The Motor System (Lecture 7) Flashcards
muscle contraction
- produced by motor neuron excitation
- basis of all movement
Motor Neuron Cell bodies
- located in ventral horn of spinal cord
- send axons via the ventral root to innervate muscle fibers
nerve
ventral root + dorsal root
flexion
- muscle contraction
- shortening of a limb
extension
- muslce contraction
- lengthening of limb
How do muscles generate force?
contraction
Types of Muscles
-flexors and extensors
Synergistic Muscles
muscles in which contraction produced the same movement
antagonistic Muscles
muscles with opposite functions
Neuromuscular Junction
- motor neuron releases ACh on muscle
- ACh activates nicotinic Ach receptors (opens Na+/K= channel
Depolarization leads to……
Muscle contraction
The Nervous system controls muscle contraction by:
- vary the firing rate of motor neurons
- increased number of motor neurons firing on synergistic motor units
Muscle Spindles
- proprioceptors deep within most skeletal muscles that provide information about muscle length
- sensory axons are wrapped around spindle
- enter dorsal root and form excitatory synapses on interneurons and motor neurons
Sensory axon
- wraps around spindle
- enter dorsal root and form excitatory synapses on internuerons and motor neurons
Mechanosensitive ion channels
-open and depolarize sensory axons when muscle is stretched
Sensory axon Excitation follow the excitation of….
motor neurons in the spinal chord
Monosynaptic Reflex Steps
- Knee jerk or patellar reflex
- tapping tendon stretches quad
- muscle spindle senses stretch
- sensory afferents activated
- motor neuron activated
- muscle contracted
Muscle State when No beverage is in glass
Muscle contracts enough to hold up the glass
Muscle State when beverage being poured into glass
-as beverage content increases weight of glass increases and muscle is stretched
Role of Muscle Spindle when Muscle is being stretched (from the increasing weight of a beverage in a glass)
- sends message to spinal cord
- synapses on and excites motor neurons
- causes more muscle contraction to support extra weight
Somatosensory Afferents
primary sensory neurons branch
- axons go through
1. dorsal colum up to brain
2. to interneurons in the spinal cord (gray matter) = important for reflexes
Reciprocal Innervation
-sensory afferents also synaps on inhibitory interneurons that connect to antagonistic motor neurons
-activation of muscle spindle leads to inhibition of antagonistic muscles
-prevents opposing muscles working against each other
37***
location of pain and temperature cell bodies
- dorsal root ganglion
- axons enter the spinal cord through the dorsal root
pain afferents divide into 2 branches
- travel a bit up and down spinal cord in Zone of Lissauer
2. Synapses, decussates and travels up the spinothalamic tract to S1
Withdrawl Reflex
- withdrwl of a limb from a painful stimulus
- polysynaptic
- pain afferents travel (up/down) in Zone of Lissauer = coordinate muscle contraction
- stimulate excitatory interneurons
- interneurons excite motor neurons
- muscles contract to withdrawal limb from painful stimulus
Motor Programs Theory
-sensorimotor system is programmed with patterns of activity = complex movements are produced by activating conbinations of these programs
Nacho Example for Motor Programs Theory
- Assocation cortex takes in the delicious site and smell of nachos and decides it is time to eat
- high level cortical programs activated (secondary motor cortex)
- lover level programs (brainstem) activated for reaching/putting chip into mouth
Brainstem programs activate spinal programs that control our muscles to complete the objective
Role of Brainstem programs
-activate spinal programs that control your muscles to complete objective
Activated programs continue
- once programs at each level are able to run without direct control from higher cortical levels
- YOU CAN EAT NACHOS WITHOUT EVEN THINKING ABOUT IT
motor equivalence
-same basic movement can be carried out in different ways using different muscles (e.g. you can eat nachos with your feet)
sensory information can be out of consciousness
-you may not have to consciously consider aroma of a food and just reach out and grab one
Practice can create motor programs
- you can learn to quickly and accurately toss a nacho into the air and carch it in your mouth
- motor programs can also develop without practice (you were born eating nachos)
The Motor System organization
-hierarchially
Motor output guided by….
Sensory input
What changes the nature and location of motor control?
Learning
Motor Association Cortex
Where movements begin
- posterior parietal
- dorsolateral prefrontal
Posterior Parietal Association Cortex
- receives info from the visual. auditory, and somatosensory areas
- determines position of body relative to objects it will interact with
- provides spatial information and attention
- sends output to –>
1. dorsolateral pfc motor association
2. secondary motor cortex
Damage: Posterior Parietal Association Cortex
- leads to apraxia
- disorder of voluntary movement
- unrelated to motor comprehension, or motivation deficit
contralateral neglect
- inabiliy to respond to stimuli contralateral to damage (left neglect)
- leads to someone neglecting half of a picture when drawing it
Dorsolateral Prefrontal Association Cortex (DLPC)
- recieves info from posterior parietal
- integrates information about stimuli
- PLANS APPROPRIATE MOVEMENT
- send information to secondary and primary motor cortex
Dorsolateral Prefrontal Association Cortex Neurons respond to….
