Chapter 7 - Movement

Animals’ and Humans’ movements are done by the way of __

Muscle Contractions

Three Categories of Vertebrate Muscles

1. Skeletal Muscles

2. Smooth Muscles

3. Cardiac Muscles

Skeletal Muscles

Vertebrate muscles that control movement of the body in relation to the environment (striated muscles/stripped-like); predominate in all muscle tissue

Smooth Muscles

Vertebrate muscles in visceral organs and blood vessels where in visceral organs: control digestive activity and in blood vessels: regulate the sizes of blood vessels

Cardiac Muscles

Vertebrate muscles in the heart that control cardiac output (pumping blood to the body) and has properties of both skeletal and smooth muscles

Bulks of Muscle (what are they comprised of?)

Comprised of many individual muscle fibers → each individual muscle fiber receives motor command from the motor neurons’s axonal terminal

Since one individual motor neuron’s axon has many branching terminals, an individual motor neuron can innervate many muscle fibers

Neuromuscular Junction

A special type of synapse where muscle contractions are triggered; and lies between an axons of the motor neuron and muscle fiber

Motor Neuron Cell Body (location)

In the ventral horn of the spinal cord gray matter

Acetylcholine

A neurotransmitter that is released from a motor axonal terminal, which causes the muscle to contract

Muscle Movement

Each muscle makes only one movement: a contraction. In the absence of muscle excitation, it relaxes

Antagonistic Muscles

Muscles that work in pairs to oppose each other by alternating contractions

i.e., one muscles contracts while the other in the pair relaxes

Agonist: the muscle that is contracting

Antagonist: the muscle that is relaxing

Flexor Muscles

Muscles that allow limbs to be flexed or raised

Extensor Muscles

Muscles that extend or straighten limbs

Bulk of Muscles (fast vs slow twitch)

Humans have fast- and slow-twitch muscles that are often mixed in a bulk of muscles having varying percentages of each

Fast-Twitch Muscle Fibers

Muscle cells that contract quickly and produce a lot of force for short periods of time; they fatigue after vigour use because the process is anaerobic

e.g., sprinting, and other quick movements

Anaerobic

A reaction that does not require oxygen

Slow-Twitch Muscle Fibers

Muscle cells that are responsible for endurance activities; produce less vigour contractions without fatiguing because the process is aerobic

e.g., long distance running

Aerobic

A reaction that requires oxygen

How can percentages of fast-twitch and slow-twitch muscles be “accommodated”?

Through training by increasing one type or the other depending on which one is used more often (plasticity)

e.g., competitive sprinters have more fast-twitch fibers for speed/acceleration instead of endurance

What is muscle contraction controlled by?

Proprioception; most purposeful

Proprioceptor (what is it? how does it control movement?)

A receptor located in muscles that monitors and controls the position and movement of the body by detecting the stretch and tension of muscles then sending messages that enable the motor neurons to adjust the controlling of muscles

Myotatic Reflex

An involuntary reflex that causes a muscle to contract when it is stretched; also known as the stretch reflex

e.g., tapping on the tendon of knee using hammer, the spinal cord sends a reflexive signal to contract

Two Kinds of Proprioceptors

1. Muscle Spindles

2. Golgi Tendon Organ

Muscle Spindles

A kind of proprioceptor that is parallel to the muscle that responds to a stretch, when muscles are stretched, their sensory nerves sends the stretch message to a motor neuron in the spinal cord. This message informs the muscles surrounding the spindle → evoking a contraction

Golgi Tendon Organ

A kind of proprioceptor that is located in the tendons at opposite ends of muscles; and responds to increases in muscle tension, which inhibits muscle contraction when it is too intense; acts as a “brake” against excessively vigorous contraction

Muscle Spindle and Golgi Tendon Organ regulate __ by the __

muscle contractions; myotatic reflex

Monosynaptic Reflex Arc

A reflex arc that facilitates direct communication between sensory and motor neurons innervating the muscle; evokes the ‘stretch’ reflex

A negative feedback loop used to maintain muscle length at a desired value

Myotatic Reflex (process from muscle stretch to the golgi tendon organ)

When a load is being added to muscles, the muscles are stretched, this invokes an afferent volley in the sensory axons to innervate muscle spindles

→ muscle spindles send the afferent volley to motor neurons in the spinal cord

→ an efferent volley returns to muscle, leading to the contraction of the muscle

→ contraction of the muscle stimulates the Golgi tendon organ, which acts as a brake to prevent contraction that is too quick or extreme

Muscle Tone

The degree of contraction of a muscle, measured as tension; (muscles are always under some degree of stretch to keep a length)

Muscle Tone (how can it be adjusted?)

By myotatic (stretch) reflex to maintain a desired length of muscles

Two Types of Movement

1. Involuntary

2. Voluntary

Involuntary Movements

Mostly reflexive movements that are automatic and consistent responses to stimuli

i.e., not affected by reinforcement, punishments, and motivations

Infant-specific Involuntary Reflexes (what are the three mentioned? what does it involve?)

