Chapter 11 Objectives

1. State the 3 functions of nervous system system.

2. Define the divisions of the nervous system (CNS, PNS, sensory or afferent, motor or efferent, somatic, autonomic, sympathetic, parasympathetic). Use chart in your book when studying.

3. Describe each of the neuroglial cells by function and location.

4. Describe the cellular structure of a neuron. (dendrites, cell body components, transportation system, axon, Nissl bodies, axon collaterals, telodendria, axon terminals, node of Ranvier)

5. Define nuclei, ganglia, tracts, and nerves.

6. Describe how the myelin sheath is formed. (Schwann cell, neurilemma, white

matter, gray matter)

7. Classify neurons structurally and functionally.

8. Define synapse, presynaptic neuron, and postsynaptic neuron.

9. Describe an electrical synapse. (structure, function, location)

10. Describe a chemical synapse. (structure, function, how information is transferred across chemical synapses, how neurotransmitter effects are terminated).

11. Describe neurotransmitters by chemical structure. (where released, general effect, match names with classification).

12. Describe acetylcholine as excitatory or inhibitory at different tissues.

13. Contrast ionotropic and metabotropic neurotransmitters.

14. Describe 4 types of neuron circuits.

15. Contrast serial and parallel processing.

NERVOUS SYSTEM

Master controlling and communicating system. With the endocrine system, it is

responsible for regulating and maintaining body homeostasis.

The cells communicate by means of electrical signals, which are rapid, specific, and cause an almost immediate response.

Three overlapping functions:

1. Uses sensory receptors to monitor changes in the environment.

2. Processes and interprets the sensory input and makes a decision.

3. Effects a response.

Organization of the Nervous System

The nervous system is divided into two principal parts:

Central Nervous System (CNS): consists of the brain and spinal cord.

Integrating and command center. It interprets in coming sensory information.

Dictates responses based on past experience, reflexes, and current conditions.

Peripheral Nervous System (PNS): consists mainly of the nerves that extend from the brain and spinal cord.

Spinal nerves carry impulses to and from the spinal cord. Cranial nerves carry impulses to and from the brain.

Two functional subdivisions of PNS:

1. Sensory or afferent: consists of nerve fibers that convey impulses to the CNS from sensory receptors located throughout the body.

Somatic afferent convey impulses from the skin, skeletal muscles, and joints.

Visceral afferent fibers carry impulses from the visceral organs.

2. Motor or efferent : transmits impulses from the CNS to effecter organs (muscles and glands).

(a). Somatic Nervous System: composed of motor nerve fibers that conduct impulses from the CNS to skeletal muscles. It is the voluntary nervous system.

(b). Autonomic Nervous System: consists of motor nerve fibers that regulate the activity of smooth muscles, cardiac muscles, and glands. It is the involuntary nervous system.

The Autonomic Nervous System (ANS) has two functional divisions: Sympathetic and Parasympathetic. Typically bring about opposite effects on the activity of the same visceral organs. What one stimulates the other inhibits.

Histology of Nervous System

Highly cellular. Both CNS and PNS are made up of just two principal types of cells: neurons and supporting cells

Supporting Cells - Form the scaffolding of nervous tissue. Generally assist, segregate, and insulate neurons. Each type of supporting cell has special functions.

Six types of cells: In the CNS are astrocytes, microglia, ependymal cells,

oligodendrocytes. In the PNS are Schwann cells and satellite cells. Depending on the neural site, supporting cells are 10 - 50 times more numerous than neurons. Glial cells retain their ability to reproduce themselves throughout life (a severe type of cancer is a gliablastoma).

Astrocytes (star - shaped): abundant in the CNS; account for nearly half of neural tissue volume. Numerous radiating projections with bulbous ends that cling to neurons and capillaries, bracing the neurons and anchoring them to their nutrient source. Form a living barrier between capillaries and neurons, and play a role in making exchanges between the two. Control the chemical environment around the neurons. Recapture and recycle released neurotransmitters.

