Neuro Week 9 - Somatosensory Pathways (Part II) & Descending Motor Control

what happens as you use more of one finger than the others

the region of your somatosensory cortex that is responsible for that finger or limb will expand

what kinds of pain is short-lasting pain

causes us to withdraw from the source, reducing further damage

what kinds of pain is long-lasting pain

promotes

behaviours that speed

recuperation (e.g., sleep,

inactivity, feeding, drinking)

what kinds of pain is Expression of pain

serves as a

social signal about potential

harm and elicits caregiving

anterolateral system

contains several defined tracts related to pain processing (e.g., spinohypothalamic tract, spinomesencephalic tract, spinoreticular tract)

spinothalamic tract

spinal cord → thalamus → somatosensory cortex

Sensory-discriminative (first pain)

pathway determines the sensory quality of the pain (e.g., throbbing, gnawing, shooting)

Affective-motivational (second pain)

pathways determines the emotional quality

of the pain (e.g., tiring, sickening, fearful)

role of free nerve endings

axons that terminates in the skin without any specialized cells

associated with it and that detects pain and/or changes in temperature

Nociception

involves specialized neurons responsive to the painful stimulus range

what is the threshold for pain from heat stimulus

43 degrees Celsius

when do Nociceptors start to fire action potentials and when do thermoreceptors start/increase

at 43 degrees celsius increasing firing rate with increasing temp whereas thermoreceptors respond at the same rate after pain reached (reaches a ceiling for action potentials)

what is the difference between first and second pain

First pain: early perceptions

of sudden, sharp pain

• Second pain: later sensation

of a duller, burning quality

Transient Receptor Potential (TRP) ion channels

respond to pain and temp

why do peppers taste “hot”

many widely used spicy peppers contain a chemical capsaicin

An isolated receptor TRPV1 on free nerve endings that responds to sudden increases in temperature, was also found to bind capsaicin

same for cool temps: • TRPM8 aka cool-menthol receptor 1 (CMR1) like when you eat menthol in minty things

what is TRPMS (CMRI) cool receptors fibers

C fibers (unmyelinated) which is why it takes longer for you to taste the cool minty taste

what are the TRPM3 free nerve ending receptors

A fibers (large and myelinated therefore detect sudden sharp pain)

where is the first synapse do nociceptive afferent neurons terminate?

directly in the spinal cord on first order neurons: (dorsal horn)

• C-fiber afferents terminate in Rexed’s laminae I and II of the dorsal horn

• Aẟ fibers terminate in laminae I and V.

what happens in second-order neurons

second-order neurons cross the midline (via the anterior white

commissure) right away and then ascend to higher levels within the anterolateral column of the spinal cord. terminates in thalamus (spinothalamic tract)

third order neurons

reside in the VPL (ventral posterior lateral nucleus) of thalamus

- Axons travel in internal capsule

- Terminates in ipsilateral postcentral gyrus (primary somatosensory cortex)

Given a unilateral lesion to lower thoracic cord (hemisection). What clinical signs would you expect? Why?

• dorsal column: loss of fine touch ipsilaterally on the lower limbs (mechanoreceptive)

• anterolateral column: contralateral pain and temperature information from the lower limbs (nociceptive afferent -

  • contralateral because the nociceptive afferent neurons cross the spinal cord immediately)

what controls the skeletal muscles

spinal cord

muscles are arranged in _____ ______

reciprocal fashion

muscle antagonists

a muscle that counteracts the effect of another muscle

neuromuscular junction

the region where a motor neuron axon terminal

and adjoining muscle fiber meet. This is the point where the nerve transmits its

message to the muscle fiber

where do motor neurons leave the spine from?

ventral horns

Muscle fiber:

a large cylindrical cell that can contract in response to

neurotransmitter released from a motor neuron (actin and myosin are mediators for muscle fibers)

motor end plates

part of neuromuscular junctions but they are very wide axon terminals

final common pathway

the sole route through which the spinal cord and brain can control our

many muscles and coordinate our behaviours

pro of end plate

floods the neuron with acetylcholine making sure this synapse will happen

lower motor neurons (LMN)

