BIO 004 · Human Anatomy
Functional Organization and Nervous Tissue
Block 5 · Module 1: Functional Organization and Nervous Tissue
A reference for the nervous tissue video and lab. This page covers the organization of the nervous system and its divisions, the three functions, the structure of a neuron, the synapse, the classification of neurons, the neuroglia, myelination, and the collections of nervous tissue. The focus is on the structures and the job each one does.
How to use this sheet Toggle the toolbar above. Notes prints the full reference for review. Study prints as a fill-in-the-blank worksheet. Print it, then write each definition while you watch the video or read your book. Quiz me is on-screen typing practice: type the term, click Reveal to check yourself. The comparison grids respond to Study and Quiz too, with a Reveal button on each row.
The Foundations video gives you a complete foundational understanding of this topic, enough on its own for a foundational course. Learn it first, then move on to the Deep dive, which adds the majors-level depth for this course.
- Outline the organization of the nervous system and its divisions.
- Identify the parts of a neuron and classify neurons by shape and by function.
- Name the neuroglia and describe myelination and the collections of nervous tissue.
Your pre-work
Work through these the evening before class. None of it is turned in. It is how you learn the material and build your spaced recall.
This is more than a checklist. Ticking these boxes is the start, not the finish. Committing this material to memory and being able to apply it takes considerable time and repeated effort. You are not done when the boxes are checked. Put in the real hours, and keep coming back for frequent recall and review until the material is genuinely yours.
The parts of a neuron
Add a labeled neuron showing the cell body, dendrites, axon, and axon terminals.
The neuroglia
Add a labeled diagram of the four CNS glia and the two PNS glia.
A myelinated axon
Add a labeled view of the myelin sheath, a node of Ranvier, and the neurolemma.
The Nervous System, an Overview
The nervous system gives the body the ability to feel, think, and act. It has two main divisions: the central nervous system and the peripheral nervous system.
- Nervous systemthe body's fast control system; it lets us feel, think, and act
- Central nervous system (CNS)the brain and the spinal cord; it processes information and issues commands
- Brainthe CNS organ in the cranial cavity, with about 85 billion neurons
- Spinal cordthe CNS cord in the vertebral canal that links the brain to the body
- Peripheral nervous system (PNS)every neural structure outside the brain and spinal cord
- Nervesbundles of axons in the PNS; 12 pairs of cranial nerves and 31 pairs of spinal nerves
- Gangliasmall masses of neuron cell bodies located outside the CNS
- Sensory receptorsstructures that monitor changes inside and outside the body
Divisions of the Peripheral Nervous System
The motor side of the peripheral nervous system is split by the kind of effector it controls. The somatic system runs skeletal muscle; the autonomic system runs the viscera; the enteric system runs the gut.
- Somatic nervous system (SNS)the voluntary division; its motor neurons carry commands from the CNS to skeletal muscle
- Autonomic nervous system (ANS)the involuntary division; its motor neurons carry commands to smooth muscle, cardiac muscle, and glands
- Sympathetic divisionthe ANS division for fight or flight, supporting exercise and emergencies
- Parasympathetic divisionthe ANS division for rest and digest, supporting routine activities
- Enteric nervous system (ENS)the brain of the gut, about one million neurons in the wall of the GI tract that can operate on their own
The Three Functions of the Nervous System
Whatever the task, the nervous system handles it in the same three steps. A stimulus comes in, the information is processed, and a response goes out.
- Sensory functionsensory receptors detect a change inside or outside the body and send the input toward the CNS
- Integrative functionthe CNS analyzes the sensory input, stores it, and decides on a response
- Motor functionthe CNS sends commands out to the effectors, the muscles and glands, which carry out the response
The Structure of a Neuron
A neuron is the signaling cell of the nervous system. It is excitable, it can carry an impulse, and it cannot divide. Every neuron has three basic parts.
- Neuronthe excitable cell that receives, conducts, and transmits nerve impulses; neurons cannot divide
- Cell bodythe soma or perikaryon; it holds the nucleus and cytoplasm and is the metabolic center of the neuron
- Dendritesthe branched, tree-like processes that receive incoming signals
- Axonthe single long process that carries the nerve impulse away toward other cells
- Nissl bodiesclusters of rough ER and ribosomes in the cell body; the site of protein synthesis
- Neurofibrilscytoskeletal fibers that give the neuron its shape and support
- Microtubulescytoskeletal tubes that transport materials along the neuron
- Axon hillockthe cone-shaped region where the cell body joins the axon
- Axoplasm and axolemmathe cytoplasm of the axon and the plasma membrane of the axon
- Axon terminalsthe fine branches at the far end of the axon that contact the target cell
- Lipofuscina harmless pigment that builds up in neurons with age
The Synapse
A synapse is the junction where a neuron passes its signal to another neuron, a muscle, or a gland. At a chemical synapse the signal crosses as a chemical message.
