Chapter 11: Fundamentals of the Nervous System and Nervous Tissue Flashcards
2 main parts of the nervous system
- Central Nervous System (CNS)
- Peripheral Nervous System (PNS)
Central Nervous System
- brain and spinal cord
- integrative and control centers
Peripheral nervous system
- cranial nerves and spinal nerves
- communication lines between CNS and rest of body
- 2 divisions: sensory (afferent) and motor (efferent) division
Sensory (afferent) division
- PNS
- somatic and visceral sensory fibers
- conducts impulses from receptors to CNS
Motor (efferent) division
- motor nerve fibers
- conducts impulses from CNS to effectors (muscles and glands)
- 2 divisions: Somatic nervous system & Autonomic system
Somatic Nervous System
- motor (efferent) division
- somatic (voluntary) motor fibers
- conducts impulses from CNS to skeletal muscles
Autonomic Nervous System (ANS)
- motor (efferent) division
- visceral (involuntary) motor nerve fibers
- conducts impulses from CNS to cardiac muscle, smooth muscle, and glands
- 2 divisions: sympathetic division & parasympathetic division
Sympathetic Division
- ANS
- mobilizes body systems during activities
Parasympathetic division
- ANS
- conserves energy
- promotes housekeeping functions at rest
Neuroglia
- glial cells
- supporting cells
- surround, wrap, and make scaffold around delicate neurons
- 6 types: 4 in CNS, 2 in PNS
Neuroglia in CNS
- outnumber neurons in CNS
- 4 kinds: astrocytes, microglial cells, ependymal cells, oligodendrocytes
Neuroglia in PNS
2 types: satellite cells and schwann cells (aka neurolemmocytes)
Astrocytes
- CNS
- provides nutrient supply line between capillaries and neurons
- guide migration of young neurons
- control chemical environment surrounding neurons
- respond to nerve impulses and neurotransmitters
- influence neuronal function
- assistant brain in processing info
Microglial Cells
- CNS
- monitor neuron health:
- migrate towards injured/damaged neurons
- transform into specialized macrophage if invading pathogen present or neuron has died: phagocytize invading pathogens or debris from dead neuron; important function bc nervous system largely isolated form systemic immune response, immune cells have limited access to CNS
Ependymal cells
- CNS
- wrapping garment
- form lining of central cavities in brain and spinal cord: separates cerebrospinal fluid in cavities from fluid surrounding cells in CNS
- many have cilia: action of cilia helps to circulate CSF
- produce and regulate flow of CSF within brains ventricle
CSF
cushions brain and spinal cord
Oligodendrocytes
- CNS
- located close to thicker nerve fibers of CNS
- processes wrap tightly around nerve fibers
- produce myelin sheath- insulating cover for nerve fibers
Satellite cells
- PNS
- surround neuron cell bodies in PNS
- function not fully understood, believed to be PNS equivalent of astrocytes in CNS
Schwann cells (aka Neurolemmocytes)
- PNS
- surround every nerve fiber in PNS
- form myelin sheath around thicker fibers
- similar function to oligodendrocytes
- very important role in regenerating PNS nerve fibers
Neuron structure
- all have a cell body (soma) and at least one process (dendrites/axon)
- myelin sheath covers axons
Neural cell body
- soma
- contains spherical nucleus + nucleous
- biosynthetic center and metabolic center of neuron : contains protein and membrane-making machinery: free ribosomes, golgi apparatus; chromatophilic substance aka Nissl bodies = name for rough ER in neurons
Neuron Process
- armlike, extend from cell body of all neurons
- 2 kinds: dendrites & axons
Neuron Process: CNS vs PNS
- CNS- has neuron cell bodies and processes
- PNS- mostly neuron processes, cell bodies lie in CNS- results in some very long neuron processes extending from CNS into periphery of the body
Dendrites
- neuron process
- in motor neurons
- short, branching extension off cell body
- hundreds branch off one cell body
- receptive- input region: receives signals from other neurons
- once a dendrite receives a signal it is transmitted toward the neurons cell body
- signals are short distance called graded potentials
Axon
- neuron process
- neurons never have more than one axon (some have no axon)
- axons vary in length: some very short, some very long
- axons may branch along their length: branches- axon collaterals
- at axon end (terminus) there are terminal branches
- axon terminals: knob-like distal endings of terminal branches
Axon Hillock
cone-shaped protrusion from cell body where axon originates
long axons
nerve fibers
bundles of axons (neuron processes) have different names based on location
- In CNS: tracts
- In PNS: nerves
Myelin Sheath
- covers nerve fibers (axons) that are long or thick in diameter
- myelin- whitish protein-lipid substance
- protects nerve fibers
- increases speed of nerve impulse transmission
- very good electrical insulators
- only axons have myelin sheath, dendrites are unmyelinated
3 unique characteristics of neurons
- Extreme longevity
- Amitotic
- High metabolic rate
Characteristics of neurons: extreme longevity
can optimally function for a lifetime
Characteristics of neurons: Amitotic
- mature neurons lose mitotic ability (no cell division)
- most neurons cannot be replaced if destroyed (exceptions: olfactory epithelium and some regions of hippocampus)
Characteristics of neurons: High metabolic rate
- need continuous, abundant supply of oxygen and glucose
- only survive a few minutes without oxygen: why starting CPR in pulseless person after heart attack ASAP is important, need to circulate by doing compressions –> oxygen to brain
Clusters of neuron cell bodies in CNS
