Neuro Exam 3

descending systems

• basal ganglia and cerebellum project to descending system upper motor neurons

• then projects to spinal cord and brainstem circuits

upper motor neurons

• motor cortex

• brainstem centers

spinal cord and brainstem circuits

descending systems (upper motor neurons) project here

• local circuit neurons (interneurons that move info) —> lower motor neurons

lower motor neurons project to

skeletal muscles

• planning movement, performing movement

descending motor control systems control:

motor planning and execution

two main groups of motor neurons

• upper motor neurons

• lower motor neurons

upper motor neurons

from motor cortex to interneuron circuits in the brainstem/spinal cord

• dont leave CNS

lower motor neurons

from cranial nerve nuclei or spinal cord to muscle

• leave the CNS —> muscles

upper motor neurons for hand

project directly to lower motor neurons instead of interneurons

descending pathway: motor neuron circuitry

upper motor nerve cells/neurons (crossed over) —> midbrain —> pons —> medulla (cross over midline to side of spinal cord neurons entered) —> spinal cord interneurons —> lower motor neuron (not crossed over) —> skeletal muscle

3 subclasses of lower motor neurons

• alpha-motor neurons

• beta-motor neurons

• gamma-motor neurons

alpha-motor neurons

project to extrafusal muscle fibers

• voluntary muscle, muscle contraction, force

beta and gamma-motor neurons

project to muscle spindles (spindle tension)

• projects to intrafusal fibers

• inside, deep in muscle (muscle length and stretch)

motor unit

one alpha motor neuron and all its postsynaptic fibers

alpha motor neuron innervation

each alpha motor neuron innervates several fibers within the same muscle

• via axon collaterals

size of alpha motor neurons depends on

size of the motor unit and fibers it innervates

3 classes of motor units

1. fast-fatigable (FF) units

2. slow (S) motor units

3. fast, fatigue resistant (FFR) units

fast-fatigable units

largest alpha-motor neurons

contact large “pale” fibers

• highest force

• fastest respond

• few mitochondria

• easily fatigued

• brief exertions (sprinting jumping)

slow motor units

smallest alpha-motor neurons

contact small “red” slow fibers

• high in mitochondria (red)

• resistant to fatigue

• lowest force

• most common in skeletal muscle

• maintaining upright posture

fast, fatigue, resistant units

intermediate size fibers

slow twitch

• not as fast/fatiguable as FF

• higher force than S

• walking, running

generation of muscle force

force of muscle contraction depends on

• number of motor units activated (more units=more force)

• type of motor unit activated

• rate of action potentials generated in motor neurons

neuromuscular junction

consists of presynaptic boutons of motor neuron and postsynaptic end plate of muscle fiber

end plate

specialization of the postsynaptic fiber with membrane “pockets” called junctional folds

active zones

zones in the presynaptic bouton opposite the junctional folds of end plate

• had vesicles where NT release occurs

NT of NMJ

acetylcholine

parts of NMJ

• active zones

• vesicles with ACh

• voltage-gated Ca 2+ channels

• postsynaptic ACh receptor

• junctional folds

• postsynaptic voltage gated Na+ channel

postsynaptic ACh receptor

ligand gated ion channels that depolarize membrane and open Na+ channels

depolarization of NMJ

• ACh binds AChR (acetylocholine receptor) and depolarizes postsynaptic membrane

• end plate potential (evoked at -90 mV) depolarizes membrane at muscle end plate using Na and K

• this depolarization activates voltage-gated Na+ channels to further depolarize postsynaptic fiber

• causes AP that activates Ca2+ channels in muscle fiber —> triggers contraction

end plate potential

synaptic potential that occurs at muscle end plate and is evoked at -90mV

• graded potentials

• decay with distance from NMJ- no active zones to depolarize along path

EPPs vs APs

• APs propagate without decrement from NMJ to ensure uniform contraction.

