10. Systems in the NS. the MOTOR system Flashcards
MUSCLE CONTRACTION is caused be BINDING of ACTIN and MYOSIN triggered by…
CA2+ RELEASE from SR
(calcium induced calcium release)
due to depolarisation
why do MUSCLE CELLS (SARCOMERES) not need synapses between them
muscle cells MERGE within the membrane to form one big joined up cell : muscle fibril called SYNCYTIUM
which is undergoing rapid depolarisation
ACH released into NEUROMUSCULAR CLEFT from a neurone binds to which RECEPTORS on the adjacent muscle
LIGAND-GATED SODIUM CHANNELS
(Nicotinic ACH Receptors)
causing DEPOLARISATION by NA+ INFLUX
what triggers the release of ACH in a SYNAPSE
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)
what happens after synapse is done
ACH REUPTAKE by axon terminal
or BREAKDOWN by ACETYLCHOLINESTERASE enzyme
MOTOR UNIT is the most basic element of output / the action unit
what does it include?
SINGLE MOTOR NEURON, ITS BRANCHES, and the MYOFIBRILS that it INNERVATES
MOTOR NEURON CELL BODY is located where
in the CENTRAL GREY of the VENTRAL (Anterior) HORN of the SPINAL CORD
MOTOR UNIT, FINAL COMMON PATHWAY includes..
- 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
MOTOR UNIT, FINAL COMMON PATHWAY includes..
- 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
MOTOR NEURONs are recruited according to their SIZE
(SIZE PRINCIPLE)
which motor neurons are the FASTEST
LARGEST ones
which motor neurones are the LARGEST & FASTEST
ALPHA motor neurons
- 13-20 μm diameter
- 80-120 msec
aka lower motor neurones, ventral horn cell
which size motor neurons reach their FIRING THRESHOLD more READILY
SMALL
which A motor neurones are the SMALLEST and SLOWEST
GAMMA
- 5-8 μm
- 4-24 msec
GAMMA motor neurones go to where in the muscle
SPINDLE
- INTRAFUSAL
ALPHA neurones are for SOMATIC MOTOR and …
PROPRIOCEPTION
which A NEURONS are for TOUCH & PRESSURE
BETA
which A NEURONS are for PAIN (fast), COLD, TOUCH
DELTA
the STRETCH (myotatic) REFLEX (aka Liddel-Sherrington reflex) uses which CIRCUIT
MONOSYNAPTIC CIRCUIT
- 1 SYNAPSE (between a sensory and a motor neuron)
- the SIMPLEST NEURAL CIRCUIT
STRETCH REFLEX.
when a muscle is stretched unexpectedly, what AFFERENT in the SPINDLE Fires
(increased tonic firing)
Ia (SENSORY) AFFERENT
- VERY FAST
where is the SYNAPSE of the STRETCH REFLEX between the Ia Sensory AFFERENT and ALPHA MOTOR neuron
in the CENTRAL GREY of the Spinal cord
STRETCH REFLEX.
Ia AFFERENT comes from the SPINDLE. where in the muscle does the ALPHA MOTOR return to
BODY, Outside the spindle - EXTRAFUSAL
what is the purpose of the STRETCH REFLEX
muscle CONTRACTS to MAINTAIN ORIGINAL LENGTH
- MAINTAINS POSITION AGAINST UNEXPECTED FORCES
why is the STRETCH REFLEX the FASTEST of all neural responses
- MONOSYNAPTIC
- uses FASTEST NEURONAL FIBRES (Ia afferent, alpha efferent)
STRETCH REFLEX
Related reflex is mediated by..
GOLGI TENDON ORGAN
STRETCH REFLEX also sends an … RESPONSE to ANTAGONIST MUSCLE
and feedback to higher centres
INHIBITORY response
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?
CHANGE IN CONFORMATION
- BECOME LAX (slack) instead of taut
and thus LOSS of SENSITIVITY of the reflex
(not taut to pick up changes)
ALPHA-GAMMA CO-ACTIVATION (step 2)
although SPINDLE fibres become lax,
how is the STRETCH REFLEX SENSITIVITY still MAINTAINED
CO-ACTIVATION of GAMMA MOTOR NEURONS
- STIMULATE the INTRAFUSAL (SPINDLE) FIBRES and so TENSION maintained
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:
RENSHAW CELL
- INHIBIT MOTOR NEURONS that target ANTAGONIST muscle
MOTOR DESCENDING PATHWAY examples that allow brain to send effect to spinal cord:
LATERAL CORTICOSPINAL TRACT (important)
also VENTRAL CORTICOSPINAL TRACT
and more
reflex that is generated more SLOWLY than MONOSYNATPIC STRETCH reflex
LATERAL LATENCY REF;ECES
DESCENDING PATHWYAS ie lateral corticospinal tract ORIGINATE where in the brain
PRIMARY MOTOR CORTEX
where in the brain is the PRIMARY MOTOR CORTEX found
and in which broadmann area
PRE-CENTRAL GYRUS
in Broadmann area 4
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
PYRAMIDAL CELLS
LARGE pyramidal cells are called BETZ CELLS
(have pyramidal shape)
there are 6 cellular layers in the Cortex.
PYRAMIDAL CELLS in which layer form CORTICOSPINAL TRACT
LAYER 5
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
PYRAMIDAL TRACT
-> corticospinal and corticobulbar
what are INTERNEURONS
SMALL, GRANULAR (grain-like) NEURONS
found in the spinal cord, relays signals between sensory and motor neurons
why does it get the name PYRAMIDAL tract
because it PASSES the PYRAMID of the MEDULLA
- where it DECUSSATES
Path of the CORTICOSPINAL (PYRAMIDAL) TRACT
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
what is meant by HOMONCULUS
the MAPPING of the brain along the CEREBRAL CORTEX
- where each part of the body is processed
does Upper or Lower MOTOR NEURON SYNDROME (damage) have Negative (WEAKNESS) and Positive effects - INCREASED TONE (SPASTICITY) and EXAGGERATED REFLEXES
UPPER MOTOR NEURON SYNDROME
why do you get INCREASED TONE (SPASTICITY) and EXAGGERATED REFLEXES in UPPER MOTOR NEURON SYNDROM
LOSS OF INHIBITION on lower motor neurons (via RENSHAW cells etc)
symptoms of LOWER MOTOR NEURON SYNDROME
- 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
GAMMA MOTOR NEURONS have what effect on the STRETCH REFLEX
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)
what are RENSHAW CELLS
INHIBITORY INTERNEURONS
- INHBIT MOTOR NEURONS that act on ANTAGONIST MUSCLE
