Somatosensory Pathways, Receptors, and Motor Control in Neuroanatomy

4 Categories of Somatosensory Pathways

1. Discriminative touch

2. Proprioception

3. Pain and (4) temp

Discriminative Touch

Includes touch, pressure, and vibration

Proprioception

Includes receptors for m stretch, joint position, tendon tension

Pain and temp also includes...

Sensations of itch and tickle

4 Skin Mechanoreceptors

1. Meissner's corpuscles

2. Pacinian corpuscles

3. Merkel's disks

4. Ruffini endings

Responsible for discriminative touch

DCML First Order Neuron

Peripheral receptor to the medulla

- Primary neuron synapses once reaches the medulla

DCML Second Order Neuron

Medulla to the thalamus

DCML Third Order Neuron

Thalamus to the cerebral cortex

DCML: Length of 1 to 2 and 2 to 3

1 to 2: Long pathway (skin to medulla)

2 to 3: Short pathway (medulla to thalamus)

ST First Order Neuron

Pain and temp info from skin to SC

ST Second Order Neuron

Crosses the midline

- SC to thalamus

ST Third Order Neuron

Thalamus to cerebral cortex

ST Length of 1 to 2 and 2 to 3

1 to 2: Short (skin to SC)

2 to 3: Long (SC to thalamus)

Where does ST tract decussate?

1-2 levels above the level of the skin receptor in the SC

Brain Areas Involved in Motor Functions (5)

- SC

- Brainstem

- Cerebellum

- Diencephalon

- Cerebral hemispheres (cortex and BG)

Functions of the SC (3)

- Relay messages

- Coordinate reflexes

- CPGs

DTR: Stimulation that Activates

Quick m stretch (tap tendon)

DTR: Afferent Fiber

1A sensory ending of m spindle

DTR: Synapse

Monosynaptic

DTR: Significance

Determines the lesion areas

TSR (tonic stretch reflex): Stimulation that Activates

Slow, continuous stretch

TSR (tonic stretch reflex): Afferent Fiber

1A and 2A sensory endings of m spindle

TSR (tonic stretch reflex): Synapse

Polysynaptic

TSR (tonic stretch reflex): Significance

Maintenance of posture

- Ex = slight flexion during standing elicits a TSR

CPGS

Neuronal circuits in the lumbar SC that elicit stepping movements of the LEs

- Each LE has a CPG: cycles of 2 CPGs are coordinated so that when one leg flexes the other extends

Clinical Application: Passive stretching exercises should be performed ___?

Slowly

- Less likely to elicit a stretch reflex

Functions of the Brainstem (3)

- Receives and integrates somatosensory, vestibular, and visual inputs for postural control

- Origin of all the descending motor pathways except the corticospinal tract (originates in M1 and S1)

- Brainstem nuclei control

6 Medial Activation Systems

1. Tectospinal

2. Medial reticulospinal

3. Lateral reticulospinal

4. Medial vestibulospinal

5. Lateral vestibulospinal

6. Anterior corticospinal

2 Lateral Activation Systems

1. Lateral corticospinal

2. Rubrospinal

Tectospinal: Decussation

Contralateral

- Crosses in midbrain

Tectospinal: Function

Orients head and eyes toward stimuli

Medial Reticulospinal: Decussation

Bilateral

- Crosses in SC

Medial Reticulospinal: Function

Inn LMNs to proximal limb mm and axial mm

Lateral Reticulospinal: Decussation

Ipsilateral

Lateral Reticulospinal Function

Activate spinal CPGs for locomotion

Medial Vestibulospinal: Decussation

Bilateral

- Crosses in upper medualla

Medial Vestibulospinal: Function

Inn LMNs to neck and upper back mm to maintain stable gaze

Lateral Vestibulospinal: Decussation

Ipsilateral

Lateral Vestibulospinal: Function

Controls antigravity mm for posture and balance

Anterior Corticospinal: Decussation

Bilateral

- Crosses in SC

Anterior Corticospinal: Function

Inn LMNs to neck, shoulder, and trunk mm

Lateral Corticospinal: Decussation

Contralateral

- Cross in the pyramids of the caudal medualla

Lateral Corticospinal: Function

Controls fine, distal limb movements

Rubrospinal: Decussation

Contralateral

- Crosses in midbrain

Rubrospinal: Function

Facilitates rapid, coordinated movement of the UEs during reaching

- Adjusts hand to object

3 Functional Regions of Cerebellum

- Vestibulocerebellum

- Spinocerebellum

- Cerebrocerebellum

Vestibulocerebellum

The flocculonodular lobe

Spinocerebellum

Consists of the vermis (midline) and paravermis (intermediate hemispheres)

Cerebrocerebellum

Made up of the lateral hemispheres

What makes up the diencephalon?

Thalamus (4/5th) and hypothalamus (1/5th)

Thalamus Function

Regulates activity level of cortical neurons

Hypothalamus Function

Role in orchestrating visceral and drive-related activities

- ANS

Basal Ganglia (5)

- Caudate

- Putamen

- Globus pallidus

- Substantia nigra

- Subthalamic nucleus

4 Functions of the BG

- Self initiated movements to SMA motor areas

- Sequencing movements

- Regulation of m tone and force

- Cognitive aspects of movement

4 Functional Groupings of the Cortex

1. Primary sensory cortex

2. Sensory association cortex

3. Motor cortex

4. Association cortex

Primary Sensory Cortex

Discriminates intensities and qualities of sensory info

Sensory Association Cortex

More complex analysis of sensation

Primary Motor

Descending control of motor output

Motor Planning Areas

Organize movements before execution

Associate Cortex

Controls behavior, interprets sensation, and processes emotions and memories

Somatotopic Representation

Medial to lateral: foot, leg, trunk, arm, hand, face, tongue

Lateral Corticospinal Tract is aka ___?

M1

Lateral Corticospinal Tract: Primary start placr

Precentral gyrus

- Aka broadmann's area 4/M1

Other areas (besides M1) that contribute to the Lateral Corticospinal Tract?

PMA, SMA, CMA, and sensory cortex

Astereognosis

Can recognize objects by touch alone

Broca's Aphasia

Inability to produce speech

Wernicke's Aphasia

Inability to comprehend speech

Apraxia

Impaired ability to carry out (plan) motor activities despite intact motor function