• EPPs are graded potentials that decay over distance

EPP contraction

can cause an all or nothing AP- if the EPP is sufficient amplitude to activate voltage gated na+ channels, muscle fibers will cause muscle contraction

relaxation vs contraction

relaxation- no muscle force/inhibited force

contractions- increased muscle force triggered by APs

reflexive motor circuits

stimulus —> receptor —> sensory neuron —> enters dorsal side spinal cord —> interneuron —> motor neuron (leaving sc) —> effector (muscle) —> response (contraction/movement)

• sensory input —> local circuit neurons —> lower motor neurons —> skeletal muscle 

monosynaptic reflex

when sensory neuron projects directly to motor neuron (ex knee jerk)

• stimulus —> receptor on sensory neuron —> lower motor neuron —> muscle contraction

3 main peripheral sensorimotor reflex pathways

1. muscle tone/length reflexes- muscle spindles

2. muscle contraction force reflex- golgi tendon organs

3. flexion reflex- pain receptors

sensorimotor reflex pathway

sensory neurons —> interneurons in spinal cord (excitatory or inhibitory) —> lower motor neurons (alpha bc muscle contraction) —> muscle

proprioceptors

provide sensory input from muscles to spinal cord

• includes muscle spindles and golgi tendon organs

muscle spindle

• integrated within the muscle

• info on muscle length

• comprised of muscle fibers and sensory afferents

• the intrafusal fibers receive gamma-motor neuron inputs from sc

• stretching muscle stretches intrafusal fibers

• activates mechanoreceptors in sensory afferents —> APs relayed to spinal cord

  • increase AP rates during stretch and decrease during contraction

golgi tendon organ

• found in tendon

• info about force of muscle contraction

reciprocal innervation in stretch reflex circuitry

• sensory afferents split in spinal cord

• one connection excites alpha-motor neurons of the same muscle as activated spindle contraction

• other connection excites inhibitory neurons that decrease firing rate of alpha-motor neurons of antagonistic muscle relaxation

  • ex. contracting bicep means relaxing tricep

stretch reflex (hammer on knee)

1. hammer strikes tendons of patella 

2. stretches quadriceps muscle spindle

3. excites quadriceps (agonistic muscle) motor neuron pool

4. inhibits hamstring (antagonistic) motor neurons thru inhibitory interneurons

5. contracts quadriceps, relaxes hamstring, —> kick

“i dont want to spill my drink” reflex

1. added soda load stretches intrafusal muscle fibers of bicep

2. increases spindle AP rate signal sent to spinal cord

3. spindle input activates bicep motor neurons

4. spindle input inhibits triceps motor neurons

• stretch by added load, contraction of bicep, relaxation of tricep to resist load

muscle spindles vs golgi tendon organs

both fire AP during passive stretch

spindles:

• decrease AP firing rate during contraction

• AP firing codes length of muscle

gto:

• increase AP firing rate during contraction

• codes force of muscle contraction

golgi tendon organ reflex circuit

positive circuit, causes relaxation of the activating muscle when overloaded

• muscle tension stretch —> spinal cord interneurons —> relaxation response by inhibition response on same muscle 

activates local neurons that inhibit motor neurons of the same muscle- causing relaxation

sensorimotor reflexes- pain

pain sensory neuron afferents only project to interneurons not motor neurons 

• sensory receptors for pain come from nociceptors

“step on a tack reflex”

1. pain afferents activate excitatory interneurons of spinal cord

3. local neurons coordinate response of injured and uninjured legs and posture

4. a) ipsilateral excitatory interneurons excite motor neurons of hamstring to contract and withdraw foot

4. b) ipsilateral interneurons send projection to motor neurons on contralateral side to extend uninjured leg

4. c) interneurons send signals up and down spinal cord to postural muscle groups

spinal cord circuit- medial to lateral maps

• alpha motor neurons are organized from medial to lateral according to muscle groups innervated

