Horner’s syndrome, named after the ophthalmologist Johann Friedrich Horner, is caused by a problem with the sympathetic nerve supply to one side of the face.
This disruption results in miosis, which is constricted pupil; ptosis, a droopy eyelid; and anhidrosis, a failure to sweat.
Broadly speaking, the autonomic nervous system is a part of the nervous system that controls involuntary body functions like the heart rate, blood pressure and digestion.
The autonomic nervous system can be subdivided into the sympathetic and parasympathetic nervous system, which have opposite effects.
The sympathetic nervous system controls functions like increasing heart rate, blood pressure, and slowing digestion. All of this maximizes blood flow to the muscles, and can help you either run away from a threat or fight it which is why it’s also called the fight-or-flight response.
The parasympathetic nervous system has the opposite effect; it slows heart rate, decreases blood pressure, and stimulates digestion - the effects can be summarized as 'rest and digest'.
Now, with regard to the face and eye, there’s an oculosympathetic pathway with three groups of neurons called first-order, second-order and third-order neurons.
The body of the first-order neuron is located in the hypothalamus, and it’s axon extends down into the spinal cord, where it synapses with the second-order neuron.
The body of the second-order neuron is located in the cervical region of the spinal cord, and it’s axon exits the spinal cord and enters the sympathetic chain, which is a structure full of sympathetic ganglions or nerve cell bodies, and it runs along both sides of the spine.