Quartz 4

Reflex Action (Involuntary Actions)

3 min read

Definition

Definition: Reflex action is an immediate response to a specific stimulus without conscious control #BiologyDefinitions

Reflex actions → rapid + automatic → are the simplest form of response in humans and can occur without any conscious effort. → not normally possible to prevent them consciously → reflex actions are examples of involuntary actions

Spinal chord and brain are reflex centres. Reflex actions might be classified as follows:

  • Cranial Reflexes
    • These are controlled by the brain (but not conscious will) + usually occur in the head region.
    • Examples: pupil reflex, blinking, salvation
  • Spinal Reflexes
    • Controlled by the spinal chord
    • Example: sudden withdrawal of hand when touching a hot object and knee-jerk reflex

What Happens When Your Hand Touches a Hot Object?

  • is an example of spinal reflex action.

  1. receptors in the skin

    • high heat from the object stimulates your nerve endings (thermoreceptors) in your skin
    • nerve impulses are produced

  2. Sensory Neurone

    • sensory neurone transmits the nerve impulses toward your spinal chord

  3. Spinal Cord

    • In your Spinal chord, the nerve impulses are transmitted first across a synapse to the relay neurone IN THE SPINAL CORD, and then across another synapse to the motor neurone. (which is found in the PNS)

  4. Motor Neurone

    • Motor Neurone transmits the nerve impulses from the spinal chord to the effector.

  5. Effector Muscle

    • Biceps muscle (effector) then contracts and causes your hand to withdraw suddenly.

Sample

HIGH heat from object stimulated THERMORECEPTORS in the skin. Nerve impulses are produced. Sensory neurone transmits the nerve impulses toward your spinal cord. In the spinal cord, the nerve impulses are transmitted ACROSS A SYNAPSE to the relay neurone in the spinal cord, and then ACROSS ANOTHER SYNAPSE to a motor neurone. The motor neurone transmits nerve impulses from the spinal cord to the effector (i.e. bicep muscle) Bicep muscle then contracts (effector muscle) ⇒ causing hand to withdraw (outcome)

memorise!

Important

sometimes relay neurone can also be in the brain. It depends whether it is a spinal reflex or cranial reflex to decide whether it is spinal cord or brain. it is a MUST to state location of this relay neurone. Location of motor neurone is not needed!

(think about what will happen if a persons sensory or motor neurone is cut)

Reflex Arc

→ this nervous pathway is known as the reflex arc

Definition

Definition: Reflex arc is the shortest pathway by which nerve impulses travel from the receptor to the effector in a reflex action.

BiologyDefinitions

Consists of:
  • A receptor or sense organ
  • Sensory neurone
  • Relay Neurone in CNS (spinal cord or brain)
  • Motor neurone
  • An Effector (muscle or gland)

Nervous Pathway in a reflex arc can be written as follows

Note

Conditioned Reflex

  • Reflex action acquired from past experience or learning with a stimulus → which is originally ineffective in producing the response

Important

describe pathway taken by nerve impulses which allows person to perceive sensation of heat. Thermoreceptors in skin → sensory neurone → relay neurone in spinal cord → BRAIN

Graph View

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