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1) Somatosensory System

2) Mechanoreceptors / Receptor Potentials

3) Muscle Spindles / GTOs

4) Central Somatosensory Processing

5) Deafferentation / Plasticity

Topic 5 – Somatosensory System

Source: Kandel et al., Principles of Neural Science, McGraw Hill, 20000

Peripheral receptor

Dorsal root ganglion cell

Peripheral axonproximal axon

Spinal cord

"First-order" sensory afferents carry info from peripheral receptors to the spinal cord.

Source: Purves et al. Neuroscience, SinauerAssociates Inc: Massachusetts, 2001.

Vertebra

Dorsal Root Ganglion

White matter

Gray matter

Spinal nerve

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cerebrum

Primary somatic sensory cortex

Ventral posterior lateral nucleus of thalamus

midbrain

mid-pons

Rostral medulla

Caudal medulla

Cervical spinal cord

Lumbar spinal cord

Medial lemniscus

Medial lemniscus

Gracile nucleus (pathways from lower body)

Cuneate nucleus (pathways from upper body)

Internal arcuate fibers

Gracile tract

Cuneate tract

Mechanosensoryreceptors from upper body

Mechanosensoryreceptors from lower body

Afferent pathway for mechanosensoryinformation:

The dorsal column - medial lemniscus (DCML) pathway

Source: Purves et al. Neuroscience, Sinauer Associates Inc: Massachusetts, 2001.

- carries cutaneous and proprioceptive information to the brain

Dorsal root ganglion

cerebrum

Primary somatic sensory cortex

Ventral posterior lateral nucleus of thalamus

midbrain

mid-pons

Rostral medulla

Caudal medulla

Cervical spinal cord

Lumbar spinal cord

Medial lemniscus

Medial lemniscus

Gracile nucleus (pathways from lower body)

Cuneate nucleus (pathways from upper body)

Internal arcuate fibers

Gracile tract

Cuneate tract

Mechanosensoryreceptors from upper body

Mechanosensoryreceptors from lower body

Source: Purves et al. Neuroscience, Sinauer Associates Inc: Massachusetts, 2001.

Dorsal root ganglion

- afferents travel through dorsal column up to the medulla

cerebrum

Primary somatic sensory cortex

Ventral posterior lateral nucleus of thalamus

midbrain

mid-pons

Rostral medulla

Caudal medulla

Cervical spinal cord

Lumbar spinal cord

Medial lemniscus

Medial lemniscus

Gracile nucleus (pathways from lower body)

Cuneate nucleus (pathways from upper body)

Internal arcuate fibers

Gracile tract

Cuneate tract

Mechanosensoryreceptors from upper body

Mechanosensoryreceptors from lower body

Source: Purves et al. Neuroscience, Sinauer Associates Inc: Massachusetts, 2001.

Dorsal root ganglion

1st order afferents synapse with 2nd order neurons in dorsal column nuclei (gracile and cuneate)

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cerebrum

Primary somatic sensory cortex

Ventral posterior lateral nucleus of thalamus

midbrain

mid-pons

Rostral medulla

Caudal medulla

Cervical spinal cord

Lumbar spinal cord

Medial lemniscus

Medial lemniscus

Gracile nucleus (pathways from lower body)

Cuneate nucleus (pathways from upper body)

Internal arcuate fibers

Gracile tract

Cuneate tract

Mechanosensoryreceptors from upper body

Mechanosensoryreceptors from lower body

Source: Purves et al. Neuroscience, Sinauer Associates Inc: Massachusetts, 2001.

Dorsal root ganglion

2nd order neurons travel through internal arcuate and medial lemniscus tracts to the thalamus

cerebrum

Primary somatic sensory cortex

Ventral posterior lateral nucleus of thalamus

midbrain

mid-pons

Rostral medulla

Caudal medulla

Cervical spinal cord

Lumbar spinal cord

Medial lemniscus

Medial lemniscus

Gracile nucleus (pathways from lower body)

Cuneate nucleus (pathways from upper body)

Internal arcuate fibers

Gracile tract

Cuneate tract

Mechanosensoryreceptors from upper body

Mechanosensoryreceptors from lower body

Source: Purves et al. Neuroscience, Sinauer Associates Inc: Massachusetts, 2001.

