Chapter 2: Structure and Function of the Nervous System Flashcards
Action potential
Book definition: “The active or regenerative electrical signal that is required for synaptic communication. Action potentials are propagated along the axon and result in the release of neurotransmitter. (p. 30)”
An action potential is a rapid depolarization and repolarization of a small region of the neuronal membrane caused by the opening and closing of ion channels.
In order to trigger an action potential, the following steps take place:
- First, multiple excitatory postsynaptic potentials (EPSPs) are generated at the synapses of the neuron’s dendrites.
- The passive electrical currents of the EPSPs are conducted through the cytoplasm of the neuron and converge at the “root” of the axon, called the axon hillock.
- The current then flows across the part of the neuronal membrane responsible for initiating the action potential – the so-called spike-triggering zone – and depolarizes the membrane.
- If the depolarization is strong enough to lower the electrical charge of the membrane from the resting membrane potential of -70 millivolts to below the threshold of approximately -55 millivolts, an action potential is triggered.
Once the action potential has been triggered, the following occurs (numbers correspond to labels on image):
- Na+-specific voltage-gated ion channels are triggered open by the depolarization of the membrane. This allows sodium ions to rapidly travel into the neuron, which further depolarizes the membrane.
- The further depolarization starts a process known as the Hodgkin-Huxley cycle; a rapid, self-reinforcing cycle of opening even more voltage-gated ion channels, which in turn let even more sodium ions pass inside, which opens more channels, and so on. The whole process lasts about 1 millisecond and changes the charge of the neuron from around -55 millivolts to around +20 millivolts.
- The change in charge now triggers K+-specific voltage-gated ion channels to open. This allows potassium ions to flow out of the neuron down their concentration gradient, which begins to shift the membrane potential back to its resting potential of -70 millivolts.
- The opening of K+ channels outlasts the closing of Na+ channels. This causes a second repolarization phase, which drives the electrical charge towards the equilibrium potential of K+ at around -80 millivolts.
- This hyperpolarization of the membrane causes the K+-channels to close, gradually returning the membrane to its resting potential. During this temporary state, known as the absolute refractory period, the neuron is unable to fire. This lasts only a couple of milliseconds, but has two consequences: 1) the neuron’s firing rate is limited to about 200 action potentials per second, and 2) the action potential cannot re-open the ion-gated channels that just generated it. It can, however, depolarize the membrane further ahead, resulting in the action potential propagating down the axon until it finally reaches the axon terminal.
In the case of myelinated axons, the myelin-sheath makes the axon super-resistant to voltage loss. The result is that action potentials need only occur at nodes of Ranvier – i.e. the little gaps in myelin along the axon – to propagate down the axon. This is called saltatory conduction (after the Latin saltare, meaning to jump) as the action potential appears to jump from node to node.
Amygdala
Book definition: “Groups of neurons anterior to the hippocampus in the medial temporal lobe that are involved in emotional processing. (p. 47)”
Association cortex
Book definition: “The volume of the neocortex that is not strictly sensory or motor, but receives inputs from multiple sensorimotor modalities. (p. 56)”
Autonomic nervous system
Book definition: “Also autonomic motor system or visceral motor system. The body system that regulates heart rate, breathing, and glandular secretions and may become activated during emotional arousal, initiating a “fight or flight” behavioral response to a stimulus. It has two subdivisions, the sympathetic and parasympathetic branches. (p. 38)”
Axon
Book definition: “The process extending away from a neuron down which action potentials travel. The terminals of axons contact other neurons at synapses. (p. 26)”
Some axons branch off into axon collaterals that can transmit signals to more than one cell.
Axons can be myelinated, meaning they become surrounded by a layer of fat that functions as electrical insulation, making the electrical signals passing through them faster.
Along the axon are evenly spaced gaps in the myelin, which are referred to as the nodes of Ranvier.
Axon collateral
Book definition: “Branches off an axon that can transmit signals to more than one cell. (p. 26)”
Axon hillock
Book definition: “A part of the cell body of a neuron where the membrane potentials are summated before being transmitted down the axon. (p. 30)”
The axon hillock is essentially the “root” or beginning of the axon. From the hillock, the axon stretches outward until ending in the terminal, which synapses unto the dendrites of other neurons.
The hillock also contains the spike-triggering zone, where excitatory postsynaptic potentials (or EPSPs) converge and accumulate in order to trigger an action potential.
Basal ganglia
Book definition: “A collection of five subcortical nuclei: the caudate, putamen, globus pallidus, subthalamic nucleus, and substantia nigra. The basal ganglia are involved in motor control and learning. Reciprocal neuronal loops project from cortical areas to the basal ganglia and back to the cortex. Two prominent basal ganglia disorders are Parkinson’s disease and Huntington’s disease. (p. 47)”
The basal ganglia are located beneath the anterior portion of the lateral ventricles, near the thalamus. The nuclei in the basal ganglia include:
- The caudate nucleus: involved with working memory, learning, emotional response to visual beauty, and the integration of spatial information with motor movement. The latter includes body and limb posture, speed and accuracy of directed movements, and goal-directed action. Makes up the striatum in combination with the putamen.
- The putamen: a non-specialized structure involved with aiding other structures in several functions, including movement regulation and various types of learning. Makes up the striatum in combination with the caudate nucleus.
- The globus pallidus: involved with the unconscious regulation of voluntary movement, it allows for smooth, controlled movement and response to sensory feedback. Damage to this area can result in tremors or jerks, such as seen in Parkinson’s disease.
- The substantia nigra: Latin for “black substance,” it is divided into the pars compacta and pars reticulata. The prior serves mainly as an input to the basal ganglia circuit, supplying the striatum with dopamine, while the ladder serves as an output, conveying signals from the basal ganglia to other brain structures.
The several neurological conditions that are associated with basal ganglia dysfunction help outline the role of these structures. These include Parkinson’s (which involves a degeneration of dopamine-producing cells in the pars compacta of the substantia nigra) and Huntington’s (which primarily involves damage to the striatum), as well as Alzheimer’s, Tourette’s, OCD, and general addiction.
Blood-brain barrier (BBB)
Book definition: “A physical barrier formed by the end feet of astrocytes between the blood vessels in the brain and the tissues of the brain. The BBB limits which materials in the blood can gain access to neurons in the nervous system. (p. 36)”
Brainstem
Book definition: “The region of the nervous system that contains groups of motor and sensory nuclei, nuclei of widespread modulatory neurotransmitter systems, and white matter tracts of ascending sensory information and descending motor signals. (p. 43)”
The brainstem is usually thought of as having three main parts: the myelencephalon (medulla), the metencephalon (pons and cerebellum), and mesencephalon (midbrain).
As the brainstem nuclei control respiration and global states of consciousness such as sleep and wakefulness, damage to this area is life threatening.
Central nervous system (CNS)
Book definition: “The brain and spinal cord. Compare peripheral nervous system. (p. 37)”
Central sulcus
Book definition: “The deep fold or fissure between the frontal and parietal cortex that separates the primary motor cortex from the primary somatosensory cortex. (p. 51)”
Cerebellum
Book definition: “Also known as “little cerebrum.” A large, highly convoluted (infolded) structure located dorsal to the brainstem at the level of the pons. The cerebellum maintains (directly or indirectly) interconnectivity with widespread cortical, subcortical, brainstem, and spinal cord structures, and plays a role in various aspects of coordination ranging from locomotion to skilled, volitional movement. (p. 44)”
The cerebellum clings to the brainstem at the level of the pons and contains most of the brain’s neurons in a tightly folded layer of tissue. It resembles the forebrain, with the cerebellar cortex consisting of gray matter, and a center of white matter.
The cerebellum is critical for maintaining posture, walking, and performing coordinated movements. Fibers arriving from the brain carry information about motor outputs and sensory inputs about body position. Outputs either ascend to the thalamus and then the motor and premotor cortex, or descend to the spinal cord.
Apart from essentially analyzing, coordinating and maintaining movement, the cerebellum has recently also been linked to aspects of language, attention, learning, mental imagery, and more.
Cerebral cortex
Book definition: “The layered sheet of neurons that overlies the forebrain. The cerebral cortex consists of neuronal subdivisions (areas) interconnected with other cortical areas, subcortical structures, and the cerebellum and spinal cortex. (p. 38)”
The cerebral cortex is between 1.5 and 4.5 mm thick and has billions of neurons arranged in layers of thin sheets, folded across the surfaces of the cerebral hemispheres like a cloth.
This allows for both more cortical surface to be packed inside the skull, as well as facilitating faster communication by bringing neurons closer together, allowing them to connect through “shortcuts” in the brain’s white matter.
The white matter is named so for its white appearance due to a high concentration of myelinated axons (i.e. the above mentioned connective shortcuts). In contrast, gray matter appears gray due to the high density of neuronal cell bodies.