1. Grasp reflex - grasps objects placed in hand

2. Babinski reflex - extends big toe and fans other when sole of foot is stroked

3. Rooting reflex - turns head and sucks when the cheek is stimulated

Allied Reflexes

Reflexes that are induced by the performance of another reflex

e.g., chewing reflex and salivation

What are many movements corrected by?

Feedback Adjustment

Ballistic Movements

A movement made as rapidly as possible; these movements are non-adjustable and not considered as purely voluntary; once initiated, this movement cannot be altered or corrected

Central Pattern Generators

Neural mechanisms in the spinal cord or brainstem that generate rhythmic patterns of motor output

e.g., breathing

Motor Programs

A fixed sequence of movements that is either learned or built into the nervous system from your life experience; once begun, the sequence is fixed, however thinking about it will interfere

e.g., yawning, or a skilled musician playing a peice

Primary Motor Cortex

A region in the precentral gyrus that initiates and executes complex voluntary movements by sending integrated (planned) motor command; and controls limb movements of the opposite side of the body

Primary Motor Cortex (how?)

Axons from the primary motor cortex (precentral gyrus) connects to the brainstem and the spinal cord, which generate impulses that control movements

Representation Areas

Stimulation at a point in the primary motor cortex evokes a movement of a part of the body indicated as representation areas, but not a contraction of an individual muscle

e.g., output of a given neuron influences a movement of the hand, wrist, or arm and not just a single muscle contraction

The motor cortex orders a command by __

Generating the right combination of muscles associated with a specific body movement

Where does planning a movement take place?

Outside of the primary motor cortex; in the posterior parietal cortex

Posterior Parietal Cortex (what does it keep track of?)

The position of the body relative to the environment and analyze information obtained; this areas just does sensory analysis without triggering movement

e.g., when a neurosurgeon stimulates this part during surgery, patients report an intention to move

Neurons in what areas are involved in the preparation and instigation of movemen?

1. Prefrontal Cortex

2. Premotor Cortex

3. Supplementary Motor Cortex

Prefrontal Cortex (what is it? relation to movement?)

The anterior part of the frontal lobe of the cerebral cortex where decision-making occurs; cognitively planning a movement

Responds to sensory signal that require the body to act; calculates predictable outcomes of actions and then plans movement

Premotor Cortex (what is it? relation to movement?)

A cortex located anterior to the primary motor cortex responsible for preparing for moving; receives information about the target muscles and integrates it according to the position and posture of the body provided by the posterior parietal cortex, plans provided by the prefrontal cortex, then organizes and directs sensory-guided movements

Supplementary Motor Cortex

A part of the motor cortex that specifically organizes rapid sequential movements in a specific order; active seconds before moving

e.g., pushing, pulling and turning a knob

Mirror Neurons

Neurons that are active during preparation of a movement and while watching someone else perform a similar movement

Readiness Potential

A particular type of activity in the motor cortices that occur before any type of voluntary movement; meaning that motor cortices produce a kind of readiness activity before any actual voluntary movement occurs

Begins at least 300 ms before the decision is reported → conscious decision is made at about 200 ms before the movement

Corticospinal Tracts

Paths from the cerebral cortex to the spinal cord

Two Corticospinal Tracts

1. Lateral

2. Medial

Lateral (or dorsolateral) Corticospinal Tract

A corticospinal tract that contains around 90% of the axons of the corticospinal tract (from the primary motor cortex and from red nucleus of the midbrain) and synapse in the contralateral spinal cord; controls FINE movements of the DISTAL limbs (e.g., hands, fingers, and toes)

Medial (or ventromedial) Corticospinal Tract

A corticospinal tract that contains the remaining 10% of the corticospinal tracts (originating from many parts of the cerebral cortex, not just the primary motor cortex); Axons of the medial cortical spinal tract go to both sides of the spinal cord (i.e., BILATERAL projections); controls the muscles of the neck, shoulders, and trunk

e.g., walking, turning, bending, standing up, and sitting down

Motor programs and motor coordination are modulated by the __ and __

Cerebellum; Basal Ganglia

The Cerebellum

The part of the brain most known for balance and coordination; contains 1) many folia, 2) four deep cerebellar nuclei (dentate, globose, emboliform, and fastigial), and 3) is subdivided into the anterior, posterior, and flocculonodular lobe

Ataxia

Incoordinating movement; the exploration of the cerebellum start with finding deficits of motor coordination due to cerebellar damage

Alternating Movement Deficit

People with cerebellar damage have trouble rapidly tapping a rhythm by clapping and flipping hands (along with other coordinating movements, such as pointing at a moving object, speaking, writing, typing)

Finger-to-nose Test Failure

A person is instructed to hold one arm straight out and then at command to touch their nose as quickly as possible

The Cerebellum functions to help execute sequential movements including:

