Plant and animal responses: (i) reflex actions

The idea

A reflex action is a rapid, involuntary and stereotyped response to a stimulus, coordinated by a reflex arc so that the brain is not required to make the decision. The simplest arc runs from receptor to sensory neurone, into the spinal cord or brain, then to a motor neurone and effector; the knee-jerk (patellar) reflex is monosynaptic, with the sensory neurone synapsing directly onto the motor neurone, so no relay neurone is involved. Most protective reflexes, such as the blinking reflex and the withdrawal reflex, are polysynaptic and do include relay neurones, which allows rapid protective responses and some modification by the brain.

What earns the marks

• Identifies a reflex action as a rapid, involuntary and stereotyped response to a stimulus.
• Describes the reflex arc as receptor, sensory neurone, coordinator (spinal cord or brain), motor neurone and effector.
• States that the knee-jerk reflex is monosynaptic, with the sensory neurone synapsing directly onto the motor neurone and no relay neurone.
• Explains that polysynaptic reflexes such as the blinking reflex and withdrawal reflex include one or more relay neurones.
• Recognises that reflex arcs bypass the need for conscious decision-making by the brain, although the brain can be informed and can modify some reflexes.
• Distinguishes the roles of sensory neurone, relay neurone and motor neurone within a named reflex pathway.

Common mistakes and how to put them right

• Inserts a relay neurone into the knee-jerk reflex.

  Put it right: Describe it instead as monosynaptic, with the sensory neurone synapsing directly onto the motor neurone.

• Treats every reflex as identical in pathway.

  Put it right: Distinguish monosynaptic reflexes such as the knee-jerk from polysynaptic reflexes such as blinking and withdrawal.

• Describes a reflex as a voluntary or consciously controlled action.

  Put it right: State that it is involuntary and stereotyped.

• Omits the effector from the reflex arc.

  Put it right: Include the muscle or gland that carries out the response.

Exam tips

• Learn one monosynaptic and two polysynaptic examples with their full pathways so you can compare them.
• When asked about a named reflex, name each neurone type in order and state whether a relay neurone is present.
• Use the terms involuntary, stereotyped and rapid precisely, since each carries a separate mark-worthy idea.