12)_Hand_Muscles.pdf

# Anatomy and functional territories of the hand The hand is a complex instrument at the terminal segment of the upper limb, capable of a vast range of movements from powerful grasping to fine manipulation. Its unique structure, defined by five digits including an opposable thumb, is essential for its function. Understanding this capability requires exploring its intrinsic muscles, which are organized into three distinct functional territories within the palm [1](#page=1). ### 1.1 The three territories of the palm The intrinsic muscles of the hand are systematically divided into three groups, each occupying a distinct region of the palm and serving a specialized role. These territories are [2](#page=2): #### 1.1.1 The middle group This group occupies the central palm and includes the lumbricals and interossei muscles. These muscles are critical for the nuanced and independent movements of the fingers, facilitating dexterous tasks [14](#page=14) [2](#page=2). #### 1.1.2 The thenar eminence Located on the lateral side of the palm, at the base of the thumb, the thenar eminence contains the muscles that power the thumb's unique strength and opposability. It provides the thumb with the power and mobility necessary for opposition, which is the cornerstone of grip [14](#page=14) [2](#page=2). #### 1.1.3 The hypothenar eminence Situated on the medial side of the palm, at the base of the little finger, the hypothenar eminence controls the movement of the little finger. It positions the little finger to create a stable medial buttress for grasping objects [14](#page=14) [2](#page=2). > **Tip:** The coordinated action of these three intrinsic muscle groups—the thenar eminence, hypothenar eminence, and middle group—transforms the hand into a sophisticated tool of unparalleled versatility, enabling everything from a firm grasp to a delicate touch [14](#page=14). --- # The middle group of intrinsic hand muscles The middle group of intrinsic hand muscles, situated deep within the palm, is responsible for the fine motor control required for intricate finger movements. This group is comprised of the lumbrical muscles and the interosseous muscles [3](#page=3). ### 2.1 Lumbrical muscles The four lumbrical muscles are unique in their origin from the tendons of the flexor digitorum profundus [4](#page=4). #### 2.1.1 Origin and insertion * **Origin:** * The 1st and 2nd lumbricals originate from the lateral edge of the corresponding deep flexor tendon [4](#page=4). * The 3rd and 4th lumbricals originate from the lateral edges of the two deep flexor tendons between which they lie [4](#page=4). * **Insertion:** They join the interosseous tendon and insert at the lateral edge of the extensor tendon at the metacarpophalangeal joint [4](#page=4). #### 2.1.2 Action The primary action of the lumbrical muscles is to flex the first phalanx (P1) of the finger while simultaneously extending the other two phalanges (P2 and P3) [4](#page=4). #### 2.1.3 Innervation The lumbrical muscles receive dual innervation: * The 1st and 2nd lumbricals are innervated by the median nerve [4](#page=4). * The 3rd and 4th lumbricals are innervated by the ulnar nerve [4](#page=4). ### 2.2 Interosseous muscles The eight interosseous muscles are located in the spaces between the metacarpal bones and are divided into two groups based on their location and function relative to the hand's anatomical axis. The anatomical axis of the hand is defined as an imaginary line passing through the middle finger, with movements of other fingers classified as either abduction (moving away from the axis) or adduction (moving toward the axis) [5](#page=5). #### 2.2.1 Palmar interossei * **Number:** There are four palmar interosseous muscles [6](#page=6). * **Location:** They are smaller muscles situated on the palmar side of the metacarpals [5](#page=5). * **Origin:** They originate from the anterior half of the lateral surface of metacarpals 1, 2, 4, and 5 – those furthest from the axis of the hand [6](#page=6). * **Insertion:** They insert at the base of the first phalanx of the corresponding finger and the extensor tendon [6](#page=6). * **Action:** Their primary action is to adduct the fingers, bringing them closer to the axis of the hand. They also share the action of flexing P1 and extending P2/P3 [5](#page=5) [6](#page=6). * **Innervation:** They are innervated by the ulnar nerve [6](#page=6). #### 2.2.2 Dorsal interossei * **Number:** There are four dorsal interosseous muscles [6](#page=6). * **Location:** These are larger muscles visible on the dorsum (back) of the hand, occupying the interosseous spaces [5](#page=5) [6](#page=6). * **Origin:** They originate from the lateral sides of the two metacarpals that form each interosseous space [6](#page=6). * **Insertion:** They insert at the base of the proximal phalanx of the 2nd, 3rd, and 4th fingers, as well as the extensor tendon [6](#page=6). * **Action:** Their primary role is to abduct the fingers, spreading them away from the axis of the hand. Similar to the palmar interossei, they also contribute to flexing P1 and extending P2/P3 [5](#page=5) [6](#page=6). * **Innervation:** They are innervated by the ulnar nerve [6](#page=6). > **Tip:** A helpful mnemonic for remembering the actions of the interosseous muscles is "PAD" for Palmar ADduct and "DAB" for Dorsal ABduct [6](#page=6). #### 2.2.3 Shared action of interossei Despite their opposing roles in abduction and adduction, both the palmar and dorsal interosseous muscles share