The Nervous System Flashcards
What two systems maintain internal coordination?
Endocrine and nervous systems
Communicates by means of chemical, messengers (hormones) secreted into the blood
Endocrine system
Employs electrical and chemical means to send messages from cell to cell
Nervous system
What are the two major anatomical subdivisions of the Nervous System?
- Central Nervous System (CNS)
- Peripheral Nervous System (PNS)
- brain and spinal cord enclosed in bony coverings
- enclosed by cranium and vertebral column
Central nervous system (CNS)
All the nervous system except the brain and spinal cord; composed of nerves and ganglia
Peripheral nervous system (PNS)
A bundle of fibers (axons) wrapped in fibrous connective tissue
Nerve
A knot like swelling in a nerve where neuron cell bodies are concentrated
Ganglion
Carries sensory signals from various receptors to the CNS
- informs the CNS of stimuli within or around the body
Sensory (afferent) division
Carries signals from receptors in the skin, muscles, bones, and joints
Somatic sensory division
Carries signals from the viscera of the thoracic and abdominal cavities
- heart, lungs, stomach, and urinary bladder
Visceral sensory division
Carries signals from the CNS to gland and muscle cells that carry out the body’s response
Motor (efferent) division
Cells and organs that respond to commands from the CNS
Effectors
Carries signals to skeletal muscles
- output produces muscular contraction as well as somatic reflexes- involuntary muscle contractions
Somatic motor division
-Carries signals to glands, cardiac muscle, and smooth muscle
- involuntary, and responses of this system receptors are viscera, reflexes
Visceral motor division (autonomic nervous system)
- tends to arouse body for action
- accelerating heart beat and respiration, while inhibiting digestive and urinary systems
Sympathetic division
- tends to have calming effect
- slows heart rate and breathing
- stimulates digestive and urinary systems
Parasympathetic division
Neurons respond to environmental changes called stimuli
Excitability (irritability)
Neurons respond to stimuli by producing electrical signals that are quickly conducted to other cells at distant locations
Conductivity
When electrical signal reaches end of nerve fiber, a chemical neurotransmitter is secreted that crosses the gap and stimulates the next cell
Secretion
What are the three general classes of neurons based on function?
Sensory, interneuron, motor
- specialized to detect stimuli
- transmit information about them to the CNS
- begin in almost every organ in the body and end in the CNS
Sensory (afferent) neurons
Conducting signals toward CNS
Afferent
-lie entirely within the CNS
- receive signals Demi many neurons and carry out the intergrative function (process, store, and retrieve information and “make decisions” that determine how the body will respond to stimuli)
- 90% of all neurons are these
- lie between and interconnect the incoming sensory pathways and the outgoing motor pathways of the CNS
Interneurons (associated neurons)
-
Send signals out to muscles and gland cells (the effectors)
- motor because most of them lead to muscles
- efferent neurons conduct signals away from the CNS
Motor (efferent) neuron
The control center of the neuron
- also called neurosoma, cell body, or perikaryon
- has a single, centrally located nucleus with large nucleolus
- cytoplasm contains mitochondria, lysosomes, a Golgi complex, numerous inclusions, and extensive rough ER and cytoskeleton
- cytoskeleton consists of dense mesh of microtubules and neurofibrils (bundles of actin filaments)
Soma
Vast number of branches coming from a few thick branches from the soma
- resemble bare branches of a tree in winter
- primary site for receiving signals from other neurons
- the more of these a neuron has, the more information it can receive and incorporate into decision making
- provide precise pathway for the reception and processing of neural information
Dendrites
Originates from a mound on one side of the soma called the axon hillock
- cylindrical, relatively unbranched for most of its length
- branch extensively on distal end
- specialized for rapid conduction of nerve signals to points remote to the soma
Axon (nerve fiber)
Branches of axon
Axon collaterals
Cytoplasm of axon
Axoplasm
Plasma membrane of axon
Axolemma
What enclose and axon?
Schwann cells and myelin sheath
Little swelling that forms a junction (synapse) with the next cell
Synaptic knob
- one axon and multiple dendrites
- most common
- most neurons in the brain and spinal cord
Multipolar neuron
- one axon and one dendrite
- olfactory cells, retina, inner ear
Bipolar neuron
- single process leading away from the soma
- sensory from skin and organs to spinal cord
Unipolar neuron
- many dendrites but no axon
- help in visual processes
Anaxonic neuron
two-way passage of proteins, organelles, and other material along an axon
axonal transport
movement down the axon away from the soma
anterograde transport
movement up the axon toward the soma
retrograde transport
- support and protect the neurons
- bind neurons together and form framework for nervous tissue
- in fetus, guide migrating neurons to their destination
-If mature neuron is not in synaptic contact with another neuron it is covered by glial cells
neuroglia or glial cells
What are the four types of Neuroglia that only occur in CNS?
- oligodendrocytes
- ependymal cells
- microglia
- astrocytes
- form myelin sheaths in CNS
- each arm like process wraps around a nerve fiber forming an insulating layer that speeds up signal conduction
oligodendrocytes
- line internal cavities of the brain
- cuboidal epithelium with cilia on apical surface
- secretes and circulates cerebrospinal fluid (CSF) (clear liquid that bathes the CNS)
ependymal cells
- small, wandering macrophages formed white blood cell called monocytes
- though to perform a complete checkup on the brain tissue several times a day
- wander in search of cellular debris to phagocytize
microglia
- most abundant glial cell in CNS
- cover entire brain surface and most nonsynaptic regions of the neurons in the gray matter of the CNS
- diverse functions: form a supportive framework; have extensions that contact blood capillaries that stimulate them to form a tight seal called the blood-brain barrier; convert blood glucose to lactate and supply this to the neurons for nourishment
astrocytes
What two types of Neuroglia only occur in PNS?
- Schwann cells
- Satellite cells
- envelope nerve fibers in PNS
- wind repeatedly around a nerve fiber
- produce a myelin sheath similar to the ones produced by oligodendrocytes in CNS
- assist in the regeneration of damaged fibers
Schwann cells
- surround the neurosomas in ganglia of the PNS
- provide electrical insulation around the soma
- regulate the chemical environment of the neurons
Satellite cells
an insulating layer around a nerve fiber
- formed by oligodendrocytes in CNS and Schwann cells in PNS
- consists of the plasma membrane of glial cells (20% protein and 80% lipid)
myelin sheath
production of the myelin sheath
- begins at week 14 of fetal development
- proceeds rapidly during infancy
- completed in late adolescence
- dietary fat is important to CNS development
myelination
What are the segmented parts of the myelin sheath?
- Nodes of Ranvier
- Internodes
- Initial segment
- Trigger zone
gap between segments
Nodes of Ranvier
myelin-covered segments from one gap to the next
Internodes
short section of nerve fiber between the axon hillock and the first glial cell
Initial segment
the axon hillock and the initial segment
- play an important role in initiating a nerve signal
Trigger zone
degenerative disorder of the myelin sheath
- oligodendrocytes and myelin sheaths in the CNS deteriorate
- myelin replaced by hardened scar tissue
- nerve conduction disrupted (double vision, tremors, numbness, speech defects)
- onset between 20 and 40 and fatal from 25 to 30 years after diagnosis
- cause may be autoimmune triggered by virus
multiple sclerosis
formed by Schwann cells, basal lamina, and the neurilemma near the injury
- guides the growing sprout back to the original target cells and reestablishes synaptic contact
regeneration tube
conduction speed of fibers: 0.5 to 2.0 m/s
small, unmyelinated fibers
conduction speed of fibers: 3 to 15.0 m/s
small, myelinated fibers
conduction speed of fibers: up to 120 m/s
large, myelinated fibers
Living cells are ______
polarized
charge difference across the plasma membrane
- about -70 mV in a resting, unstimulated neuron
- negative value means there are more negatively charged particles on the inside of the membrane than on the outside
resting membrane potential (RMP)
a rapid up-and-down shift in the membrane voltage
- sodium ions arrive at the axon hillock
- depolarize the membrane at that point
- threshold
action potential
critical voltage to which local potentials must rise to open the voltage-regulated gates (-55mV)
threshold
polarity reversed from RMP
depolarization
do not get weaker with distance
nondecremental
once started goes to completion and cannot be stopped
irreversible
the period of resistance to stimulation
refractory period
What are the two phases of the refractory period?
- absolute refractory period
- relative refractory period
- no stimulus of any strength will trigger AP
- as long as Na+ gates are open
- from action potential to RMP
absolute refractory period
- only especially strong stimulus will trigger new AP
- K+ gates are still open and any effect of incoming Na+ is opposed by the outgoing K+
relative refractory period
synapses: first neuron in the signal path
- releases neurotransmitter
presynaptic neuron
synapses: second neuron
- responds to neurotransmitter
postsynaptic neuron
synapses: may synapse with a dendrite, soma, or axon of postsynaptic neuron to form axodendritic, axosomatic, or axoaxonic synapses
presynaptic neuron
____ ______ of presynaptic neuron contains synaptic vesicles containing neurotransmitter
synaptic knob
_____ ______ membrane contains proteins that function as receptors and ligand-regulated ion gates
postsynaptic neuron
- synthesized by the presynaptic neuron
- released in response to stimulation
- bind to specific receptors on the postsynaptic cell
- alter the physiology of that cell
neurotransmitters
employs acetylcholine (ACh) as its neurotransmitter
- ACh excites some postsynaptic cells (skeletal muscle)
- inhibits others
Cholinergic synapse
any voltage change in the direction of threshold that makes a neuron more likely to fire
- usually results from Na+ flowing into the cell cancelling some of the negative charge on the inside of the membrane
excitatory postsynaptic potential (EPSP)
excitatory brain neurotransmitters that produce EPSPs
glutamate and aspartate
any voltage change away from threshold that makes a neuron less likely to fire
- neurotransmitter hyperpolarizes the postsynaptic cell and makes it more negative than the RMP making it less likely to fire
- produced by neurotransmitters that open ligand-regulated chloride gates (causing inflow of Cl- making the cytosol more negative)
inhibitory postsynaptic potential (IPSP)
occurs when a single synapse generates EPSPs so quickly that each is generated before the previous one fades
- allows EPSPs to add up over time to a threshold voltage that triggers an action potential
temporal summation
occurs when EPSPs from several different synapses add up to threshold at an axon hillock
- several synapses admit enough Na+ to reach threshold
- presynaptic neurons cooperate to induce the postsynaptic neuron to fire
spatial summation
a process in which one neuron enhances the effect of another one
- combined effort of several neurons facilitates firing of postsynaptic neuron
facilitation
process in which one presynaptic neuron suppresses another one
- opposite of facilitation
- reduces or halts unwanted synaptic transmission
- neuron 1 releases inhibitory GABA (prevents voltage-gated calcium channels from opening in synaptic knob and presynaptic neurons releases less or no neurotransmitter
presynaptic inhibitation
What are the two subdivisions of the peripheral nervous system (PNS)?
Sensory division & motor division
