Anatomy and Physiology Study Guide for the HESI Exam

Muscular System

The muscular system works with the skeletal system to facilitate movement, stability, and posture. It also plays roles in circulation, digestion, and temperature regulation. The body contains approximately 700 muscles, which can be categorized into three main types based on function and structure.

Types of Muscle Tissue

Cardiac Muscle

Cardiac muscle is an involuntary, striated muscle found only in the heart. It has a striped appearance under a microscope and is controlled by the autonomic nervous system.

• The myocardium is the layer of the heart responsible for contraction, and it is composed of cardiac muscle fibers.

• Cardiac muscles control the heartbeat, pumping blood throughout the body.

Smooth Muscle

Smooth muscle is an involuntary, non-striated muscle found in the walls of hollow organs such as blood vessels, the uterus, bladder, and digestive tract.

• Unlike skeletal and cardiac muscle, smooth muscle cells lack striations and can stretch significantly while maintaining function.

• Smooth muscle contractions are slow and rhythmic, allowing for functions such as moving food through the digestive tract and regulating blood pressure.

Skeletal Muscle

Skeletal muscles are voluntary, striated muscles that attach to bones via tendons. These muscles are under conscious control, allowing for movement and posture.

• Skeletal muscles are controlled by the nervous system, specifically the somatic (voluntary) division of the peripheral nervous system.

• Movement occurs when skeletal muscles contract and pull on bones.

• The sartorius muscle, which runs from the hip to the inner knee, is the longest muscle in the human body and helps with flexing, abducting, and rotating the thigh.

How Muscles Produce Movement

Muscle movement occurs through contraction and relaxation:

• Nerve impulses from the nervous system signal muscle contraction.
• Muscle fibers contract when actin and myosin filaments slide past one another.
• Adenosine triphosphate (ATP) must be present for muscle contraction to occur.
• Muscles shorten and pull on bones, producing movement.

During this movement, muscles work in opposing pairs:

• agonist (prime mover)—the main muscle responsible for movement
• antagonist—the muscle that relaxes while the agonist contracts

For example, when the biceps contract to bend the arm, the triceps relax. When the arm straightens, the roles reverse.

Major Muscle Groups

Muscles in the body are grouped into the following types, based on their location and function:

Muscles of the Head and Neck

• masseter—controls jaw movement for chewing
• sternocleidomastoid—rotates and flexes the head

Muscles of the Upper Body

• trapezius—moves the shoulders and neck
• pectoralis major—moves the shoulders and arms
• deltoid—raises the arm

Muscles of the Lower Body

• gluteus maximus—extends the thigh and supports posture

• quadriceps femoris—straightens the knee and supports walking

• hamstrings—bend the knee and extend the thigh

• gastrocnemius—helps with standing on tiptoes and walking

• adductor muscles—move the legs toward the midline of the body (e.g., adductor longus, adductor magnus)

• abductor muscles—move the legs away from the midline of the body (e.g., gluteus medius, gluteus minimus)

Structure of Skeletal Muscle

Skeletal muscles have a bundle-within-a-bundle structure that provides strength, flexibility, and coordination. Each muscle is surrounded by a layer of connective tissue called fascia, which supports and protects the muscle fibers while allowing smooth movement.

Muscle Layers (From Outer to Inner):

• epimysium—encases the entire muscle, providing structural support and reducing friction

• perimysium—surrounds fascicles (bundles of muscle fibers), containing blood vessels and nerves

• endomysium—surrounds individual muscle fibers (myocytes), providing additional structural support

These connective tissue layers merge at the ends of muscles to form tendons, which attach muscles to bones.

Muscle Cells (Myocytes)

Each skeletal muscle cell (myocyte) is long, cylindrical, and multinucleated, allowing for efficient protein synthesis and muscle repair. Myocytes contain specialized structures essential for contraction:

• sarcoplasm—cytoplasm of the muscle cell, containing glycogen for energy and myoglobin for oxygen storage

• sarcoplasmic reticulum (SR)—stores and releases calcium ions (Ca²⁺), which are crucial for contraction

• T-tubules—extensions of the sarcolemma (cell membrane) that transmit electrical impulses deep into the muscle fiber, ensuring coordinated contraction

Myofibrils and Sarcomeres: The Functional Units of Contraction

Inside each myocyte, myofibrils run parallel to the length of the cell. These myofibrils are composed of repeating sarcomeres, which are the fundamental units of muscle contraction.

• Sarcomeres contain thin filaments (actin) and thick filaments (myosin), which slide past each other to generate force.

• The sliding filament theory explains contraction: Myosin heads pull actin filaments inward, shortening the sarcomere and causing the muscle to contract.

• This process is powered by ATP and regulated by calcium ions.

Nervous System

The nervous system is the body’s communication network, coordinating voluntary and involuntary functions by transmitting signals between the brain, spinal cord, and peripheral nerves. It consists of two main divisions:

• central nervous system (CNS)—The CNS is composed of the brain and spinal cord and is responsible for processing and interpreting sensory information.

