Bell-Magendie law

ensory information enters the spinal cord through the dorsal root; motor commands leave through the ventral root Example: the knee-jerk reflex

Actions are not just chains of reflexes;

theyre organized sequences, planned in advance  

Movement relies on

constant feedback and on anticipation – the brain predicts what will happen before it happens  

Hierarchy of movement control

A concept referring to the structured organization of motor control, where higher brain centers plan and supervise movements while lower levels execute them. It emphasizes the coordination and integration of sensory feedback to refine motor actions.

Spinal cord

controls the body’s skeletal muscles, including reflexes, in response to sensory input  

Brainstem

relay commands to the spinal cord and directly controls muscles of the face, head, and neck 

Primary motor cortex (M1)

issues the main commands for voluntary movement  

Nonprimary motor cortex

plans movements; sends commands to M1 and also directly to the spinal cord 

Basal ganglia and cerebellum

don’t send commands themselves; they adjust the system, through loops via the thalamus back to cortex  

3 types of muscle

Skeletal muscle, smooth muscle and cardiac muscle

skeletal muscle

moves the body; under voluntary control 

smooth muscle

lines the digestive tract, blood vessels and other organs  

Cardiac muscle

the heart; striped (striated) like skeletal muscle but involuntary like smooth muscle

smooth and cardiac muscle are controlled by

the automatic nervous system

Skeletal muscles are controlled

by motor neurons from the spinal cord and brainstem

skeletal muscles move

the skeleton at joints —
Muscles can only pull, not push; to reverse a movement, the opposing muscle must contract  

Movement requires

antagonistic muscles – opposing pairs that contract in alternation (e.g., biceps and triceps)  

flexor muscle

bends a joint (e.g., biceps)

Extensor muscle

straightens a joint (e.g., triceps)  

muscles are made of

many individual fibers

how many motor neurons do each muscle fiber receives

receives input from only one motor neuron

one motor neuron can innervate how many fibers

multiple fibers at once

Neuromuscular junction –

the synapse between a motor neurons axon and a muscle fiber, works much like a synapse between neurons  

Motor unit –

one motoneuron and all the muscle fibers it innervates  

small motor units

few fibers per motor neuron - fine control e.g., eye muscles

Large motor units

(many fibers per motor neuron) - coarse, powerful control; e.g., leg muscles, hundreds of fibers per neurons  

Motor neurons release

ACh at the motor endplate, activating nicotinic ACh receptors ad depolarizing the muscle fiber  

what does the release of motor neurons trigger and what does it release

an action potential in muscle fiber, which releases Ca2+ from stores inside the fiber  

after the action potential in muscle fiber releases Ca2+ what does it do

let actin and myosin "cycle", sliding flaments past each other and contracting the muscle  

what are 2 types of skeletal muscle fibers

Extrafusal muscle fibers and intrafusal muscle fibers

Extrafusal muscle fibers do and control what

do actual work of contraction – controlled by alpha motor neurons  

Intrafusal muscle fibers

are small muscle fibers inside muscle spindles  

Muscle spindles are

sensors that respond when the muscle is stretched  

Sensory nerve endings around the muscle spindle signal what

muscle length and stretch to the spinal cord and brain  

Gamma motor neurons

contract the intrafusal fibers to adjust the spindles sensitivity  

Proprioception

sensory feedback from muscles and joints that tells you where your body is in space and how its moving

what did Ian Waterman lose

Proprioception - his motor commands still worked, but without feedback he couldn’t control his movements 

what are two major proprioceptive receptors

Muscle spindle and Golgi tendon organ

Muscle spindle

senses muscles stretch (length)

Golgi tendon organ

senses muscle tension (force)  

what do muscle spindles contain

specialized intrafusal muscle fibers

what do sensory nerve endings respond to when muscles stretch is sensed

respond to changes in spindle length and relay this to the spinal cord

what do gamma motor neurons contract when the muscle stretches

contract the intrafusal fibers, keeping the spindle sensitive as the muscle changes length  

golgi tendon organs are

sensory nerve endings in the tendon, where muscle attaches to bone

what activates Golgi tendon organ

very strong muscle tension which inhibits alpha motor neurons and helps prevent tearing and damage

how the brain controls motor output

the brainstem directly controls muscles of the face and head thought the cranial nerves

for all other muscles, the brain works through motor neurons to the spinal cord

two major descending pathways that reach the spinal cord

the pyramidal and extrapyramidal systems

pyramidal system mainly the corticospinal tract runs through and controls what

runs from motor cortex - mainly M1- to the spinal cord pass thru the pyramids of the medulla

controls fine movement, especially of the hands and feet

most axons cross through the medulla at the pyramids, so each side of the brain controls the opposite side of the body

where is primary motor cortex - M1 - located

in the precentral gyrus of the frontal lobe; just in front of the central sulcus

Main source of the pyramidal system

M1 neurons send axons down the corticospinal tract to alpha motor neurons in spinal cord  

