Recording-2025-09-03T14:55:17.337Z
Neuron: Basic Structure and Function
The brain contains around neurons (estimates vary). Neuron = a nerve cell and the primary functional unit of the nervous system.
Dendrites: tree-like branches on the left side of the neuron.
Dendrites are where neurons receive information from other neurons.
Receptors on dendrites pick up signals from neurotransmitters released by other neurons.
These signals cause electrical changes that are interpreted in the soma (cell body).
Soma (cell body): contains the nucleus with DNA; site of integration of information from dendrites.
Soma performs metabolic functions and houses the cell’s machinery.
The soma is the center of the neuron’s metabolism and contains mitochondria; glial cells support the soma.
Axon hillock: the integration site where inputs from dendrites are summed; acts as a gatekeeper.
If the signal is strong enough (surpasses a threshold), the neuron generates an action potential and sends it down the axon.
If not strong enough, the signal stops at the hillock.
Axon: conducts the electrical signal (action potential) away from the soma toward axon terminals.
Axon is insulated by myelin, which prevents signal degradation and speeds transmission.
The axon ends in axon terminals (synaptic buttons).
Myelin: an insulating white fatty layer around the axon that speeds conduction and protects the signal.
Myelin is formed by glial cells: Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS).
A single Schwann cell wraps around one axon segment to form one internode; one oligodendrocyte can form many internodes on multiple axons.
Myelinated axons appear white and constitute the brain’s white matter; unmyelinated axons contribute to gray matter.
Myelination increases conduction speed; disruptions in myelin (e.g., multiple sclerosis) impair neural communication and can cause physical/psychiatric symptoms.
Nodes of Ranvier and internodes:
Gaps in myelin are nodes of Ranvier; sections adjacent to nodes are internodes.
Nodes are rich in sodium channels; they help refresh the action potential as it travels along the axon.
The action potential appears to jump from node to node (saltatory conduction), increasing speed.
Axon terminals and synapses:
The axon terminals release neurotransmitters into the synaptic cleft when an action potential arrives.
The synaptic cleft is the gap to the next neuron’s dendrites; neurons do not touch.
Neurotransmitters bind to receptors on the postsynaptic membrane of the next neuron to propagate or modulate the signal.
Purkinje cell example (cerebellum):
Purkinje cells have highly arborized dendritic trees; the cerebellum’s wiring supports balance and reflexes.
The presence of a soma, dendrites, axon, and a complex dendritic arbor is typical of cerebellar circuitry.
Electrical and chemical signaling:
Neurons rely on electrical signals (action potentials) and chemical signaling (neurotransmitters) to communicate.
The two primary modalities coordinate to pass information rapidly and precisely.
The soma and cortex:
The soma sits in the cortex (gray matter) while the axon travels through white matter.
The cortex is about 40 Post-it notes thick in common analogies; it contains somas and dendrites involved in high-level processing.
Brief cat cerebellum note:
The cerebellum’s structure (including arborized dendrites) is associated with balance and reflexes; this is used to illustrate neural wiring in the brain.
Two-minute neuroscience snapshot on myelin:
Myelin is an insulating layer around axons; it is lipid-rich and prevents electrical current from leaking.
More myelin → faster and more efficient conduction of action potentials.
Myelin is not continuous; there are gaps (nodes of Ranvier) that help maintain signal strength.
Axon hillock and signal strength:
The axon hillock determines whether the incoming signal reaches threshold to trigger an action potential.
Signals that do not reach threshold are suppressed and do not propagate.
The synapse (revisited):
Neurotransmitters are released from presynaptic terminals, traverse the synaptic cleft, and bind to receptors on the postsynaptic neuron.
Neurotransmitters can be excitatory (increase likelihood of the postsynaptic action potential) or inhibitory (decrease likelihood).
Clear, precise communication depends on receptor binding, transmitter clearance, and receptor response.
Vesicles and neurotransmitters:
Neurotransmitters are packaged in vesicles in the presynaptic neuron.
When an action potential arrives, vesicles fuse with the presynaptic membrane and release their contents into the synaptic cleft.
Vesicles may contain thousands of neurotransmitter molecules.
Clearance from the synaptic cleft: