Anatomy Chapter 13

the nervous system

nervous system characteristics

• controls and adjusts the activity of the body

• provides swift but brief responses

two anatomical divisions of the nervous system

central nervous system (CNS) and peripheral nervous system (PNS)

the nervous system consists of _____________________

all the nervous tissue in the body

components of the CNS

the brain and spinal cord

central nervous system (CNS) functions

1) integrating, processing, and coordinating sensory input and motor output

2) acting as the seat of intelligence, memory, learning, and emotion 

CNS receives information from the _______ division and commands with the _______ division

afferent, efferent

cerebrospinal fluid (CSF)

clear, watery fluid that fills the central canal and ventricles, surrounding the CNS

components of the peripheral nervous system (PNS) 

all of the peripheral nerves and nervous tissue outside the CNS

PNS function

provides sensory information to the CNS and carries motor commands away from the CNS

divisions of the PNS

afferent division and efferent division

afferent division of the PNS

carries sensory information to the CNS; begins at receptors that monitor specific characteristics of the environment

the __________ of a receptor carries information to the CNS

stimulation

receptor examples

sensory process, specialized cells or clusters of cells, or complex sense organs 

example of a complex sense organ that acts as a receptor

the eye

efferent division of the PNS

carries motor commands from the CNS to muscles and glands; begins inside the CNS and ends at the effector 

effector 

a peripheral gland or muscle cell innervated by a motor neuron

examples of effectors

muscle cells, gland cells, other cells specialized to perform specific functions

subdivisions of the afferent division of the PNS

somatic and visceral sensory receptors

somatic sensory receptors of the afferent division

monitor skeletal muscles, joints, skin, and from the visceral sensory receptors

visceral sensory receptors of the afferent division

monitor other internal structures, such as smooth muscle, cardiac muscle, glands, and respiratory and digestive organs

special sensory receptors of the afferent division

monitor smell, taste, vision, balance, and hearing

subdivisions of the efferent division of the PNS

somatic and autonomic nervous systems

somatic nervous system (SNS) 

controls skeletal muscle contraction

autonomic nervous system (ANS)

regulates smooth muscle, cardiac muscle, and glandular activity

divisions of the ANS

parasympathetic and sympathetic divisions

activities of the somatic nervous system may be under ____________________ control

voluntary or involuntary

example of voluntary control of the SNS

voluntary contractions of the skeletal muscles

example of involuntary control of the SNS

immediately withdrawing your hand from a hot stove even before noticing the pain

examples of involuntary control of the ANS

heartbeat, digestive processes, and instinctive responses to threatening situations

concept check 13.1: what are the two subdivisions of the nervous system?

the central and peripheral nervous systems

two types of cells contained in the nervous system

neurons and neuroglia

neurons

nerve cells that are responsible for the transfer and processing of information in the nervous system

neuroglia

cells that support and protect neurons

components of neurons

cell body, axon, dendrites, perikaryon, dendritic spines, axon terminals

perikaryon

cytoplasm that surrounds the nucleus in the cell body of a neuron

dendrites

sensory processes of a neuron

dendritic spines

fine processes of dendrites that receive information from other neurons

percent of dendritic spines making up the total surface area of the neuron

80-90%

axon

elongated extension of a neuron that conducts an action potential away from the cell body and toward the synaptic terminals 

axon terminals

site of neuron communication with another cell

approximate ratio of neuroglia to neurons

5:1

are neuroglia smaller or larger than neurons?

smaller

can neuroglia divide?

yes, neuroglia can divide

concept check 13.2: what are the two distinct cell types found within nervous tissue?

neurons and neuroglia

four types of neuroglia in the CNS

astrocytes, oligodendrocytes, microglia, and ependymal cells

two types of neuroglia in the PNS

satellite cells and Schwann cells

function of neuroglia in the CNS

surround CNS neurons and hold them in place, isolate neurons from each other, supply oxygen ad nutrients to neurons, destroy pathogens, and remove dead or damaged neurons

factors that distinguish neuroglia cells from each other

size, intracellular organization, the presence of specific cytoplasmic processes, and staining properties

astrocytes

star-shaped cells with projections that anchor to capillaries

largest and most numerous neuroglia of the CNS

astrocytes

astrocyte functions

1) maintain the blood brain barrier

2) provide structural support

3) regulate ion, nutrient, and dissolved-gas concentrations

4) absorb and recycle neurotransmitters

5) form scar tissue after injury

astrocyte function in the embryonic brain

involved in directing the growth and interconnection of developing neurons through the secretion of neurotropic factors 

astrocyte pedicels

cytoplasmic processes that increase surface area for the uptake of ions, neurotransmitters, or metabolic by-products accumulating around the neurons, enabling them to control the chemical content of the interstitial space; contact the surfaces of adjacent neurons to enclose them and isolate them from changes in the chemical composition of the interstitial space 

blood brain barrier (BBB)

physical and biochemical isolation of the CNS from general circulation

why is the blood brain barrier necessary?

