Venom is a prerogative of all species of spiders except some orb weavers like Uloboridae and some trapdoor spiders like Liphistiidae. The main components are neurotoxins whose aim is to paralyse the prey with the sole object of inoculate digestive enzymes responsible of the braking down of the tissues. Most of spiders are harmless to humans but the danger grows in presence of some potent molecules:

Polyamines

They are small molecules with high efficiencies as ion-channel blockers or high affinity for other specific receptors for excitatory amino acids localized in the mammalian central nervous system. The classification takes care of functional analogy with N-methyl-D-aspartate (NMDA), quisqualate (QA), or kainate (KA) as type agonists respectively.
These compounds are object of accurate studies because of the high dose-correlated strength, for example the Fishing Spider venom containing a non-competitive antagonist of NMDA (N-methyl-D-aspartate) receptors has yielded a reversible L-and R-type voltage-sensitive calcium channels blocker with high analgesic capabilities.
Alpha-agatoxins of the American Funnel Web Spider are associated with a reversible paralysis: AGA-G1 permits the blockage of a calcium channel indifferent to the action of nifedipine and this fact opens new potential treatment strategies; AGA-G2 intravenous administration in rats cause a dose-dependent decrease in convulsive phenomenons induced by kainic acid, picrotoxins and bicuculline.
Jorotoxin, a specific postsynaptic glutamate receptors blocker take a role as investigational ligand and has good potential as anticonvulsant.

Peptides

Mu-agatoxins act as inhibitors of muscle sodium channels and trouble action potential. They are cysteine-rich polypeptides that bring on irreversible paralysis and recurring action potentials originating in presynaptic axons or nerve terminals; belong to this group mu-agatoxin-I and mu-agatoxin-IV, both of which contain 36 amino acids and four internal disulfide bonds, isolated in American Funnel Web Spider.
The omega-agatoxins from American Funnel Web Spider consist of two subtypes of neuronal calcium channel toxins with different structural characteristics and binding specificities. The brazilian Wandering Spider peptide neurotoxins has a similar distribution of cysteine residues with neurotoxins from other North American funnel web spider but is the only lethal, that may represent a case of a spider that is taxonomically divergent.

Delta Atracotoxin

Robustoxin (atracotoxin) from A. robustus and versutoxin from H. versuta venoms both are 42 amino acid peptide components; versutoxin differs from robustoxin by only 8 amino acid residues.
The effects of these presynaptic neurotoxins that bind to tetrodotoxin-sensitive sodium channels notice as spontaneous, repetitive firing of autonomic and motor neuron action potentials derived from a forced release of endogenous acetylcholine, noradrenaline and adrenaline. These toxins Binding to the outer surface of the channel, these toxins alter the conformational changes necessary for gating and the result is a voltage dependent slowing of sodium channel inactivation.
These lethal toxins are typical from Australian Funnel Web Spiders and the symptoms involve pain at the bite site, strong salivation, piloerection, muscle fasciculation, nausea, pulmonary edema and respiratory failure, tachycardia and hypertension followed by hypotension and circulatory failure.

Latrotoxin

Instead of small peptide toxins the most important bioactive constituent from the venom of the lethal American Black Widow Spider (Latrodectus mactans) is a much larger protein. Latrotoxin is synaptic activator of calcium uptake and the presence of Ca2+ ions support a massive release of acetylcholine; the symptoms are cramps and rigid paralysis as for some peptide omega-conotoxin that act on channels.