The cortex is divided into four functionally different areas, named after the overlying skull bones: the frontal, parietal, temporal, and occipital lobe.
Commissure
Book definition: “White matter tracts that cross from the left to the right side, or vice versa, of the central nervous system. (p. 39)”
Corpus callosum
Book definition: “A fiber system composed of axons that connect the cortex of the two cerebral hemispheres. (p. 39)”
Commissural fibers cross between hemispheres through bridges called commissures, most of which pass through the corpus callosum.
In a drastic procedure known as corpus callosotomy, these fibers are surgically severed as a last-ditch effort to alleviate powerful epileptic seizures. This results in so-called split-brain patients, who can experience two separate instances of perception – e.g. visual information cannot be vocally explained, if it is not processed in the hemisphere where the speech center is located. In extreme cases, movement may be impaired, with the patient’s arms each seeming to have a will of their own.
Cytoarchitectonics
Book definition: “The way in which cells differ between brain regions. (p. 8)”
By staining thin slices of brain tissue with chemical agents, it is possible to study the tissues’ cellular composition under a microscope.
Using this method to define differences in cerebral cytoarchitecture, German neurologist Korbinian Brodmann (1868-1918) divided the cerebral cortex into 52 discrete areas, which he proposed each served unique functional purposes. “Brodmann area” (or simply “BA”) is now used to refer to several such areas – e.g. BA4, which is the primary motor cortex, and BA17, the primary visual cortex.
Dendrite
Book definition: “Large treelike processes of neurons that receive inputs from other neurons at locations called synapses. (p. 25)”
Due to the tree-like shape of dendrites, their branching is called “arborization” (arbor being Latin for “tree”). Dendrites can take many varied and complex forms, depending on the type and location of the neuron they extend from. The arborizations may be simple, like the dendrites in spinal motor neurons, or resemble old oak trees, like the complex dendritic structures of the cerebellar Purkinje cells.
Many dendrites also have spines; little knobs attached by small necks to the surface of dendrites, where the dendrites receive inputs from other neurons.
Depolarization
Book definition: “A change in the membrane potential in which the electrical current inside the cell becomes less negative. With respects to the resting potential, a depolarized membrane potential is closer to the firing threshold. Compare hyperpolarization. (p. 31)”
Dura mater
Book definition: “Dense layers of collagenous fibers that surround the brain and spinal cord. (p. 38)”
The dura mater is the outermost of the three protective membranes – or meninges – covering the brain and the spinal cord. The middle layer is called the arachnoid mater, and the inner and most delicate layer is called the pia mater, which firmly adheres to the surface of the brain.
Between the middle and innermost layers, in the subarachnoid layer, is the cerebrospinal fluid (or CSF) in which the brain is suspended.
Electrical gradient
Book definition: “A force that develops when a charge distribution across the neuronal membrane develops such that the charge inside is more positive or negative than the one outside. Electrical gradients result from asymmetrical distributions of ions across the membrane. (p. 29)”
In neurons, an electrical gradient occurs when ion pumps move three sodium ions (Na+) out of the cell, and two potassium ions (K+) into the cell. As the net amount of positively charged ions outside the neuronal membrane becomes greater than on the inside, an electrical gradient – i.e. a difference in electrical charge – is created.
Electrotonic conduction
Book definition: “Passive current flow through neurons that accompanies activated electrical currents. (p. 30)”
Electrotonic conduction is also called decremental conduction, as it diminishes with distance from its origin; the maximum distance a passive current will flow is only 1 mm.
An example of such electrotonic conduction is the excitatory postsynaptic potential (or EPSP) at the synapses of neurons. The small electrical current produced by the EPSP is passively conducted through the cytoplasm of the dendrite, cell body, and axon.
In structures like the retina, neurons are close enough together to allow neuron-to-neuron communication by electrotonic conduction. In most cases however, the maximum distance of 1 mm is too short. Instead, these neurons rely on the action potential mechanism, which can travel for meters without loss in signal strength, as it continuously regenerates the signal.
Equilibrium potential
Book definition: “The membrane potential at which a given ion (e.g., K+) has no net flux across the membrane; that is, as many of the ions move outward as inward across the membrane. (p. 31)”
Frontal lobe
Book definition: “The mass of cortex anterior to the central sulcus and dorsal to the Sylvian fissure. The frontal lobe contains two principal regions – the motor cortex and the prefrontal cortex – each of which can be further subdivided into specific areas both architectonically and functionally. (p. 50)”
Glial cell
Book definition: “Also neuroglial cell. One of two cell types (along with the neuron) in the nervous system. Glial cells are more numerous than neurons, by perhaps a factor or 10, and may account for more than half of the brain’s volume. They typically do not conduct signals themselves; but without them, the functionality of neurons would be severely diminished. Tissue made of glial cells is termed glia. (p. 35)”
Neuroglia is Latin for “nerve glue,” as early anatomists believed glial cells to be simple support structures for neurons. However, glial cells also help form the blood-brain barrier (BBB) and aid in the speed of information transfer.
