neuromuscular and spinal cord control movements

Question 1

recall teh structure of the NMJ *

Answer 1

like a synapse but between a motor neuron adn a motor end plate (teh muscle cell fibre membrane)

contact ratio varies from 1:1 in muscle to 10(power3):1 in CNS

synaptic cleft is 10-50nm

receptors on the post synaptic membrane which is the muscle membrane

Question 2

what is the function of the NMJ *

Answer 2

allows for contact fro the neurone to muscle

Question 3

describe the transmission across synapses *

Answer 3

membrane potential in post-synaptic neuron can be altered in 2 directions by inputs:

it can be made less -ve, ie closer to the threshold for firing - this is an excitatory post synaptic potential (EPSP)

or

it can be made more -ve ie brought furtehr from the threshold for firing - this is inhibitory post synaptic potentials (IPSP)

these graded effects = summation

the degree of summation determines how readily a neuron can reach threshold to produce an AP

Question 4

describe activation of the neuromuscular junction

Answer 4

when an AP arrives Ca ion channels open = Ca influx = ACh release

ACh bind to receptors on motor end plate

ion channel opens - Na influx to muscle = AP in muscle fibre

at rest individual vesicles release ACh at a very low rate causing miniture end plate potentials - there is no overt muscle action

Question 5

describe the alpha motor neuron *

Answer 5

they are lower motor neurons of brainstem motor nuclei and spinal cord ventral horn

they innervate the muscle fibres of the skeletal muscles (extrafusal muscle fibres) - their activation causes muscle contraction - voluntary

a motor neuron pool contains all alpha motor neurons innervating a single muscle

Question 6

describe the organisatiomn of the motor neurons within the spinal cord *

Answer 6

extensors are more ventral

flexors are more dorsal

proximal neurons are more medial

distal neurons are lateral

Question 7

define the term motor unit *

Answer 7

a single motor neuron together with all the muscle that it innervates - it is the smallest functional unit with whcih to produce force

stimulation of 1 motor unit causes cintraction of all the muscle fibres that are in it

Question 8

how many motor neurons and muscle fibres do we have

Answer 8

420000 motor neurons, 250 million skeletal muscle fibres

therefore one average each motor neuron supplies about 600 muscle fibres

Question 9

compare the 3 types of motor units *

Answer 9

slow (S, type I) - smallest diameter cell bodies, small dendritic trees, thinnest actions- therefore slowest conduction velocity

fast fatigue resistant (FR, type IIA) - larger diameter cell body, larger dendritic trees, thicker axons, faster conduction velocity

fast fatiguable (FF, type IIB) - larger diameter cell body, larger dendritic trees, thicker axons, faster conduction velocity

Question 10

how can you see the distribution of different motor units

Answer 10

histologically - ATP myosin stain - different types of fibre stain differently resulting from different amounts of ATPase

Question 11

how are the 3 types of motor fibre classified *

Answer 11

amount of tension/force - type IIB produce largest force

speed of contraction - type IIB has the fasstest speed of contraction

fatiguability of motor unit

Question 12

list the 2 mechanisms that the brain uses to regulate the force that a single muscle can generate *

Answer 12

recruitment

rate coding

Question 13

describe how recruitment can be used to regulate the force produced by a motor unit

Answer 13

there is an order to recruitment

this is governed by the size principle - smaller units are recruited 1st - usually type 1

as more force is required larger units are recruited

this allows fine control when low levels of forces are required eg for writing

this generates a force generated over time graph as pictured

Question 14

describe how rate coding can be used to regulate the force produced by a motor unit *

Answer 14

a motor unit can fire at a range of frequencies - slow units fire at lower freq

as teh firing rate increases the force produced by the unit increases

summation occurs when the units fire so fast that ther muscle cant relax between APs - this is tetany

Question 15

what are neurotrophic factors and what do they do

Answer 15

growth factors

prevent neuronal death

promote growth of neurons after injury

Question 16

how are motor units modifyable

Answer 16

motor units are dependannt on the nerve that innervates them

if a fast twitch muscle and a slow twitch muscle are cross innervated - the slow muscle becomes fast and teh fast muscle becomes slow - contractility is modified by the motor neuron

type IIB fibres can change to type IIA following training - reduce fatigue to improve performance

type 1 -> II is possible if there is severe deconditioning or spinal cord injury or microgravity during space flight

aging = loss of type 1 and II but also preferential loss of type II fibres = larger proportion of type 1 fibres in elderly = slower contraction times

