Motor systems combined

Reflexes

• Simplest

• Largely involuntary 

• Rapid and somewhat stereotypes

• Ex. Sneezing, knee, flexor withdraw

Voluntary Movements:

• Goal-oriented and purposeful

• Initiated in response to external stimuli or will

• Largely learned and improved with practice

• Ex: playing piano, writing 

Rhythmic Motor Pattern

• Starting and stopping are voluntary 

• Once started, consist of sequence of stereotyped, repetitive movements performed in a reflex-like manner

• Ex: walking, running

List the tasks that the motor system must be able to perform

• Basic Task: muscle contraction and relaxation

  • Additional tasks:

  • Convey times commands to variety of muscles that control precise movement of a variety of joints

  • Make postural adjustments in preparation for movement 

  • Continuously modify commands during movement to adjust for changes in position of the body

• Describe the 2 organizational features of the system that allow it to perform these tasks 

• 1. Continuous inflow of sensory information about the body and environment allows selection of appropriate movement and modification of on-going movements 

• 2. Motor system organized as a hierarch (draw)

Higher Centers Function

Generates the "general intention to move"; communicates with the middle level via hypothetical "command neurons

Mid-level Function

Generate motor program with input of starting body position with commands from the highest level

Types of interneurons that synapse with motor neurons at the local level of CNS

can be excitatory or inhibitory

Synapse on motor neurons

Act as a switch as higher centers can turn movement on or off to override spinal reflexes

why motor neurons are referred to as the “final common pathway” in motor function

• All motor function ultimately depends on motor neurons that innervate skeletal muscles 

• Once a motor neuron fires and AP you will have a muscle contraction

Motor nucleus

a group of motor neurons that innervates particular group of muscles.

Arrangement of motor neurons within the SC gray matter

Cell bodies are in some cranial nerve nuclei and in the ventral horm of the spinal cord gray matter, running its length

The muscles controlled by the motor neurons located in the medial and lateral motor nuclei of the enlargements

• Medial motor nuclei: in Gray H control axial muscles and concerned with posture and balance

• Lateral motor nuclei control found only in cervical and lumbar SC and control muscles of arms and legs

Define reflex

a simple, relatively stereotyped response to a specific type of stimulus

Reflex arc

 the neuronal circuit responsible for the reflex

PNS —> interneurons (CNS) —> target cells (PNS)

Describe the structure of the muscle spindle and the type of information about a muscle that is detected by the spindle.

• Muscle spindle: a monitoring system that detects changes in muscle length

  • Encapsulated in muscle cells

  • Arranged parallel to regular muscle cells 

  • Receptor is outside the muscle instead of internal

  • Fuses at afferent terminal in tendon

  • Function: monitor muscle length 

Myotatic Reflex

• 1. Primary afferent from spindle synapses with alpha motor neuron SC

• Path A: primary afferent synapses on the alpha-motorneuron, so the stretched muscle contracts

• Path B (reciprical innervation): another branch inhibits motor neurons to the antagonist muscle. Lets the agonist contract without opposition

• Path C: activates synergistic muscles that assist the agonist

• Path D: goes to the brain with information about the adjustment

  • Agonist opens the joint

  • Antagonist closes the joint

Describe the arrangements for reciprocal innervation and how this arrangement assists the myotatic reflex.

prevents opposing muscles from fighting each other, ensuring a smooth and effective corrective movement.

Describe the inverse myotatic reflex

Uses the Golgi tendon organ for sensory input

• Circuit (Path A): the afferent synapses on an inhibitory interneuron in the cord.

  • Increased firing inhibits the motor neuron to the contracted muscle, causing relaxation.

  • Decreased firing releases that inhibition, so the muscle contracts.

• Other branch (Path B): another example of reciprocal innervation of the antagonist muscle.

effects of muscle contraction/lengthening on inverse myotatic reflex

• Primary afferent from Golgi  organ fires more rapidly when muscle contracts and tugs on tendon

• Primary afferent fires less rapidly when muscle relaxes

Describe the relationship between the myotatic and inverse myotatic reflexes in terms of maintaining an upright posture.

When the quadriceps lengthens, the spindle reflex drives contraction. The reduced Golgi firing also removes inhibition, so the muscle contracts and the leg stays extended.

