus Motor Trigeminal nucleus Mesencephalic trigeminal tract Mesencephalic trigeminal nucleus Spinal trigeminal nerve (1° axons on way to terminate in spinal trigeminal nucleus) Main sensory trigeminal nucleus Spinal Trigeminal nucleus Trigeminal nerve Trigeminal nerve Spinal trigeminal nucleus

(g) Sensory Processing in the Somatosensory System

Sensory Processing in the Somatosensory System - PAIN

Sensory Processing in the Somatosensory System - PAIN

Sensory Processing in the Somatosensory System - PAIN

Sensory Processing in the Somatosensory System - PAIN

Sensory Processing in the Somatosensory System - PAIN

Sensory Processing in the Somatosensory System - PAIN

Sensory Processing in the Somatosensory System - PAIN

Sensory Processing in the Somatosensory System - PAIN

Sensory Processing in the Somatosensory System - UNCONSCIOUS PROPRIOCEPTION

Sensory Processing in the Somatosensory System - UNCONSCIOUS PROPRIOCEPTION

Sensory Processing in the Somatosensory System - STATE DEPENDENCE (SLEEP)

Sensory Processing in the Somatosensory System - STATE DEPENDENCE Transmission (awake)

Sensory Processing in the Somatosensory System - STATE DEPENDENCE Bursting (slow wave sleep) Transmission (awake)

Sensory Processing in the Somatosensory System - STATE DEPENDENCE Bursting (slow wave sleep) Transmission (awake)

Sensory Processing in the Somatosensory System - STATE DEPENDENCE Bursting (slow wave sleep) Transmission (awake)

Sensory Processing in the Somatosensory System - HIGHER ORDER PROCESSING

Sensory Processing in the Somatosensory System - HIGHER ORDER PROCESSING

Sensory Processing in the Somatosensory System - HIGHER ORDER PROCESSING

Sensory Processing in the Somatosensory System - HIGHER ORDER PROCESSING

Sensory Processing in the Somatosensory System - HIGHER ORDER PROCESSING

Sensory Processing in the Somatosensory System - HIGHER ORDER PROCESSING