Autonomic nervous system and senses
Some of the nervous system's most important work happens without conscious thought, from regulating heartbeat to processing what we see and hear.
Sympathetic division
The sympathetic division prepares the body for 'fight or flight': it raises heart rate and blood pressure, dilates pupils and airways, diverts blood to muscles, releases glucose, and slows digestion. Its main neurotransmitter at target organs is noradrenaline, and it triggers adrenaline release from the adrenal medulla.
Parasympathetic division
The parasympathetic division promotes 'rest and digest': it slows the heart, constricts pupils and airways, and stimulates digestion and urination. It uses acetylcholine at target organs. The vagus nerve carries most parasympathetic output to the chest and abdomen. Most organs receive both divisions, which balance each other.
Cranial nerves
Twelve pairs of cranial nerves arise from the brain. Examples include the optic nerve (II) for vision, the facial nerve (VII) for facial expression and taste, the vestibulocochlear nerve (VIII) for hearing and balance, and the vagus nerve (X), which serves many organs.
Sensory receptors
Receptors convert stimuli into nerve signals. Mechanoreceptors sense touch, pressure, and sound; thermoreceptors sense temperature; nociceptors sense pain; chemoreceptors sense chemicals for taste and smell; photoreceptors sense light. Many receptors adapt, so steady stimuli like clothing fade from awareness.
The eye
Light passes through the cornea and pupil and is focused by the lens onto the retina. The retina's photoreceptors are rods, sensitive in dim light, and cones, which detect colour and fine detail and are concentrated in the fovea. The optic nerve carries signals to the brain. Where it leaves the eye, there are no receptors: the blind spot.
Vision problems
In myopia (short-sightedness), the eyeball is too long, focusing images in front of the retina; concave lenses correct it. In hyperopia, images focus behind the retina; convex lenses correct it. With age, the lens stiffens (presbyopia), making close focus harder. Cataracts cloud the lens.
The ear
The outer ear funnels sound to the eardrum, which vibrates three tiny bones (malleus, incus, stapes) in the middle ear. These pass vibrations to the fluid-filled cochlea in the inner ear, where hair cells convert them into nerve signals. The vestibular apparatus, also in the inner ear, senses head position and movement for balance.
Taste and smell
Taste buds detect five basic tastes: sweet, sour, salty, bitter, and umami (savoury). Smell receptors high in the nasal cavity detect thousands of odour molecules. Much of what we call flavour is actually smell, which is why food seems bland when you have a blocked nose.
Key takeaways
The sympathetic division drives fight or flight; the parasympathetic drives rest and digest, mainly via the vagus nerve. Twelve cranial nerves serve the head and some organs. Receptors convert stimuli to signals and often adapt. The eye focuses light on rods and cones in the retina. The ear's cochlea converts vibrations to signals; the vestibular apparatus senses balance. Flavour depends heavily on smell.
Flashcard
Fight or flight
Check your understanding
Which effect would the sympathetic nervous system produce?