Nerve Cells and Impulses

• Cortical areas

  • Brain is convoluted, has gyri and sulci (ridges and grooves)
  • Allows the brain to be as big as it is
• Cortexes of the brain

  • Frontal - voluntary movement, language, and higher cognitive skills
  • Parietal - somatosensory processing and interpretation
  • Occipital - processing and perceiving visual information
  • Temporal - processing of auditory information, encoding of memory, processing emotions, and language
• Cerebellum

  • Located at the bottom of the brain
  • Involved in fine motor control and balance, as well as some cognitive functioning
• Basal ganglia

  • Located below the cerebellum and connects to the spinal cord by the medulla oblongata
  • Contains a caudate nucleus and putamen on both the left and right hand side
  • Contains the substantia nigra and the subthalamic nucleus in the center
  • Contains the globus pallidus located near the putamen on either side
  • Interconnected structures that play an important role in motor coordination, motor flexibility, and cognitive flexibility
• Pituitary gland

  • Mediates between the brain and the endocrine system
• Limbic system

  • Hippocampus is a structure on either side of the midbrain, situated in the medial temporal lobe
  • Fornix is a bundle of white matter that connects the mammillary bodies (important for episodic memory) and the hippocampus
  • Amygdala is located in the temporal cortex and is important in processing negative emotions
  • Anterior cingulate cortex is located above the rest of these structures in the medial area of the cortex, connects the amygdala and hypothalamus
  • Hypothalamus sits below and in front of the thalamus and is important for regulating our autonomic physiological processes in the body
• Dopamine


  • Connects the midbrain with the basal ganglia, the frontal cortex, and the pituitary gland
  • Can interact with 5 different G-protein coupled receptors (metabotropic receptors)
• Tuberoinfundibular pathway

  1. Cell bodies are in the arcuate nucleus (part of the hypothalamus)

  2. Connects to the median eminence (part of the hypothalamus) with its axons

  3. Dopamine is released there into the bloodstream to inhibit the release of prolactin (hormone) from the pituitary gland
• Mesolimbic pathway

  1. Cell bodies are in the ventral tegmental area

  2. Axons travel to the nucleus accumbens
• Nigrostriatal pathway

  1. Cell bodies in the substantia nigra pars compacta of the midbrain

  2. Axons connect to the dorsal striatum in the basal ganglia
• Mesocortical pathway

  1. Cell bodies in the ventral tegmental area in the midbrain

  2. Axons travel to the cortex, mostly the prefrontal and cingulate cortex
• Dopaminergic cell bodies

  • Mainly located in the lateral area of the substantia nigra and the medial areas of the ventral tegmental area
  • Darker than most other cells due to a high concentration of neuromelanin
• Parkinson's disease


  • Occurs when the cell bodies in the substantia nigra die which leads to a reduction in dopamine neurotransmission in the basal ganglia
  • Leads to characteristic motor symptoms such as stiffness of the muscles, tremor, and slowness of movement
• Addiction

  • Drugs of abuse initially activate the mesolimbic dopaminergic system which gives a euphoric feeling
  • Prolonged exposure causes the euphoria to decrease, which causes compulsive drug use and is associated with an increase in the dopamine release from the nigrostriatal dopaminergic system
• Serotonin

  • 95% of serotonin (5-hydroxytryptamine, or 5-HT) is outside of the central nervous system, but fibers containing serotonin are widespread in the brain
  • Can interact with 14 different receptors - most belong to the metabotropic class except for the 5-HT3 receptors
• Serotonergic cell bodies

  • Located in the midbrain in a series of clusters called the raphe nuclei
  • The axons then connect to almost all other areas of the brain
• Serotonin and brain disorders

  • It is difficult to prove that there is an association between serotonin functioning and psychiatric disorders because the changes of serotonin in these disorders are very dynamic and seemingly random
  • However, serotonin changes have been seen in patients with depression, anxiety disorders, and autism spectrum disorders, and drugs that affect the serotonergic system are known to improve symptoms of these disorders
• Glutamate


