Premed · Premed · Anatomy Physiology 1
Lecture 4: Histology — Muscle and Nervous Tissues
Anatomy and Physiology I
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the general characteristics shared by all muscle tissues
- Compare and contrast the three types of muscle tissue in terms of structure, location, and control
- Identify the basic components of nervous tissue
- Distinguish between neurons and neuroglia and describe their respective roles
- Describe the basic structural classification of neurons
- Explain the concept of tissue repair and the factors affecting it
Lecture Content
I. Muscle Tissue — Overview
All muscle tissues share several fundamental characteristics. They are composed of elongated cells called muscle fibers (or myocytes) and are highly vascularized to meet their substantial metabolic demands. Muscle cells are excitable, meaning they can respond to electrical stimuli, and contractile, meaning they can shorten to produce force and movement. They are also extensible, capable of being stretched beyond their resting length, and elastic, able to return to their original shape afterward. Muscle cells use specialized terminology: the plasma membrane is called the sarcolemma, the cytoplasm is the sarcoplasm, and the smooth endoplasmic reticulum, which stores calcium, is the sarcoplasmic reticulum.
II. Skeletal Muscle Tissue
Skeletal muscle is attached to bones and, in some cases, to the skin, as with facial muscles. It is under voluntary control, directed by the somatic nervous system. Structurally, skeletal muscle fibers are very long, cylindrical, and multinucleated, with the nuclei located at the cell periphery beneath the sarcolemma. These giant cells form during embryonic development through the fusion of many myoblasts, which accounts for their multinucleated nature.
The most visually striking feature of skeletal muscle is its prominent striations, alternating light (I) and dark (A) bands visible under the microscope. These bands result from the highly organized arrangement of the contractile proteins actin and myosin into repeating units called sarcomeres. Skeletal muscle produces voluntary movement of the skeleton, maintains posture, and generates heat through thermogenesis. In adults, skeletal muscle fibers do not divide; damaged fibers are typically replaced by scar tissue, although satellite cells can contribute limited regeneration. Skeletal muscle accounts for approximately 40 percent of total body mass.
III. Cardiac Muscle Tissue
Cardiac muscle is found exclusively in the wall of the heart, forming the myocardium. It operates involuntarily, regulated by the autonomic nervous system and by intrinsic pacemaker cells that set the rhythm of contraction. Cardiac muscle cells are short and branching, typically with a single centrally located nucleus, though some cells may be binucleated. Like skeletal muscle, cardiac muscle is striated because it contains sarcomeres.
A distinguishing feature of cardiac muscle is the presence of intercalated discs, unique junctions that connect cells end to end. These discs contain desmosomes that hold cells together against the mechanical stress of constant contraction and gap junctions that allow electrical impulses to spread rapidly from cell to cell. This electrical coupling enables the heart to contract as a coordinated unit, functioning as a functional syncytium. Cardiac muscle cells exhibit autorhythmicity, generating their own electrical signals through pacemaker activity. Their regenerative capacity is very limited; damage is typically repaired by fibrosis, the formation of scar tissue.
IV. Smooth Muscle Tissue
Smooth muscle is found in the walls of hollow organs such as the stomach, intestines, bladder, uterus, blood vessels, and airways, as well as in the iris of the eye and the arrector pili muscles of hair follicles. It operates involuntarily under the control of the autonomic nervous system, hormones, and local chemical signals.
Smooth muscle cells are spindle-shaped (fusiform) with tapered ends and a single, centrally located nucleus. They contain actin and myosin but lack the organized sarcomere arrangement of skeletal and cardiac muscle, which is why they display no striations. They also lack T-tubules and have a poorly developed sarcoplasmic reticulum. Smooth muscle cells may be arranged in sheets, often in two layers — one longitudinal and one circular — and in visceral (single-unit) smooth muscle, gap junctions allow coordinated contraction, enabling wave-like movements such as peristalsis.
