Pt12 Somatic Motor System & Reflex Arc Flashcards
Describe the hierarchical organization of the motor system.
(CNS -> PNS)
1. CC (cerebellum & basal nuclei) = connections & feedback loops
2. UMN originate in brainstem & motor cortex
3. LMNs connect with interneuronal circuitry that connect to sensory nerves (reflex arc) or synapse in regions of brain
4. LMN connects w muscle
What are the functions of the spinal cord?
- Receiving & distribute info from PNS (sensory system)
- Relay afferent/sensory info to brain (sensory system)
- Integration of sensory & motor = reflex
- Relay efferent/motor info from motor center via tracts to connect with peripheral neurons (motor system)
>2 motor tracts = pyramidal & extrapyramidal
What are the 3 main functions of the motor system?
- Posture (somatic)
- Voluntary movement & locomotion (somatic)
- Controls visceral motor function (ANS)
What are the 2 divisions of the motor system?
-UMN
-LMN
Describe the organization of LMNs & UMNs of the somatic motor system.
-LMN = originate in ventral horn of spinal cord or in cranial nerve & synapse in skeletal muscle
-UMN = originate in brain & control LMN (inhibit/stimulate) located in CNS
What are the 2 divisions of the UMN?
- Pyramidal neurons
-corticospinal
-motor area of CC
-voluntary motor control in humans (skilled) - Extrapyramidal neurons
-brainstem
-gait & movements in quadrupeds
Describe UMN.
-initiate voluntary movement
-maintain muscle tone for support against gravity
-regulate posture
-loss of inhibitory UMN = increased muscle tone & spinal reflexes
-loss of facilitatory UMN = paresis or paralysis
Describe LMN.
-cell body in CNS (brainstem or spinal cord)
-axons project into PNS via cranial or spinal nerves
>striated or smooth muscle at neuromuscular junction
-when stimulated = muscle contraction
-muscle tone & bulk
-loss of LMN = paresis/paralysis with decreased/absent muscle tone & reflexes
What do the LMNs innervate?
Cervical & lumbar intumescence
What if all limbs show signs of LMN?
-lesion at neuromuscular junction
-lesions in spinal cord affect limbs caudal to lesion
If all 4 limbs are affected where is the problem at?
C1-C5 = UMN in both limbs
C6-T2 = LMN in front & UMN in hind
If only the pelvic limbs are affected where is the problem at?
T3-L3 = UMN in pelvic limb
L4-S3 = LMN in pelvic limb
KNOW THE SPINAL CORD REGIONS.
What is the function of the somatic motor system?
-supports body against gravity
-posture
-voluntary movement
COMPOSITION:
-cerebral motor cortex
-basal nuclei (habits)
-cerebellum (coordinates movement)
-thalamus (relay nucleus)
-brainstem
-spinal cord
-peripheral nerves
What do posture & movement depend on?
-voluntary = high brain center
-involuntary = spinal cord
What must the CNS do to control body movement?
-assess effect of gravity on the muscles
-determine initial position of the body parts to moved (proprioception)
-detect any discrepancy between intended movement & the actual movement
What are the 2 muscle receptors that provide the CNS with body movement info?
- Muscle spindle: special muscle fibers located in muscle mass
- Golgi tendon organ: specialized receptor located between muscle & tendon
Describe the reflex arc & its components (5).
-involuntary motor response of NS to a stimulus
-neuronal circuit directs motor response = ‘reflex arc’
1. Receptor
2. Sensory neuron
3. One or more synapses in CNS
4. Motor neuron
5. Target organ
Describe the stretch reflex/tendon reflex (ex. Patellar reflex).
-monosynaptic reflex
-afferent axons from muscle stretch receptor (muscle spindle) synapse w spinal cord alpha motor neurons = contraction
-reflex tests the integrity of the diff components of reflex arc & influence of descending UMN motor pathway
contract quad & relax hamstring
-counteracts gravity
-maintain equilibrium & posture
Describe the patellar reflex (quadriceps, knee-jerk reflex).
-tapping tendon of quadriceps femoris (distal to patella)
-tests extensor reflex mediated by femoral nerve
-reliable test tendon reflex
-cord segment: L4, L5, L6 (lumbar intumescence)
stretch reflexes & other muscle contractions depend on reciprocal inhibition - reflex phenomenon that prevents muscle from working against each other - inhibit the antagonists
Describe the golgi tendon organ role in the inverse stretch reflex.
-duration & force of muscle contraction
-receptor is sensitive to muscle tension increasing firing rate when the muscle contracts
-excites inhibitory interneuron in spinal cord
-inhibits Alfa motor neuron (causes contraction)
-leads to muscle relaxation
-prevents excessive tension on muscle
contract hamstring & relax quads
inhibits (in spinal cord) of motor neurons to extensor muscle & stimulated motor neurons to flexor muscles
What are the steps of the patellar reflex?
Stretch:
1. Muscle spindles detect stretch of muscle.
2. Sensory neurons conduct AP to the spinal cord.
3. Sensory neurons synapse with alpha motor neurons.
4. Alpha motor conduct AP to the muscle = contract & resist being stretched.
Describe the patellar & inverse stretch reflex.
- Sensory fibers:
-femoral nerve (L4, L5, L6) - Motor neurons:
-innervate muscles for both reflexes - Sensory input:
-one segment of spinal cord & influence motor neurons in the other segments
Describe the flexor reflex (withdrawal reflex).
-polysynaptic reflex
-all flexor muscles in a limb contracts
-alpha motor neurons to limb flexors stimulated
-extensors inhibited
-reciprocal innervation
-force & duration of reflex are proportional to intensity of noxious stimulus
assesses the integrity of sensory & motor reflex pathways that mediate flexion of thoracic & pelvic limbs
Describe the spinal cord segments involved in the withdrawal reflex.
-thoracic cord = C6-T2 (cervical intumescence)
-pelvic cord = L4-S1 (lumbar intumescence)
What does an intact reflex tell the clinician?
-the lesion doesn’t involve the area of the CNS or PNS
-reflex is present if neuroaxis cranial or caudal to reflex circuit has been damaged
-lack of overall inhibitory influence on LMNs
