This lecture corresponds to Chapter 12 in the Marieb 8e textbook.

Overview & Learning Objectives
Lectures Notes & Slides
Lecture Videos
Additional Resources
Noneducational Shenanigans

Overview

It is important to understand an overview of the human nervous system before focusing on the functional anatomy of the nervous tissue, especially neurons. Without a complete understanding of sensory input, integration, and motor output, basic principles of neural function are elusive.

The nervous system shares important functions with the endocrine system; you will explore the endocrine system in detail in a later chapter.

The nervous system is the master controller of the other systems and enables you to respond to internal and external environmental information.

Use the Complete Anatomy app as well as items under Additional Resources to help you form a firm grasp of the material.

Learning Objectives

List the main functions of the nervous system.
Explain the structural and functional divisions of the nervous system.
Define neuron, describe its structural components, and relate each structure to its functional role.
Describe the structure of a synapse.
Classify neurons both structurally and functionally.
List the six types of neuroglia in nervous tissue, and distinguish them by location and function.
Describe the structure of myelin sheaths.
Define nerve and describe the structural components of nerves.
Distinguish gray matter from white matter in the CNS.
Describe the structural link between the peripheral nervous system and the central nervous system.
Define reflex, and list the basic components of a reflex arc. Distinguish monosynaptic reflexes from polysynaptic reflexes.
Describe the roles of interneurons in the CNS.
Describe how multiple sclerosis relates to myelin and axon function.
Define nerve, tract, ganglion, and nucleus (not the cellular organelle).

Lecture Notes & Slides

Lec.16 Notes _ Nervous system.docx
Lec.16 Notes _ Nervous system.pdf
Lec.16 slides _ Nervous system.pdf

Lec.16 Fundamentals of the Nervous System & Nervous Tissue

Reading — Ch. 12 Fundamentals of the Nervous System & Nervous Tissue

Figure 12.1 Functions of the nervous system.

Figure 12.1 Functions of the nervous system.

Functional Organization of the Nervous System (pp. 350–353)

Functions of the Nervous System (p. 350) (Figure 12.1)

the master control and communications system of the body — controls most of the other organ systems in the body
It has the chief functions of sensory input, integration, and motor output.
- monitor, integrate, and respond to information in the environment.

Basic Divisions of the Nervous System (pp. 350–353)

Figure 12.2 Divisions of the nervous system: the central nervous system (CNS) & the peripheral nervous system (PNS).

Figure 12.2 Divisions of the nervous system: the central nervous system (CNS) & the peripheral nervous system (PNS).

basic divisions of the nervous system (p. 350, Figure 12.2)
- central nervous system (CNS) — brain and spinal cord
- peripheral nervous system (PNS) — external to the CNS; consist of nerves and ganglia.
The nervous system receives sensory inputs and dictates motor outputs.
- Sensory (afferent) signals are carried from sensory receptors through the PNS into the CNS.
  - integration of this information in the CNS dictates a response
- Motor (efferent) signals are carried away from the CNS and through the PNS to the effectors, which are muscles and glands.
- The types of sensory inputs and motor outputs are further categorized as somatic (outer body) and visceral (mainly inner body), and general (widespread) and special (localized)
Four main subdivisions of the PNS — somatic sensory, visceral sensory, somatic motor, and visceral motor. (Figure 12.3 and Table 12.1)
Proprioception refers to a series of senses that monitor the degree of stretch in muscles, tendons, and joint capsules. Proprioception thus senses the positions and movements of our body parts.

Figure 12.3 Schematic of levels of organization in the nervous system.

Figure 12.3 Schematic of levels of organization in the nervous system.

Table 12.1 Types of Sensory & Motor Fibers of the Peripheral Nervous System

Table 12.1 Types of Sensory & Motor Fibers of the Peripheral Nervous System

Nervous Tissue (pp. 353–361)


Figure 12.4 Structure of a typical neuron (a motor neuron).

The human body contains billions of nondividing neurons, or nerve cells. (p. 353)
Nervous tissue consists of
- Neurons — impulse-conducting cells that can transmit electrical signals
- Neuroglia — support cells that ensure the neurons are functioning properly. These do not conduct impulses.

