Human Anatomy & Physiology (Human Body Systems)

Unit 2: Unit 2: Communication (Nervous & Endocrine)HBS 2.1Human Body Systems: reflexes & signaling

Map a reflex arc

Trace a reflex from the stimulus through receptor, neurons, and spinal cord to the response: and explain why it skips the brain.

Builds on (2 levels back)inferred · high confidence

Neurons carry signals in one direction: A reflex arc has a fixed order, so you must know a sensory neuron carries signals toward the spinal cord and a motor neuron carries them away.
Stimulus and response: A reflex starts with a stimulus and ends with a response; naming these two endpoints anchors the whole pathway.

Prerequisites are inferred: pending teacher review.

Re-learn the skill with worked practice and clear examples.

A reflex arc has a fixed order: stimulus, receptor, sensory neuron, spinal cord (interneuron), motor neuron, effector, response. The signal goes through the spinal cord and bypasses the brain, which is why it is fast.

Step 1: Lay out the order

Stimulus to receptor to sensory neuron to interneuron in the spinal cord to motor neuron to effector (muscle) to response. The signal enters the spinal cord and is sent back out without going up to the brain to think.

Step 2: Read the pathway diagram

Follow the arrows: the change is detected, the signal travels in to the cord, crosses to a motor neuron, and travels back out to the muscle.

A flow of labeled boxes connected by arrows: Stimulus, Receptor, Sensory neuron, Spinal cord, Motor neuron, Effector (muscle), Response. The boxes are in order but the diagram does not state which box is the correct answer to any question.

Practice

Using the reflex-arc flow shown, which structure receives the signal from the sensory neuron BEFORE it reaches the motor neuron?

Reviewed

The same ordered flow of boxes: Stimulus, Receptor, Sensory neuron, Spinal cord, Motor neuron, Effector (muscle), Response, connected left to right by arrows. The diagram shows order only and does not mark a correct answer.

A.The effector muscle
B.The spinal cord
C.The receptor
D.The brain

Show the worked solution ▾

Answer: B. The spinal cord

Step 1: Find the sensory neuron in the flow: Locate the 'Sensory neuron' box and follow the arrow leaving it.
Step 2: Read the next box: The arrow from the sensory neuron points to the spinal cord box, which comes before the motor neuron.

Why it's right: In the arc, the sensory neuron carries the signal into the spinal cord, which then routes it to the motor neuron.

Why the others miss:

A: The effector comes after the motor neuron, not before it.
C: The receptor comes before the sensory neuron, not after it.
D: The diagram routes the signal through the spinal cord, not the brain, in a reflex.

Aligned to HBS 2.1: order of the reflex arc · reading level ~grade 9

Where you'd see this

A nurse who knows the arc order can tell whether a missing reflex points to a problem in the sensory side, the spinal cord, or the motor side.

Careers that use this

Registered Nurse: Checks reflexes during a neuro exam and uses the arc order to locate where a signal may be blocked.

Video library

Watch: Map a reflex arc

Components of a Reflex Arc

Dr. Veronica Foster

Guided notes

Fill these in as you work through the lesson.

Big idea: A reflex is an automatic, fast response that travels stimulus to receptor to sensory neuron to spinal cord to motor neuron to effector, ending in a response: and it bypasses the brain so it is quick.

Key terms: write the meaning

Reflex (automatic; no conscious decision):
Stimulus (the change that starts it):
Receptor (the part that detects the change):
Sensory neuron (carries the signal which way?):
Motor neuron (carries the signal which way?):
Effector (the muscle or gland that acts):

The rule

The order of a reflex arc is: stimulus to to sensory neuron to to motor neuron to to response.

Check yourself

You touch a hot stove and pull your hand back before you feel pain. Name the stimulus, the receptor, and the effector.
Why is a reflex faster than a response you have to think about?
Put these in order: motor neuron, receptor, response, spinal cord, sensory neuron, stimulus, effector.

Work one example

A doctor taps your knee and your lower leg kicks out. Walk the signal through every step of the reflex arc in order, and say where the brain is (or is not) in this path.