5.1 Cord, brainstem, and forebrain #
5.1.1 Grey, white, and a reflex arc #
The central nervous system is brain and spinal cord. The peripheral nervous system is the cranial and spinal nerves, the sensory and autonomic ganglia, and the enteric neurons of the gut. One dorsal-root ganglion cell is both, along a single axon. Grey matter is cell bodies, dendrites, and synapses. White matter is myelinated axons.
Imgnotraçat Arc Reflex Eng
Sensory axons enter the dorsal root, cell bodies in the dorsal root ganglion. Motor axons leave the ventral root. That is the BellβMagendie law. Cutting a dorsal root removes afferent drive and the reflexes that needed it; it does not, by itself, paralyse the muscles. Cutting a ventral root does the opposite. Preganglionic sympathetic cells occupy the lateral horn from about T1 to L2.
A reflex arc is receptor, afferent, one or more central synapses, efferent, and effector. The stretch reflex (Chapter 7) is monosynaptic. Withdrawal from pain is polysynaptic: excitation of flexors, inhibition of extensors, often with crossed extension of the other limb. Descending axons bias the interneurons, so a tendon jerk tests more than the muscle. The same interneurons form pattern generators that can alternate the limbs without a cortical decision on every step.
5.1.2 Regions by the computation associated with them #
The brainstem β medulla, pons, and midbrain β holds cranial-nerve nuclei, the reticular formation involved in arousal, sleep and wake, posture, and pain modulation, and the premotor circuits of respiratory rhythm and blood-pressure reflexes. The organs those circuits move are Animal and Human Physiology.
The cerebellum is associated with error correction, timing, and motor learning, rather than with starting movement or with balance alone. The hypothalamus holds set points for temperature, fluid, feeding, and circadian time, and commands autonomic and pituitary output. The thalamus is the route to cortex for every major sense except olfaction, and cortex sends more axons back than it receives. Higher-order nuclei link cortical areas, and the GABAergic reticular nucleus gates traffic. The basal ganglia select actions and habits. Neocortex has six layers. Layer IV receives thalamic input, layer V sends the main subcortical output, and layer VI returns axons to the thalamus.
5.1.3 Topographic maps #
Neighbours on a sensory surface stay neighbours in the central sheet. The somatosensory map in the postcentral gyrus enlarges the hand and mouth because innervation is dense and receptive fields are small, not because those parts are large. Motor cortex has a related, overlapping map of movements. Retinotopy gives the fovea a disproportionate share of visual cortex. Tonotopy orders frequency in auditory cortex.
Maps let local inhibition compare neighbours and so sharpen contrast. Use and disuse shift them by the synaptic rules of Chapter 9.
5.2 Autonomic outflow, meninges, and fluid #
5.2.1 Sympathetic and parasympathetic transmitters #
Autonomic outflow is visceral motor innervation, plus an enteric network that can generate gut motility on its own. Sympathetic preganglionic cells lie from about T1 to L2. Their axons are short and cholinergic, ending on nicotinic receptors in paravertebral or prevertebral ganglia. Postganglionic axons are long and usually release noradrenaline. The adrenal medulla is a sympathetic ganglion without a postganglionic axon: its chromaffin cells secrete adrenaline and noradrenaline into the blood. Sweat glands are sympathetic and cholinergic.
Parasympathetic preganglionic cells lie in the nuclei of cranial nerves III, VII, IX, and X and in the sacral cord. The preganglionic axon is long; the ganglion is near the organ; the postganglionic transmitter is acetylcholine on muscarinic receptors. Shared organs are pushed in opposite directions at identified receptors.
| Target | Sympathetic | Parasympathetic |
|---|---|---|
| Pupil | Dilates, largely Ξ±1 | Constricts, muscarinic |
| Heart rate | Increases, noradrenaline on Ξ²1 | Decreases, acetylcholine on M2 |
| Bronchioles | Relax, Ξ²2 and circulating adrenaline | Constrict, largely M3 |
| Gut motility | Decreases | Increases |
| Bladder detrusor | Relaxes | Contracts, muscarinic |
| Sweat glands | Sweating, via acetylcholine | No important supply |
| Adrenal medulla | Chromaffin release of adrenaline | None |
Both divisions work at rest. An emergency changes the pattern β widespread sympathetic discharge, circulating adrenaline, less vagal tone β and each organ answers its receptors. Adrenaline in blood reaches vessels the nerves do not, including much skeletal-muscle vasculature through Ξ²2 receptors.
5.2.2 Meninges and cerebrospinal fluid #
The dura is the tough outer coat and, in the skull, splits to form venous sinuses. The arachnoid encloses the fluid-filled subarachnoid space. The pia follows the brain surface. Blood collecting around the brain raises pressure because the skull does not yield.
Cerebrospinal fluid is secreted by choroid-plexus epithelium. It passes from the lateral ventricles through the foramina of Monro into the third ventricle, through the cerebral aqueduct β the narrowest part β into the fourth, and out into the subarachnoid space, to be absorbed at arachnoid granulations. Adult volume is about 150 mL, and production is on the order of half a litre a day, so the pool turns over several times daily. Block the aqueduct and the ventricles upstream enlarge. The fluid is not plasma: protein is low, and the ions are set by epithelial transport. It cushions the brain. It is a poor reporter of most cortical transmitters.
5.3 Key terms, summary, and self-check #
Key terms. Grey matter; white matter; dorsal root; ventral root; reflex arc; thalamus; basal ganglia; topographic map; sympathetic; parasympathetic; muscarinic; choroid plexus.
Summary. Synapses sit in grey matter; white matter connects them. Sensory axons enter dorsally and motor axons leave ventrally. Brainstem, cerebellum, hypothalamus, thalamus, basal ganglia, and cortex are associated with cranial and autonomic integration, error correction, homeostasis, routing, action selection, and layered maps, none with a single exclusive function. Both autonomic divisions use acetylcholine at nicotinic ganglia; the usual target transmitter is noradrenaline on one side and acetylcholine on the other. Cerebrospinal fluid is secreted by the choroid plexus, passed through the ventricles, and absorbed into venous blood.
Self-check.
1. A dorsal root is cut and the ventral root spared. What is lost in that segment, and what remains?
2. Why is “relay” an incomplete description of the thalamus?
3. Name the transmitter and receptor class at a parasympathetic ganglion and at its target.
4. Why is adrenal medullary secretion sympathetic even though there is no postganglionic axon?
5. Trace cerebrospinal fluid from a lateral ventricle to a venous sinus, and name the narrowest part.
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