The Neuron (pp. 353–358)

Several functions characterize neurons
- ability to conduct electrical impulses,
- extreme longevity,
- no cell division,
- high metabolic rate.
Neuron anatomy (Figure 12.4) — Each neuron is composed of three main parts: cell body, dendrites, and axon.
- Cell body (Soma)
  - cytoplasm contains all the usual organelles and chromatophilic bodies.
  - nucleus contains a dark nucleolus near its center.
  - cytoplasm contains chromatophilic substance, which consists of concentrations of rough endoplasmic reticulum and free ribosomes. The chromatophilic substance in the cell body manufactures proteins and membranes for the entire neuron, thereby continuously maintaining and renewing the contents of the cell, including the processes.
  - Most neuronal cell bodies are located within the CNS; those in the PNS are termed ganglia.
- Dendrites — branching processes extending from cell body.
  - function as receptive sites that conduct signals from other neurons toward the cell body.
  - Most neurons have multiple branched dendrites.
- Axon — the neuronal process that generates and conducts impulses away from the cell body.
  - Most neurons have one axon
  - Impulses begin at the initial segment of the axon, arising from the cone-shaped axon hillock.
  - Impulses end at the knoblike axon terminals, which participate in synapses and release neurotransmitter molecules.

Frequently used neuroanatomy terminology

	*What it means*	*Example* (click on links to see illustrations)
Tract	Bundle of neuron axons in CNS	Corticospinal tract
Nerve	Bundle of neuron axons in PNS	Radial nerve, ulnar nerve, sciatic nerve
Nucleus (plural *nuclei*)	Cluster of neuron cell bodies in CNS	Basal nuclei (often referred to as "basal ganglia" but this is a misnomer)
Ganglion (plural *ganglia*)	Cluster of neuron cell bodies in PNS	Dorsal root ganglion (DRG)

synapse — the functional junction and the site of communication between neurons (Figure 12.5)

Figure 12.5 Structure of an axodendritic synapse.
- Watch this video (1:10 through 4:15) — "How a synapse works" by HarvardX

- Synapses may be electrical (gap junctions) or, more commonly, chemical.
- The 2 main categories of synapses are axodendritic and axosomatic.
- How synapses work (how signals are transmitted from one neuron to another)
  - At synapses, information is transferred from a presynaptic neuron to a postsynaptic neuron.
  - Synaptic vesicles in the presynaptic cell fuse with the presynaptic membrane and empty neurotransmitter molecules into the synaptic cleft.
  - Taken up by the postsynaptic membrane, this neurotransmitter influences ability to generate a nerve impulse in the postsynaptic neuron.

Table 12.2 Comparison of Structural Classes of Neurons

Table 12.2 Comparison of Structural Classes of Neurons

- Neurons are classified by structure and by function. (pp. 355–357, Table 12.2)
  - Functional classification is based on the direction the nerve impulse travels;
    - sensory (afferent) neurons conduct impulses toward the CNS;
    - motor (efferent) neurons carry impulses away from the CNS;
    - interneurons lie in the CNS between motor and sensory neurons.
  - Structural classification groups neurons as multipolar, bipolar, or unipolar neurons.
    - Unipolar neurons (pseudounipolar neurons) — have a single process that extends from the cell body and divides into two processes — a central process (runs toward the CNS) and a peripheral process (extends away from the CNS and toward the receptors).
      - Unipolar neurons do not have dendrites — instead, receptive endings at the terminal end of the peripheral axon receive sensory stimuli and transmit signals through the peripheral axon toward the cell body
      - Most unipolar neurons are sensory neurons
    - Bipolar neurons — have two processes that extend from opposite sides of the cell body
      - Very rare in the human body
      - Most bipolar neurons are sensory neurons — occurring in some of the special sensory organs (inner ear, olfactory epithelium of the nose, retina of the eye).
    - Multipolar neurons — have more than 2 processes extending from the cell body.
      - Usually, multipolar neurons have numerous dendrites and a single axon. However, some small multipolar neurons have no axon and rely strictly on their dendrites for conducting signals.
      - Well over 99% of neurons in the body belong to the multipolar class.
      - Most multipolar neurons are interneurons that conduct impulses within the CNS, integrating sensory input or motor output; may be one of a chain of CNS neurons, or a single neuron connecting sensory and motor neurons.
      - Some multipolar neurons are motor neurons that conduct impulses along the efferent pathways from the CNS to an effector (muscle/gland).

Neuroglia (pp. 358–361)

Non-nervous supporting cells in the nervous system support, protect, nourish, and insulate neurons.
- These different types of support cells are collectively called neuroglia or glial cells.
These support cells are capable of division throughout life. (pp. 358–359, Figure 12.6)


Figure 12.6 Neuroglia.

