Nervous System BIOL 3310 Anatomy and Physiology 1 Lecture Exam 3

Endocrine system

communicates by means of chemical messengers, hormones, secreted into the blood

Nervous system

employs chemical and electrical means to send messages from cell to cell

What are the steps the nervous system carries out?

receive information, process information, issue commands

Central nervous system (CNS)

brain and spinal cord enclosed in bony covering

Peripheral nervous system (PNS)

all nervous system except for brain and spinal cord, composed of nerves and ganglia

Nerves

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

Ganglion

knot-like swelling in a nerve where neuron cell bodies are concentrated

Sensory (afferent) division of PNS

carries sensory signals from various receptors to the CNS

informs the CNS of stimuli within or around the body

Somatic sensory division

carries signals from receptors in the skin, muscles, bones, and joints

Visceral sensory division

carries signals from the viscera of the thoracic and abdominal cavities

Motor (efferent) of PNS

carries signals from CNS to motor gland and muscle cells that carry out the body’s response

Somatic motor division

carries signals to skeletal muscles

output produces muscular contractions

Somatic reflexes

involuntary muscle contractions

Visceral motor division (autonomic nervous system)

carries signals to glands, cardiac muscle, and smooth muscles

involuntary, and responses of the system and its receptors are visceral reflexes

Sympathetic division

tends to arouse the body for action

accelerating heart beat and respiration, while inhibiting digestive and urinary systems

Parasympathetic division

tends to have calming effect

slows heart rate and breathing

stimulates digestive and urinary systems

Excitability (irritability)

response to stimuli

Conductivity

neurons response to stimuli by producing electrical signals that are quickly conducted to other cells at distant locations

Secretion

electrical signals reach the end of nerve fibers, chemical neurotransmitter is secreted that crosses the gap and stimulates the next cell

Sensory (afferent) neurons

specialized to detect stimuli

transmit information about them to the CNS

begin in almost every organ in the body, end in CNS

Afferent

conducting signals towards the CNS

Interneurons (association) neurons

lie entirely within the CNS

receive signals from many neurons and carry out the integrative function

90% of all neurons

lie between and interconnect the incoming sensory pathways, and the outgoing motor pathways of the CNS

Integrative function

process, store and retrieve information and make decisions that determine how the body will respond to stimuli

Motor (efferent) neuron

send signals out to muscle and gland cells

Effectors

muscle and gland cells

Motor

lead to muscle

Efferent

conduct signals away from CNS

Soma (neurosoma, cell body, perikaryon)

single, centrally located nucleus with large nucleolus

What is included in the cytoplasm of a neuron?

contains mitochondria, lysosomes, Golgi complex, numerous inclusions, extensive rough endoplasmic reticulum and cytoskeleton

What is the cytoskeleton of a neuron comprised of?

dense mesh of microtubules and neurofibrils

Neurofibrils

bundles of actin filaments

Nissl bodies

compartments of rough ER, dark staining

What does the soma of a neuron lack?

centrioles, cannot divide cellularly

Inclusions

glycogen granules, lipid droplets, melanin, and lipofuscin

Lipofuscin

golden brown pigment produced when lysosomes digest worn-out organelles

What happens to lipofuscin with age?

accumulates, act as wear-and-tear granules, most abundant in old neurons

Dendrites

vast number of branches coming from a few thick branches from soma

What is the primary site for receiving signals from other neurons?

dendrites

Axon (nerve fiber)

originates from axon hillock

Axon hillock

mound on one wide of the soma

What is the shape of an axon?

cylindrical, relatively unbranched for the majority of its length

Axon collaterals

branches of axon

Axoplasm

cytoplasm of axon

Axolemma

plasma membrane of axon

How many axons can one neuron have?

one

What encloses an axon?

schwann cells and myelin sheath

Terminal arborization

distal end of axon, extensive complex of fine branches

Synaptic knob (terminal button)

little swelling that forms a junction with the next cell

Synaptic vesicles

inside synaptic knob, contain neurotransmitter

Multipolar neuron

one axon, multiple dendrites

most common

most neurons in brain and spinal cord (CNS)

Bipolar neuron

one axon, one dendrite

composes olfactory cells, retina, inner ear

Unipolar neuron

single process leading away from soma

sensory from skin and organs to spinal cord

Anaxonic neuron

many dendrites but no axon

aid in visual processes

Axonal transport

two-way passage of proteins, organelles, and other material along an axon

Anterograde transport

movement down an axon away from soma

Retrograde transport

movement up an axon towards soma

Microtubules

guide materials along axon

Fast axonal transport

20-400mm/day

Fast anterograde transport

up to 400mm/day

transports organelles, enzymes, synaptic vesicles, small molecules

Fast retrograde transport

recycled materials and pathogens, rabies, herpes simplex, tetanus, polio

delay between infection and symptoms is time needed for transport up the axon

Slow axonal transport (axoplasmic flow)

