Ch.11 Flashcards
The Nervous System
- Works in conjunction with endocrine system to control homeostasis and regulate body activities
- Nervous: uses electrical signal, fast
- Endocrine: uses hormones, slower
Integration
Basic Function of nervous system
- control centers in brain/cord
- process/interpret sensory input
- coordinate response to stimuli
Sensory
Basic Function of nervous system
- receptors detect stimulus and sensory neurons
- convey message to control centers
Motor
Basic function of nervous system
motor neurons in control center send messages to effectors to bring about response
Neuroglia
(Cells that support the neuron)
Total Neuroglia
2 in PNS + 4 in CNS = 6 total
- Most have cell body with projections, each has specific function
Types of CNS neuroglia
- Astrocytes
- Microglia
- Oligodendrocytes
- Ependymal cells
Astrocytes
CNS neuroglia
- Most abundant
- Act as scaffolding for neurons
- Anchor neurons to capillaries
- Help maintain composition of the interstitial fluid
- In embryos secrete chemicals that help neurons make connections
- Recycle neurotransmitters
Microglia
CNS neuroglia
- Small cells
- Macrophages
Oligodendrocytes
CNS neuroglia
- Fewer projections
- Projections form
myelin sheath in CNS
Ependymal cells
CNS neuroglia
- Epithelium-like
- Simple cubodial w/cilia
- Line the ventricles and central canal
- Forms part of the choroid plexus (which produces cerebrospinal fluid)
Types of PNS supporting cells
- Satellite cells
- Schwann cells
Satellite cells
PNS supporting cells
- Cling to surface of neuron cell bodies in ganglia
- Provides nutrients
Schwann cells
PNS supporting cells
- Form myelin in PNS
- Insulate neuron axons
- nodes of ranvier (1mm)
- Neurolemma (PM)
Myelnated
faster conduction
Unmyelnated
Slower conduction
Neurons
= nerve cells
Characteristics:
1. Conduct electrical impulses
2. Extreme longevity
3. Amitotic
4. Fast rate of metabolism
Cell body
Neuron
- ganglia: collections of nerve cell bodies found in the PNS
- nuclei: collections of nerve cell bodies found in the CNS
Processes
Neuron
- tracts: bundles of nerve cell processes found in the CNS
- nerves: bundles of nerve cell processes found in the PNS
– dendrites
– axons
Structural classification of neurons
Based on number of processes
Multipolar neuron
- 99% of neurons
- Major neuron type in the CNS
Bipolar Neuron
- Rare
- Found in the special sense organs (eye, olfactory mucosa
Unipolar Neuron
- Found mainly in the PNS
- Most are sensory neurons
Sensory (Afferent)
Functional classification of neurons
- Message TO CNS
- Most unipolar, bipolar
Interneurons/Integration (Association neurons)
Functional classification of neurons
- connect afferent and efferent neurons
- Mostly multipolar
Motor (Efferent)
Functional classification of neurons
- Message FROM CNS
- Most multipolar
How do neurons communicate?
via electrical impulses & neurotransmitters
Ion Channels
Allow ions to diffuse across the cell membrane
Leakage Channels
Ion Channel
- Never really completely close, allows slow diffusion of specific ion
- there are more K+ leakage channels, than there are Na+ leakage channels
Gated Channels
Ion Channel
- These open and close, and when closed do not allow any diffusion of ions
- 3 types:
1. Voltage-Gated Channels
2. Ligand-Gated Channels
3. Mechanically-Gated Channels
Voltage-Gated Channels
Type of gated channel
open when membrane potential changes
Ligand-Gated Channels
Type of gated channel
open when chemicals (like hormones or neurotransmitters) attach to them
Mechanically-Gated Channels
Type of gated channel
open & close in response to mechanical forces (like vibrations and pressure)
Na+/K+ pumps
- Are active transport membrane proteins that pump 3 Na+ out of the cell, while pumping 2 K+ into the cell
- they maintain an electrical and chemical gradient across the membrane
- Positive outside, negative inside
Resting Membrane potential
- Polarized membrane = voltage gradient across the membrane
- Average potential is -70 mV
- Leakage channels create the potential and Na+/K+ pumps maintain it.