because hormones or other chemicals in the blood could disrupt neuron function

endothelial cells lining capillaries in the CNS

quite impermeable and therefore control the chemical exchange between blood and interstitial fluid

oligodendrocyte

type of neuroglia in the CNS that forms myelin sheaths, internodes, and myelin sheath gaps

oligodendrocyte functions

1) myelinates CNS axons

2) provide structural framework

white matter

areas with mostly myelinated axons

gray matter

areas devoid of myelinated axons and composed of cell bodies and dendrites

internodes

sections of myelinated nerve fibers between two successive nodes

myelin sheath gaps (nodes of Ranvier)

small gaps between the myelin sheaths, produced by adjacent oligodendrocytes

microglia

phagocytic neuroglia in the CNS that are derived from monocytes 

stem cells that produce microglia

originate in the bone marrow and are related to stem cells that produce tissue macrophages and monocytes of the blood 

microglia function

remove cell debris, wastes, and pathogens by phagocytosis

the smallest neuroglia, possessing slender cytoplasmic processes with many fine branches

microglia

percentage of microglia in the CNS

5%, but increases upon infection or injury

ependymal cells

simple cuboidal epithelial cells that make up the ependyma; type of CNS neuroglia

ependyma

layer of cells lining the ventricles and central canal of the CNS

ependymal cells function

1) line the ventricles of the brain and the central canal of the spinal cord

2) assist in producing, circulating, and monitoring cerebrospinal fluid

how ependymal cells differ from typical epithelial cells

ependymal cells have slender processes that branch extensively and make direct contact with neuroglia in the surrounding nervous tissue 

cerebrospinal fluid (CSF)

fluid bathing the internal and external surfaces of the CNS, providing a protective cushion and transportation of dissolved gases, nutrients, wastes, and other materials

choroid plexus

secretes cerebrospinal fluid

cilia in ependymal cells

in adults, cilia and microvilli are found on the apical surface of the ependymal cells lining the spinal cord and the lateral and fourth ventricles of the brain, but those lining the third ventricle lack cilia

what is the role of cilia in ependymal cells? microvilli?

cilia help the CSF circulate and microvilli are involved in the absorption of CSF 

ganglia

collection of neuron cell bodies in the PNS

peripheral nerves

type of nerves in the PNS in which axons are bundled together and wrapped in connective tissue 

satellite cells of the PNS

surround neuron cell bodies in peripheral ganglia, regulating the exchange of nutrients and waste products between the neuronal cell body and extracellular fluid; isolate the neuron from stimuli not intended to pass from neuron to neuron sa

satellite cells surround neuron cell bodies in peripheral ganglia

functions of Schwann cells (neurolemmocytes)

1) surround all axons in the PNS

2) myelinate peripheral axons

3) participate in the repair process after injury

Schwann cells

neuroglia responsible for the neurilemma that surrounds axons in the PNS

axolemma

plasma membrane of an axon

neurolemma

cytoplasmic covering provided by Schwann cells 

satellite cell (PNS) analogue in the CNS

astrocytes

Schwann cell (PNS) analogue in the CNS

oligodendroglia

myelin improves the conduction speed of an ____________ (or nerve impulse) along the axon

action potential

axon myelination in the PNS

1) Schwann cell encloses a segment of the axon within a groove of its cytoplasm

2) Schwann cell rotates around the axon

3) as it rotates, the inner membranous layers are compressed and the cytoplasm is forced into more superficial layers

4) when completed, the myelin sheath consists only of the phospholipid bilayers of the plasma membrane, with the Schwann cell nucleus and cytoplasm at the surface

involvement of Schwann cells in a myelinated axon in the PNS

it takes many Schwann cells to myelinate an entire axon

cross section of a myelinated axon

myelin sheath appears as concentric dense lines around the axon

involvement of Schwann cells in an unmyelinated axon of the PNS

a single Schwann cell shields multiple unmyelinated axons within superficial grooves, but a chain of Schwann cells is still needed to enclose an entire axon 

cross section of unmyelinated axons

show axons in a Schwann cell in the neurolemma

concept check 13.3: specifically, what cells help maintain the blood brain barrier (BBB)?

astrocytes of the CNS

components of neurons

perikaryon, neurofilaments/neurotubules, neurofibrils, nissl bodies, axon hillocks, axoplasm, collaterals, telodendria, axon terminals, axoplasmic transport

neurons

transmit information from one part of the nervous system to another via electrical impulses

components of the cell body of a neuron

a large, round nucleus with a prominent nucleolus

perikaryon

cytoplasm of a neuron, containing organelles that provide energy and synthesize organic materials

appearance of the perikaryon

coarse and grainy due to its mitochondria, free and fixed ribosomes, and the membranes of the rough ER

neurofilaments and neurotubules

make up the neuron cytoskeleton

neurofibrils

bundles of neurofilaments that extend into the dendrites and axons, providing internal support

Nissl bodies

clusters of free ribosomes and RER, which stain a dark color in the perikaryon