Spinal Cord Anatomy
and Reflex Arcs
Explore the structure of the spinal cord, understand how reflex arcs enable rapid involuntary responses, and trace the pathway from stimulus to response.
Anatomy & Physiology
What Is the Spinal Cord?
The spinal cord is a long, cylindrical bundle of nervous tissue extending from the brainstem (medulla oblongata) through the vertebral canal. It serves as the main information highway between the brain and the rest of the body.
Spinal Cord Cross-Section
Grey vs White Matter
Grey Matter
Contains cell bodies, dendrites, and unmyelinated axons. Organized into dorsal horns (sensory), ventral horns (motor), and lateral horns (autonomic at T1-L2).
White Matter
Contains myelinated axons organized into columns (funiculi). Ascending tracts carry sensory info to the brain; descending tracts carry motor commands downward.
Dorsal and Ventral Roots
Dorsal Root
Carries sensory (afferent) signals from receptors into the spinal cord. Contains the dorsal root ganglion (DRG): a cluster of sensory neuron cell bodies.
Ventral Root
Carries motor (efferent) signals from the spinal cord to muscles and glands. Cell bodies reside in the ventral horn of grey matter.
What Is a Reflex Arc?
A reflex arc is the neural pathway that mediates a reflex action: a rapid, involuntary response to a stimulus. Reflexes protect the body by allowing responses without waiting for brain processing.
Five Components in Detail
1
Receptor: Detects the stimulus (e.g., pain receptor in the skin, muscle spindle).
2
Sensory (Afferent) Neuron: Transmits impulse from receptor to the spinal cord via the dorsal root.
3
Integration Center: In the spinal cord grey matter. May involve an interneuron or synapse directly on a motor neuron.
4
Motor (Efferent) Neuron: Carries the response signal from the spinal cord to the effector via the ventral root.
5
Effector: The muscle or gland that carries out the response (e.g., bicep contracts, hand withdraws).
Monosynaptic vs Polysynaptic Reflexes
Monosynaptic
Only one synapse in the pathway: sensory neuron synapses directly on the motor neuron. Fastest response. Example: the patellar (knee-jerk) reflex.
Polysynaptic
Two or more synapses: involves at least one interneuron between sensory and motor neurons. Slightly slower but more complex. Example: the withdrawal (flexor) reflex.
Somatic vs Autonomic Reflexes
| Feature | Somatic Reflex | Autonomic Reflex |
|---|---|---|
| Effector | Skeletal muscle | Smooth muscle, cardiac muscle, glands |
| Control | Voluntary muscle, involuntary response | Involuntary muscle, involuntary response |
| Speed | Very fast (myelinated fibers) | Slower (some unmyelinated fibers) |
| Pathway | One motor neuron to effector | Two motor neurons (pre- and postganglionic) |
| Example | Knee-jerk, withdrawal | Pupil constriction, heart rate change |
Worked Example: Patellar Reflex
Worked Example: Withdrawal Reflex
Advanced: Crossed Extensor Reflex
When you step on a sharp object, the withdrawal reflex flexes the injured leg. Simultaneously, the crossed extensor reflex extends the opposite leg to support your weight and prevent falling.
How to Trace a Reflex Arc
1
Identify the stimulus: What event triggers the response? (e.g., heat, stretch, pressure)
2
Find the receptor: Which type of receptor detects it? (e.g., thermoreceptor, muscle spindle, nociceptor)
3
Trace the sensory neuron: Impulse travels via the dorsal root to the spinal cord.
4
Check for interneuron: Is there one? If yes, it is polysynaptic. If no, it is monosynaptic.
5
Trace the motor neuron: Impulse exits via the ventral root to the effector.
6
Identify the effector and response: Which muscle contracts or which gland secretes?
Spinal Cord Regions
Common Mistakes
✕ Mixing Up Roots
Confusing dorsal (sensory) with ventral (motor). Remember: "D is for Dorsal and Detector" (sensory detects).
✕ Brain Required
Thinking reflexes require brain involvement. Spinal reflexes are processed at the spinal cord level: the brain is informed after the response.
✕ All Reflexes Are Simple
Assuming all reflexes are monosynaptic. Most reflexes are polysynaptic with interneurons. Only stretch reflexes (like the patellar) are monosynaptic.
Reflex Arc Summary Table
| Feature | Patellar (Stretch) | Withdrawal (Flexor) | Crossed Extensor |
|---|---|---|---|
| Type | Monosynaptic | Polysynaptic | Polysynaptic |
| Interneuron? | No | Yes | Yes (crosses midline) |
| Stimulus | Tendon stretch | Painful stimulus | Painful stimulus (foot) |
| Response | Leg extends | Limb withdraws | Opposite limb extends |
| Synapses | 1 | 2+ | 3+ |
Question 1 of 5
Which structure carries sensory (afferent) information into the spinal cord?
The dorsal root carries sensory (afferent) neurons into the spinal cord. The ventral root carries motor (efferent) signals out. Remember: "D is for Dorsal and Detector."
Question 2 of 5
The patellar (knee-jerk) reflex is classified as which type of reflex?
The patellar reflex is a monosynaptic somatic reflex: it has only one synapse (sensory neuron directly on motor neuron) and its effector is skeletal muscle (somatic). No interneuron is involved.
Question 3 of 5
In a spinal cord cross-section, grey matter is located where, and white matter is located where?
In the spinal cord, grey matter forms a butterfly-shaped core surrounded by white matter on the outside. This is the opposite of the brain, where grey matter (cortex) is on the surface.
Question 4 of 5
In the withdrawal reflex, what component does the interneuron represent?
The interneuron is part of the integration center in the spinal cord. It receives input from the sensory neuron and relays the signal to the motor neuron, allowing the CNS to "decide" the response without involving the brain.
Question 5 of 5
In the crossed extensor reflex, what happens to the contralateral limb when the ipsilateral limb flexes?
In the crossed extensor reflex, the contralateral (opposite) limb extends to bear the body's weight while the ipsilateral (same-side) limb flexes away from the painful stimulus. This prevents you from falling.