Class 6 Flashcards
1
Q
Cortical areas associated with motor processes and descending tracts
A
- Frontal lobe
- Precentral gyrus, primary motor cortex (M1), Area 4
- Adjacent cortex, supplementary and premotor cortex (Area 6)
- Parietal lobe
- Post-central gyrus, primary somatosensory projection area (S1), Areas 3-1-2
- Second somatosensory, Area 5
2
Q
humunculus
A
- Map of body on lateral surface of M1 and S1 cortex
- Distortion in map represents number of cortical cells projecting to muscles of a particular body part
- Much larger areas associated with muscles of face, speech apparatus, hands (precision of motor control)
- Electrical stimulation of a specific area within motor cortex yields muscle contraction of specific body part; damage (e.g. stroke) produces paralysis of those muscles
3
Q
What is the corticospinal tract?
A
Direct pathway to spinal cord; output from motor cortical areas (“pyramindal” tracts)
4
Q
Describe the corticospinal tract.
A
- Crossed: lateral corticospinal tract
- Projects to motor neurons and interneurons supplying muscles (eg. hands)
- Mediates fine motor control of limbs and hands (e.g. reaching and manipulation)
- Uncrossed: ventromedial corticospinal tract
- Projects to spinal interneurons which, in turn, project to motor neurons supplying proximal muscles
- Mediates axial movement and postural control
5
Q
Describe the indirect pathways to the spinal cord.
A
“Extrapyramidal”; more precisely designated as corticobulbar projections to brain stem nuclei which then give rise to several descending pathways (e.g. cortex to red nucleus: rubrospinal tract; cortext to reticular nuclei: reticularspinal tract)
6
Q
Describe the motor tracts originating in the motor cortex.
A
- Corticospinal tract
- Lateral corticospinal tract: fingers, hands, arms, lower leg, and feet
- Ventral corticospinal tract: trunk and upper legs
- Corticobulbar tract
- Face and tongue
7
Q
Describe the motor tracts originating in the subcortex.
A
- Ventromedia tracts
- Vestibulospinal tract: leg and lower trunk movement, control of posture
- Tectospinal tract: head and upper trunk movement, visual tracking
- Lateral reticulospinal tract: flexor muscles of the legs
- Medial reticulospinal tract: extensor muscles of the legs
- Rubrospinal tract
- Hands, lower arms, lower legs, and feet
8
Q
What are the inputs to the motor cortical areas?
A
- Cortical areas associated with sensory/perceptual processes (e.g. visual, somatosensory, auditory)
- Cortical/subcortical processes associated with motivational processes (limbic system)
- “Loops” involving the cerebellum and basal ganglion
- The basal ganglia or the cerebellum do not give rise to pathways that directly descend to the spinal cord
- Output from these structures is to cortical motor areas or brain stem nuclei
9
Q
Describe the function, parts, and disorders associated with the basal ganglia.
A
- Integrates movement and control of posture
- Caudate nucleus, putamen, globus pallidus
- Disorders: cerebral, Parkinson’s disease, Huntington’s disease, ballismus
10
Q
Describe the inputs to and outputs from the cerebellum.
A
- Inputs: climbing fibers, mossy fibers
- Outputs: purkinje cells, basket cells (inhibitory influence)
11
Q
Cerebellar damage may cause…
A
- Intentional tremor: at onset or termination of movement
- Disruption of coordination (asynergies, dysarthrias)
- Locomotor ataxia and postural disturbances
12
Q
Describe spinal cord output motor neurons.
A
- Cell bodies located in anterior gray (ventral horn)
- Axons exit cord through ventral root (motor) and join with dorsal root (sensory) to form spinal nerve (mixed) to supply muscles and sensory receptors
- Axon endings terminate on motor end plate of muscle fiber
- Release ACh, opens Na+ channels, causes AP, muscle contracts (shortening of muscle fiber brought about by interdigitation of action and myosin - sliding filament theory)
13
Q
Describe neuromuscular relationships.
A
- A given muscle fiber typically supplied only by one motorneuron
- Motor neuron pool: all motor neurons innvervating one muscle
- Motor unit: motor neurons terminate on many muscle fibers (endings diverge); all muscle fibers supplied by one motor neuron are activated as a unit when impulse travels down axon
- Innervation ratio: number of muscle fibers supplied by one axon (10:1 yields less precise force gradation than 3:1)
14
Q
What are the two types of motor neurons?
A
- alpha motor neurons: terminate on extrafusal fibers resulting in the generation of force and motion at joints
- gamma motor neurons: terminate on intrafusal fibers attached to the muscle spindle (a sensory receptor) resulting in regulating the sensitivity of this receptor (efferent control of afferent input)
15
Q
Describe the gamma motor system.
A
- Small motor neurons originating in the spinal cord
- Innervate the distal ends of intrafusal fibers
- This contraction actiates the stretch reflex
- The result is increased muscle tone
17
Q
Describe the monosynaptic reflex (myotactic stretch reflex).
A
- Muscle stretch is transduced by muscle spindle (a sensory receptor) and leads to action potentials in Group IA afferents
- Group IA afferents enter cord through dorsal roots and synapse with MNs that innervate that same muscle (as was stretched) causing contraction
- Monosynaptic stretch reflex: maintains position; intrafusal fibers serve as detectors
- When stretched, receptor activated; sends signals to motor neuron activating extrafusal fibers; their increased contraction reduces stretch; works because connected in series with extrafual fibers
- Reciprocal innervation: muscles arranged in pairs that act in opposite directions at a joint (agonists/antagonists); for joint displacement, agonist/antagonist MN pools activated in opposite direction (excitation of agonist MN, inhibition of antagonist MN)
18
Q
Describe the multisynaptic reflex.
A
- Flexor withdrawal reflex
- Sensory inputs from pain receptors (or pressure and thermal receptors at high-intensity stimulation) terminate on interneurons
- After interneuronal synapses, input to MN that broadly activate flexor muscles and inhibit extensor (on same side as input; reverse pattern on opposite side of body)
- Involves 3 or more neurons; receptor sends signal to spinal cord and synapses on interneuron; interneurons may excite and/or inhibit to accomplish a withdrawal reflex