Microglia: Small, ovoid cells win relatively long "thorny" processes. Special type of macrophage that helps protect the CNS from microorganisms and dead neural tissue.

Ependymal: Line the central cavity of the brain and spinal cord. Forms the cerebrospinal fluid (CSF). Beating of their cilia helps to circulate the CSF that cushions the brain and spinal cord.

Oligodendrocytes: Line up along the thicker neuron fibers in the CNS. Wrap their cytoplasmic extensions tightly around the nerve fibers. Produce insulating coverings called myelin sheaths.

Schwann cells: Form myelin sheaths around the larger nerve fibers in the PNS.

Functionally similar to oligodendrocytes. Act as phagocytes to rid a damaged nerve of deteriorating cell debris vital to the process of peripheral nerve fiber regeneration.

Satellite cells: Closely associated with Schwann cells. Thought to play some role in controlling the chemical environment of neurons with which they are associated in the PNS.

Neurons: Structural units of the nervous system.

Highly specialized cells that conduct messages in the form of nerve impulses from one part of the body to another. They have extreme longevity. Neurons are amitotic. High metabolic rate; require continuously abundant supplies of oxygen and glucose. Typically large, complex cells composed of a cell body and many processes. Most neurons have three functional components: receptive or input region, conducting component, secretory or output component.

Neuron Cell Body

Biosynthetic center of a neuron; contains the usual organelles with the exception of

centrioles. Protein and membrane-making machinery (free ribosomes, rough ER) is

probably the most active and best developed of any cell in the body. Rough ER referred

to as Nissl bodies. Golgi is elaborate and forms an are or complete circle around the

nucleus. Mitochrondria are scattered everywhere. Cell body is the focal point for

outgrowth of neuron processes. Plasma membrane of the cell body acts as part of the

receptive surface that receives information from other neurons. Most neuron cell bodies

are located within the CNS. Clusters of cell bodies in the CNS are called nuclei. Far

fewer collections of cell bodies in the PNS are called ganglia.

Neuron Processes

Cytoplasmic extensions called processes extend from the cell body. The PNS consists

chiefly of neuron processes. Bundles of neuron processes are called tracts in the CNS

and nerves in the PNS. Two types of neuron processes: dendrites and axons. Differ from

each other in structure and in functional properties.

Dendrites (of motor neurons) Short, diffusely branched extensions. Typically contains

hundreds of dendrites clustered close to the cell body. Receptive or input regions. Provide

an enormous surface for reception of signals from other neurons. Dendrites conduct

electrical signals toward the cell body. The electrical signals are not nerve impulses but

are short-distance signals called graded potentials.

Axon: Each neuron has a single axon. Arises from a cone-shaped region of the cell body

called the axon hillock The axon tapers to form a slender process that remains uniform in

diameter for the rest of its length. In some neurons, the axon is very short or absent. In

others it is long and accounts for nearly the entire length of the neuron (3-4 feet). Any

long axon is called a nerve fiber. Axons with the largest diameters conduct impulses the

most rapidly. Axons give off occasional branches along their length called axon

collaterals. Axons branch profusely at its end: 10,000 or more telodendria, or end

branches per neuron. The bulbous distal endings of the telodendria are called axonal

terminals. Functionally, axons are the conducting component; they generate nerve

impulses and transmit them away from the cell body. In motor neurons, the nerve

impulse is generated at the axon hillock and conducted along the axon to the axonal

terminals. Neurotransmitters excite or inhibit neurons with which the axon is in close

contact. Each neuron both receives signals from and sends signals to scores of

neurons. An axon contains the same organelles found in the dendrites and cell body

except it lacks Nissl bodies. The axon depends on its cell body to renew the necessary

proteins and membrane components and on efficient transport mechanisms. Axons

quickly decay if cut or severely damaged. Substances are able to travel in both directions

within the axon.