• muscles of the trunk and proximal limbs (ex. shoulder) are represented medially while muscles of distal limbs (arm, hands, fingers) are represented laterally

ventral horn (motor)

where distal and proximal limbs are innervated

spinal cord circuits-longitudinal maps

• motor neurons grouped along the length of spinal cord

• somas of neurons that innervate a given muscle (bicep) may be found in many sequential spinal cord segments

• this can lead to enlargements of spinal cord at regions where a lot of muscles are represented- ex arms and hands

spinal cord circuits- enlargements (cervical and lumbar)

result of longitudinal maps

• cervical enlargement motor neuron pools for arms, hands

• lumbar enlargement motor neuron pools for legs, feet

spinal cord circuits- medial and lateral local circuit neurons

local circuit neurons also interact with motor neurons in different segments of the spinal cord

• medial local circuits project over many spinal cord segments as well as bilaterally to coordinate left/right and upper/lower body movement as well as posture

• lateral local circuit neurons project to fewer segments and unilaterally to coordinate fine, independent muscle movement (finger movements on one hand) (complex, fine movement)

descending pathways from cortex to spinal cord

corticospinal tract

corticospinal tract

motor cortex —> medulla —> lower motor neuron

• 1 million axons descending from the cortex and brainstem

• 40% of the axons are from upper motor neurons of the motor cortex

• aka pyramidal tract because axons make up the pyramids in the medulla

• 90% of the axons in the corticospinal tract cross the midline at the pyramidal decussation

  • the other 10% stay and cross over in spinal cord

where do axons of upper motor neurons in corticospinal tract synapse and with what

they synapse with interneurons and lower motor neurons in the ventral horn of spinal cord

lateral corticospinal tracts

where 90% of axons crossed over in the medulla

anterior corticospinal tract

where the other 10% of axons crossed over in spinal cord

where do descending inputs project and what do they do

to local circuit and lower motor neurons

coordinate both gross and fine motor movements

descending control by:

brainstem nuclei:

• superior colliculus

• vestibular nuclei

• reticular formation

descending inputs

• upper motor neurons of brainstem

• vestibulo-, reticulo-, colliculo- spinal tracts

superior colliculus

(colliculospinal tract)

movements that orient eyes, head, and body towards sensory stimuli

vestibular nuclei 

(vestibulospinal tract)

reflexive changes in posture

reflexive eye movements

reticular formation

(reticulospinal tract)

anticipatory changes in posture

where are vestibulospinal, reticulospinal, and colliculosplinal tracts located

they make up the medial white matter

primary motor cortex

• located in precentral gyrus

• stimulation directly evokes eye movement

• contains a map for the musculature of the body

• contains a map for movements

L5 betz cells

in layer 5 of motor cortex

• upper motor neurons

• large neuron somas found in L5

• have the longest axons

• project to the spinal cord interneurons and lower motor neurons for the hand

penfield maps

electrical stimulation of the surface of the brain to map specific locations that elicit specific muscle contractions

what has more cortical representation

body regions that require fine motor control (hands/face)

• (similar to homunculus)

purposeful movements caused by (in macaque monkey)

• stimulation of precentral gyrus (primary motor cortex)

• results in sequentially distributed movements across multiple joints/muscles

mapping of purposeful movements in motor cortex

• movements that are often repeated

• movements that are important to the animal

suggestion of planning and execution for movement

planning- neural activity increases before movement

execution- neural activity continues throughout movement

neural activity when weight is added

AP increases in frequency from when there was no weight, suggesting motor cortex neurons code force of movement

motor cortex takes part in

planning, executing, and adjusting force of movement

motor cortex damage and plasticity and rehabilitation

• motor maps change as a result of learning and response to damage

• rehabilitation prevents loss of movement in hand and decrease in hand's cortical representation

autonomic nervous system (aka visceral motor system)

under efferent division (leaving spinal cord)

