Reflexs by duggybrown

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									Spinal Reflexes
What Is a Reflex?
 Neural reflexes are stereotyped, automatic, involuntary
   reactions of the Central Nervous System (CNS) to a
   specific sensory input.

     They are different to:
 -   voluntary, goal directed movements, i.e. learned
     experiences (such as reaching for a glass).
 -   rhythmic motor patterns (i.e. walking).
What Are the Functions of Reflexes?


Reflexes produce a rapid characteristic response to a
    stimulus. These responses can be:

1)   protective – e.g. withdrawal of limb from hot surface
     or a cough reflex.

2)   Important in postural control - so you don't fall over
     whenever you move a limb.
Classification of reflexes

  Reflexes can be categorised into one of the following:

     Somatic reflexes: involve somatic motor neurones
      and skeletal muscles.

     Autonomic reflexes: the responses are controlled
      by autonomic neurones.
    Further classification of reflexes
Reflexes can be further classified by considering:

   the CNS system where the reflex is integrated (spinal
    cord: spinal reflexes; brain: cranial reflexes)

   number of neurones in the pathway (N.B: All autonomic
    reflexes are polysynaptic, whereas somatic reflexes can
    be either monosynaptic or polysynaptic).
Somatic Reflexes
- Somatic motor neurones supply skeletal muscles.
- Skeletal muscle reflexes are involved in almost everything
   we do.
-The components of skeletal muscle reflexes form the
   Reflex arc.
- The reflex arc is the most basic unit of integrated activity
   in the nervous system.
There are 5 components to the reflex arc:

   (1)
RECEPTOR            (2)
                 SENSORY
                 AFFERENT
                                   (3)
                                CENTRAL
                                NEURONES
                     (4)
                   MOTOR
   (5)            EFFERENT
EFFECTOR
1) Receptors
 - detect stimuli which include light, sound, smell, pain muscle
    tension.
 - act as signal transducers, i.e. convert stimulus into electrical
    signals.

 Location:
 - on the surface of the skin e.g. nociceptors, which detect painful
   stimuli or thermoreceptors, which detect changes in temperature

 -  within the muscle: receptors found in skeletal muscle include:
 a) Muscle spindles: detect changes in muscle length
 b) Golgi tendon organs: detect changes in muscle tension
2) Sensory Afferents
Sensory afferents carry information from the receptors to
  the CNS.

Various types of sensory afferents include:
 Group 1a
 Group 1b
 3) Central Neurones
Sensory afferent fibers enter the spinal cord and synapse
onto either:
- Interneurones: e.g. Ia and Ib interneurones; or
- Cell body (soma) of efferent motor neurons :

        Sensory (afferent) input




       Motor (efferent) output
   ‘Lower Motor Neuron’ / Alpha Motor Neuron
 4) Motor Efferent
 Efferent motor neurons are of various types:
 * Alpha – extrafusal (bulk of muscle) or
 * Gamma- intrafusal fibres of muscle spindles.


5) Effector
 Effectors convert electrical signals from efferent
 signals into an appropriate response.
 - i.e. smooth muscle, skeletal muscle.
Examples of skeletal muscle reflexes


   Monosynaptic: stretch reflex

   Polysynaptic: Golgi tendon reflexes, crossed-extensor
    reflex
a) The Muscle Spindle
   Detects muscle length
    changes.

   Specialised skeletal muscle
    fibres contained in a fibrous
    capsule.

   Group 1a sensory afferents
    wrap around the swollen
    middle of the capsule.
Role of the Muscle spindle
   Any movement that increases muscle length also
    stretches the muscle spindle and causes its sensory
    fibers to fire rapidly, e.g placing a load on a hand
    stretches the muscle and the spindles.
   This creates a reflex contraction of the muscles, which
    prevents the muscle from over-stretching. (i.e. arm
    position is restored)
   This pathway is known as the stretch reflex.
The Stretch Reflex




 Motor neuron
      Sensory afferent




Alpha motor neuron
 Stretch   sensory activity  
Motor activity
Contraction
b) The Golgi Tendon Organ
   Located at the junction of
    muscle and tendon.

   Acts like a strain gauge.

   Monitors tension.

   Innervated by 1b sensory
    axons.
Golgi Tendon Reflexes

 The Golgi tendon reflex protects the arm from
   excessively heavy loads by causing the muscle to
   relax and drop the load.

 -   prevents damage due to overwork.

 -   Slows muscle contraction.
Golgi Tendon Organ (Muscle Tension)


    muscle contraction
      muscle and tendon tension
      GTO activation
      sensory neuron activity
      inhibitory neuron activity
      motor activity
Golgi tendon organ
Sensory
                     Inhibitory
neuron
                     interneuron




                 Motor neuron
Reciprocal Inhibition
   In addition to the excitation of the effector muscle during
    the stretch reflex, the antagonist muscle is inhibited at
    the same time.

   This is achieved by Reciprocal Inhibition caused by
    activation of the Ia inhibitory interneuron.
Reciprocal Inhibition of Flexors and Extensors
Complex Stretch Reflexes
    Polysynaptic reflexes: slower.

    Involves interneurons: inhibitory, excitatory, Renshaw cells.

    Receptor and effector may be in different parts of the body.

    For example, the Flexion (Withdrawal) reflexes are poly-
     synaptic reflexes that cause an arm or leg to move away
     from a painful stimulus e.g. pinprick or a hot stove.
    Flexion Reflex
Sensory fibers carry information from the nociceptors to the spinal
Cord, where they branch. These branches activate multiple excitatory
interneurones at different levels:

-   Some of these interneurones
    excite alpha motor neurons
    leading to contraction of the
    flexor muscles.

-   Other interneurones
    simultaneously activate inhibitory
    interneurones that cause
    relaxation of the antagonistic
    muscle groups (extensors). So
    the limb is flexed, withdrawing it
    from the stimulus.
Crossed-Extensor Reflex

The quick withdrawal of the right foot from a painful
  stimulus is matched by extension of the left leg so that
  it can support the sudden shift in weight.

This is brought about by the action of the Crossed-
  extensor Reflex: a postural reflex that helps maintain
  balance when one foot is lifted off the ground.
      The Crossed Extensor Reflex
The activation of flexors and the
inhibition of extensors on one
side of the body is accompanied
by the inhibition of flexors and
activation of extensors on the
other side.

Involves sensory input to
multiple excitatory and inhibitory
interneurones.
Central Pattern Generators
   The most complicated spinal reflex pathways are
    controlled by networks of neurones in the CNS called
    Central pattern generators (CPGs).
   Once activated, CPGs create spontaneous repetitive
    movement.
   In humans, rhythmic movements controlled by CPGs
    include locomotion and the unconscious rhythmicity of
    quiet breathing.
Higher Centres and the Control of Movement

    Motor Cortex
             Planning and coordinating complex movements.
    Brain Stem
             Posture, hand and eye movements.
    Basal Ganglia
             Motor planning.
    The cerebellum
             Adjustment of fine movements.

								
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