Dorsal root ganglion

- 2nd order neurons synapse on 3rd order in VPL nucleus of thalamus

- 3rd order neurons travel to S1

Primary somatosensory cortex (S1)

Source: Kandel et al., Principles of Neural Science, McGraw Hill, 2000

Primary Somatosensory CortexPostcentralgyrus

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3a – proprioceptive inputs3b – cutaneous inputs1 – cutaneous inputs2 – proprioceptive inputs

S1 is comprised of sub-areas:

Source: Kandel et al., Principles of Neural Science, McGraw Hill, 2000

S1 - Connectivity

Source: Kandel et al., Principles of Neural Science, McGraw Hill, 2000

Central sulcus

Postcentral gyrus (S1)Postcentral sulcus

thalamus

Receptive Fields

Source: Kandel et al., Principles of Neural Science, McGraw Hill, 2000

S1 neurons have RFs

- smaller RFs in area 3 cells than area 1 and 2 cells

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Topographic Maps -

The arrangement of receptive fields in the somatosensory cortex forms a map of the entire body.

Topographic Maps

Source: Kandel et al., Principles of Neural Science, McGraw Hill, 2000

Each S1 region has its own map

Source: Purves et al. Neuroscience, Sinauer Associates Inc: Massachusetts, 2001.

The more important an area is for tactile discrimination, the larger the representation…

Sensory Homunculus

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Source: Haines, Fundamental Neuroscience, Churchill-Livingstone, 2002

• receptor groups are separated in representation in columns

Source: Kandel et al., Principles of Neural Science, McGraw Hill, 2000

Information in S1 is arranged by: 1. receptor type (columns)

2. location of origin (homunculus)

In general -The primary somatosensory cortex integrates info from somatosensory input

- Lesions affect position sense, ability to discriminate size, texture, shape etc…

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Somatosensory information is important in the control of movement -

• This can be demonstrated using muscimol…

A. Muscaria

Mushroom species.

Source of muscimol

GABA

Muscimol

Source: Kandel et al., Principles of Neural Science, McGraw Hill, 2000

Ipsilateral hand

Contralateral hand

muscimol injected into area 2

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Secondary Somatosensory Area

Source: Kandel et al., Principles of Neural Science, McGraw Hill, 2000

Secondary Somatosensory Area

- overall function is largely unknown

- large, bilateral receptive fields

- may have a role in integration of submodalities of somatosensation

Sensory Gating -

The ability to focus on relevant sensory information while disregarding less important information.

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Sensory Gating -- The CNS can control ascending pathways

- achieved by inhibition

- inhibition can occur along the DCML pathway

1) Somatosensory System

2) Mechanoreceptors / Receptor Potentials

3) Muscle Spindles / GTOs

4) Central Somatosensory Processing

5) Deafferentation / Plasticity

Topic 5 – Somatosensory System

Plasticity

- Sensory and motor maps are modifiable by training or use.

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Somatosensory maps are modifiable by training or use.

Monkeys trained to obtain food using only tips of middle 3 fingers 1 hour / day for 3 months.

Source: Kandel et al., Principles of Neural Science, McGraw Hill, 2000

Somatosensory maps are modifiable by training or use.

Training

Source: Kandel et al., Principles of Neural Science, McGraw Hill, 2000

Representations of digits in area 3b before and after training.

Deafferentation and Plasticity

Peripheral lesions/ amputations result in modified somatosensory maps.

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Source: Ramachandran, PNAS, 1993, 90: 10413-10420.

Amputation

• referred sensations between body parts ("phantom limbs")

Source: Purves et al. Neuroscience, Sinauer Associates Inc: Massachusetts, 2001.

Re-mapping after amputation

Hand region

Face region

Arm region

Source: Haines, Fundamental Neuroscience, Churchill-Livingstone, 2002

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Peripheral Lesions

Source: Kandel et al., Principles of Neural Science, McGraw Hill, 20000

Other sources of sensory information (incl vision, vestibular and auditory) are also important for movement!