0.5-10mm/day

always anterograde

moves enzymes, cytoskeletal components, new axoplasm down the axon during repair and regeneration of damaged axons

damaged nerve fibers regen at pace of slow axonal transport

Neuroglia (glial cells)

support and protect the neurons

bind neurons together and form framework for nervous tissue

covers neurons not in contact with other neurons to prevent incorrect signaling

Which neuroglia are found in the CNS?

oligodendrocytes, ependymal cells, microglia, astrocytes

Which neuroglia are found in the PNS?

schwann cells, satellite cells

Oligodendrocytes

form myelin sheaths in CNS

each arm-like process wraps around a nerve fiber forming an insulating layer that speeds up signal conduction

Ependymal cells

lines internal cavities of the brain

cuboidal epithelium with cilia on apical surface

secretes and circulates cerebrospinal fluid

Microglia

small, wandering macrophages formed with white blood cells called monocytes

thought to perform a complete checkup on the brain tissue several times a day

wander in search of cellular debris to phagocytize

Astrocytes

most abundant glial cell in CNS

cover entire brain surface and most non synaptic regions of the neurons in the gray matter of the CNS

What are the functions of astrocytes?

form supportive framework for nervous tissue

have extensions that contact blood capillaries that stimulate them for form a tight seal

convert blood glucose to lactate and supply nourishment

secrete nerve growth factors

communicate electrically with neurons and may influence synaptic signaling

regular chemical composition of tissue fluid by absorbing excess neurotransmitters and ions

astrocytosis/sclerosis

Perivascular feet

extensions of astrocytes that form tight seal on blood capillaries

Blood-brain barrier

seal astrocytes form between sarcoplasm and blood capillaries from rest of the body

Lactate

nourishes neurons

Nerve growth factors

secreted by astrocytes, promote neuron growth and synapse formation

Astrocytosis (sclerosis)

hardened scar tissue that fills space formally occupied by now damaged neuron

Schwann cells

envelope nerve fibers in PNS, wind repeatedly around a nerve fiber to form myelin sheath

assist in regeneration of damaged fibers

Satellite cells

surround the neurosomas in ganglia of PNS

provide electrical insulation around the soma

regulate chemical environment of the neurons

Brain tumors arise from

meninges

metastasis of non-neuronal tumors in other organs of the body

most come from glial cells that are mitotically active through life

Gliomas

rapid growing, highly malignant

cannot be treated with chemo, require direct radiation or surgery

Meninges

protective membrane of CNS

Myelin sheath

insulating layer around a nerve fiber

formed by oligodendrocytes in CNS and Schwann cells in PNS

consists of plasma membrane of glial cells, 20% protein, 80% lipid

Myelination

production of the myelin sheath

begins in 14th week of fetal development, proceeds rapidly during infancy and completed in late adolescence

What is the process of myelination in the PNS?

Schwann cells spiral around single nerve fiber, lay down ~100 layers of membrane

no cytoplasm between the layers

neurilemma thickest layer, contains nucleus and most of the cytoplasm and endoneurium

What is the process of myelination of the CNS?

oligodendrocytes myelinate several fibers nearby

cannot migrate

myelination spirals inward

no neurilemma or endoneurium

Nodes of Ranvier

gap between segments

Internodes

myelin covered segments from one gap to another

Initial segment

short section of nerve fiber between the axon hillock and the first glial cell

Trigger zone

the axon hillock and the initial segment

Multiple sclerosis

oligodendrocytes and myelin sheaths in the CNS deteriorate

myelin replaced by hardened scar tissue

nerve conduction disrupted

onset between 20-40, fatal 25-30 years post diagnosis

may be autoimmune triggered by virus

Tay-Sachs disease

hereditary disorder of infants, typically of eastern European Jewish ancestry

abnormal accumulation of glycolipid called GM2 in the myelin sheath

• normally decomposed by lysosomal enzyme

• enzyme missing in individuals homozygous for Tay-Sachs allele

• accumulation of ganglioside (GM2) disrupts conduction of nerve signals

• blindness, loss of coordination, and dementia

fatal before age 4

What factors influence the speed at which a nerve signal travels?

diameter of the fiber or presence/absence of myelin

How does the diameter of a fiber influence speed of conduction?

larger fibers have more surface area and conduct signals more rapidly

How does myelin influence speed of conduction?

speeds up signal conduction

Where are slow conduction speed signals found?

stomach and dilation of the pupils

Where are fast conduction speed signals found?

skeletal muscles and transport sensory signals for vision and balance

Speed of small, unmyelinated fibers

0.5-2.0m/sec

Speed of small, myelinated fibers

3.0-15.0m/sec

Speed of large, myelinated fibers

up to 120m/sec

What part of the nervous system is capable of regeneration?

PNS

What are the steps to nerve regeneration?

Axon distal to injury degrades, macrophages clean up

cell body swells, ER breaks up, nucleus moves off center

axon stump sprouts multiple growth processes

Schwann cell neurolemma, endoneurium, and basal lamina form a regeneration tube towards original destination

once contact is reestablished with original target, the neurosoma shrinks and returns to original appearance

How long does nerve regeneration take?

very slow, may take up to 2 years

some may connect with the wrong muscle fibers

damaged CNS axons usually unable to regenerate