- nuclei
- protected by bones of skull and vertebral column
Clusters of neuron cell bodies in PNS
- ganglia
- lie along the nerve fibers
- ganglion = “knot on a string” “swelling”
Electrical charge on side of neuron vs. outside
Inside cells near plasma membrane: negative
Outside cells near plasma membrane: positive
Resting potential
- all cells have resting membrane potential: voltage across plasma membrane
- concentration gradient of ions across plasma membrane dictates resting potential
What 2 ions impact resting membrane potential
- Na+
- K+
Major extracellular cation
Na+
Major intracellular cation
K+
* most important role in generating resting membrane potential
4 steps in generating an action potential
- Resting state: voltage-gated Na+ and K+ channels closed
- Depolarization: voltage-gated channels open, allowing Na+ entry
- Repolarization: Na+ channels inactivate, voltage-gated K+ channels open, allowing K+ to exit
- Hyperpolarization: some K+ channels stay open, Na+ channels reset
Graded potential: location, distance, length
Location: cell body and dendrites
Distance: short distance– typically within cell body to initial segment of axon (0.1-1.0mm)
Length: a few milliseconds to seconds (dissipates fast)
Action potential: location, distance, length
Location: axon (beginning at initial segment)
Distance traveled: long distance– from trigger zone at initial segment through entire length of axon (a few mm to over a meter)
Length: relatively short (3-5ms)
Structural components of synapse: send information
at synapse: presynaptic neuron- neuron conduction impulse towards synapse
Structural components of synapse: receive information
- postsynaptic neurons: neuron conducting impulse away from synapse
- in PNS can be another neuron or effector cell (organ, muscle, gland)
Which structure in the synapse releases the molecule that travels across the synapse
Axon terminal: releases neurotransmitters into extracellular space
Synaptic vesicles: store neurotransmitters
Action potential triggers their release into synaptic cleft
Neurotransmitters bind to receptors on the postsynaptic cell to transmit the signal
What are the 2 different kinds of synapses
- electrical synapse
- chemical synapse
Electrical synapse
- less common form of synapse
- ions and small molecules travels across synapse
- few in adults, mostly in embryonic tissue, replaced by chemical synapses during nervous system development
Chemical synapse
- most common type of synapse
- neurotransmitters travel across it
- synaptic cleft separates pre and post synaptic neurons
EPSPs
- causes a depolarization of the postsynaptic neuron’s membrane potential, making it more likely to fire an action potential by bringing membrane potential closer to threshold for firing
- neurotransmitter binding opens chemically gated ion channels, allowing Na+ and K+ to pass simultaneously , the electrochemical gradient for Na+ is bigger than for K+ so more Na+ enters than K+ exits and the result is depolarization
- when EPSP dominant over IPSP action potential generated
IPSPs
- hyperpolarization of postsynaptic neuron- inhibitory postsynaptic potential
- makes a postsynaptic neuron less likely to generate action potential by hyperpolarizing cell membrane
- neurotransmitter binding opens chemically gated ion channels, allowing either K+ or Cl- to mass –> hyperpolarization
Temporal Summation
- one postsynaptic neuron has rapid successive EPSP impulses, before an EPSP dissipates another one is triggered, each rapidly successive EPSP brings potential close to threshold for depolarization and generation of action potential
- a rapidly firing presynaptic neuron causes EPSPs that are close in time
- 1 EPSPs close in time add together (temporal = time)
- summation brings the axon’s initial segment to threshold and an AP fires
- like someone shouting fire quickly over and over
Spatial summation
- EPSP impulses are additive, can reach threshold for depolarization and generating of active potential
- if more than one presynaptic neuron fires at the same time, EPSPs are generated at different locations of a neuron
- 2 EPSPs at the same time from different location add together (spatial = location)
- summation brings axons’ initial segment to threshold and AP fires
No summation
- a slowly firing presynaptic neuron causes EPSPs that are far apart in time
- 2 EPSPs separated in time do not add together
- like someone casually shouting fire every few minutes
Spatial summation of EPSPs and IPSPs
- if a presynaptic neuron creates an IPSP, it can override the EPSP created by another neuron
- an EPSP brings the neuron closer to the threshold
- an IPSP brings the neuron farther from the threshold
- together, nearly cancel each other out
- like one person shouting fire while another shouts don’t fire
Acetylcholine
- can be excitatory or inhibitory stimulator depending on what receptor it binds to (nicotinic- excitatory; muscarinic- inhibitory)
- all neurons that stimulate skeletal muscle release ACh (found at neuromuscular junction)- many in ANS some in CNS
- stimulating muscle contractions, regulating heart rate and bp, controlling glandular secretions, cognitive functions (memory and learning)
Nitric Oxide
- excitatory or inhibitory depending on receptor
- in brain involved in forming new memories
Serotonin
- mainly inhibitory
- made from tryptophan (amino acid)
- mood stabilizer, “happy chemical”
Endorphins
- mainly inhibitory (inhibit pain)
- natural opiates, reduce perception of pain, “runners high”
- ex. beta endorphin, dynorphin, enkephalins- increased activity in females during labor and delivery
