Reflexes

What is a reflex?

Automatic, involuntary actions that occur in response to a stimulus
Typically a predictable response that occurs before the impulse reaches the brain
Each has a specific function but all circuits share common structural characteristics

Reflex arc

Stimulus → Receptor → Afferent (sensory) pathway → (Integration) → Efferent (motor) pathway → Effector → Response

Classification - development

Innate - genetically determined
Acquired - learned

Classification - response

Somatic - control skeletal muscle contractions, include superficial and stretch reflexes
Visceral (autonomic) - control actions of smooth and cardiac muscles and glands

Classification - complexity of circuit

Monosynaptic - one synapse
Polysynaptic - multiple synapses

Classification - processing site

Spinal reflexes - processing in spinal cord
Cranial reflexes - processing in brain

Monosynaptic spinal reflex

E.g. stretch reflex
Somatic and proprioceptive

Stretch reflex - reflex arc

Stimulus (muscle stretch) → Receptor (muscle spindle) → Afferent pathway (Ia and group II fibres) → Integration (synapse in the spinal cord - monosynaptic) → Efferent pathway (alpha motor neurone) → Effector (skeletal muscles) → Response (muscle contraction)

Function of stretch reflex

Helps maintain muscle tone and tension for readiness
Can prevent overstretching and injury to an extent

Muscle spindle

Intrafusal muscle (non-contractile proteins) which are next to extrafusal muscle (contractile proteins)
Fibrous capsule that spindle sits in - sensory neurone wraps around to detect movement and send information to the spinal cord
Sensory afferent fibres (Ia and II) detect change in muscle length and rate of change in length (velocity)

Flexor withdrawal reflex

Protective function of rapidly withdrawing a limb from a potentially harmful stimulus by contracting flexor muscles to pull limb away from danger

Flexor withdrawal and crossed extensor - reflex arc

Pain → Detected by nocioceptor → Afferent pathway → Spinal cord (inter neurone) → Efferent pathway → Flexor muscles → Contraction of flexors away from stimulus, contraction of extensors

Crossed extensor

Enhances postural support during withdrawal of an affected limb from a painful stimulus

Modulation of reflexes

Done by sending descending signals from different regions of the spinal cord
Responses can be inhibited
Brain receives and sends messages to and from spinal cord which controls, inhibits and regulates these messages

Ascending pathway

Information going to the brain and spinal cord - sensory information
Enters spinal cord at dorsal root

Descending pathway

Information leaving the spinal cord - motor information
Leaves spinal cord at ventral root

Diminished reflex

Less than expected
Damage anywhere along the reflex arc - mostly lower motor neurone or peripheral nerve problem

Absent reflex

No response

Brisk reflex

Exaggerated response
Could be caused by an upper motor neurone problem (brain or spinal cord)
Information from the brain sends information to a gamma motor neurone which innervated the pole of muscle spindles (this is what sets the tone of muscle)
Inhibition of this is lost - short circuit, information not going to the brain

Reciprocal innervation

Contracting muscle group causes opposing muscle group to relax, causing smoother and more coordinated movement