The central nervous system (CNS) has three main types of glial cells:
- Astrocytes are large and round or radially symmetrical cells which surround neurons and are in close contact with the brain’s vasculature. They make contact with blood vessels at specializations called end feet, which permit them to transport ions across the vascular wall. Astrocytes also create a blood-brain barrier between the tissues of the CNS and the blood.
- Microglial cells are small and irregularly shaped phagocytic cells; these devour and remove particles such as bacteria, parasites and damaged cells, essentially taking on the role as the brain’s janitors.
- Oligodendrocytes create myelin by wrapping their cell membranes around the axon in a concentric manner during development and maturation. The cytoplasm in that portion of the cell is squeezed out, leaving the lipid layer to sheathe the membrane.
Schwann cells are similar to oligodendrocytes, but are instead found in the peripheral nervous system.
Gray matter
Book definition: “Regions of the nervous system that contain primarily neuronal cell bodies. Gray matter includes the cerebral cortex, the basal ganglia, and the nuclei of the thalamus. Gray matter is so called because, in preservative solution, these structures look gray in comparison to the white matter where myelinated axons are found (which look more white). (p. 39)”
Gyrus (pl. gyri)
Book definition: “A protruding rounded surface of the cerebral cortex that one can see upon gross anatomical viewing of the intact brain. Compare sulcus. (p. 49)”
Hippocampus (pl. hippocampi)
Book definition: “The “seahorse” of the brain. A layered structure in the medial temporal lobe that receives inputs from wide regions of the cortex via inputs from the surrounding regions of the temporal lobe, and sends projections out to subcortical targets. The hippocampus is involved in learning and memory, particularly memory for spatial locations in mammals and episodic memory in humans. (p. 47)”
Hyperpolarization
Book definition: “A change in the membrane potential in which the electrical current inside of the cell becomes more negative. With respect to the resting potential, a hyperpolarized membrane potential is farther from the firing threshold. Compare depolarization. (p. 31)”
Hypothalamus
Book definition: “A small collection of nuclei that form the floor of the third ventricle. The hypothalamus is important for the autonomic nervous system and the endocrine system, and it controls functions necessary for the maintenance of homeostasis. (p. 45)”
The hypothalamus is the main site for hormone production and control, and is the main link between the autonomic nervous system and the endocrine system (i.e. a collection of glands secreting hormones into the circulatory system). It both produces and regulates the production of hormones in other parts of the brain.
The hypothalamus controls a variety of functions necessary for maintaining homeostasis, i.e. the “normal” state of the body:
- The circadian rhythm (i.e. the light-dark sleep cycle) is controlled with inputs from the mesencephalic reticular formation, amygdala, and retina.
- Behavior to alleviate feelings such as thirst, hunger, and fatigue as well as the adjustment of body temperature is accomplished via the endocrine system and the pituitary gland.
Ion channel
Book definition: “A passageway in the cell membrane, formed by a transmembrane protein that creates a pore, through which ions (charged atoms in solution) of sodium, potassium, and chloride (Na+, K+, and Cl–) might pass into or out of the cell. (p. 28)”
The three-dimensional protein structures known as ion channels selectively permit only certain types of ions to pass through the neuronal membrane. The ion channels present in the membranes of neurons are selective for either sodium (Na+), potassium (K+), calcium (Ca2+) or chlorine (Cl–) ions, giving neuronal membranes the attribute of selective permeability.
As there are more K+-selective channels than other types of channels, the neuronal membrane is more permeable to K+ than other ions. This, along with ion pumps, creates a negative electrical charge of -70 millivolts on the inside of the membrane – also known as the resting membrane potential.
Neurons are excitable, meaning they can change the permeability of their membranes based on changes in voltage, or chemical or physical stimuli. Ion channels able to change their permeability for a particular type of ion are called gated ion channels, while ion channels unable to do this are called nongated.
