Basically, the nervous system is made up of cells that are able to communicate with each other. A huge amount of communication takes place between a countless number of these special cells. This complex communication is what allows us to think, sense, and control our bodies.
Messages sent between these cells are called impulses. The impulses are "electrical" but not like electrical wires. Instead, impulses are made using ionic imbalances (also called membrane potentials) that are created using sodium-potassium pumps that maintain a resting potential of -70mV. You can think of these impulses (aka action potentials) as small waves that flow between cells on narrow rivers.
Organization of the Nervous System
- Central Nervous System (CNS)
- Brain and spinal cord.
- Peripheral Nervous System (PNS)
- Nervous tissue in the outer regions of the nervous sytem.
- Cranial nerves: originate in the brain.
- Spinal nerves: originate from the spinal cord.
- Central fibers: extend from cell body towards the CNS.
- Peripheral fibers: extend from cell body away from CNS.
- bundles of parallel neurons in the PNS.
- bundles of parallel neurons in the CNS.
- surrounds each nerve fiber.
- surrounds fascicles (bundles of nerve fibers).
- surrounds a complete nerve (PNS) or tract (CNS).
- regions of gray matter in the PNS.
Afferent vs Efferent
- incoming information (sensory). Towards CNS.
- outgoing information (motor). Towards muscles and glands.
- connect afferent and efferent neurons. In CNS.
Some people remember these using mnemonics like "Efferent = Exit from brain" and "Afferent = Away from receptor", but these can be confusing. So just remember that afferent is "in" and efferent is "out".
Somatic vs Autonomic Nervous System
- Autonomic nervous system (ANS): carries information to the autonomic or visceral effectors (smooth muscles, cardiac muscles, and glands)
- Afferent: visceral sensory.
- Sympathetic: "fight or flight". Dilates pupils, inhibits saliva flow, accelerates heartbeat, dilates bronchi, inhibits peristalsis, converts glycogen to glucose, secretes adrenaline/noradrenaline, inhibits bladder contraction, etc.
- Parasympathetic: "rest and digest, and repair". Encourages the opposite of sympathetic.
Cells of the Nervous System
- Neurons: excitable cells that conduct information.
- Neuroglia: aka glia or glial cells. These numerous, tiny cells support the neurons and don't conduct any information themselves. There are five major types of neuroglia:
- Astrocytes: star-shaped, largest, most numerous. Connect to neurons and capillaries. Transfer nutrients from blood to neuron while helping to form the blood-brain barrier (BBB).
- Microglia: the macrophages of the brain, they engulf and destroy cellular debris (phagocytosis).
- Ependymal cells: similar to epithelial cells, they line fluid-filled cavities of the brain and spinal cord. Some ependymal cells produce cerebrospinal fluid (CSF). Sometimes cilia is present on ependymal cells to help circulate CSF.
- Oligodendrocytes: hold nerve-fibers together and produce myelin sheaths in CNS.
- Schwann cells: like oligodendrocytes they produce myelin sheaths but Schwann cells are only present in the PNS. Satellite cells are a type of of Schwann cell that covers a neuron's cell body.
All neurons have three parts:
- Cell body (aka soma)
- Nissl bodies are part of the rough endoplasmic reticulum (RER) in neuron cell bodies.
- Axons take impulses away from the neuron to another neuron's dendrites.
- One or more dendrites
- Dendrites bring impulses towards the neuron from other neuron's axons.
- The junction between two neurons (or a neuron and an effector such as a muscle). Neurons are not physically connected to each other. Instead an axon of one neuron comes very close to the dendrites of another neuron where communication between the two can occur by chemical substances called neurotransmitters. This point is called a synapse.
- Myelin sheath
- insulation around nerve fibers that helps speed-up nerve impulses. Areas of the nervous system that are white have more myelin sheathing and areas that are gray are unmyelinated.
Structural Classification of Neurons
Neurons can be classified into three types based on structure:
- One axon, several dendrites.
- Most common (99% of neurons)
- One axon, one dendrite.
- Least numerous.
- Found in retina, inner ear, and olfactory pathway.
- One axon that branches into central process (towards CNS) and peripheral process (towards PNS). Dendrites come out of the distal end of the peripheral process.
- Sensory neurons and ganglia of PNS.
- Water, oxygen, CO2, glucose, and alcohol pass freely.
- Regulates passage of ions between brain and blood.
- Astrocytes wrap around capillaries in the brain to form the BBB.
- Distal axon and myelin sheath degenerates.
- Remaining neurilemma and endoneurium forms a "tunnel" from the injury to the effector.
- Proteins produced in the Nissl bodies help extend a new axon down the tunnel to the effector.
- An action potential is a nerve impulse.
- The resting membrane potential is -70 mV.
- A stimulus that causes an increase of voltage to at least -55 mV triggers the action potential. -55 mV is also called the threshold.
- The membrane potential then rapidly depolarizes from -55 mV to +30 mV.
- Then, it repolarizes back down towards -70 mV but ends up hyperpolarizing.
- Hyperpolarizing is when the membrane potential keeps decreasing beyond -70 mV (to -80 mV or -90 mV). During the hyperpolarization phase, no new action potentials can be generated. This is why it is also called the refractory period.