Commands for movement, whether reflexive or voluntary, are ultimately conveyed

to the muscles by the activity of LMN

how are cell bodies in the ventral horn are distributed in a ______ ________

somatotopic fashion

stretch reflex

causes muscle contraction in response to stretch of the muscle

how does your arm respond to sudden weight drop on your hand?

bicep is flexed and motor nerve A sends excitatory signals to bicep to continue flexing to hold the weight whereas another nerve, nerve B sends inhibitory signals to the triceps to not contract

lower motor neuron syndrome

lesion of the LMN causes flaccid paralysis

• characterized by a loss of muscle tone (hypotonia) and areflexia (loss of reflexes)

  due to the interruptions of LMN input and to the sensorimotor reflex arcs

• Long-term effect: atrophy (wasting away) of the affected muscles due to long term denervation and disuse

hypotonia

loss of muscle tone

areflexia

loss of sensory reflexes

muscle atrophy

wasting away of the muscle because of no use

when there is a legion to the ventral horn, is the damage ipsilateral or contralateral?

ipsilateral 

flaccid paralysis

Damage to LMN (lower motor neuron) cell bodies, or their peripheral axons going out to skeletal muscles, results in a type of paralysis/paresis

what layer of primary motor cortex are first-order neurons found in the pyramidal system and what are they called?

layer 5 called Betz cells referred to as Upper Motor Neurons

corticobulbar tract

cortex → brainstem

non-primary motor cortex

frontal lobe regions adjacent to the primary motor cortex that contribute to motor control and modulate the activity of the primary motor cortex

• includes the SMA and premotor cortex

supplementary motor area (SMA)

lies mainly on the medial aspect of the hemisphere; important for planning movements that are internally generated

premotor cortex

anterior to M1(precentral gyrus) (Brodmann’s area 6); important for directing movements in response to external cues

describe the corticospinal and corticobublar patwats

first order neurons: cell bodies in motor cortex

corticospinal: cortex → internal capsule → cerebral peduncles → pons → medulla (pyramids) then decussate and form lateral corticospinal tract in the spine (through lateral white matter tracts) → spinal cord

bulbar: cortex → internal capsule → cerebral peduncles (end up in brainstem as cranial nerves through the red nucleus or reticular formation

lateral corticospinal tract

90% of axons that cross midline

• skilled movement and distal limb muscles

• largest in vertebrates 

anterior (ventral) corticospinal tract

• 10% of uncrossed axons

• terminate bilaterally in ventral horns (branches cross within ventral white commissure of the spinal cord to reach the opposite ventral horn).

• posture, balance, locomotion of axial and proximal limb muscles

where do first synapse occur for lateral cortical spinal neurons compared to anterior cortical spinal neurons?

Lateral: doesn’t occur until axons terminate on lower motor neurons within ventral horn of spinal cord (after decussation in pyramids)

Anterior: doesn’t occur until axons meet the area where they will synapse on a lower motor neuron (is bilateral so doesn’t really decussate)

second-order neurons of lateral and anterior corticospinal tract

lateral: ventral horn lower motor neurons that send peripheral axons to contract muscles particularly in distal limbs like arms and legs)

anterior: synapse bilaterally through the anterior commissure in the spinal cord to reach the opposite ventral horn

upper motor neuron syndrome

1. immediate syptoms, spinal shock)

• Flaccid paralysis of affected muscles similar to LMN syndrome

2. Later symptoms (several days):

   • Spastic paralysis of affected muscles (constant contraction)

   • Hyperactive stretch reflexes

   • Hypertonia

   • Impairment in fine movements,

   gross movements recover

How would facial expression be affected by lesion A, B, and C

the pyramidal system

carries descending signals from motor neurons in cortex to the brainstem or spinal cord

• function: voluntary skilled movements