CH 4 – The Nervous System Flashcards
Neurons
specialized cells capable of transmitting electrical impulses and then translating those electrical impulses into chemical signals.
The nucleus is in the __ or __, which is also location of endoplasmic reticulum and ribosomes
cell body; soma
Cell has many appendages emanating from the soma called __, which receive incoming messages from cells. Information received from the __ is transmitted through the cell body before it reaches the __, which integrates the incoming signals
dendrites; axon hillock
Axon hillock plays important role in __, or transmission of electrical impulses down the axon. The signals arrive from the dendrites, and the axon hillock sums up these signals. If the result is excitatory enough, it will initiate __
action potentials
__ is a long appendage that terminates near a target structure (muscle, gland, other neuron). __ carry neural signals away from the soma; dendrites carry signals toward the soma.
Axon(s)
Most mammalian nerve fibers are insulated by __, a fatty membrane, to prevent signal loss or crossing over of signal. Sort of like insulation of wires, __ maintains the electrical signal within one neuron. __ also increases the speed of conduction in the axon.
myelin; myelin sheath; Myelin
Myelin is produced by:
oligodendrocytes in the CNS and Schwann cells in the peripheral nervous system
At certain intervals along the axon, there are small breaks in the myelin sheath with exposed areas of axon membrane called __. These are critical for rapid signal conduction
nodes of Ranvier
At the end of the axon is the __ or __. This structure is enlarged and flattened to maximize transmission of the signal to the next neuron and ensure proper release of __, the chemicals that transmit information between neurons.
nerve terminal; synaptic bouton (knob); neurotransmitters
Immune response against own myelin:
Sometimes the body mounts an immune response against its own myelin, leading to destruction of this insulating substance. Common demyelinating disorder is MS, where myelin of the brain and spinal cord is selectively targeted. This can cause weakness, lack of balance, vision problems, and incontinence.
Neurons are not physically connected to each other. Between neurons there is a small space into which the terminal portion if the axon releases neurotransmitters, which bind to dendrites of the adjacent neuron (postsynaptic neuron). This space known as the __. Together the __, __, and __ are known as a __.
synaptic cleft;
nerve terminal; synaptic cleft; postsynaptic membrane; synapse
Multiple neurons may be bundled together to form a __ in the peripheral nervous system. Nerves may be __, __, or __ in reference to the type of information they carry. The cell bodies of neurons of the same type are clustered together into __.
nerve;
sensory; motor; mixed
ganglia
Pathway of neural signals
Dendrites->Soma->Axon hillock->Axon->nerve terminal->synaptic cleft
Description of neuron structure and function:
Axon: Transmits electrical signals (action potential) from the soma to the synaptic knob
Axon hillock:Integrates excitatory and inhibitory signals from the dendrites and fires an action potential if the excitatory signals are strong enough to reach threshold.
Dendrites: Receive incoming signals and carry them to soma
Myelin sheath: Acts as insulation around the axon and speeds conduction
Soma: Cell body and contains the nucleus, ER, and ribosomes.
Synaptic bouton: lies at the end of the axon and releases neurotransmitters
In the CNS, axons may be bundled together to form __. Unlike nerves, __ only carry one type of information. Cell bodies of neurons in the same tract are grouped into nuclei.
tracts; tracts
nuclei
Other neural cells
Neurons must be supported and myelinated by other cells. These cells are often called glial cells, or neuroglia. Glial cells play both structural and supportive roles
- Astrocytes nourish neurons and form the blood-brain barrier, which controls the transmission of solutes from the bloodstream into nervous tissue
- Ependymal cells line the ventricles of the brain and produce cerebrospinal fluid, which physically supports the brain and serves as a shock absorber
- Microglia are phagocytic cells that ingest or break down waste products and pathogens in the CNS
- Oligodendrocytes (CNS) and Schwann cells (PNS) produce myelin around axons
action potentials
all or nothing messages neurons use to relay electrical impulses down the axon to the synaptic bouton. Action potentials cause release of neurotransmitters into the synaptic cleft.
resting membrane potential - explain ion as well
A cells resting membrane potential is the net electric potential difference that exists across the cell membrane, created by movement of charged molecules across that membrane. For neurons, this potential is ~ -70mV, with the inside of the neuron being negative relative to the outside.
2 most important ions in generating and maintaining the resting potential are K+ and Na+. Concentration of K+ is higher inside the cell than outside, making it favorable for K+ to move outside the cell due to K concentration gradient. To facilitate this movement, cell membrane has transmembrane potassium leak channels, which allow the slow leak of potassium outside the cell. As K+ continually leaks out, the inside of the cell loses positive charge leaving behind negative charge and making the outside of the cell positively charged.