M1codes what

movement and their direction; not just individual muscles

neuronal responses in the M1 precede

movement — M1 neurons start firing before the arm moves - they’re issuing a command not reacting

Nonprimary motor cortex assists

complex behaviors

premotor cortex

movements guided by external cues (reaching for something you see)

Supplementary motor area (SMA)

movement sequences that are internally driven; active seconds before the movement begins

what in the nonprimary motor cortex sends commands to M1 and directly to the spinal cord

Premotor cortex and supplementary motor areas - not purely motor system

Extrapyramidal tracts

descending tracts that run outside the pyramids of the medulla, mostly starting in the brainstem  

Lesions of the extrapyramidal systems do not typically interfere with

individual movement, but instead interfere with other aspects of motor function

it disrupts posture, muscle tone, and the smoothness of movement

The extrapyramidal system is actually

a misnomer - it has several separate tracts, each with its own job

3 tracts of the extrapyramidal system

Reticulospinal tract, Rubrospinal tract, and the Vestibulospinal tract

reticulospinal tract

controls posture through trunk and leg muscles

from reticular formation in brain stem to spinal cord

Reticular formation

a network of neurons running through the core of the brainstem (medulla pons and midbrain); involved in posture, arousal, and basic function like breathing

Rubrospinal tract

controls arm movement; receives input from M1 and the cerebellum — it helps control limbs but in humans the pyramidal system does this the most

from the red nucleus to the spinal cord

Vestibulospinal tract

uses vestibular input to stabilize the head and adjust posture; from the brainstem

Basal ganglia

a group of nuclei deep in the brain that influence movement

Basal ganglia receives input from

much of the cortex and send output through the thalamus back to motor cortex — together they form a loop: cortex→ basal ganglia→ thalamus → cortex

3 parts of Basal ganglia

Caudate nucleus, putamen, and globus pallidus

2 important associated regions near basal ganglia

subthalamic nucleus and substantia nigra

The caudate and putamen (together make the striatum) receive and send what

receive input from M1 and other cortical areas

send inhibitory projections to the globus pallidus

After the globus pallidus receive inhibitory projections from the striatum, it does what

The globus pallidus inhibits the thalamus reducing its excitation of M1 and the SMA

to select a movement, the striatum inhibits

the globus pallidus which releases the thalamus so the cortex can act. This is inhibition of inhibition (disinhibition)

Function of the Basal Ganglia

are best known for starting self-initiated movements – those not triggered by a stimulus (like the SMA)

Critical for learning motor skills and organizing sequences of movement 
Also involved in "automatic" and habitual behaviors 

Parkinsons disease

Loss of dopamine neurons in the substantial nigra leaves the basal ganglia's brake stuck on, so movements are hard to start

Parkinsons onset and treatment

L-DOPA (a dopamine precursor) relieves symptoms but doesnt stop the disease

symptoms appear only after much of the dopamine system is lost; the brain compensates for years - late onset

Cerebellum

coordination, precision, and timing of movement

receives proprioceptive input from muscle spindles and Golgi tendon organs plus copies the motor commands and compares the intended movement with the actual one, correcting errors.

Fine-tunes motor skills with practice

the BG choose the action and the cerebellum makes it smooth

Damage to the cerebellum

causes clumsy, poorly timed movement and tremor during movement — recent evidence also suggest roles in attention language and emotion

Granule cells

receive input from the spinal cord, sensory systems and cortex: their axons, the parallel fibers, synapse on Purkinje cells

Purkinje cells are

the only output of the cerebellar cortex and they inhibit the deep cerebellar nuclei

The deep nuclei send

the cerebellums output on to motor area, through the thalamus to cortex and separately, to the brainstem  

This inhibition sculpts the output, fine-tuning movement