Characteristics of Neurons, Part 1
Characteristics of Neurons:
- Neurons –> receive information and transmit it to other cells; the adult brain has about 100 billion but actual number varies
- Main founders of neuroscience: Charles Sherrington, Santiago Ramon y Cajal (Cobined art and medicine by becoming an anatomical researcher and illustrator, made detailed drawings of the nervous system).
- Camillo Golgi found a way of looking at a single cell by staining the nerve cells with silver salts
- Cajal used that method on infant brains and found that the cells are seperate
Structures of an animal cell:
- Membrane = edge of cell, separates inside from outside, made of 2 layers of fat molecules
- Protein channels = specialized openings in the membrane, only a few charged ions (sodium, potassium, calcium, chloride) can pass through
- Nucleus = structure contianing chromosomes
- Mitochondrion = structure performing metabolic acticities, provides the cell with energy, requires fuel and oxigen
- Ribosomes = sites where the cell synthesizes new protein molecules
- Protein = provide building material for the cell and facilitate various chemical reactions
- Endoplasmic reticulum = a network of thin tubes that transport newly synthesized proteins, some ribosomes are attached to it
Structure of a Neuron:
- Contain the same things as an animal cell
- Larger neurons have: dendrites, soma (cell body), axon and presynaptic terminals
- Motor neuron: has the soma in the spinal cord, receives excitation from other neurons through the dendrites and conducts impulses along ist axon to a muscle
- Sensory neuron: specialized at one end to be highly sensitive to a particular type of stimulation, different sensory neurons have different structures
- Dendrites = branching fibers that get narrower near their ends. It’s surface is lined wih specialized synaptic receptors where it receives information from other neurons (the greater its surface the more info it can receive). Some have dendritic spines (chort outgrowths that increase the surface areas)
- Cell body/Soma: contains the nucleus, mitochondria, ribosomes, etc. metabolic work of neurons happens here. It’s covered with synapses
- Axons: thin fibre of constant diameter, mostly longer than dentrites. Information sender of the neuron, conveys impulse to neurons/glands/muscles. Every neuron only has one, it has various lengths.
- Myelin sheath: insulating material that coveres many vertebrate axons
- Nodes of Ranvier: interruptions in the myelin sheath
- Presynaptic terminal/end bulb/ bouton: swelling on the tip of a branch of the axon. This is where the axon releases chemicals
- Afferent axon: brings information into a stucture. (every sensory neuron is afferent to the nervous system)
- Efferent axon: carries information away from a stucture. (every motor neuron is efferent to the nervous system)
- Inerneuron/intrinsic neuron: if a cells dendrites and axons are entirely contained within a single structure (ex. Contained within the thalamus)
Variation among neurons:
- The function of a neuron is closel related to its shape
- Glia/neuroglia: cellular component of nervous system that does not transmit information like neurons, but they do exchange chamicals with adjacent neurons. They are smaller but more numerous than neurons.
- Astrocytes: star-shaped type of glia, wraps around the presynaptic terminals of a group of functionally related axons, it takes up chemicals released by those axons and later releases them back. It helps synchronize the activity of the axons, enabling them to send messages in waves. Also remove waste maerial from neurons that die, and help control bloods flow to a brain area.
- Microglia: very small cells, also remove waste and viruses, fungi and other microorganisms.
- Oligodendrocytes: in brain, spinal cord and are specialized types of glia that build myelin sheaths.
- Schawnn cells do what radial glia do in the periphery, guiding a regenerating axon to the appropriate target. And build myelin sheaths like oligo’s.
- Radial glia: type of astrocyte, guides the migration of neurons and the growth of their axons and dndrites during embryonic development.
- Blood-brain barrier: mechanism that keeps most chemicals out of the vertebrate brain (so there’s no brain damage if a harmful substance enters the body)
- Endothelial cells for the walls of the capillaries, they are joined very tightly together so nothing passes through them. So chemicals only enter the brain by crossing the membrane.
- Two categories of molecules pass the blood-brain barries passively(without the expenditure of energy): small uncharged molecules (oxygen, carbon dioxide and water(through special protein channels)) and molecules that dissolve in the fats of the membrane (ex. Vitamins A and D, drugs that effect the brain-> heroin, marijuana, antidepressant drugs)
- Active transport: a protein mediated process that expends energy to pump chemicals from the blood into the brain. The chemicals doing this are: glucose, amino acids, some vitamines and hormones.
Nourishment of Vertebrate Neurons:
- They depend almost entirely on glucose, it is the only nutrient that crosses the blood-brain barrier in adults
- Metabolic pathway that uses glucose requires oxygen (-> they consume a large amount of oxygen)
- The liver makes glucose
- Vitamine B1 (thiamine) is a chemical necessary for the use of glucose
- Axons regenerate impulses at each point -> the impulse won’t weaken
- Brain isn’t set up to register small differences in the time of arrival of touch messages -> we feel a touch on our foot at the same time as on the shoulder, eventhough it takes longer from the foot to get to the brain
- But, in vision axons further to the brain send impulses faster -> we can see small differences in time