However as negative charge builds up in the cell, some potassium ions will be drawn back into the cell due to the attraction between positive K ions and the negative potential building inside the cell. As this potential difference grows, K will also be more strongly drawn back into the cell. At a certain potential, each K cation pushed out the cell due to the concentration gradient will be matched by a K cation pulled back in the due to electric potential. At this point there is no net movement of the ion, cell is in equilibrium with respect to K. The potential difference of this is called equilibrium potential of potassium. This is ~ -90mV. Negative sign due to convention, and because a positive K ion is leaving the cell. Equilibrium potential of potassium is established almost instantly, as only a small amount of K need to exit the cell before resulting electrostatic force equals force of the concentration gradient.
Na concentration gradient is the revere of K. There is a driving force pushing Na into the cell. This movement is facilitated by sodium leak channels. The slow leak of Na into the cell causes a buildup of electric potential. The equilibrium potential of sodium is ~60mV, positive b/c Na is moving into the cell.
In a living system, Na and K are flowing across the cells membrane at the same time. In a certain sense, Na undoes the effect of K movement. The resting potential is thus a tug-of-war: K movement pulls the cell potential towards -90mV while Na movement pulls the cell potential towards +60mV. Neither ion wins, a balance is reached ~ -70mV for average nerve cell. This balance, the net effect of Na and K equilibrium potentials, is the resting membrane potential. The resulting potential is significantly closer to K equilibrium b/c the cell is more permeable to K.
Given the continual ion leaking at the membrane, Na+/K+ ATPase continually pumps Na and K back against their gradients to maintain the resting potential and the ions respective gradients. In a person’s body more ATP is spend by the Na+/K+ ATPase than for any other single purpose.
Neurons can receive both excitatory and inhibitory input.
Excitatory input causes depolarization (raising the membrane potential from its resting potential) and makes the neuron more likely to fire an action potential. If the axon hillock receives enough excitatory input to be depolarized to the threshold value (between -55 to -40), an action potential will be triggered.
Inhibitory input causes hyperpolarization (lowering membrane potential from its resting potential), and thus makes the neuron less likely to fire an action potential.
Not every stimulus generates a response. A postsynaptic neuron may receive information from several different pre-synaptic neurons, both excitatory and inhibitory. The additive effects of multiple signals is known as summation.
2 types of summation:
temporal and spatial.
Temporal summation is the integration of multiple signals close to each other in time. Several small excitatory signals firing at nearly the same moment could bring a post-synaptic cell to its threshold.
Spatial summation is the integration of multiple signals close to each other in space. Many inhibitory signals firing directly on the soma will cause more profound hyperpolarization of the axon hillock then the depolarization caused by a few excitatory signals firing on the dendrites of a neuron.
If the cell is brought to threshold:
voltage-gated Na channels open in the membrane. These ion channels open in response to the change in potential of the membrane (or depolarization of the membrane) and permit passage of Na ions. There is a strong electrochemical gradient that promotes the migration of Na into the cell. From an electrical standpoint, interior of the cell more negative than exterior , which favors the movement of + charged Na cations into the cell. From a chemical standpoint, higher concentration of Na outside the cell than inside, which favors the movement of Na into the cell. As Na passes through these ion channels, the membrane potential becomes more positive (cell rapidly depolarizes). Na channels can be opened or inactivated by changes to membrane potential. When Vm approaches +35mV, Na channels are inactivated and will have to be brought back near the resting potential to be deinactivated.
The + potential inside the cell triggers Na channels to inactivate and triggers voltage-gated K channels to open. Once Na has depolarized the cell, there is an electrochemical gradient favoring the efflux of K from the neuron. As K+ cations are driven from the cell, negative membrane potential will be restored called repolarization. The efflux of K+ cause on overshoot of the resting membrane potential, hyperpolarizing the neuron. This makes the neuron refractory to further action potentials
Na channels exists in 3 states:
Closed: before the cell reaches the threshold, and after inactivation has been reversed
Open: from threshold to ~ +35mV
Inactive: ~ 35mV to resting potential
2 types of refractory periods:
During absolute refractory period, no amount of stimulation can cause another action potential to occur. During relative refractory period, there must be a greater than normal stimulation to cause an action potential b/c the membrane is starting from a potential that is more negative than its resting value.
Na+/K+ ATPase acts to restore both resting potential and Na & K gradients that have been partially dissipated by the action potential. Action potential relies on these electrochemical gradients
impulse propagation
For a signal to be conveyed to another neuron, the action potential must travel down the axon and initiate neurotransmitter release. As Na rushes into one segment of the axon, it will cause depolarization in the surrounding regions of the axon. This depolarization will bring subsequent segments of the axon to threshold, opening the Na channels in those segments. Each of these segments then continues through the rest of the action potential in a wave-like fashion until the action potential reaches the nerve terminal. After the action potential has fired in one segment of the axon, that segment becomes momentarily refractory. Thus, information only flows in one direction.
