10. Systems in the NS. the MOTOR system

Question 1

MUSCLE CONTRACTION is caused be BINDING of ACTIN and MYOSIN triggered by…

Answer 1

CA2+ RELEASE from SR
(calcium induced calcium release)

due to depolarisation

Question 2

why do MUSCLE CELLS (SARCOMERES) not need synapses between them

Answer 2

muscle cells MERGE within the membrane to form one big joined up cell : muscle fibril called SYNCYTIUM

which is undergoing rapid depolarisation

Question 3

ACH released into NEUROMUSCULAR CLEFT from a neurone binds to which RECEPTORS on the adjacent muscle

Answer 3

LIGAND-GATED SODIUM CHANNELS
(Nicotinic ACH Receptors)

causing DEPOLARISATION by NA+ INFLUX

Question 4

what triggers the release of ACH in a SYNAPSE

Answer 4

VOLTAGE GATED CA2+ CHANNELS OPEN

CA2+ INFLUX

causes vesicles to move and merge with membrane

(action potential by Na+ influx causes the ca2+ channels to open)

Question 5

what happens after synapse is done

Answer 5

ACH REUPTAKE by axon terminal
or BREAKDOWN by ACETYLCHOLINESTERASE enzyme

Question 6

MOTOR UNIT is the most basic element of output / the action unit
what does it include?

Answer 6

SINGLE MOTOR NEURON, ITS BRANCHES, and the MYOFIBRILS that it INNERVATES

Question 7

MOTOR NEURON CELL BODY is located where

Answer 7

in the CENTRAL GREY of the VENTRAL (Anterior) HORN of the SPINAL CORD

Question 8

MOTOR UNIT, FINAL COMMON PATHWAY includes..

Answer 8

• motor neuron CELL BODY in central grey of ventral horn
• MOTOR AXONS transverse the surrounding WHITE matter as a FASCICLE
• the axons EXIT the cns as the VENTRAL RAMUS OF THE ROOT
• the root transverses the FLUID of the SPINAL CANAL, and EXITS axial skeleton through INTERVERTEBRAL FORAMEN

Question 9

MOTOR UNIT, FINAL COMMON PATHWAY includes..

Answer 9

• motor neuron CELL BODY in central grey of ventral horn
• MOTOR AXONS transverse the surrounding WHITE matter as a FASCICLE
• the axons EXIT the cns as the VENTRAL RAMUS OF THE ROOT
• the root transverses the FLUID of the SPINAL CANAL, and EXITS axial skeleton through INTERVERTEBRAL FORAMEN
• roots corresponding to the spinal levels of the arm and leg INTERTWINE to form a PLEXUS from which the names PERIPHERAL NERVE ARISES (usually mixed sensory and motor nerve)
• over the trunk the anatomy is more clearly segmented (rather than plexus)
• the motor axons of a particular spinal root supply a certain group of muscles: MYOTOME

Question 10

MOTOR NEURONs are recruited according to their SIZE
(SIZE PRINCIPLE)
which motor neurons are the FASTEST

Answer 10

LARGEST ones

Question 11

which motor neurones are the LARGEST & FASTEST

Answer 11

ALPHA motor neurons

• 13-20 μm diameter
• 80-120 msec

aka lower motor neurones, ventral horn cell

Question 12

which size motor neurons reach their FIRING THRESHOLD more READILY

Answer 12

SMALL

Question 13

which A motor neurones are the SMALLEST and SLOWEST

Answer 13

GAMMA

• 5-8 μm
• 4-24 msec

Question 14

GAMMA motor neurones go to where in the muscle

Answer 14

SPINDLE

• INTRAFUSAL

Question 15

ALPHA neurones are for SOMATIC MOTOR and …

Answer 15

PROPRIOCEPTION

Question 16

which A NEURONS are for TOUCH & PRESSURE

Answer 16

BETA

Question 17

which A NEURONS are for PAIN (fast), COLD, TOUCH

Answer 17

DELTA

Question 18

the STRETCH (myotatic) REFLEX (aka Liddel-Sherrington reflex) uses which CIRCUIT

Answer 18

MONOSYNAPTIC CIRCUIT

• 1 SYNAPSE (between a sensory and a motor neuron)
• the SIMPLEST NEURAL CIRCUIT

Question 19

STRETCH REFLEX.
when a muscle is stretched unexpectedly, what AFFERENT in the SPINDLE Fires
(increased tonic firing)

Answer 19

Ia (SENSORY) AFFERENT

• VERY FAST

Question 20

where is the SYNAPSE of the STRETCH REFLEX between the Ia Sensory AFFERENT and ALPHA MOTOR neuron

Answer 20

in the CENTRAL GREY of the Spinal cord

Question 21

STRETCH REFLEX.
Ia AFFERENT comes from the SPINDLE. where in the muscle does the ALPHA MOTOR return to

Answer 21

BODY, Outside the spindle - EXTRAFUSAL

Question 22

what is the purpose of the STRETCH REFLEX

Answer 22

muscle CONTRACTS to MAINTAIN ORIGINAL LENGTH

• MAINTAINS POSITION AGAINST UNEXPECTED FORCES

Question 23

why is the STRETCH REFLEX the FASTEST of all neural responses

Answer 23

• MONOSYNAPTIC
• uses FASTEST NEURONAL FIBRES (Ia afferent, alpha efferent)

Question 24

STRETCH REFLEX
Related reflex is mediated by..

