Muscles of the Human Body

Muscles of the Head & Neck

The muscles of the head and neck are responsible for facial expressions, mastication (chewing), and movement of the head and neck. Understanding their locations and functions is essential for comprehending human movement and communication.

• Frontalis: Raises eyebrows and wrinkles the forehead.

• Platysma: Tenses skin of the neck and depresses the mandible.

• Orbicularis oculi: Closes the eyelids; involved in blinking and squinting.

• Orbicularis oris: Closes and protrudes lips; the "kissing muscle."

• Zygomaticus major: Elevates the corners of the mouth (smiling).

• Buccinator: Compresses cheek; assists in chewing.

• Masseter: Elevates mandible; primary muscle of mastication.

• Temporalis: Elevates and retracts mandible.

• Triangularis (Depressor anguli oris): Depresses the corners of the mouth (frowning).

• Sternocleidomastoid: Flexes and rotates the head.

Anterior Trunk Muscles

These muscles are involved in movements of the shoulder, chest, and respiration, as well as maintaining posture and protecting internal organs.

• Pectoralis major: Adducts and medially rotates the arm.

• Pectoralis minor: Draws scapula forward and downward.

• Deltoid: Abducts the arm.

• Serratus anterior: Protracts and stabilizes scapula.

• External intercostals: Elevate ribs during inspiration.

• Internal intercostals: Depress ribs during forced expiration.

• Diaphragm: Main muscle of respiration; contracts to increase thoracic cavity volume.

• Abdominal muscles: Include external oblique, internal oblique, transverse abdominis, and rectus abdominis; involved in trunk movement and compression of abdominal contents.

Posterior Trunk Muscles

These muscles support the back, move the shoulders, and stabilize the spine.

• Trapezius: Elevates, retracts, and rotates scapula.

• Latissimus dorsi: Extends, adducts, and medially rotates the arm.

• Infraspinatus, Supraspinatus, Teres Major, Teres Minor: Rotator cuff muscles; stabilize and move the shoulder joint.

Muscles of the Upper Limb

These muscles control movements of the arm, forearm, and hand, enabling complex tasks and manipulation.

• Coracobrachialis: Flexes and adducts the arm.

• Biceps brachii: Flexes elbow and supinates forearm.

• Brachialis: Flexes elbow.

• Triceps brachii: Extends elbow.

• Brachioradialis: Flexes forearm at elbow.

• Pronator teres: Pronates forearm.

• Supinator: Supinates forearm.

• Extensor digitorum: Extends fingers.

• Palmaris longus: Flexes wrist.

• Flexor carpi radialis: Flexes and abducts wrist.

Muscles of the Lower Limb

These muscles are essential for locomotion, posture, and balance.

• Gluteus maximus: Extends and laterally rotates thigh.

• Gluteus medius: Abducts and medially rotates thigh.

• Adductor longus, Adductor magnus: Adduct thigh.

• Sartorius: Flexes, abducts, and laterally rotates thigh; flexes knee.

• Tensor fasciae latae, Iliotibial tract: Stabilize and abduct thigh.

• Gracilis: Adducts thigh, flexes knee.

• Gastrocnemius, Soleus: Plantar flex foot.

• Tibialis anterior: Dorsiflexes and inverts foot.

• Fibularis (Peroneus) longus and brevis: Evert and plantar flex foot.

• Achilles tendon: Common tendon for gastrocnemius and soleus; attaches to calcaneus.

• Hamstring group (Biceps femoris, Semimembranosus, Semitendinosus): Flex knee and extend hip.

• Quadriceps group (Rectus femoris, Vastus lateralis, Vastus medialis, Vastus intermedius): Extend knee.

The Brain and Spinal Cord

Brain Structure and Regions

The brain is divided into several major regions, each with specialized functions. Understanding these regions is crucial for studying the central nervous system.

• Forebrain (Prosencephalon): Includes the cerebrum (with five lobes: frontal, parietal, temporal, occipital, insula), lateral ventricles, choroid plexus, corpus callosum, septum pellucidum, sulci (grooves), gyri (ridges), cortex (outer gray matter), and medulla (inner white matter).

• Longitudinal fissure: Deep groove separating the two cerebral hemispheres.

• Lateral fissure: Separates temporal lobe from frontal and parietal lobes.

• Central sulcus: Separates frontal and parietal lobes.

• Primary somatosensory area (Postcentral gyrus): Receives sensory input.

• Primary motor area (Precentral gyrus): Controls voluntary movements.

Meninges

The brain and spinal cord are protected by three connective tissue membranes called meninges:

• Pia mater: Innermost, delicate layer adhering to the brain and spinal cord.

• Arachnoid mater: Middle, web-like layer.

• Dura mater: Outermost, tough layer.

Diencephalon

• Thalamus: Relay station for sensory information.

• Hypothalamus: Regulates homeostasis, endocrine functions, and autonomic nervous system.

• Interventricular foramina: Connect lateral ventricles to third ventricle.

