bsc 2085 lesson 12

endocrine system

communicates by means of chemical messengers (hormones) secreted into the blood

Nervous system

utilizes neurons to send messages from cell to cell by electrical and chemical means

Steps of neuron communication

1. receives stimuli from external environment and transmits messages to central nervous system

2. the CNS processes the information and determines the response

3. CNS issues commands to muscle and/or gland cells to carry out the response

Central nervous system (CNS)

brain and spinal cord

Peripheral nervous system (PNS)

nerves and ganglia

Nerve

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

Ganglion

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

Sensory (afferent) division

carries signals from receptors (sense organs) to CNS

Somatic sensory division

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

• outside the body sensory

Visceral sensory division

carries signals from the viscera (heart, lungs, stomach, and urinary bladder)

• inside the body sensory

Motor (efferent) division

carries signals from CNS to the effectors (glands and muscles that carry out the body’s response)

Somatic motor division

carries signals to skeletal muscles; causes voluntary muscle contraction and automatic reflexes

Visceral motor division (autonomic nervous system, ANS)

carries signals to glands, cardiac, and smooth muscle; no voluntary control; responses called visceral reflexes 

Sympathetic division of ANS

stimulates and prepares the body for action (fight or flight)

Parasympathetic division of ANS

has a calming effect on the body

Enteric plexus (enteric nervous system) of ANS

within digestive tract wall, enables coordination and communication within the digestive tract

Tract

a bundle of axons in the CNS

Nerve

a bundle of axons in the PNS 

Nucleus

a cluster of neuronal cell bodies in the CNS

Ganglion

a cluster of neuronal cell bodies in the PNS

excitability

ability to respond to stimuli

conductivity

produce electrical signals that are conducted to other cells

secretion

when signal reaches end of the neuron’s axon, the neuron secretes a neurotransmitter that signals the next cell

Interneurons

receive signals from other neurons, process this information, and make resulting decisions; entirely within CNS

Sensory (afferent) neurons 

detect stimuli and transmit information about the stimuli toward the CNS

Motor (efferent) neurons

send signals out to muscles and gland cells (the effectors)

Dendrites

most numerous neurites, resemble branching of a tree; primary sites for receiving signals from other neurons

axon (nerve fiber)

long extension, relatively unbranched but give off axon collaterals, specialized for rapid conduction of nerve signals

Axon hillock

where axon originates, mound on one side of cell body

axoplasm

cytoplasm

axolemma 

axon membrane

terminal arborization

where axon branches profusely at its end

Axon terminal (terminal bouton)

bulbous end of each brand of arborization, forms a synapse with next cell

Multipolar neuron 

one axon and multiple dendrites, most common in body, most neurons in CNS

Bipolar neuron

one axon and one dendrite, most uncommon

unipolar neuron

single process leading away from cell body, splits into peripheral process and central process

anaxonic neuron

many dendrites but no axon 

Axonal transport

two-way passage of materials along an axon

Anterograde transport

movement away from cell body, down the axon

• driven by motor protein kinesin

Retrograde transport

movement up the axon toward the cell body

• driven by motor protein dynein

Why can’t neurons undergo mitosis?

they are stuck in G0

Neuroglia (Glial cells)

binds neurons together, prevents neurons from touching each other by maintaining synaptic cleft

Oligodendrocytes

form myelin sheaths in CNS

Ependymal cells

secrete and circulates cerebrospinal fluid

• line internal cavities of brain

Microglia

macrophages

Astrocytes

most abundant type; variety of functions

Schwann cells

envelop axons of PNS, form myelin sheath, and assist in regeneration of damaged fibers

Satellite cells

surround nerve cells bodies in ganglia of PNS, provide insulation around cell body

Gliomas

tumors of glial cells

Myelin sheath

spiral layers of insulation around an axon

• Schwann cells in PNS

• Oligodendrocytes in CNS

Neurilemma 

thick outermost coil of myelin

Myelination in CNS

each oligodendrocyte extends several processes that wrap around small portions of many axons in immediate vicinity

Myelin sheath gap (node of Ranvier)

gap between segments

Internodal segments

myelin-covered segments

Initial segment

bare section of axon between the axon hillock and first glial cell

Trigger zone

axon hillock and initial segment of axon

• important role in initiating nerve signal (action potential)

Multiple sclerosis

Oligodendrocytes and myelin sheaths in CNS deteriorate 

• myelin replaced by hardened scar tissue 

Tay-sachs disease

Abnormal accumulation of glycolipid called GM2 in the myelin sheath

• normally decomposed by lysosomal enzyme

• enzyme missing in individuals homozygous for Tay-Sachs allele

Unmyelinated axons

In PNS, schwann cells hold small unmyelinated axons in surface grooves

Regeneration of damaged PNS axon can only occur if 

nerve cell bodies are intact and at least some neurilemma remains

Regeneration of PNS axon step 1

Axon distal to injury degenerates, macrophages clean up tissue debris

Regeneration of PNS axon step 2

Cell body swells, ER breaks up, and nucleus moves off center

Regeneration of PNS axon step 3

Axon stump sprouts multiple growth processes

Regeneration of PNS axon step 4

Schwann cell neurolemma, endoneurium, and basal lamina form regeneration tube

Regeneration of PNS axon step 5

Regeneration tube guides the growing axonal sprout back to original target cells to re-establish their synapses

Regeneration of PNS axon step 6

Once contact is re-established with original target, the neurosoma shrinks and returns to its original appearance

Can damaged CNS axons regenerate?

no because astrocytes with produce scar tissue that physically interferes with regrowth of axons