Question 17

describe the organisation of the motor tracts in the spinal cord *

Answer 17

lateral and anterior cortical spinal tracts are for volunatry movement

extrapyramidal tracts, modify motor func:

rubrospinal tract - automatic movement of arms in response to posture/balance change. this is from the red nucleus in the brainstem to the spinal cord

retculospinal tract - coordinated automated movements of locomotion and posture - eg painful stimuli

vestibulospinal tract - regulates posture to maintain balance, and fascilites mainly a motoneurons of the postural, anti-gravity extensor muscles - maintain the centre of gravity

Question 18

what is a reflex *

Answer 18

an automated and often inborn response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre and then outward to an effector (muscle/gland) without reaching the level of consciousness

an involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli eg pain and temp

the magnitude and timing are determined repectively by the intensity and onset of the stimulus eg if the bicep is tapped the reflex occurs quickly and is related in size to how hard the biceps were hit

reflex differ from voluntary movements in that once they are released they cannot be stopped

Question 19

describe the components in the reflex arc *

Answer 19

sensory receptor responds to a stimulus by producing a generator or receptor potential

sensory neuron axon conducts impulse from receptor to integrating centre (one or more regions in the CNS that relay impulses from sensory to motor neurons)

motor neuron conducts impulses from integrating centre to effector via the ventral horn

effector

Question 20

illustrate how pain nerves need afferents *

Answer 20

tehy need the sensory neurons and input in order to start the reflex arc

if the dorsal roots are cut = no snesory input = no force from reflex

Question 21

describe the monosynaptic (stretch) reflex ie the patella reflex *

Answer 21

intrafusal fibres sense that the muscle has stretched so sensory neuron is excited - sensory inpout goes into the dorsal horn

within the intergrating centre the snesory neuron activates the motor neuron - motor neuron exites and causes contraction of the extensor

there is also an inhibitory interneuron in the integrating centre that recieves the sensory input - inhibits teh outflowto teh antagonistic muscle in the leg

Question 22

what is the Hoffmann reflex *

Answer 22

it is where you stimulate electrically the monosynaptic reflex

Question 23

describe polysynaptic reflexes - flexion, withdrawal reflex *

Answer 23

more than one motor unit involved to move 1 muscle - so so there are many reflex arcs at different levels of the spinal cord

eg when you stand on a pin

the effector muscles are flexors that contract and withdraw the leg

Question 24

describe the flexion withdrawal and crossed extensor reflex *

Answer 24

there is the same reflex arc as for flexion withdrawal - in that a lot of spinal cord levels are involved

here the inhibitory interneurons cross the midline so that the motor neurons on the contralateral side are excited - causing contraction of extenser muscles in the other leg - this allows you to flex away from the painful stimulus without falling over

Question 25

summarise the concept of supraspinal control of reflexes *

Answer 25

there is top down influences on reflexes

eg clenching teeth and making hook with hands potentiates the patella reflex = hyperexcitability - get bigger leg swing - this is the Jendrassik manoeuvre

higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex

inhibitory control dominates in normal conditions

decerebration reveals the excitatory control from supraspinal area

rigidity and spasticity can result from brain damge giving over active or tonic stretch reflex

Question 26

how do higher centres control reflexes *

Answer 26

they activate a motor neurons

activate inhibitory neurons

activate propriospinal neurons - give proprioceptive info coming from body about where we are in space

activates gamma motor neurons - they control muscle fibre length based on muscle movement - this is innate and based on inputs - these are intrafusal muscle fibres

activating terminals of afferent fibres

Question 27

what are the higher centres that are involved in ssupraspinal control of reflexes *

Answer 27

cortex - corticospinal - fine control of limb movements and body adjustments

red nucleus - rubrospinal

vestibular nuclei - vestibulospinal

tectum - head of the midbrain down to the spinal cord - tectospinal - head movements in response to visual information

Question 28

what happens if the knee is entended and the muscle goes slack and why *

Answer 28

the spindle is shortened to maintain the sensitivity - this is the gamma reflex loop - the gamma neurons react to the shortening or not of the muscle

Question 29

describe hyper-reflexia and the causes *

Answer 29

due to a stroke = loss of desceinding inhibition

cause exaggerated reflexes eg patella reflex

clonus - muscular spasm including repeated contractions

babinski sign - stimulus run across lateral foot and then across ball of foot. normally all toes extend, in hyper-reflexia big toe flex and smaller toes fan

Question 30

describe hyporeflexia *

Answer 30

below normal or absent reflexes

mostly associated with lower motor neuron disease

Question 31

what are a motor neurons *

Answer 31

lower motor neurons of the brainsstem and the spinal cord

innervate the extrafusal muscle fibres of skeletal muscles

activation causes contraction