Neuroglia in the CNS
- astrocytes
  - the most numerous cells in the CNS — a few folds more numerous than neurons!
  - star-shaped cells that perform many functions
    - Provide structural support
    - Maintain blood-brain barrier (BBB)
    - Regulate ion, nutrient, and dissolved gas concentrations
    - Absorb and recycle neurotransmitters
    - Form scar tissue after injury
- microglial cells
  - remove cell debris, wastes, and pathogens by phagocytosis
- ependymal cells
  - line the ventricles (in brain) and central canal (in spinal cord)
  - assist in producing, circulating, and monitoring cerebrospinal fluid (CSF)
- oligodendrocytes
  - myelinate CNS axons
  - provide structural framework
Neuroglia in the PNS

Figure 12.7 Schwann cells on myelinated & nonmyelinated axons in the PNS.
- Schwann cells
  - Myelinate axons in PNS
  - Participates in repair process after injury
- Satellite cells
  - Surround neuron cell bodies in ganglia
  - Regulate O₂, CO₂, nutrient, and neurotransmitter levels around neurons in ganglia

Myelin sheaths
- cover axons and provide insulation and increased speed of impulse conduction. Thick axons are myelinated.
- myelinated axons are present in the CNS and PNS. (pp. 359–361, Figures 12.7, 12.9)
- The myelin sheath is a coat of neuroglial cell membranes wrapped in layers around the axon.
- This sheath is formed by Schwann cells in the PNS and by oligodendrocytes in the CNS.
- Nodes of Ranvier — the gaps formed between myelin sheath cells along the axon.
- Nonmyelinated axons are surrounded by glial cells, but they are not wrapped by layers of membrane.

Gross Anatomy of the Nervous System — An Overview (pp. 361–362)

Gray and White Matter of the CNS (p. 361)

Figure 12.8 Cross section through the spinal cord.

Figure 12.8 Cross section through the spinal cord.

Gray matter — gray colored and surrounds the central cavities of the CNS. (p. 361, Figure 12.8)
- composed primarily of neuronal cell bodies, neuroglia, and nonmyelinated axons.
- All CNS synapses occur in gray matter
- In the cerebral cortex and cerebellum, an additional layer of gray matter surrounds the white matter of CNS.
White matter — lies external to the gray matter of the CNS.
- composed of myelinated axons passing (carrying information) between specific regions of the CNS.
- Tracts are bundles of axons traveling to similar destinations. (p. 361, Figure 12.8)
Gray matter white matter is a distinction only made in reference to the CNS and not PNS.
The extreme center of the spinal cord and brain is a hollow central cavity.

Nerves (pp. 361–362)

Nerves are bundles of axons in the PNS. (pp. 361–362, Figure 12.9)

Figure 12.9 Structure of a nerve.
- Endoneurium — each axon is surrounded by Schwann cells. Covering the Schwann cells is a delicate layer of loose connective tissue called endoneurium
- Perineurium — connective tissue surrounding nerve fascicles.
  - Fascicle — a group of axons bound together into a bundle.
- Epineurium — a tough connective tissue surrounding the entire nerve.
Because they contain more than one kind of tissue, nerves are organs.

Neuronal Integration (pp. 363–367)

The CNS and PNS are functionally interrelated — The nerves of the PNS carry sensory and motor information between the periphery of the body and the CNS.
The CNS is composed of interneurons that
- process sensory information,
- direct information to specific CNS regions,
- initiate appropriate motor responses,
- transport information from one region of the CNS to another.

Reflex Arcs (pp. 363–364)


Figure 12.10 Components of a reflex arc.

Reflexes — rapid, automatic motor responses to external as well as internal stimuli. (p. 363)
- They can be either somatic or visceral.
- mediated by chains of neurons called reflex arcs.
reflex arc — a simple chain of neurons that determines the basic structural plan of the nervous system;
- there are five basic components to a reflex arc (pp. 363–364, Figures 12.10, 12.11)
  - receptor,
  - sensory neuron,
  - integration center,
  - motor neuron,

The basic simplified plan of the nervous system emphasizes the reflex arcs associated with the spinal cord; sensory neurons enter the spinal cord dorsally, and motor neurons exit the spinal cord ventrally. (pp. 365–366, Figure 12.13)
The minimum number of elements in a reflex arc is five — a receptor, a sensory neuron, an integration center, a motor neuron, and an effector.


Figure 12.11 Types of reflex arcs.

Monosynaptic reflexes
- Only have a sensory neuron and a motor neuron, with no interneuron in between; thus, as its name implies, there is only one synapse in this reflex arc.
- Faster than polysynaptic reflexes — this is because chemical signals (across synapse) is slower than electrical signals (down the axon), so there is a slowing down every time the signal crosses a synapse. This means there more synapses there are in a reflex arc, the more time it takes.
Polysynaptic reflexes
- The majority of reflexes in humans
- The simplest of these, such as withdrawal reflexes, have three neurons — a sensory neuron, an interneuron, and a motor neuron.

Integration Between the PNS and CNS (pp. 366–367)

Simple reflex arcs reflect the structural plan of the entire nervous system.
Sensory signals are processed by neuronal circuits in the CNS to produce a reflexive response.
Cortical processing through interneurons in the CNS enable conscious sensation and nonreflexive motor actions.

Lecture videos

Additional Resources

Additional resources available on 🧪 Lab 19 Brain: Meninges, External and Internal structures

Noneducational Shenanigans

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