• peripheral nervous system (PNS)—The PNS is composed of nerves and sensory organs, which relay information between the CNS and the rest of the body.

The Brain

The brain is the control center of the nervous system, containing approximately 100 billion neurons. It regulates higher mental functions such as memory, judgment, and emotion, as well as lower body functions like breathing, digestion, and reflexes.

Major Regions of the Brain

• cerebrum—This is the largest part of the brain, responsible for higher cognitive functions, voluntary movement, and sensory processing. It is divided into two hemispheres, which communicate via the corpus callosum, with each hemisphere containing four lobes:

  • The left hemisphere is generally responsible for language, logic, and analytical thinking.

  • The right hemisphere is more involved in creativity, spatial awareness, and intuition.

  • frontal lobe—This lobe controls judgment, planning, impulse control, problem-solving, and social behavior. It also regulates voluntary movements through the primary motor cortex.

  • parietal lobe—This lobe processes touch, temperature, and pain sensations and helps with spatial awareness.

  • temporal lobe—The temporal lobe is responsible for processing auditory information, memory formation, and language comprehension.

  • occipital lobe—This lobe processes visual information from the eyes.

• diencephalon—Located deep within the brain, the diencephalon contains:

  • thalamus—This is the brain’s relay station, directing sensory signals to the appropriate parts of the brain.

  • hypothalamus—This portion regulates body temperature, hunger, thirst, and emotions, and serves as the control center of temperature regulation.

• brainstem—The brainstem connects the brain to the spinal cord and regulates autonomic functions such as breathing and heart rate and includes:

  • midbrain—This is involved in vision, hearing, and motor control.

  • pons—This assists in motor control and sensory analysis.

  • medulla oblongata—This controls respiratory rate, heart rate, and blood pressure; it is the respiratory control center of the brain.

• cerebellum—Located at the base of the brain, the cerebellum controls balance, coordination, and fine motor movements.

The Spinal Cord

The spinal cord is a thick bundle of neurons extending from the medulla oblongata to the lumbar region of the spine. It serves as the main communication pathway between the brain and the rest of the body, carrying sensory and motor signals through the following pathways:

• ascending (sensory) pathways—transmit sensory information from the body to the brain
• descending (motor) pathways—carry motor commands from the brain to muscles and glands

The spinal cord is protected by:

• vertebrae—the bony spine encasing the spinal cord
• meninges—protective membranes surrounding the brain and spinal cord
• cerebrospinal fluid (CSF)—cushions and nourishes the spinal cord

Nerve Tissue and Neurons

The functional unit of the nervous system is the neuron (nerve cell), which transmits electrochemical signals throughout the body. Neurons consist of three main parts:

• cell body (soma)—contains the nucleus and organelles; processes incoming information

• dendrites—branch-like extensions that receive incoming signals from other neurons

• axon—a long projection that moves nerve impulses away from the cell body toward other neurons, muscles, or glands. Many axons are covered by a myelin sheath, a fatty insulating layer that speeds up electrical signal transmission.

Nerve impulses move quickly along the axon by jumping from one gap in the myelin sheath to the next. These gaps are called nodes of Ranvier, and they help speed up signal transmission. In the peripheral nervous system (PNS), the myelin sheath is made by Schwann cells, which wrap around the axon to provide insulation.

Synapses and Neurotransmitters

Neurons communicate with each other through synapses, which are specialized junctions between nerve cells. Synapses are classified into two types:

• chemical synapses—These use neurotransmitters (chemical messengers) to relay signals across a small gap (synaptic cleft) between neurons. Neurotransmitters such as dopamine, serotonin, and acetylcholine regulate brain function, muscle movement, and mood.

• electrical synapses—These allow direct electrical communication between neurons through gap junctions, enabling rapid signal transmission.

At chemical synapses, neurotransmitters are released from axon terminals, diffusing across the synaptic cleft and binding to receptors on the receiving neuron. This process enables signal transmission throughout the nervous system.

Types of Neurons

• afferent (sensory) neurons—transmit sensory information (e.g., touch, temperature, pain) from the body to the CNS

• efferent (motor) neurons—carry signals from the CNS to muscles and glands to control movement and organ function

• interneurons—connect afferent and efferent neurons, forming neural circuits that process and relay information.

Peripheral Nervous System (PNS)

The PNS connects the CNS to the body and is divided into these two major parts with the listed functions:

Somatic Nervous System (SNS)

• controls voluntary movements of skeletal muscles

• transmits sensory information from the skin, muscles, and joints to the CNS

Autonomic Nervous System (ANS)

• regulates involuntary functions like heart rate, digestion, and respiration

• is divided into two parts:

  • sympathetic nervous system—triggers the “fight-or-flight” response, increasing heart rate, dilating pupils, and redirecting blood flow to muscles

  • parasympathetic nervous system—controls the rest-and-digest response, slowing heart rate, constricting pupils, and promoting digestion