Fun fact: toxin called tetrodotoxin(TTX) is found in pufferfish. TTX blocks voltage gated Na+ channels, blocking neuronal transmission. This causes the phrenic nerves innervating the diaphragm to not be able to depolarize thus leading to paralysis of the muscle and cessation of breathing.
Fun fact: Local anesthetics work by blocking voltage gated Na+ channels. These drugs work particularly well on sensory neurons and thus block transmission of pain. They favor pain neurons b/c these neurons have small axonal diameters and little myelin, allowing for easy access to the Na channels.
Speed at which action potentials move depends on the length and cross-sectional area of the axon:
Myelin helps maximize speed of transmission and prevents dissipation of the electrical signals. Membrane is only permeable to ion movement at the nodes of Ranvier. The signal thus hops from node to node known as __.
Increased length results in higher resistance and slower conduction. Greater cross-sectional area allows for faster propagation due to decreased resistance.
saltatory conduction
Neurons not actually in direct physical contact. Neuron preceding the synaptic cleft is the __. Neuron after the synaptic cleft is the __ If a neuron signals to a gland or muscle instead of another neuron, it is known as an __. Most synapses are chemical in nature; they use small molecules called __ to send messages.
presynaptic neuron; postsynaptic neuron; effector; neurotransmitters
Specific mechanism of neurotransmitter release
Prior to release, neurotransmitter molecules are stored in membrane bound vesicles in the nerve terminal. When the action potential reaches the nerve terminal, voltage-gated Na channels open, allowing for calcium to flow into the cell. This sudden increase in intracellular calcium triggers fusion of the membrane bound vesicles with the membrane at the synapse, causing exocytosis of the neurotransmitter. Within a neuron, electricity is used to pass signals down the length of the axon. Between neurons, chemicals are used to pass signals to the subsequent neuron.
Once released into the synapse, the neurotransmitter molecules diffuse across the cleft and bind to receptors on the postsynaptic membrane.
Distinction between excitatory and inhibitory comes at the level of the neurotransmitter receptors. If receptor is ligand-gated ion channel, the postsynaptic cell will either be depolarized or hyperpolarized. If it is a G protein-coupled receptor, it will cause changes in the levels of the cyclic AMP (cAMP) or an influx of calcium.
Neurotransmission must be regulated; neurotransmitter must be removed from the synaptic cleft. There are 3 main mechanisms to accomplish this goal:
- Neurotransmitters can be broken down by enzymatic reactions. Breakdown of acetylcholine (ACh) by acetylcholinesterase is a classic example.
- Neurotransmitters can be brought back into the presynaptic neuron using reuptake carriers. The re-uptake of serotonin (5-HT) is a classic mechanism. Dopamine (DA) and norepinephrine (NE) also use re-uptake carriers.
- Neurotransmitters may diffuse out of the synaptic cleft. Nitric oxide (NO), a gaseous signaling molecule, is a classic example.
Fun fact: Many common drugs modify process that occur in the synapse. For instance, cocaine acts by blocking neuronal reuptake carriers, thus prolonging the action of neurotransmitters in the synapse. There are clinically useful drugs that inhibit acetylcholinesterase thereby elevating levels of Ach. Nerve gasses like those used in warfare are potent acetylcholinesterase inhibitors. Such gasses prevent relaxation of skeletal muscles (such as diaphragm) leading to respiratory arrest
Nervous system governs both voluntary and involuntary behavior, while also maintaining __.
Functions include:
homeostasis;
- Sensation and perception
- Motor function
- Cognition and problem solving
- Executive function and planning
- Language comprehension and creation
- Memory
- Emotion and emotional expression
- Balance and coordination
- Regulation of endocrine organs
- Regulation of heart rate, breathing rate, vascular resistance, temperature, and exocrine glands
Generally speaking, 3 different cell types in the nervous system:
Sensory neurons (afferent neurons) transmit sensory information from sensory receptors to the spinal cord and brain.
Motor neurons (efferent neurons) transmit motor information from the brain and spinal cord to muscles and glands.
Interneurons are found between other neurons and are the most numerous of the 3 types. Interneurons located predominately in the brain and spinal cord and are often linked to reflexive behavior.
Processing of stimuli may happen at the level of the spinal cord or may require input from the brainstem or cerebral cortex. Ex: when a hammer hits the patellar tendon, the sensory information goes to the spinal cord, where a motor signal is sent to the quadriceps muscle causing the leg to jerk. No input from the brain required. __ circuits require input from brain or brainstem.
Supraspinal
Nervous system divisions
CNS & PNS
CNS -> brain & spinal cord
PNS -> somatic & autonomic
Autonomic -> sympathetic & parasympathetic
Grey and white matter
Brain consists of white and grey matter. White matter consists of axons encased in myelin sheath. Grey matter consists of unmyelinated cell bodies and dendrites. White matter lies deeper than the grey matter in the brain.