Ion pump
Book definition: “Proteins in the cellular membrane of neurons that are capable of transporting ions against their concentration gradient. The sodium-potassium pump transports sodium ions out of the neuron and potassium ions into the neuron. (p. 28)”
While ion channels allow specific ions to passively cross the neuronal membrane down their concentration gradient – i.e. from high concentration to low – ion pumps actively transport ions against their concentration gradients using energy.
This energy comes from hydrolyzing adenosine triphosphate (ATP), and is used by the pump proteins to move three sodium ions (Na+) out of the cell, and two potassium ions (K+) into the cell.
As the net amount of positively charged ions outside the cell becomes greater than the inside, a difference in electrical charge – called the electrical gradient – is created alongside the ion concentration gradient. These two gradients are opposites: while the electrical gradient pulls K+ back inside the cell, the ion concentration gradient pushes K+ out through the ion channels.
When the two gradient forces are equal, an electrochemical equilibrium is reached. The result is an electrical charge around -70 millivolts on the inside of the neuronal membrane –also known as the resting membrane potential.
Limbic system
Book definition: “Several structures that form a border (limbus in Latin) around the brainstem, named the grand lobe limbique (“limbic lobe”) by Paul Broca. The limbic system is the emotional network that includes the amygdala, orbitofrontal cortex, and portions of the basal ganglia. (p. 47)”
The limbic system is a group of brain structures mainly involved in emotion, memory, and smell. The structures include:
- The hippocampus, involved with memory, learning, and recollection of spatial relationships.
- The parahippocampal gyrus, involved with place-recognition and modification of emotional expression.
- The amygdala, involved with behavior and emotional response, including sexual interest, anger, and fear.
- The mammillary bodies (parts of the hypothalamus), involved with transmitting information to and from the fornix and thalamus.
- The anterior thalamic nuclei (parts of the thalamus), involved with modulation of alertness, learning, and episodic memory.
- The cingulate gyrus, involved in memory and regulating emotion.
- The olfactory bulbs, directly involved with smell. Their close connection to structures related to emotion and memory helps explain the power of smell to evoke emotion and memories.
The limbic system surrounds another group of nuclei, called the basal ganglia, almost like a cage (filling in much of the “empty space” in the image).
Although the limbic system was first considered a functionally unified system, that idea is now being abandoned due to the many different functions of its structures.
Medulla
Book definition: “Also myelencephalon. The brainstem’s most caudal portion. The medulla is continuous with the spinal cord and contains the prominent, dorsally positioned nuclear groups known as the gracile and cuneate nuclei, which relay somatosensory information from the spinal cord to the brain, and the ventral pyramidal tracts, containing descending projection axons from the brain to the spinal cord. Various sensory and motor nuclei are found in the medulla. (p. 43)”
The medulla is essential to life: it houses the cell bodies of many of the 12 cranial nerves, providing sensory and motor innervations to face, neck, abdomen, throat, and heart. It also controls vital autonomic functions such as respiration, heart rate, blood pressure, and digestive and vomiting responses.
All ascending somatosensory information from the spinal cord passes through the medulla via two bilateral nuclear groups: the gracile and cuneate nuclei.
Midbrain
Book definition: “The part of the brain containing the tectum (meaning “roof,” and representing the dorsal portion of the mesencephalon), tegmentum (the main portion of the midbrain), and ventral regions occupied by large fiber tracts (crus cerebri) from the forebrain to the spinal cord (corticospinal tract), cerebellum, and brainstem (corticobulbar tract). (p. 44)”
The midbrain, also called the mesencephalon, lies superior to the pons. It surrounds the cerebral aqueduct, a channel of cerebrospinal fluid which connects the CSF-filled third and fourth ventricles.
The midbrain also contains the superior colliculus, which plays a role in visually perceiving objects in the periphery and orienting our gaze toward them, and the inferior colliculus, which does the same for auditory stimuli.
Much of the midbrain is occupied by the mesencephalic reticular formation, a set of interconnected nuclei involved in maintaining consciousness and arousal, which runs up the brainstem.
Myelin
Book definition: “A fatty substance that surrounds the axons of many neurons and increases the effective membrane resistance, helping to speed the conduction of action potentials. (p. 26)”
Neural circuit
Book definition: “Groups of interconnected neurons that process specific kinds of information. (p. 37)”
Although they come in many forms, neural circuits share some basic features: they take in information (called afferent inputs); they evaluate the input either at a synapse or within one or a group of neurons (called local circuit neurons); and they convey the results to other neurons, muscles, or glands (called efferent outputs).
An example of a neural circuit is the patellar tendon reaction (or “knee jerk”) triggered by a tap on the kneecap: this sends a sensory signal to the spinal cord, which stimulates motor neurons to fire action potentials leading to muscle contraction in the leg.
Some neural circuits show plasticity, i.e. the ability to change, which is what happens with learning and during development.
Neural circuits can combine in order to form neural systems – e.g. the visual system or auditory system.
Neuron
Book definition: “One of two cell types (along with the glial cell) in the nervous system. Neurons are responsible for processing sensory, motor, cognitive, and affective information. (p. 24)”
Neurons receive, evaluate, and transmit information – a process known as neuronal signaling.
Within a neuron, this information transfer involves changes in the electrical state of the neuron; between neurons, information transfer is usually mediated by chemical signaling molecules known as neurotransmitters.
In regards to information flow, neurons are referred to as either presynaptic (when their axon connects to other neurons) or postsynaptic (when other neurons connect to their dendrites); most neurons are both.
A neuron consists of a cell body (also called soma), a cable-like axon that carries information to other neurons, and tree-shaped (or arborized) dendrites that the axons of other neurons connect to.
The dendrites and axon are extensions of the cell membrane and contain cytoplasm continuous with that in the cell body.
The cytoplasm is a salty intracellular fluid made up of a combination of ions (predominantly of potassium, sodium, chloride, and calcium) and molecules such as proteins.
A typical cortical neuron has between 1,000 and 5,000 synapses, while a Purkinje cell in the cerebellum may have up to 200,000 synapses.
Neurotransmitter
Book definition: “A chemical substance that transmits the signal between neurons at chemical synapses. (p. 33)”
Neurotransmitters are synthesized and localized within the presynaptic neuron, and stored in the presynaptic axon terminal before release.
There are two broad categories of neurotransmitters: small-molecule transmitters and neuropeptides. The former is synthesized in the axon terminal, while the latter is made in the soma, after which storage vesicles transport it to the terminal.
Over 100 neurotransmitters have been identified. Some of the most common transmitters and their function include:
- Glutamate: memory.
- GABA (or gamma-aminobutyric acid): calming.
- Epinephrine (also called adrenaline): “fight or flight” responses.
- Norepinephrine (or noradrenaline): concentration.
- Dopamine: pleasure.
- Serotonin: mood.
- Endorphin: euphoria.
- Acetylcholine (or ACh): muscle activation.
Some neurons produce only one transmitter, while others produce several. In the latter case, transmitters may be released together or separately, depending on the conditions of stimulation.
The effect of a neurotransmitter is not determined by the transmitter itself, but by the postsynaptic receptor. Depending on the receptor, a certain transmitter may cause either an increase or a decrease in neuronal firing.
Transmitters can still be classified by their typical effect. Excitatory neurotransmitters include acetylcholine, glutamine, histamine and serotonin. Inhibitory neurotransmitters include GABA, glycine, and some of the peptides.
Conditional neurotransmitters act in concert with other factors, such as the presence of another transmitter or activity in the neuronal circuit. This allows for complex modulations of information processing in the nervous system.
Node of Ranvier
Book definition: “A location at which myelin is interrupted between successive patches of axon, and where an action potential can be generated. (p. 26)”
In myelinated axons, a fatty substance known as myelin covers the axon in long, cylinder-shaped layers. Between these shapes are the exposed patches of axon membrane known as nodes of Ranvier.
When an action potential propagates down a myelinated axon, the myelin-sheath makes the axon super-resistant to voltage loss, causing the action potential to occur only at the nodes of Ranvier. This is called saltatory conduction (after the Latin saltare, meaning to jump) as the action potential appears to jump from node to node.
Nucleus (pl. nuclei)
Book definition: “1. In Neuroanatomy, a collection of cell bodies in the central nervous system – for example, the lateral geniculate nucleus. 2. In biology, a cellular organelle where DNA is stored. (p. 38)”
In neuroanatomy, nuclei are relatively compact arrangements of nerve cell bodies and their connections, ranging from hundreds to millions of neurons, with functionally similar inputs and outputs. Nuclei are located throughout the CNS – i.e. the brain and spinal cord.
Peripheral nervous system (PNS)
Book definition: “A courier network that delivers sensory information to the CNS and then conducts the motor commands of the CNS to control muscles of the body; anything outside the brain and spinal cord. Compare central nervous system. (p. 37)”
The peripheral nervous system (PNS) is comprised of two systems: the somatic nervous system, which controls the voluntary muscles of the body, and the autonomic nervous system, which controls visceral (or instinctive/automatic) functions.
