11. Nervous System + Flashcards
1
Q
The basic structural unit of the nervous system:
A
- nerve cell or neuron
2
Q
Neuron consists of:
A
- cell body: contains nucleus and other cellular organelles
- dendrite: typically short, abundantly branched, extension of cell body that RECEIVES stimuli
- axon: typically long, extension of cell body that SENDS impulses. axon = away
3
Q
Path of nerve impulse
A
tips of dendrite branches—-> cell body—-> axon (terminates at branches of axon
4
Q
Three general groups of Neurons:
A
- Sensory neurons (afferent). receive initial stimulus. ex. sensory neurons in retina of eye stimulated by light, ex. sensory neurons in skin stimulated by touch.
- Motor neurons (efferent). stimulate effectors (target cells that produce some kind of response). ex. efferent neurons stimulate muscles (create movement to maintain balance or avoid pain), ex. sweat glands (cool body), ex. cells in stomach secrete gastrin in response to smell of food (perhaps).
- Association neurons (interneurons). located in spinal cord or brain and receive impulses from sensory neurons and send impulses to motor neurons. Interneurons are integrators, evaluating impulses for appropriate responses.
- 99% of nerves are interneurons
5
Q
Membrane of unstimulated neuron:
A
- polarized –> there is a difference in electrical charge between outside and inside. In particular, inside is negative w/ /respect to outside.
- polarization established by maintaining excess Na+ outside and excess K+ inside. A certain amount is always leaking, but Na+/K+ pumps in membrane actively restore the ions to the appropriate side.
- other ions, such as large, negatively charged proteins and nucleic acids, reside inside the cell. It is these large, negatively charged ions that contribute to the overall negative charge on the inside of the cell membrane.
- additionally, neuron membranes are selectively permeable to K+ as opposed to Na+, which helps to maintain the polarization.
6
Q
Transmission of a nerve impulse steps:
A
- RESTING POTENTIAL. unstimulated, polarized state (about -70 millivolts)
- ACTION POTENTIAL. stimulus—> gated ion channels in membrane suddenly open and permit Na+ on outside to rush into the cell —> as the (+) Na+ rush in, membrane becomes depolarized (more + on inside, from -70 to 0 mv)—> if stimulus is strong enough (above certain threshold level), more Na+ gates open, causing action potential or complete depolarization (about +30 millivolts)—> stimulates neighboring Na+ gates to open—>action potential travels down the length of the neuron as opened Na+ gates stimulates neighbors to open. Action potential is an ALL-OR-NOTHING event: when stimulus fails to reach threshold value, no action potential occurs, but when when threshold value is exceeded, complete depolarization occurs.
- REPOLARIZATION. In response to inflow of Na+ —–> another kind of gated channel opens, allowing K+ on inside to rush out. movement of K+ out causes repolarization by restoring original membrane polarization. Unlike resting in that K+ is on outside and Na+ on inside. Soon after K+ gates open, Na+ gates close.
- HYPERPOLARIZATION. by time K+ gates close, more K+ moved out than is necessary to establish original polarized potential. Thus, membrane is hyperpolarized (about -80 millivolts)
- REFRACTORY PERIOD. Now, the Na+ and K+ are on the wrong sides of membrane. During this refractory period, the neuron will not respond to a new stimulus. Na+/K+ pumps establishes correct location by pumping 3 Na+ out for every 2 K+ in. Once ions returned to correct location, the neuron is ready for another stimulus.
7
Q
Myelin Sheath
A
- Insulating cells (schwann/oligodendrocytes) that encircle the axon
- act as insulators and are separated by gaps of unsheathed axon called NODES OF RANVIER.
- instead of traveling continuously down the axon, the action potential jumps from node to node (SALTATORY CONDUCTION), thereby speeding propagation of impulse.
8
Q
Synapse or Synaptic Cleft
A
- gap that separates adjacent neurons
- transmission of impulse across synapse, from PRESYNAPTIC CELL to POSTSYNAPTIC CELL may be electrical or chemical. In electrical synapses, action potential travels along the membranes of gap junction, small tubes of cytoplasm that connect adjacent cells. In most animals however, most synaptic clefts are traversed by chemicals.
9
Q
Synaptic Cleft Chemicals
A
- Calcium (Ca+2) gates open. When action potential reaches end of axon, depolarization of membrane causes gated channels to open and allow Ca+2 to enter cell.
- Synaptic Vesicles release neurotransmitter. Influx of Ca+2 causes SYNAPTIC VESICLES to merge with presynaptic membrane, releasing NEUROTRANSMITTERS into synaptic cleft.
- Neurotransmitter binds with postsynaptic receptors. diffusion via brownian motion. neurotransmitter diffuses across synaptic cleft and binds proteins on postsynaptic membrane. Different proteins are receptors for different neurotransmitters.
- Postsynaptic membrane is EXCITED or INHIBITED. Depending on neurotransmitter/membrane receptors, two possible outcomes for postsynaptic membrane. 1. Na+ gates open —> depolarization —> EXCITATORY POSTSYNAPTIC POTENTIAL (EPSP)—> if threshold is exceeded—> action potential generated. 2. K+ gates open—> membrane hyperpolarized—> INHIBITORY POSTSYNAPTIC POTENTIAL (IPSP)–> difficult to generate action potential on this membrane
- Neurotransmitter is degraded and recycled. After neurotransmitter binds postsynaptic membrane receptors, it is broken down by enzymes in the synaptic cleft. ex. common neurotransmitter acetylcholine is broken down by CHOLINESTERASE. degraded neurotransmitters are recycled by presynaptic cell.
10
Q
Common Neurotransmitters
A
- Acetylcholine. commonly secreted at neuromuscular junctions, gap of motor neurons and muscle cells, where it stimulates muscles to contract. Inhibitory everywhere else. Parasympathetic Nervous System.
- Epinephrine, norepinephrine, dopamine, and serotonin. derived from amino acids and mostly secreted between neurons of CNS. Sympathetic Nervous System.
- Gamma aminobutyric acid (GABA). usually an inhibitory neurotransmitter among neurons in the brain.
11
Q
Nervous System
A
- CNS. brain and spinal cord
- PNS. consist of sensory neurons that transmit impulses to CNS and motor neurons that transmit impulses from CNS to effectors. Motor system divided into 1. SOMATIC NERVOUS SYSTEM- directs contractio of skeletal muscles 2. AUTONOMIC NERVOUS SYSTEM-controls activities of organs and various involuntary muscles (ex. cardiac/smooth)
12
Q
Divisions of Autonomic Nervous System
A
- SYMPATHETIC NERVOUS SYSTEM. (“fight or flight”). prepare body for action–>increase heart rate, increase release of sugar from liver into blood.
- PARASYMPATHETIC NERVOUS SYSTEM. activates tranquil (free from disturbance/calm) functions, such as stimulating secretion of saliva or digestive enzymes into the stomach. “rest and digest” (lower HR, digestion, relaxation, sexual arousal)
- Generally, both systems target same organs but often work antagonistically. ex. sympathetic speeds cardiac cycle, while parasympathetic slows it down. each system is stimulated as is appropriate to maintain homeostasis.
13
Q
Reflex Arc
A
- rapid involuntary response to a stimulus.
- consists of two or three neurons. sensory and motor and sometimes, an interneuron.
- Although neurons may transmit info about reflex response to brain, the brain does not actually integrate the sensory and motor activities.
14
Q
Nervous vs. Endocrine Systems
A
- neuron communication is rapid/direct/specific
- hormonal is slower/spread through body/many cells/tissues in different ways/longer lasting
15
Q
Dendrites
A
- receive info and transfer it TO cell body
