Dermatomes and Cutaneous fields

Summary

In this detailed lesson, Dr. Morton explores the concepts of dermatomes and cutaneous fields, highlighting how sensory deficits occur from injuries to the brachial plexus roots versus peripheral nerves. Using the analogy of Google Maps, he explains how sensory neurons travel from specific dermatomes through the brachial plexus to the spinal cord. The video also distinguishes between the effects of injuring a brachial plexus root (which causes loss of sensation across an entire dermatome) and injuring a peripheral nerve (which affects only the associated cutaneous field). The video offers clear examples and practice questions to reinforce these anatomical concepts.

Takeaways

• 😀 Dermatomes are areas of skin supplied by sensory neurons from a single spinal cord level, and they can be transmitted through multiple peripheral nerves.
• 😀 The brachial plexus acts as the pathway for sensory neurons to travel from the dermatome to the spinal cord, much like how roads guide you from a starting point to a destination on Google Maps.
• 😀 Injury to the ventral ramus or root of the brachial plexus results in the loss of sensation across the entire dermatome associated with that spinal cord level.
• 😀 Cutaneous fields refer to areas of skin supplied by sensory neurons from a single peripheral nerve and may cover more than one spinal cord level.
• 😀 Sensory neurons from different dermatomes may be transmitted through a single peripheral nerve, such as the median nerve which serves both the C6 and C7 dermatomes.
• 😀 Injuries to peripheral nerves, like the musculocutaneous nerve, result in the loss of sensation in the entire cutaneous field associated with that nerve.
• 😀 The c5, c6, c7, and c8 dermatomes are mapped on the brachial plexus, and each corresponds to different regions of the upper limb.
• 😀 An injury to the upper trunk of the brachial plexus, which involves the c5 and c6 roots, affects both the C5 and C6 dermatomes in the upper limb.
• 😀 Injuries to the lower trunk, including c8 and t1, lead to loss of sensation in the ulnar nerve distribution and the medial cutaneous nerve of the arm and forearm.
• 😀 Clawing of the hand, as seen in conditions like Klumpke's palsy, occurs when there is damage to the lower trunk of the brachial plexus, affecting sensory and motor function in the corresponding dermatomes.

Q & A

What is a dermatome?

A dermatome is an area of skin supplied by sensory neurons from a single spinal cord level. These neurons may be transmitted through more than one peripheral nerve.

How do dermatomes relate to the brachial plexus?

Dermatomes are mapped onto the brachial plexus. Sensory neurons from a dermatome travel along the brachial plexus to reach specific spinal cord levels, with different nerves transmitting these sensory neurons.

What happens when a root of the brachial plexus is injured?

Injury to a root of the brachial plexus can lead to loss of sensation in the entire associated dermatome, as the sensory neurons from that dermatome are no longer able to transmit signals.

What is the analogy used to understand sensation in the brachial plexus?

The analogy used is understanding sensation in the brachial plexus is like using Google Maps. Just as a map shows the starting point, destination, and routes, the brachial plexus shows where sensory neurons start and end, with the nerves acting as routes.

What is the difference between a dermatome and a cutaneous field?

A dermatome is an area of skin supplied by sensory neurons from a single spinal cord level, while a cutaneous field is an area of skin supplied by sensory neurons from a single peripheral nerve.

How does a peripheral nerve injury affect sensation?

Injury to a peripheral nerve results in the loss of sensation in its associated cutaneous field, which may involve multiple spinal cord levels as the nerve carries sensory neurons from various dermatomes.

What would happen if the ventral ramus or a root of the brachial plexus is injured?

If the ventral ramus or a root of the brachial plexus is injured, the entire associated dermatome would lose sensation, as the sensory neurons cannot reach the spinal cord.

What is the role of the musculocutaneous nerve in sensation?

The musculocutaneous nerve transmits sensory neurons from the lateral cutaneous nerve of the forearm, contributing to sensation in that specific cutaneous field.

How do cutaneous fields overlap with dermatomes?

Cutaneous fields can overlap with dermatomes since the sensory neurons from a single peripheral nerve may span more than one spinal cord level, thus covering parts of multiple dermatomes.

What happens in the case of a Klumpke's palsy?

In Klumpke's palsy, the injury affects the lower trunk of the brachial plexus, leading to sensory loss in the C8 and T1 dermatomes, typically resulting in hand and arm deficits.