  • The major excitatory neurotransmitter in the brain
  • Widespread throughout the central nervous system
  • The main neurotransmitter for pathways from the cortex to other brain regions (corticofugal pathways), and it also connects the thalamus to the cortex and the striatum, and connects various nuclei in the basal ganglia together
  • Can interact with 8 G-protein coupled receptors, and 3 ionotropic receptors, NDMA, AMPA, and Kainate
• Glutamate and brain disorders

  • Glutamate dysfunction has been related to epilepsy, as an overactive glutamatergic system could contribute to increased activation of brain regions
  • There has also been some data showing a reduction in glutamate in schizophrenia
• Resting membrane potential


  • Julius Bernstein was the first to theorize this
  • Walter Nernst was the first to prove and calculate this
• Cell membrane

  • Made up of phospholipids
  • The phospholipids have a polar head, which is hydrophilic, and non polar tail, which is lipophilic
  • Creates a bilayer with a hydrophilic outside and lipophilic inside
  • Because the inside of the cell is more negatively charged than the outside, there is a difference in voltage which we call the resting membrane potential (usually -70mV)
• Ions

  • The cell membrane has selective permeability, which means some chemicals can pass through easier than others
  • Chemicals such as sodium, potassium, chloride, and calcium have to cross through membrane channels that are sometimes open and sometimes closed
  • Every cell has a leaky K+ channel that allows some K+ ions to travel out of and into the cell at any time
• Sodium-potassium pump


  1. A protein complex that requires a lot of energy

  2. It repeatedly transports 3 sodium ions out of the cell and brings 2 potassium ions into the cell

  3. Because of this, sodium ions are more concentrated on the outside and potassium ions are more concentrated on the inside
• Electrical force and concentration forces

  • The sodium ions pumped out of the cell stay out, but some of the potassium ions in the cell leak back out
  • When the neuron is at rest, the electrical force is trying to push sodium into the cell, because sodium ions are positively charged and the inside of the cell is negatively charged
  • The concentration force is also trying to push sodium into the cell, as there is a higher concentration of sodium outside than in
  • Despite this, no sodium actually flows into the cell because of the closed sodium channels when the membrane is at rest
  • Potassium ions are subject to opposing forces, as the electrical gradient wants to pull the ions inside the cell, while the concentration force wants to push it out
• Action membrane potential

  1. When a stimulating depolarizing current large enough to reach the cell's specific threshold (usually around -60mV) is applied, the cell opens its sodium channels and allows sodium ions to flow into the cell

  2. The new potential is generally around +30mV
• All or none law

  • A stimulus larger than the threshold will not result in a larger or faster action potential
  • While the amplitude, velocity, and shape of an action potential is consistent for a given axon, they can vary from one neuron to another, as thicker axons can create greater velocities and more action potentials per second
  • Because of this law, axons must change the timing of axon potentials to differentiate between a weak and a strong stimulus (similar to morse code)
• Voltage gated K+ and Na+ channels

  • At the resting potential, voltage gated channels will always remain closed
  • As the membrane becomes depolarized, both channels will open allowing a free flow of Na+ and K+
  • At first, the opening of the K+ channels does not do much, because the concentration force and electrical force on the potassium are relatively balanced
  • However the opening of the Na+ channels creates a large difference, because both forces are pushing Na+ into the cell - the Na+ ions flow in rapidly until the the electric potential reaches above zero to a reversed polarity
  • At the peak of the action potential, the Na+ gates snap shut
  • After so many sodium ions have entered the cell, the inside of the cell ends up with a slight positive charge, which results in both the EF and CF pushing K+ outside of the cell
  • Because the leaky K+ channels remain open, enough K+ ions leave the cell to return the membrane potential to its resting state, and then even slightly lower (hyperpolarization) until it returns again back to normal resting state
  • Eventually, the sodium-potassium pump restores the original distribution of sodium and potassium ions
• Action potential propagation

  1. When a sodium ion enters a spot on the axon during action potential, that spot is positively charged compared to neighboring spots along the axon temporarily

