The scalp is a multi-layered soft tissue structure covering the skull. It extends from the eyebrows (superior orbital margins) anteriorly to the nape of the neck (superior nuchal lines) posteriorly and laterally to the temporal lines on both sides.[1] The scalp consists of five layers: the skin, connective tissue, aponeurosis, loose areolar tissue, and the pericranium, which collectively protect the skull and support the hair.
Location
The scalp is located on the upper part of the head, covering the cranial bones. It stretches from the forehead and eyebrows anteriorly, over the crown, and down to the back of the head at the superior nuchal line.[4]
Structure and Anatomy
The scalp is a complex soft tissue structure composed of five distinct layers. These layers work together to protect the skull and provide a framework for hair growth.[8] The layers can be remembered using the acronym SCALP: Skin, Connective tissue, Aponeurosis, Loose areolar tissue, and Pericranium.
Skin
- Description:
- The outermost layer of the scalp, thick and durable.
- Contains numerous hair follicles, sebaceous (oil) glands, and sweat glands.
- Features:
- Richly vascularized, ensuring a robust blood supply.
- Contains sensory nerve endings, making it highly sensitive to touch and pain.
Connective Tissue (Dense Subcutaneous Tissue)
- Description:
- A thick layer of dense connective tissue beneath the skin.
- Features:
- Contains a rich network of blood vessels and nerves.
- The vascular supply is tightly bound within the connective tissue, making the scalp prone to profuse bleeding when injured.[6]
- Clinical Note:
- This layer’s firmness prevents vessels from retracting when cut, contributing to persistent bleeding in scalp injuries.
Aponeurosis (Galea Aponeurotica)
- Description:
- A strong, fibrous sheet of connective tissue forming the third layer of the scalp.
- Also referred to as the epicranial aponeurosis.[7]
- Features:
- Connects the frontalis muscle at the front of the head to the occipitalis muscle at the back.
- Facilitates movement of the scalp, such as raising the eyebrows.
- Attachments:
- Laterally attached to the temporalis fascia, providing structural stability.
Loose Areolar Tissue
- Description:
- A layer of loose connective tissue beneath the aponeurosis.
- Features:
- Allows free movement of the upper layers (skin, connective tissue, and aponeurosis) over the underlying pericranium and skull.
- Contains small blood vessels and veins that communicate with the intracranial venous system via emissary veins.
- Clinical Note:
- This layer is referred to as the “danger area” of the scalp because infections here can spread into the cranial cavity through emissary veins.
Pericranium
- Description:
- The innermost layer of the scalp, consisting of the periosteum of the cranial bones.
- Features:
- Tightly adherent to the surface of the skull bones.
- Provides a vascular supply to the outer surface of the cranial bones.[5]
- Capable of forming new bone during healing or in response to injury.
Blood Supply
The scalp has an extensive vascular network supplied by branches of the external and internal carotid arteries:
- External Carotid Artery: Superficial temporal, posterior auricular, and occipital arteries.
- Internal Carotid Artery: Supratrochlear and supraorbital arteries.
Innervation
- Anterior Scalp: Supplied by the ophthalmic division of the trigeminal nerve (supratrochlear and supraorbital nerves).
- Lateral Scalp: Supplied by the auriculotemporal nerve.
- Posterior Scalp: Supplied by the greater occipital and lesser occipital nerves.
Function
Protection
The scalp provides a protective covering for the skull and the underlying brain, shielding them from physical trauma, temperature changes, and environmental factors.
Support for Hair Growth
The scalp supports the growth of hair, which offers additional protection against sunlight and helps regulate body temperature by providing insulation.[3]
Facilitates Mobility
The loose areolar tissue layer allows the scalp to move freely over the skull, reducing the risk of injuries from external forces.
Blood Supply to the Skull
The vascular network within the scalp nourishes the overlying skin and underlying periosteum, contributing to the overall health and repair of tissues.
Sensory Perception
The scalp contains numerous sensory nerve endings, enabling the detection of touch, pain, and temperature changes.
Expression and Communication
Muscles attached to the aponeurosis, such as the frontalis, contribute to facial expressions like raising the eyebrows, which play a role in non-verbal communication.
Clinical Significance
The scalp is clinically significant due to its complex structure and susceptibility to various injuries and conditions:
- Scalp Lacerations:
- Scalp injuries can bleed profusely due to the dense vascular network. The connective tissue layer holds blood vessels open, preventing retraction and prolonging bleeding.
- Scalp Infections:
- Infections in the loose areolar tissue layer can spread rapidly and may enter the cranial cavity via emissary veins, potentially leading to intracranial complications like meningitis or abscess formation.
- Alopecia (Hair Loss):
- Conditions like androgenetic alopecia, alopecia areata, or scalp infections (e.g., tinea capitis) can cause partial or complete hair loss, impacting appearance and self-esteem.
- Scalp Tumors:
- Benign (e.g., sebaceous cysts) or malignant (e.g., squamous cell carcinoma) lesions may develop on the scalp, requiring medical or surgical intervention.[2]
- Trauma:
- Severe scalp injuries, such as avulsion (scalp detachment), may require reconstructive surgery to restore function and appearance.