Autonomic control of erection, emission and ejaculation
Parasympathetic ("rest and digest") nerves drive erection, whereas sympathetic ("fight or flight") nerves drive emission (the delivery of semen into the urethra) and are involved in ejaculation (expelling semen).
Remember
Parasympathetic fibres from the sacral cord (S2 to S4, the second to fourth sacral levels) give erection, whereas sympathetic fibres from T11 to L2 (the lower thoracic and upper lumbar levels) give emission and are involved in ejaculation. With sympathetic outflow lost and the sacral segments intact, a touch-triggered erection survives but ejaculation fails.
Memory aid
point and shoot
The parasympathetic system points (erection) and the sympathetic system shoots (emission), though the saying simplifies, because the final expulsion also needs the somatic pudendal nerve.
The divisions of the autonomic nervous system share the work of sexual function. The parasympathetic division ("rest and digest") drives the engorgement of excitement, whereas the sympathetic division ("fight or flight") predominates at orgasm.
An erection is a vascular event. Parasympathetic fibres from S2 to S4 reach the penis as the cavernous nerves and release nitric oxide. This raises cyclic GMP (cGMP, a small signalling molecule inside the cell), which relaxes the smooth muscle of the helicine arteries (the small coiled arteries supplying the erectile tissue). The corpora cavernosa (the two columns of spongy erectile tissue) then fill with blood.
Sympathetic fibres from T11 to L2 produce emission. The smooth muscle of the vas deferens (the tube carrying sperm), the seminal vesicles and the prostate contracts and places semen in the urethra, while the internal urethral sphincter (the muscle ring at the bladder outlet) closes so that semen does not pass back into the bladder. Ejaculation follows. Somatic fibres (nerves to voluntary skeletal muscle) in the pudendal nerve (a somatic nerve that supplies the muscles that expel semen) make the muscles at the base of the penis contract rhythmically and expel the semen.
Erection, ejaculation and orgasm are under separate neurological control, so an injury can affect one and spare another. After a complete injury at T12, the sacral reflex arc (an automatic loop through the cord) lies below the injury, so touch can still produce an erection. The sympathetic outflow needed for emission is disrupted, so the man can have an erection but cannot ejaculate.
Key facts
- Erection
- parasympathetic, S2 to S4, with nitric oxide released by the cavernous nerves.
- Emission
- sympathetic, T11 to L2, with contraction of the vas deferens, seminal vesicles and prostate and closure of the internal urethral sphincter.
- Expulsion
- the somatic pudendal nerve, S2 to S4, drives the rhythmic contraction of the muscles at the base of the penis.
- Orgasm phase
- sympathetic outflow predominates, whereas excitement is driven mainly by parasympathetic outflow.
- Reflexogenic and psychogenic erection
- a reflexogenic erection (triggered by genital touch) relies on S2 to S4, whereas a psychogenic erection (triggered by thoughts and sights) relies on brain input and T11 to L2.
- Retrograde ejaculation
- if the internal urethral sphincter fails to close, sperm passes back into the bladder.
Exam traps
- Parasympathetic fibres from S2 to S4 do not explain failed ejaculation with a preserved erection, because they release nitric oxide to produce erection, and a preserved erection shows they are working.
- Somatic pudendal motor outflow does not explain lost emission after a complete spinal cord injury above the sacral cord, because emission itself is the sympathetic step and the pudendal motor neurones lie in the sacral cord, below the injury.
- A selective sacral parasympathetic injury does not stop the internal urethral sphincter closing, because it closes through sympathetic activity, which stops semen passing backwards into the bladder.
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