Myelin Sheath and Neurilemma: Many nerve fibers are covered with a whitish, fatty

segmented sheath called the myelin sheath which protects and electrically insulates

fibers from one another. Myelinated fibers conduct nerve impulses rapidly. Myelin

sheaths are associated only with axons; dendrites are always unmyelinated.

Myelin sheaths in the PNS are formed by Schwann cells. The Schwann cells first become

indented to receive the axon and then wrap themselves around it in a jelly roll fashion.

The nucleus and most of the cytoplasm of the Schwann cell end up just beneath the

outermost part of its plasma membrane, external to the myelin sheath. This portion of the

Schwann cell, which surrounds the myelin sheath is the neurilemma. Adjacent Schwann

cells along an axon do not touch one another, so there are gaps in the sheath called nodes

of Ranvier. At these nodes axon collaterals can emerge from the axon.

Oligodendrocytes form the myelin sheaths in the CNS. Whereas Schwann cells can only form one segment of a myelin sheath, the oligonendrocytes have multiple flat processes than can coil around as many as 60 different axons at the same time. Nodes of Ranvier are present. Regions of the brain and spinal cord containing dense collections of myelinated fibers are referred to as white matter and are primarily fiber tracts. Gray matter contains mostly nerve cell bodies and unmyelinated fibers.

Classification of Neurons

Classified structurally and functionally.

Structurally they are grouped according to the number of processes extending from the

body. Three major neuron groups:

multipolar: 3 or more processes

bipolar: 2 processes - an axon and a dendrite extend from opposite sides

unipolar: single process; very short and divides t-like.

Functional classification according to the direction in which the nerve impulse travels

relative to the CNS.

Sensory or afferent: transmit impulses from sensory receptors in the skin or internal

organs toward the CNS.

Motor or efferent: carry impulses away from the CNS to the effecter organs (muscles,

glands).

Virtually all sensory neurons of the body are unipolar and their cell bodies are located in

sensory ganglia outside the CNS. Motor neurons are multipolar, and their cell bodies are

located in the CNS.

The Synapse

The operation of the nervous system depends on the flow of information through circuits

consisting of chains of neurons connected by synapses. Most synapses occur between the

axonal endings of one neuron and the dendrites or the cell bodies of other neurons. Less

common are the synapses between axons, between dendrites, or between dendrites and

cell bodies. The neuron conducting impulses toward the synapse is called the

preslynaptic neuron (information sender). The neuron that transmits the electrical

activity away from the synapse is the postsynaptic neuron (information recipient).

Most neurons function as both presynaptic and postsynaptic neurons, receiving

information from some neurons and dispatching it to others. A typical neuron has

thousands of axonal terminals making synapses and is stimulated by an equal number of

other neurons. In the body periphery, the postsynaptic cell may be either another neuron

or an effecter cell. Synapses between neurons and muscle cells are neuromuscular

junctions. Synapses between neurons and gland cells are neuroglandular junctions.

There are two varieties of synapses: electrical and chemical.

Electrical Synapses: Bridge junctions that correspond to the gap junctions. They contain

protein channels that interconnect the cytoplasm of adjacent neurons. Neurons are

electrically coupled, and transmission across these synapses is very rapid.A key feature is

that they provide a simple means of synchronizing the activity of all interconnected

neurons. In adults, these synapses are found in regions of the brain responsible for certain

stereotyped movements, such as jerky movement of the eyes. Electrical synapses are

abundant in non-nervous tissues, such as cardiac and smooth muscle, where they allow

sequential and rhythmic excitation.

Chemical Synapses: Specialized for release and reception of chemical neurotransmitters.

The neurotransmitters function to open or close ion channels that influence membrane

permeability and membrane potential. A typical chemical synapse is made up of two

parts: (I) a knoblike axonal terminal of the presynaptic neuron, containing many

membrane-bound sacs called synaptic vesicles containing neurotransmitter molecules.

(2) a receptor region on the membrane of a dendrite or the cell body of the postsynaptic

neuron, which bears neurotransmitter receptors.

