1. Brain Basics Flashcards
Two functions of spinal cord
Reflexes and highway between brain and body
What distinguishes the human nervous system from others?
Large size of brain relative to body
3 types of neurons
Sensory, motor, interneurons
Sensory neurons
Coupled to receptors specialized to detect attributes of the environment. When stimuli to the skin exceed a certain intensity, they can cause tissue damage and nociceptors are activated, giving rise to protective reflexes and the sensation of pain
Motor neurons
Control muscle activity, responsible for all forms of behaviour.
Interneurons
Interposed between sensory and motor neurons. Most numerous in the brain. Mediate simple reflexes and are responsible for the highest functions of the brain
Cell body
Also called the soma. Contains the nucleus, most of its cytoplasm, and molecular machinery for building and transporting proteins
Dendritic spines
Tiny protuberances that stick out from dendrites, where incoming axons make their connections
Growth factors
Factors taken in by axons and transported to cell body where they influence expression of neuronal genes and manufacture of new proteins. Enable neuron to grow.
Glial cells
Previously thought to have a purely supporting function, now known to contribute to the development of the nervous system. Much more numerous than neurons, but don’t transmit information in the way neurons do. Previously thought to outnumber neurons 10:1, but recent research suggests that in some regions the ratio is closer to 1:1.
Four types of glial cells
Astrocytes: regulate ion concentrations around neurons, provide them with nutrients, regulate formation of new connections between neurons
Microglia: immune cells of brain, function as phagocytes and regulate formation of new neuronal connections
Ependymal cells: make cerebrospinal fluid (cushions brain)
Oligodendrocytes: wrap axons in myelin sheaths
Resting potential in mammals
-70 millivolts, more negative inside cell
Depolarized
Making a cell less negative
Hyperpolarized
Making a cell more negative
Action potential
If the sum of the signals at dendrites reaches the threshold, a series of ion channels (sodium and potassium) opens, triggering an action potential. Starts at the cell body, sodium channels open first and a new equilibrium is established within a millisecond. Membrane potential increases by about 100 mV, which opens potassium channels causing the membrane potential to change back to its original negative value
Over very quickly, can race along at 100 metres per second. Followed by a brief refractory period
Time elapsed between arrival of action potential at synapse and generation of new signal at next neuron
1/1000 of a second
Mechanism by which an action potential is propelled
Wave of depolarization
Old experiments regarding neurons were conducted with…?
Large neurons from sea creatures
Modern day technique for study of the movement of ions in neurons
Patch-clamping
Gaps in myelin sheath
Where ion channels are concentrated, function as amplifiers
Cerebrum
Largest part of the brain. Divided into two hemispheres, left and right, which are connected by bundles of nerve fibres.
Corpus callosum
Largest of nerve bundles bridging cerebral hemispheres
Cerebral cortex
Surface of the cerebrum. Deeply folded to increase the area, creating more space for neurons.
Frontal lobe
Front of brain. Coordinates voluntary movements, speech, memory, emotion, higher cognitive skills, and personality
Parietal lobe
Top of brain. Integrates sensory signals from skin, processes taste and some types of visual information
Occipital lobe
Back of brain. Processes visual information, responsible for recognition of colours/shapes and integrating them into complex visual understanding
Temporal lobe
Sides of brain (bottom). Some visual processing and interprets auditory information
Hippocampus
In the temporal lobe. Curved structures lying beneath cerebral cortex. Encodes new memories
Amygdala
Deep structure within temporal lobe. Integrates memory and emotion
Thalamus
Integrates sensory information and relays it to other parts of the brain (cerebral cortex) which sends messages back to the thalamus
Hypothalamus
Sends hormonal signals to rest of body through pituitary gland. Controls functions such as eating and drinking. Regulates hormones involved in sexual functions
Limbic system
Group of structures deep within brain that helps regulate emotion and motivation. Includes hippocampus, amygdala, thalamus, hypothalamus
Diencephalon
Divided into two areas: thalamus and hypothalamus.
Forebrain
Cerebral cortex and limbic system
Midbrain
Beneath thalamus. Includes groups of neurons that coordinate eye movements and trigger reflexes to sounds. Other regions inhibit unwanted body movements and help coordinate sensory input and motor output to manage fine motor control. Each group of neurons seems to predominantly use a particular type of chemical messenger, but all of them project up to cerebral hemispheres. Thought that these can modulate the activity of neurons in higher centres of brain to mediate functions like sleep, attention, or reward.