Pituitary gland
Book definition: “Controlled by the hypothalamus, the pituitary gland helps maintain the normal state of the body (homeostasis). (p. 46)”
Pons
Book definition: “A region in the brain that includes the pontine tegmental regions on the floor of the fourth ventricle, and the pons itself, a vast system of fiber tracts interspersed with pontine nuclei. The fibers are continuations of the cortical projections to the spinal cord, brainstem, and cerebellar regions. The pons also includes the primary sensory nuclear groups for auditory and vestibular inputs, and somatosensory inputs from, and motor nuclei projecting to, the face and mouth. Neurons of the reticular formation can also be found in the anterior regions of the pons. (p. 44)”
Latin for “bridge,” the pons is the main connection between brain and cerebellum. Located anterior to the medulla, it is made up of a vast system of fiber tracts interspersed with nuclei.
As many of the cranial nerves responsible for sensory and motor information synapse here, the pons is important for movement of face, mouth and eyes – including the generation of rapid eye movement (REM) sleep.
Some auditory information is channeled through a pontine structure named the superior olive.
Postsynaptic
Book definition: “Referring to the neuron located after the synapse with respect to information flow. Compare presynaptic. (p. 27)”
Regarding the flow of information, a neuron is referred to as postsynaptic when other neurons make a connection onto its dendrites.
If imagining neural information flow as a road with synapses as intersections, a postsynaptic neuron is further ahead, after crossing the intersection, while a presynaptic neuron is further back, before crossing the intersection.
Presynaptic
Book definition: “Referring to the neuron located before the synapse with respect to information flow. Compare postsynaptic. (p. 27)”
Regarding the flow of information, a neuron is referred to as presynaptic when its axon makes a connection onto the dendrites of other neurons.
If imagining neural information flow as a road with synapses as intersections, a presynaptic neuron is further back, before crossing the intersection, while a postsynaptic neuron is further ahead, after crossing the intersection.
Resting membrane potential
Book definition: “The difference in voltage across the neuronal membrane at rest, when the neuron is not signaling. (p. 27)”
When a neuron is not signaling – i.e. in its resting state – the inside of the neuron is more negatively charged than the outside. This voltage difference, which is typically -70 millivolts inside, is known as the resting potential or resting membrane potential. This functions as a sort of battery, able to use the stored energy to do signaling work.
The electrical potential of a neuron is defined as the difference in voltage across the neuronal membrane – or simply put, the voltage inside the neuron versus outside the neuron. These two voltages depend on the concentration of electrically charged potassium, sodium, and chloride ions as well as charged protein molecules both inside and outside the cell.
Ions are moved across the cell membrane by two types of transmembrane proteins: ion channels, which allow certain ions to flow from high to low concentration gradients, and ion pumps, which use energy to actively transport ions against their concentration gradients.
Soma (pl. somata)
Book definition: “The cell body of a neuron. (p. 25)”
The soma – or cell body – of the neuron contains the metabolic machinery that maintains the neuron: a nucleus, endoplasmic reticulum, a cytoskeleton, mitochondria, Golgi apparatus, and more.
These structures are suspended in cytoplasm: a salty intracellular fluid made up of a combination of ions (predominantly of potassium, sodium, chloride, and calcium) and molecules such as proteins.
Spike-triggering zone
Book definition: “The location, at the juncture of the soma and the axon of a neuron, where currents from synaptic inputs on the soma and distant dendrites are summed and where voltage-gated Na+ channels are located that can be triggered to generate action potentials that can propagate down the axon. (p. 30)”
The spike-triggering zone is located at the axon hillock – i.e. the “root” of the axon. Here, excitatory postsynaptic potentials (or EPSPs) converge and accumulate in order to trigger an action potential. This happens when the EPSPs lower the charge of the membrane from the resting membrane potential of -70 millivolts to below the firing threshold of approximately -55 millivolts.
Sulcus (pl. sulci)
Book definition: “Also fissure. An invaginated region that appears as a line or crease of the surface of the cerebral cortex. Compare gyrus. (p. 49)”
Synapse
Book definition: “The specialized site on the neural membrane where a neuron comes in close position to another neuron to transmit information. Synapses include both presynaptic (e.g., synaptic vesicles with neurotransmitter) and postsynaptic (e.g., receptors) specializations in the neurons that are involved in chemical transmission. Electrical synapses involve special structures called gap junctions that make direct cytoplasmic connections between neurons. (p. 26)”
There are two major kinds of synapses involved in synaptic transmissions: chemical and electrical.