The presynaptic and postsynaptic membranes are separated by the synaptic cleft.

Chemical synapses prevent a nerve impulse from being directly transmitted from one

neuron to another. The transmission of nerve impulses along an axon is an electrical

phenomenon. The transmission of signals across chemical synapses is a chemical event

that depends on the release, diffusion, and receptor binding of neurotransmitter molecules

and results in unidirectional communication. Depending on the types of neurotransmitters

released and the receptor proteins to which they bind, the result may be either excitation

or inhibition.

Termination of Neurotransmitter Effects

As long as neurotransmitter is bound to a receptor, it continues to produce its effects and

block the reception of additional messages coming in. The effects of neurotransmitters

appear to last a few milliseconds before being terminated by one of three mechanisms:

1. degradation by enzymes in the synapse.

2. removal from the synapse by re-uptake into presynaptic terminals.

3. diffision of the neurotransmitter away from the synapse.

Neurotransmitters

Neurotransmitters are the means by which each neuron communicates with others to

process information and send messages to the rest of the body. Regulate many body

activities and states such as sleep, hunger, memory, anger, joy, etc. Over 100 different

chemicals are either known neurotransmitters or candidates.

Neurotransmitters are classified either chemically or functionally.

Chemical Structure:

1. Acetylcholine (ACh): Released at neuromuscular junctions of the somatic nervous

system and also by neurons of the autonomic nervous system. Synthesized and enclosed

in synaptic vesicles within axonal terminals.

2. Biogenic amines: Include catecholamines (dopamine, norepinephrine, epinephrine)

and indolamines (seratonin, histamine). Broadly distributed in the brain, where they

appear to play a role in emotional behavior and regulation of the biological clock.

3. Amino acids: Difficult to prove a neurotransmitter role because they occur in ail cells,

and are important in many biochemical reactions.

4. Peptides: Strings of amino acids. Include a broad spectrum of molecules with diverse

effects. Include beta-endorphins and enkephalins which act as natural opiates or

euphorics, reducing perception of pain under stressful conditions.

Functional Classification:

1. Excitatory and Inhibitory: Some are excitatory, some are inhibitory, some are both.

ACh is excitatory at neuromuscular junctions with skeletal muscle and inhibitory on

cardiac muscle.

2. Ionotropic and Metabotropic:

Neurotransmitters that open ion channels are ionotropic. ACh and amino acids are

ionotropic. Metabotropic promote broader, longer-lasting effects by acting through

intracellular second messenger molecules, such as cyclic AMP. Intracellular second

messengers trigger the stimulation of genes to produce proteins that bring about the

longer-lasting effect.

Neural Integration

Neurons function in groups. Each group contributes to still broader neural functions.

There must be integration so that the parts fuse into a smoothly operating whole.

Types of Circuits:

The patterns of synaptic connections in the pools are called circuits. Circuits determine

functional capabilities. Four basic types of circuits:

1. Diverging circuits: One incoming fiber triggers responses in ever increasing numbers

farther and farther along in the circuit. Amplifying circuits.

2. Converging circuits: Opposite that of diverging circuits. Pool receives inputs from

several presynaptic neurons and funnels or concentrates them.

3. Reverberating circuits: Incoming signal travels through a chain of neurons, each of

which makes collateral synapses with neurons in the previous part of the pathway.

Impulses reverberate, giving a continuous output.

4. Parallel after-discharge circuits: Incoming fiber stimulates several neurons arranged

in parallel that eventually stimulate a common output cell.

Patterns of Neural Processing

Processing of inputs in the various circuits is both serial and parallel.

Serial processing: The input travels along a single pathway to a specific destination.

Whole system works in a predictable all or nothing manner. One neuron stimulates the

next in sequence eventually causing a specific response.

Parallel processing: The input travels along several different pathways to be integrated

in different CNS regions. Inputs are segregated into many different pathways and

information is dealt with simultaneously by different parts of the neural circuitry. Not

repetitious because the circuits do different things with the information.