ephe 380 lecture 5 sensory and motor neurons...cortex or brainstem nuclei: from frontal cortex: i....
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OVERVIEW Today
Sensory and Motor Neurons
Thursday
Parkinsons Disease Administra7on
Exam One Bonus Points Slides Online
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7 major descending motor control pathways from Cerebral Cortex or Brainstem Nuclei:
From Frontal Cortex:i. Lateral Corticospinal tractii. Ventral Corticospinal tractiii. Corticobulbar tract (à cranial nerve motor nuclei).
From Brainstem Nuclei:i. Rubrospinal tractii. Reticulospinal tractiii. Vestibulospinal tractiv. Tectospinal tract
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Interneuron Connections: (sometimes an intermediary before motor neurons)
1. Segmental interneurons – short branches within single spinal cord segment
2. Propiospinal neurons – projects for multiple spinal segments before synapsing onto motor neuron - long projections coordinating movements of upper and lower limbs
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Somatotopic Organization
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Lateral Corticospinal Tract:
Starts at the 1° motor cortexdecussation zone and lateral motornuclei of cervical and lumbrosacralCord.
Primary control of distal muscles
LATERAL DESCENDING PATHWAYS
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Rubrospinal Tract
Red nucleus (magnocellular) à midbrain decussation à lateral intermediate zone and ventral horn.
Provides additional control
LATERAL DESCENDING PATHWAYS
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Medial (Ventral) Corticospinal Tract
Axial and girdle muscles.
Ipsilateral ventral column à bilateral projections to medial grey matter.
Controls especially for head, shoulder, and upper trunk muscles.
MEDIAL DESCENDING PATHWAYS
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Reticulospinal Tract Many terminate in cervical cord.But, 2° projections to propiospinal neurons may influence lower axial muscles also.Pontine à ventral column.Medullary reticular formation à lateral column Autonomic movements: posture and repetitivemovements.
Tectospinal Tract From superior coliculus à coordinates head and eye movements.
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Vestibulospinal Tracts:Lateral vestibular nucleus à lateral vestibulospinal tract (to all spinal limbs)
Maintains balance.
Medial nucleus à medial tracts for control of head position (cervical only).
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Organization of Tracts ���Note the descending and ascending tracts and their relative
positions.
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Beta Motor Neurons Innervate intrafusal muscle fibers of muscle spindles, with collaterals to extrafusal fibres.
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Why is the motor neuron system set up this way?
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Sensory Pathways
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Sensory Receptors Transduce s1muli into electrical signals Three major divisions: 1. Interoceptor
Informa1on from within the body
2. Exteroceptor Informa1on from the environment
3. Proprioceptor Informa1on about the rela1ve orienta1on of body segments
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Receptors have a wide range of complexity…
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How do we get informa7on from mechanoreceptors?
S1mulus transduced to change in membrane poten1al
Receptor responds to specific energy form; but can respond to others…
Need adequate s1mulus to reach AP threshold
Specialized cells code strength
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S7mulus characteris7cs are coded S1mulus aOributes are preserved by 4 proper1es: 1. Modality –
Type of neuron
2. Loca1on – Which neurons fire? Somatosensory cortex very organized (topographical
map)
3. Intensity – Number of ac1vated receptors; frequency of APs
larger s1mulus = ↑’d receptor poten1al, ↑’d AP firing frequency
4. Dura1on – Coded by dura1on of AP trains in sensory neuron Longer a s1mulus persists, the longer the train of APs
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Coding for intensity and dura7on of s7mulus…
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Soma7c Senses 1. Touch
Cutaneous
2. Propriocep1on Muscle & joint receptors
3. Nocicep1on (pain, etc)
4. Temperature 1 & 2 play important roles in motor control
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Cutaneous Receptors
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Two major classes of cutaneous receptors: Tonic & Phasic
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Sensory receptors associated with muscles & joints…
Muscle: Muscle spindles
Golgi tendon organs
Joint: Joint capsule mechanoreceptors
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Types of Sensory Neurons
Type 1a Primary Rate of change of muscle Type 1b N/A in GTO Type II Secondary Fire when muscle sta7c
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Sensory Receptors in Muscle
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Muscle Spindles
Stretch-‐sensi7ve mechanoreceptors Provide info about muscle length & velocity of contrac7on to CNS
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Golgi Tendon Organ (GTO)
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Golgi Tendon Organ (GTO)
Receptors at junction of tendons & muscle fibers
Innervated by Group Ib afferents
stretching a muscle: pinching of sensory fibers -- neuron firing
contracted muscle: pulls tendon during contraction -- sensory neuron fires
GTOs are our major “force sensors” in skeletal
muscle
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How does the sensory system “know” about sensory feedback?
Segrega1on of informa1on… • Organiza1on based on the
anatomical loca1on of an input & the proximity of relay nuclei
Topographic organiza1on... • Motor & sensory maps
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Different groups of afferents have different projec7ons in spinal cord
• Size determines termina1on point within spinal cord
• Some inputs make monosynap1c connec1ons with MNs (remember muscle spindle pathways)
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Sensory Processing Brain is broken into different areas
Brodmann’s areas speech, touch, vision, motor areas, etc...
Specialized processing within different cerebral areas; e.g…
Rapidly adap1ng skin mechanoreceptors – Area 1
Slowly adap1ng skin mechanoreceptors – Area 3b
Thalamus receives & relays informa1on
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Func7onal areas of the cerebral cortex…
Bear et al. (1996) EPHE 380 -‐ Motor Control
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Dorsal Column Pathway
• Major somatosensory ascending pathway – Group I muscle afferents – Tac1le mechanoreceptors
• Terminate in the cuneatous & gracilis nuclei in medulla
• Eventually arrives in somatosensory cortex
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Somatosensory Cortex
• Subdivided into dis1nct regions, based on anatomical connec1ons & func1on – e.g. inputs from similar regions & receptors are grouped together
– Columnar organiza1on
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Somatosensory Cortex
• Organized into columns based on anatomy – e.g. first, second digit
• Organized into subcolumns based on modality – e.g. SA & RA receptors
• Organized into layers based on connec1ons in brain – e.g. thalamic inputs/outputs
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Mapping the cortex…
• There are both sensory AND motor maps... – HOMUNCULUS (see also Kandel et al. Fig 38-‐1)
• Distor1ons in size representa1on – why?
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Sensory receptors: What’s important?
Cutaneous Receptors
-‐ Meissners Corpuscles
-‐ Merkel Discs
-‐ Ruffini Endings
-‐ Pacinian Corpuscles
Nociceptors
Chemoreceptors
Muscle Receptors
-‐ Muscle Spindles
-‐ Golgi Tendon Organs
Joint Receptors
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Bo^om line?
• Feedback from joint receptors, muscle spindles, GTOs, & cutaneous afferents all play roles in kinesthesia & propriocep1on
• Interac1on between movement goal & feedback