- Synapsethe site where a neuron communicates with another neuron, a muscle cell, or a gland
- Axodendritic synapsean axon ending on a dendrite
- Axosomatic synapsean axon ending on a cell body
- Axoaxonic synapsean axon ending on another axon
- Chemical synapsea synapse with no direct contact; the signal crosses as neurotransmitter molecules
- Synaptic cleftthe narrow gap between the two cells that the neurotransmitter diffuses across
- Synaptic end bulbsthe swellings at the axon terminal that hold the signal-sending machinery
- Synaptic vesiclesthe tiny sacs inside the end bulbs that store neurotransmitter
- Neurotransmittersthe chemical messengers that carry the signal across the cleft to the next cell
Classification of Neurons
Neurons are classified two ways: by their shape, which is the structural classification, and by the direction they carry signals, which is the functional classification.
Structural classification, by shape
- Multipolar neuronmany dendrites and one axon; the most common type, found in the brain and cord and as motor neurons
- Bipolar neuronone dendrite and one axon; found in the retina, the inner ear, and the olfactory area
- Unipolar neurona single fused process; the typical shape of a sensory neuron
- Purkinje cellsa distinctive multipolar neuron found in the cerebellum
- Pyramidal cellsa distinctive multipolar neuron found in the cerebral cortex
Functional classification, by direction
- Sensory (afferent) neuronscarry sensory input toward the CNS; most are unipolar
- Motor (efferent) neuronscarry motor commands away from the CNS; these are multipolar
- Interneuronsalso called association neurons; they connect sensory and motor neurons within the CNS and are multipolar
Neuroglia
Neuroglia, or glial cells, are the support cells of nervous tissue. They are smaller and far more numerous than neurons, and unlike neurons they can still divide.
- Neurogliathe support cells of nervous tissue; they nourish, support, and protect neurons and make up about half the volume of the CNS
Glia of the central nervous system
| Glial cell | Role |
|---|---|
| Astrocytes | star-shaped cells that give structural support, help form the blood-brain barrier, guide neuron growth, and maintain the chemical environment |
| Oligodendrocytes | cells that form the myelin sheaths of the CNS |
| Microglia | small phagocytic cells that remove debris and microbes |
| Ependymal cells | cells that line the ventricles and central canal and help produce and circulate cerebrospinal fluid |
Glia of the peripheral nervous system
| Glial cell | Role |
|---|---|
| Schwann cells | cells that form the myelin sheath around PNS axons and aid axon regeneration |
| Satellite cells | cells that surround and support the cell bodies of PNS neurons in ganglia |
Myelination
Myelin is a fatty wrapping around an axon. It insulates the axon and makes the nerve impulse travel much faster.
- Myelin sheatha multilayered covering of lipid and protein wrapped around an axon
- Function of myelinit insulates the axon and speeds up the nerve impulse
- Nodes of Ranvierthe gaps in the myelin sheath, spaced along the axon
- Saltatory conductionthe fast conduction in which the impulse jumps from node to node
The cell that builds the myelin differs between the PNS and the CNS, and that difference decides whether a damaged axon can regrow. Compare them.
| Feature | PNS myelination | CNS myelination |
|---|---|---|
| Cell that makes the myelin | a Schwann cell, one per axon segment | an oligodendrocyte, one cell for up to 15 axons |
| Neurolemma | present; it is the outer layer of the Schwann cell | absent |
| Axon regeneration | possible; the neurolemma guides the axon as it regrows | very limited, because there is no neurolemma |
Myelination increases from birth to maturity, so an infant's nerve conduction is slower than an adult's.
Collections of Nervous Tissue
The same kind of structure has a different name depending on whether it sits in the CNS or the PNS. Learn these as pairs.
| Structure | In the CNS | In the PNS |
|---|---|---|
| A cluster of cell bodies | nucleus | ganglion |
| A bundle of axons | tract | nerve |
- White matternervous tissue made of myelinated axons
- Gray matternervous tissue made of cell bodies, dendrites, unmyelinated axons, and neuroglia
- In the spinal cordwhite matter is on the outside, gray matter is on the inside
- In the braingray matter is on the outside, white matter is on the inside
See also: Histology: The Four Tissue Types for nervous tissue among the four, and Gross Anatomy and Neuronal Integration, the next page in this block.
Study questions
Work on answering these in writing, in your own words. They are the questions to bring to class, and good practice for the reasoning the exams ask for.
- Name the parts of a neuron and the direction a signal travels through them.
- Compare neurons and neuroglia by what each one does.
- Classify neurons by structure and by function, and give an example of each.
- Explain what myelin is, which glia produce it, and what it does for a signal.
Step 2 . Retrieval check
Now explain it back, in your own words.
In 60 words or more, pull together what the video just taught you. Include the key concepts. This is the point where the learning actually sticks. After you submit, your spaced-recall cards for this topic unlock.