Download Neural Integration: Control of Body Movement

Control of Body Movement
A General Scheme For Motor System Organization
1
Major Motor Pathways
Neuronal Reflexes
•The integration of sensory information into a involuntary response is the
hallmark of a reflex.
•All neuronal reflex begin with the activation of sensory receptors by a
stimulus.
Reflexes Require Feedback
•In a negative feedback pathway, the response of a system removes or
opposes the stimulus signal in order to keep the system at or near a set
point. -ve feedback keeps the CNS informed about the state of sensory
receptors.
•Some reflexes have feedfoward component which allows the body to
anticipate the stimulus and begin the response. In +ve feedback loops,
the response reinforces the stimulus rather than remove or decrease it.
Eg. contraction of the uterus during childbirth.
Classification of Neuronal Reflexes
•Reflex pathways consist of an interconnected network of neurons
which link sensory receptors to muscles and glands. They can be
classified in the following ways:
Monosynaptic and Polysynaptic Reflex
•Monosynaptic reflex formed by two neurons forming a single synapse.
•Most reflexes are polysynaptic, consisting of three or more neurons.
Sensory Receptors in Skeletal Muscle
•Skeletal muscle reflexes are
involved in almost everything we do.
•Sensory inputs are required to
control these reflexes.
•Motor neurons (MNs) to skeletal
muscles are excitatory, and when
activated, cause muscles to contract.
If the muscles need to be relaxed, the
CNS needs to inhibit MN activity.
•Sensory receptors in muscles sense
changes in tension and length, and
activate muscle reflexes.
•Three types of sensory receptors in muscle: a) muscle spindles; b) Golgi
tendon organs; c) joint capsule mechanoreceptors (proprioceptors).
•These three receptors send information to the CNS about the relative
positioning of bones linked by flexible joints.
Two Different Types of Motor Neurons Carry
Information to Muscle Following Sensory
Information Processing
•Once sensory information is processed in the CNS, it is sent back to
muscles by two different types of motor nurons.
•Alpha motor neurons (-MNs): Innervate normal skeletal muscle
fibres (also known as extrafusal muscle fibres). APs in these neurons
produce muscle contraction.
•Gamma motor neurons (-MNs): are smaller motor neurons
associated with specialized muscle fibres within sensory receptors.
Muscle Spindles Respond to Stretch
•Muscle spindles send
information about muscle length
to the CNS.
•They are small elongated
structures arranged in parallel
along extrafusal muscle fibres.
They are scattered throughout
the muscle.
•Each spindle consists of
connective tissue ensheathing a
group of intrafusal muscle
fibres.
•Intrafusal fibres are modified fibres which lack myofibrils at the
centre, but contain contractile ends. They contract when -MNs fire.
•Sensory neurons wrap around the middle of the fibres and project to the
spinal cord. They fire when the ends of the intrafusal fibres stretch.
Muscles Maintain Muscle Tone
•When muscles are at resting length, the spindles are slightly stretched
causing sensory neurons to generate tonic activity. As a result of
sensory neurons activating -MNs resulting in muscles maintaining a
certain level of tension, or tone at rest.
Length of Intrafusal Fibres Affects Sensory
Neuron Activity
•Muscle stretch also
stretches the spindles,
causing sensory neurons
to fire.
•This in turn causes the
muscle to contract.
•Contraction relieves the
stretch on spindles, and
reduces sensory neuron
AP firing.
•The contraction acts as a negative feedback to diminish the reflex.
•The reflex pathway in which muscle stretch initiate a contraction is known
as a stretch reflex.
-MNs Keep Spindles Functioning During Contraction
•Normally,  and
 MNs fire
together during
muscle
contraction.
•This causes both
intrafusal and
extrafusal fibres to
contract together.
•Contraction of
intrafusal fibres
causes their
middle portion to
stretch, causing
sensory neurons to
fire.
Example of Muscle Spindle Function During A
Stretch Reflex
Myotatic Stretch Reflex And Reciprocal Inhibition
•The synergistic and
antagonistic muscles
that control a single
joint are known as a
myotatic unit.
•The simplest reflex
in a myotatic unit is
a monosynaptic
stretch reflex.
•The knee jerk reflex
is an example of a
monosynaptic stretch
reflex.
•In addition to contraction, there’s relaxation of antagonistic muscles
(reciprocal inhibition) upon inhibition of their -MNs from firing.
Summary
•Integration of sensory information into involuntary responses is the hallmark of a
reflex. Feedback regulates reflexes.
•Reflexes can be both monosynaptic and polysynaptic.
•Sensory receptors in the muscle send information into the CNS to in turn regulate
motor neuron activity.
•- and -MNs to muscles are activated following sensory information processing.
•Sensory receptors in muscle sense tension and length.
•Muscle spindle receptors respond to stretch. Their length is affected by muscle length.
•- and -MNs are activated together during muscle contraction. -MNs keep spindle
functioning during muscle contraction.
•The knee jerk reflex involves contraction of quadriceps and relaxation of hamstrings
(reciprocal inhibition). Sensory neurons from quadriceps muscle spindle excite  -MNs
to the quadraceps, and activate inhibitory neurons which suppress AP firing in  -MNs
to hamstrings.
My Lab Website:
http://www.neurobio.arizona.edu/faculty/karunanithi/index.php
References
1. Tortora, G.J. & Grabowski, S.R (2003). Principles of
Anatomy & Physiology.New Jersey: John Wiley & Sons. Ch.
13, pp.426-433; Ch.15, pp.505-506; .
2. Silverthorn, D.U (1998). Human Physiology: An Integrated
Approach. New Jersey: Prentice Hall. Ch.13, pp.362-374.