Smooth muscle propels substances through hollow organs, regulates blood vessel diameter, controls pupil size, and erects hairs. Unlike skeletal and cardiac muscle, smooth muscle retains the ability to divide and regenerate, increasing in cell number (hyperplasia) or cell size (hypertrophy) as needed.
<image>A three-panel comparison of muscle tissue types under the microscope. Panel A: Skeletal muscle — long, cylindrical, multinucleated fibers with clearly visible striations and peripheral nuclei. Panel B: Cardiac muscle — short, branching cells with central nuclei, striations, and intercalated discs marked with arrows at cell junctions. Panel C: Smooth muscle — spindle-shaped cells with single central nuclei, no striations, arranged in a sheet. A summary table beneath the panels compares each type by location, striations (yes/no), nuclei (number and position), control (voluntary/involuntary), and intercalated discs (yes/no).</image>
V. Nervous Tissue — Overview
Nervous tissue is found in the brain, spinal cord, and peripheral nerves. It is highly specialized for generating and conducting electrical impulses, known as nerve impulses or action potentials. The tissue contains two main cell types: neurons, the excitable cells that generate and transmit these impulses, and neuroglia (glial cells), the supporting cells that protect, insulate, and nourish the neurons.
VI. Neurons
Neurons are the functional units of the nervous system. They are extremely long-lived, potentially lasting a lifetime, and in most cases are amitotic, meaning they do not divide in adults. They have a high metabolic rate and require a continuous supply of oxygen and glucose to function.
Neuron Structure
The cell body (soma or perikaryon) contains the nucleus with a prominent nucleolus, abundant Nissl bodies (rough ER) for protein synthesis, and neurofibrils — cytoskeletal elements made of intermediate filaments and microtubules that maintain cell shape and transport materials. Most cell bodies are located in the CNS, where clusters of cell bodies are called nuclei; in the PNS, such clusters are called ganglia.
Dendrites are short, highly branched extensions of the cell body that receive incoming signals in the form of graded potentials from other neurons or sensory receptors. They conduct impulses toward the cell body and greatly increase the surface area available for receiving signals.
The axon (nerve fiber) is a single, long process that extends from the cell body at a specialized region called the axon hillock. It conducts action potentials away from the cell body toward the axon terminals, which are bulb-shaped endings (synaptic knobs or boutons) containing vesicles filled with neurotransmitters. Axons can be extraordinarily long — up to a meter or more in the sciatic nerve neurons. Many axons are covered by a myelin sheath, a fatty insulating layer formed by oligodendrocytes in the CNS and Schwann cells in the PNS. Myelination dramatically speeds up nerve impulse conduction through a process called saltatory conduction. Gaps in the myelin sheath, called nodes of Ranvier, are the sites where action potentials are regenerated. Axons may also give off branches called axon collaterals, allowing a single neuron to communicate with multiple targets.
Structural Classification of Neurons
Multipolar neurons have one axon and many dendrites and are the most common type in the CNS, including motor neurons and interneurons. Bipolar neurons possess one axon and one dendrite and are rare, found in special sense organs such as the retina and olfactory epithelium. Unipolar (pseudounipolar) neurons have a single process that extends from the cell body and divides into a peripheral branch and a central branch; most sensory neurons of the PNS are of this type.
Functional Classification of Neurons
Sensory (afferent) neurons transmit impulses from sensory receptors toward the CNS. Motor (efferent) neurons carry impulses from the CNS to effectors such as muscles and glands. Interneurons (association neurons) lie entirely within the CNS, connecting sensory and motor neurons and participating in integration and processing.
<image>A detailed diagram of a typical multipolar neuron. The cell body (soma) is shown in the center with a large nucleus, prominent nucleolus, Nissl bodies (rough ER), and neurofibrils. Multiple dendrites branch from the cell body. A single long axon extends from the axon hillock, wrapped in a myelin sheath with segments formed by Schwann cells (in PNS). Nodes of Ranvier are shown as gaps between myelin segments. The axon terminates in multiple axon terminals (synaptic knobs) containing synaptic vesicles. An inset shows the three structural types: multipolar (many dendrites, one axon), bipolar (one dendrite, one axon), and unipolar (single process that bifurcates).</image>
VII. Neuroglia (Glial Cells)
Neuroglia far outnumber neurons, though recent estimates suggest the ratio may be closer to equal rather than the traditional 10:1. Unlike most neurons, glial cells can divide throughout life. They do not transmit nerve impulses but perform essential support functions.