Basal ganglia
Parts of the midbrain and forebrain which form a collection of structures that helps regulate complex body movements (initiation and control of movement).
Hindbrain
Glucose regulation, sleep, movement control, control of vital functions. Extension of the spinal cord.
Cerebellum
Underneath occipital lobe at back of brain, arises from roof of hindbrain. Second largest part of brain in volume, contains over half the brain’s neurons. Deeply folded, divided into two hemispheres. Coordinates voluntary movements (control and timing of movements), helps brain learn new motor skills, spatial and temporal perception. Patient with cerebellar damage might have a jerky gait or be unable to accurately touch his nose.
Pons
Below cerebellum. Influences breathing and posture
Medulla
Part of hindbrain. Carries nerve pathways connecting brain to spinal cord, helps control basic functions (swallowing, heart rate, breathing)
Brainstem
Midbrain, pons, medulla. Divided into hindbrain, midbrain, and diencephalon (between brain)
Synaptic cleft
Space between a dendrite and an axon at a synapse. 20 nanometres. Only verified by electron microscopy in 1950s.
When an action potential arrives at the axon terminal, the voltage change triggers…
The voltage change triggers ion channels to open, allowing calcium ions to flow into the cell and bind to synaptic vesicles, causing them to fuse with the membrane at the axon terminal and empty their contents into the synaptic cleft (done by activating enzymes that act on proteins called “snare”, “tagmin”, and “brevin”). Afterwards, pieces of the membrane cycle back into the soma as new vesicles re-filled with neurotransmitters
Substances that act as neurotransmitters
Amino acids, gases, small organic chemicals, short peptides
Production of neurotransmitters
Small non-peptides (dopamine, acetylcholine) can be made in the axon terminal. Proteins are built in the soma
Vesicles come from ______, binding to proteins called _______ that travel down the _____ along _______
Golgi apparatus, kinesins, axon, microtubules
Acetylcholine
Neurotransmitter, acts on both ionotropic and metabotropic receptors. First neurotransmitter to be discovered. Uses ionic mechanisms to signal across neuromuscular junctions. Can also function as a neuromodulator when you want to focus on something
Dopamine
Neuromodulatory. Reward system.
Noradrenaline
Released in response to forms of stress, organizes responses to these stresses. Neuromodulatory. Only 1600 noradrenaline neurons in brain, but they reach everywhere. Located in locus coeruleus. Axons distributed throughout midbrain: hypothalamus, cerebellum, cerebral cortex
Outer surface of dendrite with high concentration of receptors
Postsynaptic density/membrane
Two types of receptors
Ionotropic and metabotropic
Ionotropic receptors
Neurotransmitter binds directly to part of an ion channel. Receptor changes shape of channel, widening the tunnel
Metabotropic receptors
Receptor and channel are different proteins linked by a series of biochemical steps that are triggered by the attachment of a neurotransmitter. May open a channel some distance away or activate other intracellular molecules. Slower and more long-lasting. Neuromodulation.
Mechanism by which neurotransmitters attach to receptors
Lock and key
Process by which neurotransmitters are broken down or reabsorbed
Reuptake
Excitatory neurons
Make neurotransmitters that open ion channels to depolarize membrane. 80% of neurons in brain are excitatory. Send signals that push other neurons to fire. Typically pass signals forward through a circuit and eventually send outputs to rest of brain.
Inhibitory neurons
Make neurotransmitters that hyperpolarize membrane. Suppress activity of other neurons. Regulate circuit activity. Typically local and loop responses back to earlier sections of a circuit.
