Course 17.2Lesson 3 of 3

The spinal cord and the reflex arc

The spinal cord carries messages between the brain and the body, and it also runs reflexes. A reflex is a quick and involuntary response to a stimulus that does not need conscious effort. The reflex arc has at least one afferent neuron and one efferent neuron. In the knee jerk the impulse goes in through the dorsal root and out to the muscle.

about 30 min1 simulation1 practice question

Lesson 3 of 3Show all lessons

Try the code and the simulations beside the lesson.

A response without thinking

Your hand jerks away from a hot pan before you have decided to move it. This kind of response is a reflex. The entire process of response to a peripheral nervous stimulation that occurs involuntarily, without conscious effort or thought, and that needs a part of the central nervous system, is a reflex.

The reflex arc

The path of a reflex is the reflex arc. It has at least:

a receptor, such as a sense organ, that receives the stimulus;
one afferent neuron (the sensory neuron). It receives the signal from the sense organ and carries the impulse into the CNS through a dorsal nerve root, at the level of the spinal cord;
one efferent neuron (the motor neuron). It carries the signal from the CNS to the effector;
an effector, a muscle or a gland, that responds.

The two neurons are arranged in a series. The stimulus and the response together make the reflex arc. The knee jerk is a reflex that uses such an arc.

A diagram of a reflex arc. A receptor in the skin on the left receives a stimulus. A sensory neuron, which is the afferent neuron, carries the impulse along a nerve and through the dorsal root into the spinal cord, shown as a cross section in the middle. In the grey matter it makes a synapse with a motor neuron, which is the efferent neuron. The motor neuron leaves through the ventral root and carries the impulse along a nerve to a muscle on the right, which is the effector. An optional interneuron between the two neurons is drawn with a dashed line. Arrows show the direction of the impulse.

What the spinal cord looks like (not in the NEET syllabus)

NCERT does not describe the structure of the spinal cord. This part is for A-Level and IB.

The spinal cord lies inside the vertebral column.
A cross section shows an inner butterfly shaped region of grey matter and an outer region of white matter. This is the opposite of the cerebrum, where the grey matter is outside.
The dorsal root carries sensory fibres into the cord. It has a swelling, the dorsal root ganglion, which holds the cell bodies of the sensory neurons.
The ventral root carries motor fibres out of the cord.
The two roots join to form a spinal nerve. A human has 31 pairs of spinal nerves and 12 pairs of cranial nerves.

A cross section of the spinal cord. The outer part is the white matter. Inside it lies a butterfly shaped region of grey matter, with two dorsal horns pointing up and two ventral horns pointing down, and a small central canal in the middle. On the left, the dorsal root, which carries sensory fibres into the cord, has a swelling called the dorsal root ganglion. The ventral root, which carries motor fibres out of the cord, leaves from the bottom of the grey matter. The two roots join to make a spinal nerve. A note says that this structure is taught for A-Level and IB and is not in the NEET syllabus.

You should find that a reflex is much faster than a voluntary action, because the path is shorter, it has fewer synapses and the brain does not spend time deciding. A faster fibre helps both, but the brain's decision time is the biggest part of the voluntary action.

Reflex and feeling

When you touch something hot, two things happen. The reflex arc pulls the hand away through the spinal cord. At the same time, other impulses climb to the brain, and you feel the pain. So the movement comes first, and the feeling comes slightly later. NCERT does not say this, but it follows from the arc.

Cutting different parts of the arc gives different results.

If the afferent path is cut, the reflex is lost and the person does not feel the stimulus.
If the efferent path is cut, the reflex movement is lost, but the person still feels the stimulus.

Try it in Python

The program adds up the time of a reflex arc.

Practise with new numbers

Common mistakes

Saying that a reflex does not involve the CNS. A reflex needs a part of the CNS, such as the spinal cord.
Saying that the efferent neuron enters the cord through the dorsal root. The afferent neuron enters through the dorsal root. The efferent neuron leaves through the ventral root.
Saying that all reflexes go through the brain. A reflex such as the knee jerk is completed in the spinal cord.
Saying that a reflex needs a decision. A reflex is involuntary, and it needs no conscious effort.
Mixing up grey matter and white matter in the cord. In the cord the grey matter is inside. In the cerebrum it is outside.

Remember these

Reflex: an involuntary response to a stimulus that needs a part of the CNS.
Reflex arc: at least one afferent neuron and one efferent neuron in a series, as in the knee jerk.
The afferent neuron enters the CNS by a dorsal nerve root. The efferent neuron carries the signal to the effector.
A reflex is faster than a voluntary action, because the path is short and the brain does not decide.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

What is a reflex action?

What is the smallest number of neurons in a reflex arc, and what are they called?

Through which root does the afferent neuron enter the spinal cord?

Give one reason why a reflex is faster than a voluntary action.

A person cannot move a finger away from a hot object but feels the heat. Which part of the reflex arc is most likely damaged?

References

Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 35, The Nervous System, section 35.3.
Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 13, Anatomy of the Nervous System.
NCERT (2023). Biology, Textbook for Class XI, chapter 18.