Organisms reach the brain and spinal cord in a variety of ways, with blood being one of the most obvious. Because there is extensive vascularization in the CNS, any organism that circulates in the blood and can enter the cerebrospinal fluid can cause meningitis (infection of the meninges). In addition, the skull contains various sinuses and mastoid air spaces that are separated from the brain by the bone of the skull. Infections in these areas can eventually cause the erosion of the bone, and then organisms can enter the brain and cause abscesses to form.
Inflammation is one of the first and most powerful responses to infection, causing an increase in vascular permeability and therefore swelling, but it is problematic for the CNS. The brain is enclosed by a hard case (the skull) which cannot expand, so any swelling will cause an increase in intracranial pressure, which can have disastrous consequences. An inflammatory response in the subarachnoid space, with the requisite release of cytokines, results in loosening of the tight junctions between vascular endothelial cells. The loosened junctions allow albumin into the cerebrospinal fluid and the result is the form of swelling known as vasogenic edema. Swelling can also result from the toxic substances produced by bacteria and from neutrophil invasion, both of which result in oxygen and nutrient deficiencies. This produces another kind of swelling, cytotoxic edema.
Tetanus spores require areas of low oxygen content if they are to germinate, and the area of necrosis around a tissue injury is perfect for the initiation of this process. The spores germinate, and Clostridium begins to grow in that location. The bacteria do not cause any damage to the tissue where they reside but they produce a neurogenic toxin, which then enters the presynaptic terminals of the lower motor neurons and from there they gain access to the CNS. In the spinal cord, the toxin acts at the level of the anterior horn cells and blocks postsynaptic inhibition of the spinal motor reflexes. This produces spasmodic contraction of the muscles, contractions that occur locally at first but then may extend up and down the spinal cord. The muscles responsible for respiration and swallowing can be compromised, and in severe cases patients can also result in opithotonos, where the head and heels move toward each other and the body bows out dramatically. Mortality for untreated tetanus is 15% to 60% and death usually occurs due to exhaustion and respiratory failure.