Premed · Premed · Anatomy Physiology 1
Lecture 13: Nervous Tissue — Neurons and Glia
Anatomy and Physiology I
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the overall organization of the nervous system (CNS vs. PNS, somatic vs. autonomic)
- List the functions of the nervous system
- Review the structure of a neuron and the types of neuroglia
- Explain how myelination occurs in the CNS and PNS
- Describe the classification of nerve fibers by myelination and conduction speed
- Describe the process of nerve regeneration in the PNS
- Describe the organization of neurons into nerves, ganglia, nuclei, and tracts
Lecture Content
I. Organization of the Nervous System
Structural Divisions
The nervous system is divided structurally into two parts. The central nervous system (CNS) comprises the brain and spinal cord, serving as the integration and command center that processes information and issues responses. The peripheral nervous system (PNS) consists of the cranial nerves (12 pairs) and spinal nerves (31 pairs) plus ganglia, functioning as communication lines that link the CNS to the rest of the body.
Functional Divisions of the PNS
The PNS is functionally divided into sensory and motor divisions. The sensory (afferent) division carries impulses from receptors to the CNS and includes somatic sensory fibers (from skin, muscles, and joints, conveying conscious sensation) and visceral sensory fibers (from visceral organs, usually conveying subconscious information).
The motor (efferent) division carries impulses from the CNS to effectors (muscles and glands) and is subdivided into two systems. The somatic nervous system (SNS) provides voluntary control of skeletal muscles via a single motor neuron extending from the CNS to the skeletal muscle; its effect is always excitatory, causing contraction. The autonomic nervous system (ANS) provides involuntary control of smooth muscle, cardiac muscle, and glands via a two-neuron chain from the CNS to the effector. The ANS is further divided into the sympathetic division (fight-or-flight) and the parasympathetic division (rest-and-digest), and its effects can be either excitatory or inhibitory.
II. Functions of the Nervous System
The nervous system performs three overlapping functions. Sensory input involves monitoring internal and external environments via sensory receptors. Integration is the processing and interpretation of sensory information and the making of decisions. Motor output is the activation of effectors (muscles and glands) to produce responses.
III. Neuron Structure (Review and Expansion)
The cell body (soma) contains the nucleus, Nissl bodies (rough ER), Golgi apparatus, mitochondria, and neurofibrils, and serves as the biosynthetic center of the neuron. Most cell bodies are located in the CNS. Collections of cell bodies are called nuclei in the CNS and ganglia in the PNS.
Dendrites are the receptive input regions of the neuron, conducting graded potentials toward the soma. The axon is the conducting region, generating and propagating action potentials away from the soma. The axon hillock is the cone-shaped area where the axon emerges from the cell body and serves as the trigger zone (initial segment) where action potentials are initiated. At the distal end, the axon terminals (synaptic boutons) release neurotransmitters. Axons may branch into axon collaterals. Bundles of axons are called tracts in the CNS (forming white matter) and nerves in the PNS.
IV. Neuron Classification (Review)
By Structure
Multipolar neurons have many dendrites and one axon and are the most common type, including most CNS neurons and motor neurons. Bipolar neurons have one dendrite and one axon and are found in special sense organs such as the retina, olfactory epithelium, and inner ear. Unipolar (pseudounipolar) neurons have a single process that bifurcates into peripheral and central branches; most sensory neurons of the PNS are of this type. Anaxonic neurons have no distinguishable axon, are found in the brain and retina, and function in local signal processing.
By Function
Sensory (afferent) neurons carry impulses toward the CNS. Motor (efferent) neurons carry impulses away from the CNS. Interneurons (association neurons) lie entirely within the CNS, are the most numerous type, and form integrative circuits.
V. Neuroglia (Review and Expansion)
CNS Glia
Astrocytes are the most abundant glial cells. They contribute to blood-brain barrier (BBB) formation, regulate ions and neurotransmitters, provide metabolic support, modulate synapses, and form scars (gliosis) after injury. Oligodendrocytes form myelin sheaths in the CNS, and a single oligodendrocyte can myelinate segments of up to 60 axons. Microglia provide immune defense through phagocytic activity and are derived from monocytes. Ependymal cells line the ventricles and central canal, circulate CSF with their cilia, and some form the choroid plexus that produces CSF.
PNS Glia
Schwann cells (neurolemmocytes) form myelin sheaths in the PNS, with one Schwann cell myelinating one segment of one axon. They are essential for nerve regeneration. Satellite cells surround neuron cell bodies in ganglia and regulate the chemical environment, performing functions analogous to those of astrocytes.
VI. Myelination
The myelin sheath is a segmented covering of lipid-rich membrane around many axons. It insulates the axon, increases conduction speed, and conserves energy by confining ion fluxes to the nodes.
Myelination in the PNS
In the PNS, a Schwann cell wraps concentrically around a single axon segment. The cytoplasm is squeezed out, and the concentric layers of Schwann cell membrane form the myelin sheath. The outermost layer of Schwann cell cytoplasm and membrane constitutes the neurilemma (neurolemma). Nodes of Ranvier are gaps between adjacent Schwann cells where the axon membrane is exposed, serving as sites of action potential regeneration. Unmyelinated PNS axons are still enclosed by Schwann cells but are not wrapped; multiple axons may rest in grooves of a single Schwann cell.