Most common excitatory neurotransmitter
Glutamate. Amino acid, used by half of excitatory synapses in brain. can bind to several ionotropic receptors, including AMPA and NMDA receptors. AMPA is quicker, NMDA slower. Important in learning and memory
Most common inhibitory neurotransmitter
GABA: binds to both types of receptors. Ionotropic: lets chloride ions into cell. Metabotropic: lets potassium ions out. In both cases, makes potential more negative
Hormones
Send brain cues about condition of distant tissues in body
Neuromodulators
Endocannabinoids that suppress neurotransmitter release
Prostaglandins
Small lipids that change brain’s response to pain and inflammation (increase pain sensitivity)
If a receptor is on the surface of the cell…
Molecule changes receptor’s shape and starts chain of intracellular reactions, ultimately modifying neuronal function by shifting ion balance or enzyme activity
If molecule can diffuse through membrane…
Receptor may be inside soma. When bound, complex can transform into a transcription factor that enters nucleus, binding to genes and altering their activity. E.g., steroid hormones
Gene expression depends on…
Chemical changes to chromatin
Tay-Sachs disease
Fatal degenerative neurological condition caused by mutations in a gene that codes for the fat-metabolizing enzyme beta-hexosaminidase A. Fats build up in neurons and become toxic
In early vertebrates, the brain end of the nerve cord developed into ___ bulges. These bulges became…
3 bulges: forebrain, midbrain, hindbrain
Forebrain (brain evolution)
Able to detect chemicals. Expanded to form olfactory bulbs. With the evolution of image-producing eyes, light-sensing regions expanded and processed more complex visual signals
Cerebellum (brain evolution)
Cerebellum appeared as the hindbrain and expanded regions that control escape movements and orient the body
Nerve tracts
Distinct bundles of nerve fibers of region-spanning neurons. E.g., corpus callosum, smaller anterior commissure transmitting signals between left and right temporal lobes
Neural networks
Group of nerve tracts connecting a series of regions in the brain
Describe the neural network involved in watching a movie
Photoreceptors trigger electrical signals in response to specific wavelengths of light → signals reach optic nerve → travel through optic tract to thalamus → passes signals to primary visual cortex in occipital lobe → integrates these signals to create a 3D representation of the world → image refined as signals are sent down two parallel streams → one to temporal lobe (identifies objects) and one to parietal lobe (detects spatial location)
Thalamocortical loop
Visual cortex sends signals to thalamus to be integrated with other sensory information
EEG
Electroencephalograph, detects brain waves: signals that travel through loops, producing oscillating electrical patterns
Four types of brain waves
Alpha waves: originate in parietal and occipital lobes when brain is relaxed and eyes are closed. Between 8 and 13 Hz. Produced by awake brain.
Beta waves: produced by frontal and parietal lobes when processing sensory input or concentrating on a task. 14 to 30 Hz. Produced by awake brain.
Theta waves: 4 to 7 Hz. Produced during sleep.
Delta waves: less than 3.5 Hz. Occur during deep sleep.
Alpha and delta waves have higher amplitudes (are stronger) but all four signals are in the microvolt range (20-200 microvolts for alpha and delta, 5-10 microvolts for beta and theta)
Spinal tracts
Another example of a neural network. Chains of neurons that pass signals through brainstem and spinal cord. Signals either travel upward from sensory receptors to thalamus and parts of cortex, or travel downward from brain regions that induce movement through medulla and spinal cord to muscles
5 other neural networks
Basal ganglia part of a feedback loop that takes information from cortical areas that elicit movement and makes signals that feed back to cortex to excite/inhibit movements
Loops connecting brainstem and cerebellum influence timing and strength of motor signals
Networks looping hippocampus into sensory cortex pathways help brain analyze whether environmental signals are familiar or new
Networks linking hippocampus to thalamus and hypothalamus allow memory to influence conscious behaviour and unconscious physiological responses
Reflex loops elicit action before thoughts. Usually controlled locally by information going in/out of spinal cord or subcortical regions of brain, never reach cortex
Arrangement of neurons in neural circuits
Neurons organized into a stack of distinct layers that span the thickness of the cortex. Circuits arranged in columns as each neuron forms connections with cells in layers above and below. Neurons in a column form a single chain, signals travel down that chain. Each time the signal moves forward, it is modified in some way, building outputs that encode complex information.
Most common type of excitatory neuron
Pyramidal cell: cone-shaped cell body, two sets of dendrites - one at apex, another set of shorter ones at base. Multi-branched axon that sends a single signal to multiple destinations.
__________ may be caused by imbalances in activity of excitatory and inhibitory neurons
Seizure disorders
Feed-forward inhibitory circuit
Inhibitory interneurons connect neighbouring circuits in a way that excitatory signals in one column simultaneously send inhibitory signals to adjacent columns, reducing their activity
Feedback inhibition
Neurons send signals to downstream excitatory neighbours and to interneurons that reach back and inhibit preceding layers of the same circuit.
Recurrent neural networks
Neurons inside interconnected circuits send feedback signals to one another. Examples: feed-forward inhibitory circuit, feedback inhibition