Spinal cord also consists of white and grey matter. Grey matter deeper than white matter in spine.
Spinal cord extends down from the brainstem and can be divided into 4 regions: Almost all structures below the neck receive sensory and motor innervation from the spinal cord.
The spinal cord is protected by the __, which transmits nerves at the space between adjacent vertebrae. The axons of motor and sensory neurons are in the spinal cord. The sensory neurons bring information in from the periphery and enter in the dorsal (back) side. Cell bodies of these sensory neurons are found in the __. Motor neurons exit the spinal cord ventrally, or the side closest to the front of the body.
cervical, thoracic, lumbar, and sacral
vertebral column
dorsal root ganglia
The PNS is made up of:
PNS connects the CNS to the rest of the body and can itself be subdivided into the somatic and autonomic nervous system
nervous tissues and fibers outside the brain and spinal cord, including all 31 pairs of spinal nerves and 10 of 12 pairs of cranial nerves (olfactory and optic are outgrowths of the CNS).
Somatic nervous system:
consists of sensory and motor neurons distributed throughout the skin, joints, muscles. Sensory neurons transmit information through afferent fibers. Motor impulses travel along efferent fibers.
Autonomic nervous system (ANS):
generally regulates heartbeat, respiration, digestion, and glandular secretion. ANS manages the involuntary muscles associated with many internal organs and glands. The ANS also helps regulate body temperature by activating sweating or piloerection.
One primary difference between somatic and autonomic nervous system is that the peripheral component of the autonomic system contains two neurons. By contrast, a motor neuron in the somatic nervous system goes directly from the spinal cord to the muscle without synapsing. In the ANS, 2 neurons work in series to transmit messages from the spinal cord. The 1st neuron is known as the preganglionic neuron, while the 2nd is the postganglionic neuron. The Soma of the preganglionic neuron is in the CNS, and its axon travels to a ganglion in the PNS, where it synapses on the soma of the postganglionic neuron, which then stimulates the target tissue.
Main role of the parasympathetic nervous system is to:
conserve energy. It is associated with resting and acts to reduce heartrate and constrict the bronchi. Also responsible for managing digestion by increasing peristalsis and exocrine secretions. Ach is the neurotransmitter responsible for parasympathetic responses in the body and is released by both preganglionic and postganglionic neurons. The vagus nerve (cranial nerve X) is responsible for much of the parasympathetic innervation of the thoracic and abdominal cavities.
Sympathetic nervous system:
is activated by stress. Includes mild stressors to extreme stressors. Closely related to flight-or-flight reactions. Preganglionic neurons release ACh, while most postganglionic neurons release norepinephrine. When activated sympathetic nervous system:
- Increases heart rate
- Redistributes blood to muscles of locomotion
- Increases blood glucose concentration
- Relaxes the bronchi
- Decreases digestion and peristalsis
- Dilates the eyes to maximize light intake
- Release epinephrine into bloodstream
reflex arcs
Neural circuits that control reflexive behavior. Ex: Person steps on a nail, receptors in the foot detect pain and pain signal is transmitted by sensory neurons up to the spinal cord. Sensory neurons connect with interneurons, which relay pain impulses to the brain. Rather than waiting for the brain to send a signal, interneurons in the spinal cord can also send signals to the muscles of both legs directly, causing the person to lift the foot while supporting weight with the other foot. The original sensory information makes its way to the brain, but by the time it arrives the muscles have already responded to the pain thanks to the reflex arc.
2 types of reflex arcs, monosynaptic and polysynaptic.
monosynaptic reflex arc
a single synapse between the sensory neuron that receives stimulus and the motor neuron that responds to it. Ex is the knee-jerk reflex: when the patellar tendon is stretched, information travels up the sensory (afferent, presynaptic) neuron to the spinal cord, where it interfaces with the motor (efferent, postsynaptic) neuron that causes contraction of the quadriceps muscles. The net result is extension of the leg, which lessens the tension of the patellar tendon. Reflex is simply a feedback loop and a response to potential injury.
Polysynaptic reflex arc
has at least one interneuron between the sensory and motor neurons. Ex: stepping on a nail involving the withdrawal reflex. The leg which steps on the nail will be stimulated to flex, using the hip muscles and hamstring muscles, pulling the foot away from the nail. This by itself is a monosynaptic reflex. However, if a person is to maintain balance, the other foot must be planted firmly on the ground. For this to occur, the motor neuron that controls the quadriceps muscle on the contralateral leg must be stimulated, extending it. Interneurons in the spinal cord provide the connections from the incoming sensory information to the motor neurons in the supporting limb.