Myelination in the CNS
In the CNS, oligodendrocytes extend flat processes that wrap around axon segments, and a single oligodendrocyte can myelinate segments of multiple axons. There is no neurilemma in the CNS. Nodes of Ranvier are present but are wider and less regular than in the PNS.
Classification of Nerve Fibers by Conduction Speed
Type A fibers are large in diameter and heavily myelinated, conducting impulses the fastest at up to 130 m/s; they include somatic sensory and motor neurons. Type B fibers are medium in diameter and lightly myelinated, conducting at moderate speeds of 3 to 15 m/s; they include autonomic preganglionic neurons. Type C fibers are small in diameter and unmyelinated, conducting the most slowly at 0.5 to 2 m/s; they include autonomic postganglionic neurons and pain fibers carrying slow, chronic pain.
<image>A comparison of myelination in the PNS and CNS. Panel A (PNS): A Schwann cell wrapping around a single axon in cross-section, showing the progressive wrapping stages from loose to tight concentric layers of membrane forming myelin. The outermost layer of cytoplasm is labeled as the neurilemma. A longitudinal view shows three Schwann cells in series with nodes of Ranvier (gaps) between them. Panel B (CNS): An oligodendrocyte in the center extending multiple flat processes to myelinate segments of several different axons simultaneously. Nodes of Ranvier are visible between myelinated segments. A small table compares: PNS myelin (Schwann cells, one cell per segment, neurilemma present, regeneration possible) vs. CNS myelin (oligodendrocytes, one cell for multiple segments, no neurilemma, regeneration very limited).</image>
VII. Nerve Regeneration
PNS Regeneration (possible under favorable conditions)
When a peripheral nerve is damaged, the axon distal to the injury site degenerates in a process called Wallerian degeneration, and the myelin sheath also breaks down. Macrophages phagocytize the debris. Schwann cells then proliferate and form a regeneration tube (band of Bungner), a channel of aligned Schwann cells that guides regrowth. The proximal axon stump sprouts new growth processes called growth cones, which follow chemical signals — neurotrophic factors such as nerve growth factor (NGF) — and the Schwann cell tube toward the target. The axon regrows at a rate of approximately 1 to 5 millimeters per day, and Schwann cells remyelinate the new axon. Success depends on the proximity of the severed ends, alignment of the tube, absence of scar tissue, and speed of reinnervation.
CNS Regeneration (very limited)
CNS regeneration is severely limited for several reasons. Oligodendrocytes do not form regeneration tubes. The CNS environment contains growth-inhibiting molecules such as Nogo protein and myelin-associated glycoprotein. Astrocytes form a glial scar that acts as both a physical and chemical barrier to axon regrowth. Current research aims to promote CNS repair through the use of growth factors, stem cells, and blocking of inhibitory molecules.
VIII. Organization of Neural Tissue
Nerves (PNS)
A nerve is a bundle of axons (nerve fibers) in the PNS, wrapped in connective tissue. The endoneurium is delicate connective tissue around each individual axon. The perineurium wraps a fascicle, which is a bundle of axons. The epineurium is tough connective tissue covering the entire nerve. Nerves may be sensory (afferent), carrying only sensory fibers; motor (efferent), carrying only motor fibers; or mixed, carrying both sensory and motor fibers. Most nerves are mixed.
Gray Matter and White Matter
Gray matter contains neuron cell bodies, dendrites, unmyelinated axons, and synapses. In the brain, gray matter is found on the surface (cortex) and in deep nuclei. In the spinal cord, gray matter is in the center, forming a butterfly- or H-shaped region. White matter contains myelinated axon tracts (bundles), and the myelin gives it its white appearance. In the brain, white matter is deep to the cortex. In the spinal cord, white matter surrounds the gray matter.
<image>A cross-sectional diagram of a peripheral nerve showing its organizational layers. The outermost layer is the epineurium (dense connective tissue sheath surrounding the whole nerve). Inside, several fascicles are visible, each wrapped in perineurium. Within each fascicle, individual axons (some myelinated with visible myelin sheaths, some unmyelinated) are each surrounded by endoneurium. Blood vessels (vasa nervorum) are shown running within the epineurium. An inset compares the arrangement of gray and white matter in the brain (gray cortex on the surface, white matter deep) versus the spinal cord (gray matter as a central butterfly shape, white matter on the outside).</image>
IX. Simple Neuronal Circuits
Several basic circuit patterns are found in the nervous system. In a diverging circuit, one neuron stimulates multiple neurons, allowing a single stimulus to have widespread effects, as when one sensory signal is sent to many brain regions. In a converging circuit, multiple neurons synapse on a single neuron, allowing integration from multiple sources, such as multiple sensory inputs converging on a single motor neuron. In a reverberating circuit, neurons form a feedback loop in which the signal is recycled, producing a sustained output, as seen in breathing rhythm and short-term memory. In a serial (parallel after-discharge) circuit, one neuron stimulates several neurons in a chain that eventually converge on a single output neuron, producing a prolonged burst of output useful for complex, precise movements.
