Scalp Anatomy

• The scalp is the cutaneous region covering the head, superior to the nuchal region, and laterally between the temporal regions.
• It is a multi-layered structure possessing unique characteristics, particularly in its vascularity and innervation.

Layers of the Scalp

• The scalp consists of five layers, often remembered by the acronym "SCALP":
  • Skin: The outermost layer, containing hair follicles, sebaceous glands, and sweat glands.
  • Connective Tissue: A dense layer of vascularized subcutaneous tissue, richly supplied with nerves.
  • Aponeurosis: The epicranial aponeurosis (galea aponeurotica), a tough, tendinous sheath connecting the frontalis and occipitalis muscles.
  • Loose Connective Tissue: A loose areolar layer that allows the superficial three layers to move as a unit over the pericranium.
  • Pericranium: The periosteum of the skull bones providing nutrition to the bone and the capacity for repair.

Skin

• The skin of the scalp is thicker than most other areas of the body.
• It contains a large number of hair follicles and sebaceous glands.
• The abundance of these structures makes the scalp prone to certain dermatological conditions.

Connective Tissue

• This layer is composed of dense connective tissue that tightly binds the skin to the epicranial aponeurosis.
• It is highly vascular, containing numerous arteries and veins.
• The vessels are supported by the dense connective tissue, preventing them from collapsing when cut, which can result in profuse bleeding from scalp wounds.
• This layer is richly innervated, containing sensory nerves that provide sensation to the scalp.

Aponeurosis

• The epicranial aponeurosis (galea aponeurotica) is a strong, tendinous sheet that spans the calvaria.
• It serves as an attachment point for the frontalis muscle anteriorly and the occipitalis muscle posteriorly.
• The aponeurosis allows for movement of the scalp and eyebrows.
• Laterally, it is connected to the temporalis fascia.

Loose Connective Tissue

• This layer consists of loose areolar tissue that separates the pericranium from the aponeurosis.
• It creates a plane of movement, allowing the superficial layers of the scalp to slide over the underlying pericranium.
• This layer is also referred to as the "danger zone" due to the ease with which infection or fluid can spread through it.
• Blood vessels and nerves course through this layer to reach the more superficial layers.

Pericranium

• The pericranium is the periosteum covering the outer surface of the skull bones.
• It is firmly attached to the skull, particularly at the sutures.
• The pericranium provides nourishment to the bone and has the capacity to form new bone during injury repair.
• It is continuous with the endosteum, the inner periosteal layer of the skull bones.

Arterial Supply

• The scalp has a rich arterial supply from both the external and internal carotid arteries.
• The arteries run in the connective tissue layer, providing extensive collateral circulation.
• The main arteries supplying the scalp are:
  • Supratrochlear artery: A branch of the ophthalmic artery (internal carotid).
  • Supraorbital artery: Also a branch of the ophthalmic artery (internal carotid).
  • Superficial temporal artery: A terminal branch of the external carotid artery.
  • Posterior auricular artery: A branch of the external carotid artery.
  • Occipital artery: A branch of the external carotid artery.

Venous Drainage

• The veins of the scalp generally accompany the arteries and have similar names.
• Venous drainage occurs through the following veins:
  • Supratrochlear and supraorbital veins: Drain into the angular vein, which connects to the facial vein.
  • Superficial temporal vein: Drains into the retromandibular vein.
  • Posterior auricular vein: Drains into the external jugular vein.
  • Occipital vein: Drains into the internal jugular vein or vertebral venous plexus.
• There are also emissary veins that pass through the skull and connect the scalp veins with the intracranial venous sinuses.
• These connections can provide a pathway for infections to spread intracranially.

Nerve Supply

• The scalp is richly innervated by both sensory and motor nerves.
• Sensory innervation is provided by branches of the trigeminal nerve (CN V) anteriorly and cervical spinal nerves posteriorly.
• The main sensory nerves are:
  • Supratrochlear nerve: Branch of the ophthalmic nerve (V1).
  • Supraorbital nerve: Branch of the ophthalmic nerve (V1).
  • Zygomaticotemporal nerve: Branch of the maxillary nerve (V2).
  • Auriculotemporal nerve: Branch of the mandibular nerve (V3).
  • Lesser occipital nerve: Branch of the cervical plexus (C2-C3).
  • Greater occipital nerve: Branch of the dorsal ramus of C2.
• Motor innervation to the frontalis and occipitalis muscles is provided by the facial nerve (CN VII).

Clinical Significance

• Lacerations: Scalp lacerations can bleed profusely due to the rich vascular supply and the inability of vessels to constrict within the dense connective tissue.
• Scalp Infections: Infections can spread easily through the loose connective tissue layer.
• Subgaleal Hematoma: Bleeding into the loose connective tissue layer can result in a subgaleal hematoma, which can be extensive due to the potential space available.
• Avulsion Injuries: The scalp can be avulsed (torn away) in severe trauma, typically occurring in the plane of the loose connective tissue.
• Seborrheic Dermatitis: A common inflammatory condition affecting the sebaceous gland-rich areas of the scalp, leading to scaling and itching.
• Scalp Tumors: Both benign and malignant tumors can occur on the scalp, requiring careful diagnosis and management.