Nervous System - Spinal Reflexes and Control of Movement Flashcards
skeletal muscle structure
- skeletal muscles
- skeletal muscle fibers
- muscle fascicle
skeletal muscles
- muscles connected to the skeleton
- muscle fibers contain myofibrils, organized in sarcomeres
myofibrils
- bundles of contractile proteins called myosin and actin
skeletal muscle fibers
elongated cylindrical multinucleate cells
- connective tissues surround muscle fibres and nerves that innervate them
muscle fascicle
bundle of skeletal muscle fibers
spinal nerve
- connected to the spinal cord by a dorsal root
- mixed nerve (carries both sensory and motor info)
fibers in a dorsal root
sensory
fibers in a ventral root
motor
types of muscle fibers
- extrafusal fibers
2. intramural fibers
extrafusal muscle fibers
- majority of skeletal muscle fibers
- generate force to move or rigidly fix body parts
- innervated by efferent motor fibers (alpha motor neurons)
intrafusal muscle fibers
- specialized fibers (muscle spindle)
- inform CNS about muscle length and its rate of change
- innervated by both sensory and motor nerves
- afferent sensory fibers Ia, II- efferent motor fibers: gamme motor neuron
muscle spindle
- spindle-shaped organ
- composed of intrafusal muscle fibers
- intramural muscle fibers are arranged in // with extrafusal muscle fibers
- detect muscle stretch
types of intrufsal muscles fibers
- nuclear bag fibers
2. nuclear chain fibers
co-activation of the gamma and alpha motor neurons
co-activation of the gamma efferents occur to maintain muscle spindle sensitive to changes in muscle length, even as the muscle contracts and shortens
events occurring when a muscle is stretched
- muscle stretched
- extrafusal muscle fibers lengthened
- intrafusal muscle fibers lengthened
- sensory afferent fibrs (Ia/II) are activated
- alpha motoneurons are activates (innervate extrafusal fibres)
- muscle contracts
- gamma motoneurons are co-activated (innervate intrafusal fibers)
purpose of muscle spindle reflexes
operate to return muscle to its resting length after has been shortened or lengthened
principles of the skeletal muscle (5)
- motor unit
- motoneuron pool
- size principle
- population code
- frequency code
neuromuscular junction
a chemical synapse formed by the contact between a motor neuron and a muscle fiber
junction: pre-synaptic cell, type of synapse, neurotransmitter, class of receptors, type of receptor, post-synaptic cell
- alpha motor neuron
- chemical
- acetylcholine
- ionotropic
- nicotinic
- muscle fiber
motor end plate = neuromuscular junction
- motor neurons typically branch to innervate multiple muscle fibers
- each branch ends in a neuromuscular junction or motor end plate
- motor end plate consists of a number of terminal enlargements of the nerve (terminal buttons)
terminal buttons
- terminal enlargements of nerve
- make up motor end plate
motor unit
- single motoneuron and the muscle fibers it innervates
- alpha motor neuron (lower motor neuron) and the set of muscle fibers it innervates
producing more force
- activation of more motor neurons
- activation of more muscle fibers
- produces more force
motor neuron pool
- set of alpha motor neurons innervating fibers within the same muscle
where are lower motor neurons located?
ventral horn of the spinal cord
where are upper motor neurons located?
cortex of the brain
gradation of muscle force
3 points
muscle force can vary from delicate control to maximum activation
- size principle
- population code
- frequency code
size principle
- successive engagement of larger populations of motor units occurs according to their size
- smaller motor units (with smaller motoneurons) recruited first
- large motor units are recruited last
population code
- successively larger fractions of the motor units are engaged
- encodes the strength of the muscle force
frequency code
- motor units activated with higher frequencies
- increases the muscle force but also increases its duration
somite
body segment
dermatome
- skin part supplied by a single spinal nerve
- enabling the clinician to determine the level of a lesion of the spinal cord or nerve roots
myotome
- muscle part of a body segment
- controlled by areas of the motor cortex adjacent to the sensory cortex
spinal reflex
- stereotyped muscular response to a specific sensory stimulus
- “hard wired”
- reflection of the stimulus onto the musculature
- may be suppressed or enhanced, but not eliminated
- no conscious deliberation
reflex arc
type of conduction pathway that allows impulses to travel in only one direction
reflex arc components (5)
- receptor
- sensory (afferent) neuron
- integration center
- motor (efferent) neuron
- effector
spinal reflexes (4)
- myotatic
- inverse myotatic
- withdrawal
- crossed-extensor
myotatic reflex
- muscle stretch reflex (knee-jerk reflex = patellar reflex)
- monosynaptic
- causes muscle contraction in response to muscle stretch
- involves muscle spindles which detect stretch)
inverse myotatic reflex
- golgi tendon reflex (clasp-knife reflex)
- disynaptic
- causes muscle relaxation in response to muscle contraction
- prevents damage to muscles due to excessive force production
withdrawal reflex
- flexor-withdrawal reflex
- polysynaptic
- occurs in response to painful or noxious stimulus
- causes movement of limbs towards the body (activates flexors, inhibits extensor)
(pain afferent fibers)
crossed-extensor reflex
- activate to maintain balance in response to pain
- polysynaptic
- limb on opposite side of stimulus extended (flexion on ipsilateral side, extension on contralateral side)
(pain afferent fibers)
motor control
- posture and movement depend on a combination of:
- voluntary actions controlled by higher brain centers
- involuntary reflexes coordinated by the spinal cord (spinal cord reflexes + central pattern generators)
all reflexes and voluntary control of muscles act through their effect on what type of motorneurons
lower motoneurons
major descending motor pathways
- higher control of muscles originates in the cerebrum and is coordinated with inputs and from other parts of the brain
- descending tracts contain axons that make contact with the lower motor neurons and help control them
how descending tracts are named
- first from their point of origin and secondly from their termination
ex. rubrospinal tract, ventral corticospinal tract