Establishing and running motor programs via motor learning and training

Retrieving and then executing the learned and stored motor programs

When voluntary movements are initiated by the primary cortex, the cerebellar activity allows the execution of movements smoothly and coordinately

Attention Shift

The ability to shift attention and attend to visual stimuli

Purkinje Cells

Flat (two-dimensional) cells in sequential planes parallel to one another and the major type of cerebellar neurons that’s responsible for collecting motor data and integrating data received from the spinal cord, sensory system and cerebral cortices → with coordinated movements as the outcome

fact: more numbers here than in the rest of the brain combined (> 50%)

Parallel Fibers

Axons from the brainstem relaying information of spinal cord and cerebral cortices to the cerebellum; they are parallel to one another and perpendicular to planes of the Purkinje cells

Brain Mechanisms of Movement (from spinal cord to Purkinje cells)

Information of the spinal cord and cerebral cortices is relayed in the form of action potentials and are conveyed along parallel fibers → which excites Purkinje cell’s via cells tree-like dendrites

→ these activated Purkinje cells then transmit inhibitory messages to the cells in the four deep nuclei of the cerebellum and the vestibular nuclei in the brain stem

→ the messages are sent back to cerebral cortices and spinal cord, but are now integrated (“sculpted”)

Purkinje Cells

The greater the number of excited Purkinje cells, the greater their collecttive duration of response; as a result, the messages they receive are highly integrative and last longer → they can modulate motor activity in a temporal and spatial manner

The integrated and longer output are important for controlling the timing of movement, including onset and offset

The Basal Ganglia

A group of large subcortical structures in the forebrain comprised of caudate nucleus; putamen; and globus pallidus and is responsible for motor control

Two additional structures that join basal ganglia to control voluntary motor functions

1. Substantia Nigra

2. Subthalamic Nucleus

Basic loop for movement initiation

Excitatory Input from the cerebral cortex going to the caudate nucleus and putamen

→ Inhibitory Output from the caudate nucleus and putamen goes to the globus pallidus (releasing GABA, an inhibitory neurotransmitter, constantly inhibiting the thalamus)

→ then goes to the thalamus, which relays it back to the cerebral cortex, especially motor areas and the PFC

Basal Ganglia (function)

Selects a movement ceasing to inhibit the thalamus → motor cortex

This circuit produces an action called selection of motor program which is planned and initiated by the cerebral motor cortex

The circuit not only facilitates the selection of a motor program, but also suppress competing motor programs

When cortical signals are received and processed by the basal ganglia, the suppression of competing motor programs is reinforced and simultaneously, the activation of the particular thalamocortical circuits is facilitated that underlie the intended movement

PLAYS A PERMISSIVE ROLE

Brain Mechanisms of Movement

The globus pallidus is constantly inhibiting the thalamus when a motor command is not sent

Input from the caudate nucleus and putamen tells globus pallidus which movements to stop inhibiting and which not to

Huntingon’s Disease

A disease with involuntary, jerky movements resulting from extensive damage to the globus pallidus

Long-term Learning

Activity initiating in neurons of the motor cortex adjusts their responses as a person or animal learns a motor skill; after prolonged training, movement patterns become consistent from trial to trial — implicit memory

Motor Program

The pattern of activity becomes more consistent as a specific skill has been learned and increases neuronal firing when running the program

Parkinson’s Disease

Characterized by muscle tremors, rigidity, slow movements, and difficulty initiating physical and mental activity

Caused by a gradual and progressive death of neurons occurring especially in dopamine-containing neurons of the substantia nigra (decreased output from the substantia nigra due to DA neuron death means less excitation of the caudate nucleus and putamen, therefore less inhibition of the globus pallidus, which leads to stronger inhibition of the thalamus — slower movement

Parkinson’s Disease (genetic factors)

Studies suggest that early-onset Parkinson’s has a genetic link but not for the more common form that begins later in life

Parkinson’s Disease (Environment)

People exposed to hazardous chemicals, like in herbicides and pesticides, suffer from neurotoxic action on DA neurons

Parkinson’s Disease (Treatment)

L-Dopa is an immediate precursor to DA, commonly used for treatment

Parkinson’s Disease (Treatment Downsides)

1) The effectiveness varies with different patients (variation in intestinal bacteria that degrade L-dopa is suggested as reason)

2) its effectiveness is limited to early and intermediate stages

3) L-Dopa does not prevent the continued loss of DA-containing neurons (even contributes to the death fo neurons)

4) Produces harmful side effects

Huntington’s Disease

A hyperkinetic movement disorder that usually occurs between age 30-50 (early onset) and is associated with gradual and extensive damage to the basal ganglia, especially the globus pallidus

Initial symptoms include involuntary arm jerks and facial twitches → then progress to tremors and writhing that affect walking, speech and other voluntary movements

(failure to gate unwanted movements due to damage to the basal ganglia)

Huntington’s Disease (cause)

A gene mutation in an autosomal dominant gene known as Huntingtin; can be identified with almost 100% accuracy