• sympathetic NS

• parasympathetic NS

  • smooth muscle, cardiac muscle, exocrine glands, some endocrine glands

sympathetic and parasympathetic divisions

• project to same regions but have opposite actions

• innervation of most tissue by both divisions

• 2 neuron chain from CNS to target tissue

2 neuron chain

sympathetic/parasympathetic projections from CNS to target tissue

• parasympathetic preganglionic neurons synapse onto parasympathetic postganglionic neurons which goes to tissue

  • the preganlgionic neurons are longer than post

• same for sympathetc preganglionic and postganglionic except here, the postganglionic neurons are longer than the pre

autonomic nervous system maintains

• homeostasis

• challenges to the body move the body away from homeostasis, and ANS maintains and replenishes resources

  • metabolic

  • respiratory

• controls involuntary functions of

  • smooth muscles

  • cardiac muscles

  • glands

parasympathetic and sympathetic

• para: rest and digest

• sympathetic: fight or flight

sympathetic system (skip)

• receives output from spinal cord

  • output comes from ventral root of spinal cord

• mobilization of body’s energy resources

• pupil dilation

• hairs stand

• bronchi dilate

• increased heart rate

• increased blood flow

• decrease digestion

• release NE and Epi to bloodstream

parasympathetic motor system (skip)

• receives projections from brainstem/spinal cord

• restoration of energy resources

• increases metabolic activity/digestion

• decreases blood flow to skeletal muscles

• constrict pupils

• slow heart rate

• bladder emptying

• innervation of genitals

visceral sensory neurons

receives sensory info- stretch, pressure, nociception from target tissue

• project to dorsal horn, interneurons (referred pain), and lateral horn (autonomic reflexes)

preganglionic neurons

spinal neurons that project to visceral motor neurons (the postganglionic neurons) in ganglia

sympathetic motor system neurons (visceral motor neurons)

• lower motor neurons found in sympathetic chain ganglia- the postganglionic neurons

sympathetic chain ganglia

run the length of the spinal cord

prevertebral ganglia

farther from spinal cord (sympathetic preganglionic neurons pass thru here but do not synapse)

input to sympathetic chain ganglia process from spinal cord

from spinal cord via ventral root to white rami

output of chain ganglia (sympathetic chain ganglia —> muscle) via

via gray rami to spinal nerve

parasympathetic motor system LMNs

lower motor neurons found in ganglia and plexuses nearer to target

• ganglia (groups of cell bodies) and plexuses (network of nerves) receive CNS projections from brainstem or sacral spinal cord

sympathetic nervous system 2 chain neuron

preganglionic neuron from spinal cord

• myelinated “white rami” —> ACh released to ionotropic receptors —> lower motor neurons 

ganglion (sympathetic chain ganglion)

• not myelinated “gray rami” —> postganglionic axon —> norepinephrine adrenergic receptors —> smooth muscle

parasympathetic division 2 chain neuron

spinal cord/brainstem

• preganglionic neuron myelinated —> Ach receptors 

ganglion

• lower motor neuron postganglionic neuron unmyelinated —> ACh receptors —> smooth muscle

autonomic innervation of smooth muscle (skip)

• smooth muscle lacks defined NMJs

• axons of symp and parasymp motor neurons make varicosities w smooth muscle cells (not well defined synapses)

• less organized synapses and NTs can diffuse a great distance from cleft

location of LMNs

somatic: spinal cord

visceral: ganglia outside CNS

somatic and visceral target organs

somatic: skeletal muscle

visceral: smooth and cardiac muscle, glands

# connections between spinal cord and targets

somatic: 1 myleinated neuorn

visceral: 1 myleinated and 1 unmyelinated

neuromuscular synapses

somatic: developed endplates and defined NMJs

visceral: no end plate, less defined NMJs

neurotransmitter

somatic: ACh

visceral: ACh, NE, peptides

postsynaptic receptors

somatic: ionotropic

visceral: ionotropic and metabotropic

descending control

somatic: premotor and motor cortex

visceral: central autonomic network

effects

somatic: stereotyped for all skeletal muscle'

visceral: varies by structure and contingency

refinement of movement

• basal ganglia

• cerebellum