CNS Neuroglia
Astrocytes are star-shaped cells and the most abundant glial type. They brace and anchor neurons to capillaries, help form the blood-brain barrier by wrapping their end-feet around capillaries, control the chemical environment around neurons by regulating potassium levels and recycling neurotransmitters, and participate in synapse formation and neural signaling. Microglia are small, motile cells with thorny processes that function as the macrophages of the CNS, phagocytizing microorganisms, dead neurons, and debris. They are derived from monocytes and therefore have a mesodermal origin, unlike other glia, which are ectodermal. Ependymal cells are ciliated cells lining the ventricles of the brain and the central canal of the spinal cord. Their cilia help circulate cerebrospinal fluid, and some form the choroid plexus, which produces CSF. Oligodendrocytes have fewer, thicker processes than astrocytes and form myelin sheaths around CNS axons, with a single oligodendrocyte capable of myelinating portions of multiple axons.
PNS Neuroglia
Schwann cells (neurolemmocytes) form myelin sheaths around PNS axons, with each Schwann cell wrapping around a single segment of one axon. They also enclose unmyelinated PNS axons, with multiple small axons resting in grooves of a single Schwann cell. Schwann cells are vital for nerve regeneration in the PNS. Satellite cells surround neuron cell bodies in PNS ganglia and perform functions analogous to astrocytes, protecting neurons and regulating their local environment.
<image>A labeled diagram of the six types of neuroglia. Panel A (CNS glia): An astrocyte with star-shaped processes wrapping around a capillary and contacting a neuron; a small microglia cell with thorny processes near a neuron; ependymal cells lining a ventricle with cilia beating toward the lumen; an oligodendrocyte extending processes to myelinate segments of multiple axons. Panel B (PNS glia): A Schwann cell wrapping concentrically around an axon to form myelin, with a cross-section showing the layers of myelin; satellite cells surrounding a neuron cell body in a ganglion.</image>
VIII. Tissue Repair (Wound Healing)
When tissue is damaged, repair occurs through one of two processes. Regeneration replaces destroyed tissue with the same type of cells, restoring normal function. Fibrosis replaces the damaged area with dense connective tissue (scar tissue), which, while strong, does not restore the original specialized function.
Steps in Tissue Repair
The first stage is inflammation, the immediate response to injury. Blood vessels dilate and become more permeable, allowing fluid, clotting proteins, and immune cells to flood into the injured area. A blood clot forms to stop bleeding and hold wound edges together, while white blood cells such as neutrophils and macrophages clean up debris and fight infection.
During the second stage, organization, granulation tissue forms to replace the blood clot. This delicate, pink tissue is rich in new capillaries (formed through angiogenesis) and fibroblasts that produce collagen to bridge the gap. Macrophages continue their cleanup work.
In the third stage, regeneration and fibrosis occur simultaneously. The surface epithelium regenerates beneath the scab, while fibroblasts produce scar tissue in the deeper areas. The resulting scar is strong but lacks the flexibility and specialized function of the original tissue.
Factors Affecting Tissue Repair
The capacity for repair depends on the tissue type. Epithelia, bone, and areolar connective tissue regenerate readily. Skeletal muscle and dense connective tissue regenerate poorly. Cardiac muscle and nervous tissue in the CNS have virtually no functional regenerative capacity and are primarily repaired by fibrosis. Beyond tissue type, adequate nutrition is essential, particularly protein, vitamin C for collagen synthesis, vitamin A, and zinc. A robust blood supply accelerates healing, since well-vascularized tissues receive more oxygen and nutrients. Age slows healing because of decreased circulation and metabolic rate. Finally, infection delays repair and can worsen tissue damage.
