Chapter 13 A&P II

neurobiology

study of nervous system

neurobiology subdisciplines

neuroanatomy and neurophysiology

central nervous system

brain and spinal cord which are enclosed by cranium and vertebral column

peripheral nervous system

all of the nervous system except the central nervous system

what is peripheral nervous system composed of

nerves and ganglia

nerve

bundle of nerve fibers (axons) wrapped in fibrous connective tissue

ganglion

A cluster of nerve cell bodies in the peripheral nervous system, often resembling a knot in a string

how is PNS functionally divided

sensory and motor

what does sensory division of PNS do?

carry sensory signals from receptors which informs CNS of stimuli from within and around body

what does somatic sensory division do?

carries signal receptors in the skin, muscles, bones, and joints

what does visceral sensory division do?

carries signals from viscera of thoracic and abdominal cavities such as heart, lungs, stomach, and urinary bladder

what does motor division do?

carries signals from CNS to gland and muscle cells for carrying body’s responses

effector

A molecule, cell, or organ that carries out a response to a stimulus

what does somatic motor division do?

carries signals to skeletal muscles which results in voluntary muscular contractions and somatic reflexes

what does visceral motor division do?

carries signals to glands, cardiac muscle, and smooth muscle

what does sympathetic division which arouses body for action do?

• accelerating heartbeat and respiratory airflow

• inhibits digestion and urine production

what does parasympathetic division which adapts body for energy intake and conversation do?

• stimulates digestion

• slows down heartbeat and respiratory airflow

enteric plexus

An extensive division of the autonomic nervous system throughout the digestive tract’s wall, serving to monitor conditions within the tract, control GI motility and secretion, and coordinate digestive physiology from end to end

3 properties of nervous system

• excitability

• conductivity

• secretion

excitability

The ability of a cell to respond to a stimulus, especially the ability of nerve and muscle cells to produce membrane voltage changes in response to stimuli; irritability

conductivity

nerves send electrical signals to quickly reach cells at distant locations

secretion

neurotransmitter is released when electrical signal reaches end of a nerve fiber

3 functional classes of neurons

• sensory

• interneurons

• motor

sensory

detects light, heat, chemicals, and pressure to transmit information to the CNS

interneurons

process, store, and retrieve information and make decisions about how body responds to stimuli

most common type of neurons

interneurons

motor neurons

send signals mainly to muscle and gland cells which carry out body’s responses to stimuli

cell body

control center of the neuron where nucleus is located

what is the significance of lipofuscin

wear and tear granules that are abundant in old neurons and associated with the eye and spinal cord’s degenerative diseases

dendrites

process of a neuron that receives information from other cells or from environmental stimuli and conducts signals to the cell body; shorter, more branched, and more numerous than the axon, and incapable of producing action potentials

what does having more neurons allow for a dendrite

it can receive more information from other cells and enhance neural integration

axon hillock

a mound on a side of the cell body where the axon originates

axon

A process of a neuron that conducts action potentials away from the cell body

what are axons specialized for?

rapid conduction of nerve signals to points remote from the cell body

trigger zone

axon hillock and initial segment where the neuron first generates an outgoing signal

axoplasm

axon’s cytoplasm

axolemma

axon’s membrane

dendritic spines

smaller protrusions on the dendrites that are one of the primary sites as well as dendrites for receiving signals from other neurons

terminal arborization

an extensive complex of fine branches at distal end of axon

terminal bouton (axon terminal)

a dilated tip at the end of each branch from terminal arborization

multipolar neurons

1 axon and at least 2 dendrites

characteristics of multipolar neurons

• most common type of neuron

• most neurons in brain and spinal cord

bipolar neurons

one axon and dendrite

bipolar neuron examples

• nasal cavity olfactory cells

• retina neurons

• inner ear sensory neurons

unipolar neurons characteristics

• carry signals to spinal cord for senses of touch and pain

• one long fiber that bypasses cell body and carries nerve signals directly to spinal cord

anaxonic neurons

multiple dendrites without an axon

anaxonic neurons characteristics

• communicate over short distances via dendrites

• without action potential

• brain, retina, and adrenal medulla

how much volume of nervous tissue is neurons

50%

neuroglia

• all cells of nervous tissue except neurons

• cells that perform various supportive and protective roles for the neurons

glial cells

Any of the six types of supporting cells of the nervous system; constitute most of the bulk of the nervous system and perform various protective and supportive roles for the neurons

examples of glial cells

• oligodendrocytes

• astrocytes

• microglia

• ependymal in CNS

• schwann and satellite in PNS

what’s one role of glial cells

• forms scaffold to guide young migrating neurons in fetus

• covers mature neuron when it isn’t in synaptic contact with cell to only allow contact during signal transmission

oligodendrocytes

bulbous body with as many as 15 armlike processes which reach out to a nerve fiber and spiral around it

ependymal cells

exhibit rootlike processes that penetrate into underlying tissue and connect with other cells

what do ependymal cells do?