  2. These positive areas slightly depolarize the next area of the membrane which then reaches its threshold and open its channels

  3. Action potential is regenerated by this, and the chain reaction continues along the axon

  4. Action potentials also "back-propagate" back to the cell body and dendrites, which do not conduct action potentials but passively register the event from the nearby axon

  5. When an action potential back-propagates to dendrites, they become more susceptible to the structural changes responsible for learning
• Myelin sheaths and saltatory conduction

  • Action potentials travel at less than 1m/s in thin axons, and up to 10m/s on thicker axons
  • The myelin sheath is broken periodically along the axon called nodes of Ranvier, and the axon potential always starts at the first node in myelinated axons
  • The axon potential can not regenerate along the axon between nodes because there are almost no Na+ channels between them - instead the Na+ ions enter the axon itself and diffuse, which pushes a chain of positive charge along to the next node where the action potential regenerates
  • Saltatory conduction conserves energy, because a myelinated axon admits sodium ions only at its nodes rather than any and every point along the axon
• Refractory period

  • The Na+ ion channels snap shut at the peak of the action potential so that another action potential does not occur while the voltage is still above threshold, and so that action potentials do not continue back and forth along the axon for ever
  • During an absolute refractory period, no amount of stimulation can produce an action potential - usually lasts 1ms
  • During a relative refractory period, the stimulus must be stronger than usual to initiate an action potential - lasts another 2-4ms
  • This period depends on the fact that the Na+ channels are closed, and K+ is flowing out of the cell faster than normal
• Cell structures

  • The surface of a cell is its membrane which separates the intra- and extracellular space, and most chemicals cannot cross the membrane without the help of protein channels
  • Most cells also have a nucleus which contains our DNA
  • The mitochondrion performs metabolic activities and provides energy to the rest of the cell - mitochondria also have their own DNA
  • Ribosomes are either freely floating within the cell or attached to the endoplasmic reticulum, and they synthesize new proteins
• Neuron structure

  • The cell body contains the nucleus, ribosomes, and mitochondria
  • The dendrites are branching fibers extending from the cell body and are lined with specialized synaptic receptors which is where they receive information from other neurons (some dendrites have dendritic spines which increase the surface area and therefore increases the amount of information it can receive)
  • The axon is a thin fiber which conveys an impulse to other neurons, an organ, or a muscle via the presynaptic terminal
  • Some axons are insulated with a myelin sheath, which has periodic breaks along it called nodes of Ranvier
  • Axons can be afferent, in which they bring information into a structure, or efferent, where they carry information away from a structure - every sensory neuron is an afferent to the rest of the system, and every motor neuron is efferent to the rest of the system - within the system, a neuron is afferent from one structure and efferent to another structure
  • Interneurons or intrinsic neurons are neurons that are contained to one single structure, such as a part of the brain
• Astrocytes


  • Wrap around the synapses of related axons, shielding the connections from surrounding harmful chemicals
  • Help synchronize closely related neurons by taking up and re-releasing ions and transmitters released by the axon
  • Dilate blood vessels which brings more nutrients to brain areas with heightened activity
• Oligodendrocytes


  • Found in the brain and spinal cord and build the myelin sheaths on insulated axons
  • Supply nutrients necessary for functioning to the axons
• Microglia


  • Part of the immune system, removes viruses and fungi from the brain
  • Help contribute to learning by getting rid of the weakest synapses
• Radial glia


  • Guide the migration of neurons during embryonic development
  • After embryonic development, most radial glia differentiate into neurons, but some into astrocytes and oligodendrocytes
• We have the blood-brain barrier because our brain cannot replace damaged brain cells/neurons
• Some viruses can cross the blood-brain barrier, but microglia are effective at treating these
• Blood-brain barrier

  Depends on the tightly packed endothelial cells that form the walls of the capillaries, which block out viruses, bacteria, and most other harmful chemicals
• Blood-brain barrier

  Also stops some useful chemicals from entering the brain such as amino acids - the body has special mechanisms to allow these chemicals in