the action of flexing the first phalanx (P1) and extending the second and third phalanges (P2 and P3) [5](#page=5) [6](#page=6). --- # The thenar eminence and thumb mobility The thenar eminence is a crucial anatomical region on the palm's lateral side, comprising four muscles that enable the thumb's complex and dexterous movements, vital for hand function [7](#page=7). ### 3.1 The four thenar muscles The thenar eminence is composed of four distinct muscles, arranged in layers, that collectively control thumb adduction, abduction, flexion, and opposition. These muscles, from deepest to most superficial, are the Adductor Pollicis, Flexor Pollicis Brevis, Opponens Pollicis, and Abductor Pollicis Brevis [7](#page=7). ### 3.2 Muscle profiles and actions #### 3.2.1 Adductor pollicis The Adductor Pollicis is a deep, triangular muscle with two heads: the oblique head originating from the trapezium and capitate bones, and the transverse head originating from the base of the second metacarpal and the anterior edge of the third metacarpal. It inserts onto the medial sesamoid bone and the medial side of the base of the thumb's first phalanx. Its primary action is to adduct the thumb. The Adductor Pollicis is innervated by the Ulnar Nerve [8](#page=8). #### 3.2.2 Opponens pollicis The Opponens Pollicis is situated deep to the abductor and flexor muscles. It originates from the trapezium bone and the flexor retinaculum. Its insertion is on the lateral edge of the anterior surface of the first metacarpal. The action of the Opponens Pollicis is to bring the first metacarpal forward and inward, a motion essential for opposition. This muscle is innervated by the Median Nerve [8](#page=8). #### 3.2.3 Flexor pollicis brevis The Flexor Pollicis Brevis consists of both a superficial and a deep bundle. The superficial head originates from the trapezium and flexor retinaculum, while the deep head arises from the trapezoid and capitate bones. It inserts onto the lateral sesamoid bone and the lateral base of the thumb's first phalanx. This muscle's actions include flexion and adduction of the thumb. The superficial head is innervated by the Median Nerve, and the deep head by the Ulnar Nerve [9](#page=9). #### 3.2.4 Abductor pollicis brevis The Abductor Pollicis Brevis is the most superficial muscle of the thenar eminence group. It originates from the scaphoid tubercle and the flexor retinaculum. Its insertion is at the lateral tubercle of the base of the thumb's first phalanx. The primary actions of the Abductor Pollicis Brevis are abduction and flexion of the first phalanx. It is innervated by the Median Nerve [9](#page=9). > **Tip:** The combined actions of these four muscles, particularly the Opponens Pollicis, are critical for the thumb's ability to oppose the fingertips, enabling precise grip and manipulation of objects [7](#page=7). --- # The hypothenar eminence and little finger control This section delves into the muscular anatomy of the hypothenar eminence and its crucial role in controlling the nuanced movements of the little finger, enhancing hand grip. ### 4.1 The hypothenar eminence: an overview The hypothenar eminence is the prominent, rounded muscular area situated on the medial (ulnar) side of the palm. This region houses four distinct muscles, all of which attach to the little finger (digiti minimi). These muscles are arranged in layers and collectively orchestrate the fine motor control of the fifth digit, contributing significantly to the ability to achieve a full and secure grip [10](#page=10). #### 4.1.1 The four hypothenar muscles The muscles of the hypothenar eminence, from deepest to most superficial, are: 1. Opponens digiti minimi (deepest) [10](#page=10). 2. Flexor digiti minimi brevis [10](#page=10). 3. Abductor digiti minimi [10](#page=10). 4. Palmaris brevis (most superficial, cutaneous) [10](#page=10). ### 4.2 Muscular actions of the little finger The primary movements of the little finger facilitated by the hypothenar muscles include opposition, flexion, and abduction [11](#page=11). #### 4.2.1 Opponens digiti minimi * **Origin:** The hook of the hamate bone and the flexor retinaculum [11](#page=11). * **Insertion:** The medial edge of the fifth metacarpal bone ($M5$) [11](#page=11). * **Action:** This muscle is responsible for bringing the little finger forward and outward, enabling it to oppose the thumb [11](#page=11). #### 4.2.2 Flexor digiti minimi brevis * **Origin:** The hook of the hamate bone and the flexor retinaculum [11](#page=11). * **Insertion:** The base of the first phalanx of the fifth finger [11](#page=11). * **Action:** This muscle's primary role is the flexion of the little finger [11](#page=11). #### 4.2.3 Abductor digiti minimi * **Origin:** The pisiform bone and the flexor retinaculum [11](#page=11). * **Insertion:** It merges with the short flexor muscle at the base of the first phalanx of the little finger [11](#page=11). * **Action:** This muscle abducts the fifth finger, moving it away from the longitudinal axis of the hand [11](#page=11). > **Tip:** All three of the aforementioned muscles—Opponens digiti minimi, Flexor digiti minimi brevis, and Abductor digiti minimi—share the same innervation, which is the Ulnar Nerve [11](#page=11). ### 4.3 The palmaris brevis: a unique cutaneous muscle The palmaris brevis is a distinct muscle with a flattened, quadrilateral shape, located within the subcutaneous tissue of the palm. It is anatomically separated from the deeper hypothenar muscles by the palmar aponeurosis [12](#page=12). #### 4.3.1 Palmaris brevis muscle profile * **Origin:** The medial edge of the palmar aponeurosis [12](#page=12). * **Insertion:** The deep surface of the dermis, extending along the medial edge of the hypothenar eminence [12](#page=12). * **Action:** This muscle performs two key functions: it wrinkles the skin of the hypothenar eminence and contributes to "hollowing" the palm. These actions collectively improve the effectiveness and security of a grip [12](#page=12). * **Innervation:** Like the other hypothenar muscles, the palmaris brevis is innervated by the Ulnar Nerve [12](#page=12). --- # Neurological control of intrinsic hand muscles The intrinsic muscles of the hand are primarily innervated by the ulnar and median nerves, a distinction crucial for understanding hand function and diagnosing neurological deficits [13](#page=13). ### 5.1 Nerve innervation of intrinsic hand muscles The distribution of innervation by the ulnar and median nerves is specific to individual intrinsic hand muscles. #### 5.1.1 Ulnar nerve innervation The ulnar nerve is the dominant nerve for the majority of intrinsic hand muscles. It innervates [13](#page=13): * All interossei muscles (palmar and dorsal) [13](#page=13). * The 3rd and 4th lumbricals [13](#page=13). * All hypothenar muscles, including: * Opponens digiti minimi [13](#page=13). * Flexor digiti minimi [13](#page=13). * Abductor digiti minimi [13](#page=13). * Palmaris brevis [13](#page=13). * Adductor pollicis [13](#page=13). * The deep head of the flexor pollicis brevis [13](#page=13). #### 5.1.2 Median nerve innervation The median nerve innervates a smaller subset of the intrinsic hand muscles, primarily those involved in thumb movement and the first two lumbricals. These include [13](#page=13): * The 1st and 2nd lumbricals [13](#page=13). * Opponens pollicis [13](#page=13). * Abductor pollicis brevis [13](#page=13). * The superficial head of the flexor pollicis brevis [13](#page=13). > **Tip:** Memorizing this muscle distribution is key for identifying the neurological source of hand weakness or sensory loss. A deficit in abduction or adduction of the fingers, for example, might point towards ulnar nerve involvement, while difficulty with thumb opposition could suggest median nerve pathology. --- ## Common mistakes to avoid - Review all topics thoroughly before exams - Pay attention to formulas and key definitions - Practice with examples provided in each section - Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Intrinsic muscles of the hand | Muscles located entirely within the hand, responsible for fine motor control, grip, and manipulation of the digits. | | Thenar eminence | The fleshy mound at the base of the thumb, formed by four intrinsic muscles that control the thumb's movement, including opposition. | | Hypothenar eminence | The fleshy mound at the base of the little finger, formed by four intrinsic muscles that control the little finger's movement and contribute to grip. | | Middle group | The intrinsic muscles located within the palm between the metacarpals, including the lumbricals and interossei, responsible for intricate finger movements. | | Lumbrical muscles | Four small, intrinsic muscles of the hand originating from the tendons of the flexor digitorum profundus; they contribute to finger flexion and extension. | | Interosseous muscles | Intrinsic muscles of the hand situated in the spaces between the metacarpal bones, divided into palmar and dorsal groups, responsible for finger adduction and abduction. | | Palmar interossei | Four intrinsic muscles located on the palmar side of the hand that adduct the fingers toward the anatomical axis of the hand. | | Dorsal interossei | Four intrinsic muscles located on the dorsal side of the hand that abduct the fingers away from the anatomical axis of the hand. | | Adduction (of fingers) | The movement of fingers toward the anatomical axis of the hand (the middle finger). | | Abduction (of fingers) | The movement of fingers away from the anatomical axis of the hand. | | Metacarpophalangeal joint | The joint located between the metacarpal bone and the proximal phalanx of a digit, commonly known as the knuckle. | | Opposition (of the thumb) | The complex movement of bringing the thumb's tip to touch the tips of the other fingers, facilitated by the thenar muscles. | | Flexor Digitorum Profundus | A deep muscle of the forearm whose tendons insert into the distal phalanges of the fingers and are involved in flexing the digits. | | Palmar aponeurosis | A strong, triangular sheet of fibrous tissue in the palm of the hand that provides an attachment site for some muscles and protects deeper structures. | | Digiti minimi | The little finger. | | Anatomical axis of the hand | An imaginary line passing through the middle finger, used as a reference point for defining finger abduction and adduction. | | Phalanx (plural: phalanges) | One of the bones that make up a finger or toe. P1 refers to the proximal phalanx, P2 to the middle phalanx, and P3 to the distal phalanx. | | Innervation | The supply of nerves to a part of the body or to a particular organ or tissue. | | Median Nerve | A major nerve of the upper limb that originates from the brachial plexus and innervates muscles of the forearm and hand, as well as providing sensory information. | | Ulnar Nerve | A major nerve of the upper limb that originates from the brachial plexus and innervates muscles of the forearm and hand, as well as providing sensory information. |