Stimulus that triggers the flexion withdrawal reflex

Painful stimulus

events that occur in the ipsilateral flexor and extensor muscles during flexion withdrawal reflex

• Flexor withdrawal reflex (the most powerful flexion reflex): on the stimulated side, the ipsilateral flexor muscles contract (pull away) and the ipsilateral extensors are inhibited (relax).

• Crossed extensor reflex (activated simultaneously): on the contralateral side, extensors contract and flexors are inhibited. The slides don't state the purpose. The standard answer is that it supports body weight on the opposite leg while the stimulated limb withdraws.

Importance of the spinal and supraspinal postural reflexes to voluntary movements

• Supraspinal reflexes respond to visual, proprioceptive, vestibular and other sensory information to help maintain posture, in addition to the spinal postural reflexes.

• Collectively they provide a foundation of automatic responses on which more complicated voluntary movements are built

function of sensorimotor complex

• Designates all areas of the cerebral cortex involved in muscle movement

• Plays critical role in planning and on-going control of normal voluntary movements 

• Functions at both the highest and middle levels of the motor control hierarchy 

inputs and outputs of sensorimotor cortex

• Inputs:

  • Somatosensory information directly from thalamus

  • Processed sensory information (visual, auditory, touch/pain) from association cortices

  • Other parts of the motor control hierarchy (cerebellum, basal ganglia)

• Outputs:

  • Descending motor pathways (lateral corticospinal, ventral, corticospinal, etc.)

  • Other parts of motor control hierarchy (cerebellum and basal ganglia)

motor homunculus

• Principle: the finer the movements performed by a part of the body, the greater the area of the primary motor cortex devoted to it (huge on lips and hands, smaller on trunk)

Premotor Cortex function

• Gives rise to medial descending pathway that controls trunk and head muscles

• Concerned with posture

• Participates in the preparation for movement 

Supplementary Motor Cortex function

• Complex movements (not simple)

• Planning of the sequents of complex movements

• Coordination of voluntary movements and 2 sides of the body for bi-manual coordination

Parietal Association Cortex function

Provides processed multisensory information to other regions of sensorimotor

Motor Potentials

Wave of electrical activity in primary motor cortex that occurs after readiness potential and before voluntary movement begins

Readiness Potential

Wave of electrical activity in supplementary motor cortex that is earliest indication a voluntary movement is about to occur (preparation for movement)

Basal Ganglia Disorders

Parkinsons

Huntington’s

Parkinsons Disease

• Caused by dopaminergic neurons in Substantia nigra

• Effects Basal Ganglia by increasing inhibitory GABAergic outputs to thalamus and motor cortex

• Result: suppression of motor behavior

• Signs:

  • Reduced amount/slow movement 

  • Muscular rigidity

  • Unemotional facial expression

  • Difficulty stopping ongoing activity and starting new behavior

  • tremor

• Treatments:

  • Dopamine precursor

  • Deep brain stimulation 

  • Experimental transplantation (genetically engineered cells driving dopamine synthesis)

Huntington’s Disease:

• Inherited → loss of GABAergic and colinergic neurons of basal ganglia

• Degeneration of cerebral cortex → dementia

• Flicking movements or extremities and facial muscles

• Facial grimacing 

• No treatments

Function of Cerebellum

• Motor planning by integrating information about intended movement with information about starting position and surroundings

• Comparator to compare information about actual movements with what was intended 

• Corrects discrepancies between actual and intended movements by sending an error message to motor and subcortical centers to modify on-going movement

Inputs of cerebellum

• Sensorimotor cortex

• Higher senses (eyes)

• Receptors in joints, skin via brainstem and integrates these inputs to fine-tune motor activity

Outputs of cerebellum

• Brain stem nuclei

• Sensorimotor cortex

Corticospinal Pathway

• Corticospinal pathways originate in motor cortex, cross midline. Controls muscles on the opposite side of the body 

• Cell bodies in sensorimotor cortex

• Axons descend and form 2 tracts as pass through medulla

  • 80% cross to contralateral side of CNS to form later CST in the spinal cord

  • Remaining 20% continue on the same side to form ventral CST in SC

Corticobulbar Pathway

• Originates in motor cortex and terminates on target neurons in the brainstem

• Control motor neurons that innervate muscles of the eye, face, tongue, and throat in addition to head and neck movement

• Performs same functions for the muscles of the head and neck as corticospinal does for the muscles of the rest of the body