1) Resting state
All gated Na+ and K+ channels are closed
2) Depolarization
When cation (like Na+) channels open, cations diffuse into the cell and brings the voltage difference closer to 0
3) Repolarization
Na+ channels are inactivating, and K + channels open
4) Hyperpolarization
When anion channels open, or K+ channels open, ions diffuse across the membrane and bring the voltage difference further from 0
Graded potential
- Localized, short lived change in membrane potential occurs on dendrites and cell bodies
- Only initial stimulus opens ion channel
- Becomes less effective as it moves away from contact point (like rock in a pond)
- Big stimulus causes more channels to open, bigger waves
- If this change in potential reaches the axon hillock at enough strength, it may cause an action potential
Graded potential process
- Stimulus (like heat, pressure, neurotransmitter binding, etc) detected on the cell body or dendrites
- causes ligand-gated or mechanically gated channels to open
- depolarization or hyperpolarization of the membrane at that point
- change in potential ripples out from the contact point.
Action potential
- Nerve Impulse on axon
1. Graded potential reaches hillock voltage-gated channels open
2. Na+ rushes into cells
3. Membrane depolarizes at that location
4. At threshold of -55 mV potential propagates (Na+ channels on adjacent membrane open)
5. Na+ channel opening is immediately followed by Na+ channel closing and K+ channel opening
6. K+ rushes out of cells
7. Membrane repolarizes
8. Propagation of repolarization (potassium channels on adjacent membrane open)
Graded potential vs action potential
Graded potential:
- Produced by gated channels on dendrites and soma
- May be a positive (depolarizing) or negative (hyperpolarizing) voltage change
- Graded: proportional to stimulus strength
- Reversible; returns to RMP if stimulation ceases before threshold is reached
- Local; has effects for only a short distance from point of origin
- Decremental; signal grows weaker with distance
Action Potential:
- Produced by voltage regulated gates on the trigger zone and axon
- Always begins with depolarization
- All or none; either does not occur at all or exhibits peak voltage regardless of stimulus strength
- Irreversible; goes to completion once it begins
- Self propagating; has effects a great distance from point of origin
- Nondecremental; signal maintains same strength regardless of distance
speed of transmission depends on
- Axon diameter
- Bigger diameter = faster transmission
- A fibers = large diameter (300 mi/hr)
– go to skin, skeletal muscles, joints
- B fibers = smaller diameter (40 mi/hr)
– Autonomic nervous system fibers
- C fibers = smallest diameter (2 mi/hr)
– Autonomic nervous system fibers - Myelin sheath
- A & B myelinated
- C is unmyelinated
Multiple Sclerosis
- Autoimmune disease
- Immune system attacks myelin sheaths
- Messages sent much slower
- Symptoms: weakness, clumsiness, blurred vision, speech impairment, poor skeletal muscle control, poor balance
Synapses
Where neurons communicate with each other (usually thousands per neuron)
Presynaptic neuron
sends message TO synapse
Postsynaptic neuron
carries message
away FROM synapse
Electrical synapse
- Neurons connected via gap junctions
- Found in developing embryos and limited areas of adult brains
Chemical Synapse
contains:
1. Presynaptic Neuron: Axon terminal w/vesicles containing neurotransmitters
2. Synaptic Cleft
3. Postsynaptic Neuron: Receptor region
Excitatory Synapse
- Na+ and K+ channels open
- Since there is higher gradient for Na+ than K+/ Na+ diffuses in faster
- membrane depolarizes
- graded potential
Inhibitory Synapse
- K+ channels open and K+ diffuses out OR
Cl- channels open and Cl- diffuses in - hyperpolarization
Neurotransmitters
Chemicals released by a presynaptic neuron into the synaptic cleft and attaches to postsynaptic neuron where it causes some effect
Acetycholine
- used at skeletal neuromotor junctions and by
autonomic and interneurons neurons - binds briefly to the postsynaptic neuron and then quickly degraded by enzyme into acetic acid and choline, which are then recaptured by the presynaptic neuron
Dopamine (a catecholamine)
- Important in integrating emotions and skeletal muscle tone
- Too little = Parkinson’s
- Too much = schizophrenia
Serotonin = (an indolamine)
- Involved in sensory reception, mood, appetite, sleep
- Loss of receptor is a hallmark of Alzheimer’s Disease
- Prozac (and other selective serotonin reuptake inhibitor - SSRI’s) block its uptake to relieve
anxiety & depression