- characteristics of object
- location of object
- both object and location
- decides what action are to be taken
DLPC : Planning Movement
-DLPC fires FIRST in the motor system = first step in motor planning
Prefrontal Cortex
-important for executive functions- select appropriate movement based on info from visual, somatosensory, and auditory input
Secondary Motor Cortex
-premotor cortex
-supplementary motor areas and 2 cingulate motor area
-receive info from motor association cortex
-programs patterns of movement after taking instruction from DLPC
neurons fire just before movement initiation and fire continuously throughout movement
-send info to primary motor cortex
-contains mirror neurons
Mirror neurons
- fire when an individual performs a certain goal directed hand movement or when they observe the same movement performed by another (Giacomo Rizzolati)
- posible mechanism for social cognition
- little known about human mirror neurons due to lack of non-invasive techniques ( fMRI)
Primary Motor Cortex
-also known as M1, precentral gyrus, brodmann's Area 4 the major (but not only) departure point of motor signals
Penfield’s Montreal Procedure
- stimulate cortex and watch muscles twitch
- showed that the cortex for sensorimotor system is somatotopic
Somatotopic
- more cortex devoted to body parts that make complex movements
- motor homunculus
Primary motor cortex conventional view
-neurons fire maximally to a certain direction of movement
Primary motor cortex current view
- stimulation of neurons elicits complex behaviors (feeding)
- neurons fire maximally to a particular endpoint of an action
- the route neural signals follow from m1 is extremely plastic and determined by somatosensory feedback
Neural signal route to m1
- plastic
- determined by somatosensory feedback
Cerebellum
- little brain
- 10% of brain mass but more than 50% of its neurons
- input from primary and secondary motor cortexes
- sensory input from Pons
- feedback from motor responses
- Projects to thalamus—->M1
- involved in timing, fine tuning, motor learning
- may also do the same for cognitive responses
Cerebellar Damage (cerebellar ataxia)
-uncoordinated, jerky movements
-intention tremor
-slurred speech
-lack of balance
-uncoordinated gate
(like being drunk)
Basal Ganglia 3 parts
Comprised of:
- caudate,
- putamen
- globus Pallidus
Basal Ganglia output/input
- input from cortex/ substania nigra (dopamine cells in midbrain)
- outputs to thalamus (which then brings information back to motor cortex
Basal Ganglia function
- amplitude
- direction
- initiation of movement
Basal Ganglia Damage
in Parkinson’s Disease:
- dopamine cells die decreasing dopamine input to the basal ganglia
- —> in balance in excitatory and inhibitory signals within the basal ganglia
- causes tremors, muscle rigidity, difficulty initiating and terminating movements
Substantia Nigra
- “black substance”
- dopaminergic cell bodies in the midbrain which project to the caudate + putamen (striatum)
Descending motor pathways
-carry information from the brain down the spinal cord
Dorsolateral Pathways
-deal with voluntary movements of distal limbs (fingers and toes)
ventromedial pathways
- posture and movement of trunk
- proximal limbs (shoulder/hip)
Control dista (far from center) muscles
= movement of the limbs
Dorsolateral cortico spinal tract
- begins at M1
- decussates at medulladescends in dorsolateral spinal cord white matter
- movement of individual fingers
Dorsolateral cortico-rubro spinal tract
- begins in M1
- synapses and decussates at red nucleus (midbrain)
- some neurons synapse on crabial nerves that innervate the face
- the rest descend in dorsolateral white matter
control proximal
-near to center muscles
trunk and proximal limbs
-posture and walking (whole body movements)
ventromedial cortico spinal tract
- begins in M1
- remains ipsilateral
- descends in ventro-medial spinal cord white matter
ventromedial cortico-brainstem-spinal tract
- begins in M1
- synapses at various brainstem nuclei (tectum, vesicular nucleus, cranial nerve motor nuclei)
- descends bilaterally
Motor Cortex Control Efferent Paths
- all descending paths are under control of M1
- M1 has direct projection to spinal motor neurons (dorsolateral and ventromedial corticospinal tracts)
- M1 also projects to other (indirect) descending pathways (red nucleus and brainstem nuclei)
smooth muscle
- in digestive tract and arteries
- automated by fibers of the ANS
striated muscle
- cardias muscle
- contracts rhythmically even without innervation
- has striped appearance
skeletal muscle
- a type of striated muscle
- allows movement of bones and eyes, respiration, facial expressions, speech
Innervation of Striated Muscle
- muscle fibers are sparsely innervated by axons from the spinal cord in a non overlapping fashion
- motor neuron excitation produces muscle contraction
the only way a muscle can generate force is with a…
contraction