• secrete and circulate CSF with cilia

• line internal cavities of brain and spinal cord

microglia

develop along with the CNS in the embryo and fetus to serve as self-renewing population of immune cells for life

what do microglia do

phagocytize dead tissue, microorganisms, and other foreign matter

how do microglia aid in synaptic remodeling

changing connections between neurons during nervous system development

astrocytes

the most abundant glial cells in the CNS and constitute over 90% of the tissue in some areas of the brain

role of astrocytes

• supportive framework for nervous tissue

• blood glucose converts to lactate which nourishes neurons

• communicate electrically with neurons; may influence synaptic signaling with neurons

• forms hardened scar tissue and fills space when neurons are damaged

• perivascular feet contact blood capillaries to stimulate them to form blood-brain barrier to isolate blood from brain tissue and limit where substances are able to get to brain cells

• neuron activity is monitored and signal blood vessels to constrict or dilate

• nerve growth factors promote neuron growth and synapse formation; fine tune neutral circuitry of brain and spinal cord

• astrocytes absorb neurotransmitters and potassium ions to prevent excess in the tissue fluid

how is myelin sheath formed?

• oligodendrocytes in CNS and schwann cells in PNS

• multiple plasma membrane layers from the 2 cells

myelination

The process in which an oligodendrocyte or Schwann cell deposits myelin around a nerve fiber

process of myelination

• schwann cell spirals around nerve fiber and laying down as many as 100 compact layers of its own membrane with minimal cytoplasm

• schwann cells spirals outward when wrapping nerve fiber which creates neurolemma

neurolemma

thick outermost coil of myelination with nucleus and most of cytoplasm

myelin sheath gaps

unmyelinated section between internodal segments of a myelinated nerve fiber; point where action potentials are generated in saltatory conduction

internodal segments

myelin-covered segments from one gap to the next

how many nerve fibers do schwann cells hold in grooves in its surface?

1-12

what affects the speed at which a signal travels along a nerve fiber

• fiber diameter

• presence or absence of myelin

how does the size of a nerve fiber affect it?

larger fibers have more surface area and conduct signals more rapidly

how fast nerve signals travel in unmyelinated vs myelinated nerve fibers of the same size

• 0.5-2 m/s

• 3-15 m/s and 120 m/s

what is the purpose of slow unmyelinated fibers?

processes in which quick responses aren’t important such as stomach acid and pupil dilation

what are fast myelinated fibers for?

• motor commands to skeletal system

• sensory signals for vision and balance

difference in requirements for large nerve fibers vs small ones

• large cell bodies and expenditure to maintain

• energy efficient

what does the evolution for myelin allow for

more complex and responsive nervous systems with smaller and more energy efficient neurons

what kind of disorders are multiple sclerosis and tay-sachs disease

degenerative ones of myelin sheath

multiple sclerosis

oligodendrocytes and myelin sheaths of the CNS deteriorate and are replaced by scar tissue

symptoms of MS

• numbness

• double vision

• blindness

• speech defects

• neurosis

• tremors

all patients experience cycles of milder and worse symptoms with MS until they’re bedridden. True or false?

true

tay-sachs disease

hereditary disorder mainly seen in infants of eastern European Jewish ancestry

what is tay-sachs caused by

abnormal accumulation of ganglioside in myelin sheath due to no lysosomal enzyme

tay-sachs symptoms

• blindness

• loss of coordination

• dementia

how can a nerve fiber regenerate?

if the cell body remains intact an axon can regenerate but needs neurolemma and endoneurium

regeneration tube

guides the growing axon to its destination

why does CNS suffer less trauma than PNS

it’s encased in bone

how does regeneration happen in the PNS

• schwann cells of neurolemma secrete nerve growth factors to stimulate axon’s regrowth

• schwann cells and endoneurium form regeneration tube

synapse

meeting point between a neuron and any other cell

examples of cells synapses join together

• epithelial

• muscular

• glandular

• sensory

• mostly neurons

what do synapses allow for

neural integration which allows for letting a second cell respond to signals from the first

what would happen without synapses

• signals would transmit from receptors to effectors

• effectors respond to every stimulus

• nervous system can’t make decisions

true or false? One neuron can have many synapses and a great ability for information-processing

true

presynaptic neuron

what a nerve signal goes through to arrive at a synapse

postsynaptic neuron

where a nerve signal goes through after the presynaptic neuron

axodendritic synapse

forms when a presynaptic axon ends at the dendrite of a postsynaptic neuron

axosomatic synapse

formed when the presynaptic axon terminates on the cell body of the next cell

axoaxonic synapse *****

forms when presynaptic axon terminates on the axon of the next cell

chemical synapse

a point where the presynaptic neuron releases a neurotransmitter to stimulate postsynaptic cell

synaptic cleft

gap between presynaptic neuron and postsynaptic cell about 20-40 nm wide

what prevents neurons from falling apart at the synapse

• tightly packed

• cell adhesion molecules in membrane

how do cell adhesion molecules in neurons

reach out across the cleft to CAMs of the other cell

what other function besides preventing neurons from falling apart the synapses do CAMs have

holding neurons in alignment for proper communication