Most neurons use chemical transmissions by way of releasing neurotransmitters into the synaptic cleft. This mechanism works as follows (numbers correspond to labels on image):
- The arrival of the action potential at the axon terminal depolarizes the membrane at the terminal, causing Ca2+-specific voltage-gated ion channels to open and calcium ions to flow into the cell.
- Inside the axon terminal, the presence of calcium ions triggers bubble-like cell structures named vesicles to fuse with the membrane at the synapse.
- The vesicles carry neurotransmitter, which they release into the synaptic cleft.
- The transmitter diffuses across the cleft and binds with specific receptors embedded in the postsynaptic membrane. Here, ligand-gated-ion-channels allow specific neurotransmitters – which are a type of ligand – to pass through the membrane.
The binding of neurotransmitters causes a change in the receptor, which opens specific ion channels, resulting in an influx of ions.
Depending on the neurotransmitters, this either results in a depolarization (leading to excitation) or a hyperpolarization (leading to inhibition) of the postsynaptic cell.
Excitation produces an excitatory postsynaptic potential (EPSP), triggering a positive change in the electrical charge of the neuron’s membrane, which could initiate a new action potential. Inhibition produces an inhibitory postsynaptic potential (IPSP), triggering a negative change in electrical charge, which could inhibit a new action potential.
Thalamus
Book definition: “A group of nuclei, primarily major sensory relay nuclei for somatosensory, gustatory, auditory, visual, and vestibular inputs to the cerebral cortex. The thalamus also contains nuclei involved in basal ganglia – cortical loops, and other specialized nuclear groups. It is a part of the diencephalon, a subcortical region, located in the center of the mass of the forebrain. Each hemisphere contains one thalamus, and they are connected at the midline in most humans by the massa intermedia. (p. 45)”
The diencephalon is made up by the hypothalamus and the thalamus. The latter, located at the top of the brainstem, is the larger of the two structures and divided into two hemispherical halves connected by a bridge of gray matter called the massa intermedia.
The thalamus serves as relay station: all input from the sensory modalities – except the olfactory (smell) – synapse in the thalamus before continuing to their primary cortical sensory areas.
It also receives input from the basal ganglia, cerebellum, neocortex, and medial temporal lobe and sends projections back to create circuits involved in many different functions.
The thalamus is divided into several nuclei which act as specific relays for incoming sensory information – e.g.:
- The lateral geniculate nucleus, which receives information from the ganglion cells of the retina and sends axons to the primary visual cortex.
- The medial geniculate nucleus, which receives information from the inner ear via other brainstem nuclei in the ascending auditory pathway, and sends axons to the primary auditory cortex.
- The ventral posterolateral and posteromedial nuclei, which receive somatosensory information and project it to the primary somatosensory cortex.
Tract
Book definition: “A bundle of axons in the central nervous system. (p. 39)
Tracts, also called fascicles, connect the nuclei of the central nervous system (CNS). Tracts exist in three distinct groups:
- Association fibers are tracts which connect cortical areas within the same hemisphere, with long fibers connecting different lobes and short fibers connecting different gyri within a lobe.
- Commissural fibers connect corresponding cortical areas across hemispheres. They cross between hemispheres through bridges called commissures, most of which pass through the corpus callosum.
- Projection fibers connect the cerebral cortex with the diencephalon, corpus striatum, brainstem and spinal cord. These tracts carry information like motor signals from the cerebrum to the spinal cord, or signals upward to the cortex. Superior to the brainstem, the tracts form a dense sheet called the internal capsule, from which the tracts radiate like a fan to specific areas of the cortex.
White matter
Book definition: “Regions of the nervous system composed of millions of individual axons, each surrounded by myelin. The myelin is what gives the fibers their whitish color – hence the name white matter. Compare gray matter. (p. 39)”
Spinal cord
Book definition: “None (see page 40)”
The spinal cord runs from the brainstem to its termination in the cauda equina, and consists of a butterfly-shaped center of gray matter surrounded by white matter. It is enclosed in the bony vertebral column – a stack of separate bones called vertebrae – which is divided into five separate sections: cervical, thoracic, lumbar, sacral, and coccygeal.
The spinal cord is similarly divided into 31 segments, each with a right and left spinal nerve that enters and exits the spine through openings called foramen.
Each spinal nerve has both sensory and motor axons, allowing afferent neurons to carry sensory input into the spinal cord, and efferent neurons to carry motor output from the brain to the body.
