The spinal column is the central support structure of the body, composed of 33 individual bones called vertebrae that house and protect the spinal cord. It is divided into three main sections: cervical (C), thoracic (T), and lumbar (L). The thoracic spine, which forms the mid-back, contains twelve vertebrae, numbered T1 through T12. The T4 vertebra is the fourth bone in this sequence, positioned in the upper portion of the mid-back.
Locating T4 within the Spinal Column
The T4 vertebra is situated directly below T3 and above T5, placing it in the upper quarter of the twelve thoracic bones. The thoracic region sits inferior to the seven cervical vertebrae and superior to the five lumbar vertebrae. Thoracic vertebrae are unique because they articulate with the ribs, forming the protective rib cage.
T4 is distinguished by its heart-shaped body, which is larger than the cervical vertebrae but smaller than the lumbar vertebrae. Like most thoracic vertebrae, T4 has specific facets on its sides for articulation with the heads of the fourth and fifth ribs. Its spinous process, the bony projection felt under the skin, is long and angled downward, overlapping the vertebra below it.
To begin an anatomical count to T4, one typically starts at the prominent bump at the base of the neck, which is the spinous process of the seventh cervical vertebra (C7). Moving one bone down from C7 reaches the first thoracic vertebra (T1). Counting three more bones down from T1 (past T2 and T3) identifies the T4 vertebra’s position.
Practical Identification: Finding T4 on Your Body
Physically locating the T4 vertebra involves using surface anatomy markers on the shoulder blades (scapulae). The most common method begins by finding the superior angle of the scapula. This point generally aligns with the T2 spinous process.
From the T2 level, a healthcare professional can palpate and count down two segments to find the T4 spinous process. Alternatively, the spine of the scapula—the horizontal ridge of bone—often aligns with the T3 spinous process. Counting one segment down from the T3 landmark identifies the T4 vertebra.
An anterior landmark for the T4 level is the sternal angle (Angle of Louis), the ridge where the manubrium and body of the sternum meet. This anterior structure aligns with the intervertebral disc space between the T4 and T5 vertebrae. Since the T4 spinous process projects downward, finding these anterior and posterior landmarks helps triangulate the location of the T4 segment.
Functional Significance of the T4 Segment
The T4 vertebra houses and protects the spinal cord, from which the T4 spinal nerves emerge. These nerves, specifically the anterior ramus, are known as intercostal nerves because they travel between the ribs in the fourth intercostal space. This segment is responsible for motor and sensory function in the upper chest wall.
The sensory area of skin innervated by the T4 spinal nerve is known as the T4 dermatome. This dermatome encompasses the nipple line on the front of the chest. This sensory innervation is often used by clinicians to test the integrity of the T4 nerve root.
The T4 segment contributes to the sympathetic nervous system, which controls involuntary functions like heart rate, breathing, and blood pressure. Nerve fibers from the T4 area influence the heart and lungs, as well as sympathetic outflow to the upper limbs. Because of these connections, irritation at the T4 level can cause symptoms that feel distant from the mid-back.
Common Conditions and Injuries Involving T4
The T4 segment is a common site for mid-thoracic pain, often caused by muscle strain, joint dysfunction, or postural stress. One specific presentation is T4 Syndrome, a poorly understood condition where dysfunction in the upper thoracic spine (T2-T7) causes a cluster of symptoms. These symptoms often include upper back stiffness accompanied by pain, numbness, or a “pins and needles” sensation in the arms and hands.
The thoracic spine is generally stable, but T4 and T5 are frequently injured segments in the upper thoracic region. This area is susceptible to compression fractures, especially in individuals with weakened bones due to conditions like osteoporosis. These fractures occur when the vertebral body collapses, often resulting from trauma or a fall.
Due to neural connections to the chest organs, irritation of the T4 nerve root can result in referred pain, meaning the pain is perceived away from the actual source. This referred pain can be misinterpreted as a heart or lung issue, making accurate identification of the T4 segment’s location important for diagnosis.