• Optic chiasma: Crossing of optic nerves.

• Olfactory tract and bulb: Involved in sense of smell.

• Pituitary (hypophysis): Major endocrine gland.

• Third ventricle: Contains cerebrospinal fluid (CSF).

• Choroid plexus: Produces CSF.

Midbrain (Mesencephalon)

• Mammillary body: Involved in memory.

• Cerebral aqueduct: Connects third and fourth ventricles.

• Cerebral peduncles: Motor tracts.

• Corpus quadrigemina: Contains superior (visual reflexes) and inferior (auditory reflexes) colliculi.

Hindbrain (Rhombencephalon)

• Cerebellum: Coordinates movement and balance; includes cortex and arbor vitae (white matter).

• Pons: Relays signals between cerebrum and cerebellum.

• Medulla oblongata: Controls vital functions (breathing, heart rate); contains pyramids (motor tracts).

• Fourth ventricle: Contains CSF; choroid plexus produces CSF.

Spinal Cord Anatomy

The spinal cord transmits neural signals between the brain and the body. It is organized into regions and contains both gray and white matter.

• Regions and Nerve Pairs:

  • Cervical: 8 pairs

  • Thoracic: 12 pairs

  • Lumbar: 5 pairs

  • Sacral: 5 pairs

  • Coccygeal: 1 pair

• Central canal: Contains CSF.

• Anterior median fissure & Posterior median sulcus: Grooves on the spinal cord surface.

• Dorsal root ganglion: Contains sensory neuron cell bodies.

• Anterior (ventral) root: Motor neuron axons.

• Posterior (dorsal) root: Sensory neuron axons.

• Spinal nerve: Mixed nerve (sensory and motor fibers).

• Afferent nerve: Carries sensory information to CNS.

• Efferent nerve: Carries motor commands from CNS.

• Gray matter: Contains neuron cell bodies; organized into anterior, lateral, and posterior horns, and gray commissure.

• White matter: Contains myelinated axons; organized into posterior, lateral, and anterior columns.

• Conus medullaris: Tapered end of spinal cord.

• Cauda equina: Bundle of spinal nerves below conus medullaris.

Major Nerve Plexuses and Nerves

Nerve plexuses are networks of intersecting nerves. Major plexuses and their key nerves include:

• Cervical plexus (C1-C5): Phrenic nerve (innervates diaphragm).

• Brachial plexus (C5-T1): Axillary, median, musculocutaneous, ulnar, and radial nerves (innervate upper limb).

• Thoracic nerves: Intercostal nerves (innervate intercostal muscles).

• Lumbar plexus (L1-L4): Femoral and obturator nerves (innervate anterior and medial thigh).

• Sacral plexus (L4-S4): Sciatic nerve (largest in body; splits into common peroneal/fibular and tibial nerves).

The Special Senses

The Eye

The eye is a complex organ responsible for vision, composed of three tunics (layers) and several internal structures.

• Fibrous tunic: Sclera (white, protective outer layer) and cornea (transparent, refracts light).

• Vascular tunic: Choroid (vascular, pigmented), ciliary body (contains ciliary muscle and suspensory ligaments), and iris (controls pupil size).

• Nervous tunic: Retina (contains photoreceptors).

• Suspensory ligament: Holds lens in place.

• Optic nerve: Transmits visual information to brain.

• Lens: Focuses light on retina.

• Pupil: Opening in iris; regulates light entry.

• Lacrimal gland: Produces tears.

• Extrinsic eye muscles: Control eye movement (superior, inferior, lateral, medial rectus; superior, inferior oblique).

• Cavities and humors:

  • Anterior cavity (anterior and posterior chambers): contains aqueous humor.

  • Posterior cavity: contains vitreous humor.

• Macula and fovea centralis: Area of sharpest vision.

• Optic disc (blind spot): No photoreceptors; where optic nerve exits eye.

The Ear

The ear is responsible for hearing and balance, divided into external, middle, and inner regions.

• Auricle (pinna): Collects sound waves.

• External auditory meatus: Canal leading to tympanic membrane.

• Tympanic membrane: Vibrates in response to sound.

• Ossicles: Malleus, incus, stapes; transmit vibrations to inner ear.

• Round window: Relieves pressure in cochlea.

• Cochlea: Spiral organ for hearing; contains scala vestibuli, scala tympani (perilymph), vestibular membrane, cochlear duct (endolymph), spiral organ (Organ of Corti) with tectorial membrane and hair cells, oval window, and basilar membrane.

• Vestibule: Contains utricle and saccule; detects linear acceleration.

• Semicircular canals: Detect rotational movement.

Table: Major Muscles and Their Functions

Table: Major Brain Regions and Functions

Table: Eye Structures and Functions

Table: Ear Structures and Functions

Additional info: For each muscle and structure, students should be able to identify location, function, and clinical relevance. For the nervous system, understanding the flow of information (sensory vs. motor) and the organization of gray and white matter is essential. For the special senses, knowledge of the pathway from stimulus to perception is key.