Answer 24

GOLGI TENDON ORGAN

Question 25

STRETCH REFLEX also sends an ... RESPONSE to ANTAGONIST MUSCLE and feedback to higher centres

Answer 25

INHIBITORY response

Question 26

ALPHA-GAMMA CO-ACTIVATION (step 2) when higher centres trigger stimulation of ALPHA MOTOR neuron and EXTRAFUSAL Muscle fibres CONTRACT what happens to the MUSCLE SPINDLE?

Answer 26

CHANGE IN CONFORMATION - BECOME LAX (slack) instead of taut and thus LOSS of SENSITIVITY of the reflex (not taut to pick up changes)

Question 27

ALPHA-GAMMA CO-ACTIVATION (step 2) although SPINDLE fibres become lax, how is the STRETCH REFLEX SENSITIVITY still MAINTAINED

Answer 27

CO-ACTIVATION of GAMMA MOTOR NEURONS - STIMULATE the INTRAFUSAL (SPINDLE) FIBRES and so TENSION maintained

Question 28

when an ALPHA motor neuron fires to trigger target muscle (agonist), it also stimulates INHIBITORY CELLS (to the ANTAGONIST muscle) the best known SPINAL INHIBITORY INTERNEURON is which cell:

Answer 28

RENSHAW CELL - INHIBIT MOTOR NEURONS that target ANTAGONIST muscle

Question 29

MOTOR DESCENDING PATHWAY examples that allow brain to send effect to spinal cord:

Answer 29

LATERAL CORTICOSPINAL TRACT (important) also VENTRAL CORTICOSPINAL TRACT and more

Question 30

reflex that is generated more SLOWLY than MONOSYNATPIC STRETCH reflex

Answer 30

LATERAL LATENCY REF;ECES

Question 31

DESCENDING PATHWYAS ie lateral corticospinal tract ORIGINATE where in the brain

Answer 31

PRIMARY MOTOR CORTEX

Question 32

where in the brain is the PRIMARY MOTOR CORTEX found and in which broadmann area

Answer 32

PRE-CENTRAL GYRUS in Broadmann area 4

Question 33

name of CELLS that form CORTICOSPINAL TRACT / neurons in the brain that have large/long axons that project to other parts of the CNS : - UPPER MOTOR NEURON cells

Answer 33

PYRAMIDAL CELLS LARGE pyramidal cells are called BETZ CELLS (have pyramidal shape)

Question 34

there are 6 cellular layers in the Cortex. PYRAMIDAL CELLS in which layer form CORTICOSPINAL TRACT

Answer 34

LAYER 5

Question 35

UPPER MOTOR NEURONS are found in the cerebral cortex and brainstem. The primary tract which carries signals for voluntary movement is known as the ... tract

Answer 35

PYRAMIDAL TRACT -> corticospinal and corticobulbar

Question 36

what are INTERNEURONS

Answer 36

SMALL, GRANULAR (grain-like) NEURONS found in the spinal cord, relays signals between sensory and motor neurons

Question 37

why does it get the name PYRAMIDAL tract

Answer 37

because it PASSES the PYRAMID of the MEDULLA - where it DECUSSATES

Question 38

Path of the CORTICOSPINAL (PYRAMIDAL) TRACT

Answer 38

from PRIMARY MOTOR CORTEX - PRECENTRAL GYRUS, BA 4 - traverses GREY matter DEEP in the hemisphere (forms capsule around nuclei) in the INTERNAL CAPSULE - forms ANTERIOR part of MIDBRAIN (CEREBRAL PEDUNCLE) - passes through PONS - referred to as the PYRAMID in the ANTERIOR MIDBRAIN where it DECUSSATES before passing to the SPINAL CORD

Question 39

what is meant by HOMONCULUS

Answer 39

the MAPPING of the brain along the CEREBRAL CORTEX - where each part of the body is processed

Question 40

does Upper or Lower MOTOR NEURON SYNDROME (damage) have Negative (WEAKNESS) and Positive effects - INCREASED TONE (SPASTICITY) and EXAGGERATED REFLEXES

Answer 40

UPPER MOTOR NEURON SYNDROME

Question 41

why do you get INCREASED TONE (SPASTICITY) and EXAGGERATED REFLEXES in UPPER MOTOR NEURON SYNDROM

Answer 41

LOSS OF INHIBITION on lower motor neurons (via RENSHAW cells etc)

Question 42

symptoms of LOWER MOTOR NEURON SYNDROME

Answer 42

- WEAKNESS - MARKED WASTING - loss of muscle bulk (due to loss of trophic stimulation) - ABSENT REFLEXES - FASCICULATION (muscle twitches) (lower motor neuron drops out so visible firing of large motor units which innervate a larger than normal number of muscle fibres to compensate. resting tone of muscles become more erratic) - FIBRILLATION on electrograph assessment. based on chemical hypersensitivity

Question 43

GAMMA MOTOR NEURONS have what effect on the STRETCH REFLEX

Answer 43

they are INTRAFUSAL (act on fibres in SPINDLE) - cause TENSION of these fibres / causes them to TIGHTEN when muscle contracts and thus MAINTAINS THEIR SENSITIVITY (when alpha motor neurons contract so do gamma)

Question 44

what are RENSHAW CELLS

Answer 44

INHIBITORY INTERNEURONS - INHBIT MOTOR NEURONS that act on ANTAGONIST MUSCLE