Chapter 18: Neural Control and Coordination (Human Physiology)

The brain lies in the skull, covered by three meninges (dura mater, arachnoid, pia mater). It has three parts. (i) Forebrain: cerebrum of two hemispheres joined by the corpus callosum, with an outer folded cortex (grey matter: motor, sensory, association areas) and inner white matter; thalamus (sensory and motor coordinating centre); hypothalamus (temperature, hunger, thirst; hypothalamic hormones); limbic system (emotions, motivation, sexual behaviour). (ii) Midbrain: between thalamus and pons, with the cerebral aqueduct and four corpora quadrigemina; integrates visual, tactile, auditory inputs. (iii) Hindbrain: pons (connecting tracts), cerebellum (convoluted; balance) and medulla (respiration, cardiovascular reflexes, gastric secretion), joined to the spinal cord. Midbrain, pons and medulla form the brain stem.

(a) CNS: brain and spinal cord; inside skull and vertebral column; site of information processing and control. PNS: all nerves (cranial, spinal) linking the CNS with the body; afferent and efferent fibres; somatic and autonomic divisions; carries information to and from the CNS. (b) Resting potential: potential difference across a resting (polarised) membrane, outside positive and inside negative; kept by the Na⁺-K⁺ pump and the membrane's greater K⁺ permeability. Action potential: the reversed potential at a stimulated (depolarised) site, outside negative and inside positive, due to rapid Na⁺ influx; it is the nerve impulse and travels along the axon.

(a) Polarisation: at rest the membrane is more permeable to K⁺ and nearly impermeable to Na⁺ and to the negative proteins inside. The Na⁺-K⁺ pump sends 3 Na⁺ out for 2 K⁺ in. So inside has high K⁺ and negative proteins, outside has high Na⁺; the outer surface becomes positive and the inner negative: polarised (resting potential). (b) Depolarisation: a stimulus makes the membrane at that site freely permeable to Na⁺; Na⁺ rushes in, reversing polarity (outside negative, inside positive); this is the action potential. (c) Chemical synapse: the impulse at the axon terminal makes synaptic vesicles fuse with the membrane and release neurotransmitters into the cleft; these bind receptors on the post-synaptic membrane, opening ion channels; ion entry creates a new potential (excitatory or inhibitory) in the post-synaptic neuron.

(a) See the neuron in the figure on page 2: dendrites, cell body with nucleus and Nissl's granules, axon, Schwann cells/myelin sheath, nodes of Ranvier, axon terminals with synaptic knobs. (b) See the brain on page 4: cerebrum, corpus callosum, thalamus, hypothalamus, midbrain, pons, cerebellum, medulla oblongata, spinal cord.

(a) Neural coordination: the neural system gives quick, point-to-point coordination of organs through neurons; e.g. during exercise it raises breathing and heart rate together. (b) Forebrain: cerebrum (hemispheres, corpus callosum, cortex with motor, sensory, association areas), thalamus, hypothalamus and limbic system. (c) Midbrain: between thalamus and pons; cerebral aqueduct; corpora quadrigemina; integrates visual, tactile, auditory inputs. (d) Hindbrain: pons (tracts), cerebellum (balance), medulla (respiration, cardiovascular reflexes, gastric secretion). (e) Synapse: junction between pre- and post-synaptic neurons; electrical (direct current, fast, rare) or chemical (cleft, neurotransmitters).

At a chemical synapse, the pre- and post-synaptic membranes are separated by a fluid-filled synaptic cleft. When an action potential reaches the axon terminal, the synaptic vesicles move to and fuse with the plasma membrane, releasing neurotransmitters (e.g. acetylcholine) into the cleft. These diffuse across and bind specific receptors on the post-synaptic membrane, opening ion channels; the entry of ions produces a new potential, either excitatory (an impulse starts) or inhibitory. At an electrical synapse current passes directly and faster, but such synapses are rare.

At rest Na⁺ is high outside and low inside, and the membrane is nearly impermeable to it; the Na⁺-K⁺ pump keeps it so. A stimulus makes the membrane at that site freely permeable to Na⁺. Na⁺ rushes into the axon along its gradient, making the inside positive and the outside negative: depolarisation, i.e. the action potential. The local current then depolarises the next site. The Na⁺ permeability is short-lived and K⁺ outflow restores the resting state.

(a) Myelinated: Schwann cells form a myelin sheath with nodes of Ranvier; faster conduction; spinal and cranial nerves. Non-myelinated: Schwann cell without a myelin sheath; slower; autonomous and somatic systems. (b) Dendrites: short, many, branched, with Nissl's granules; carry impulses towards the cell body. Axon: single, long, no Nissl's granules, branched at the end with synaptic knobs; carries impulses away. (c) Thalamus: wrapped by the cerebrum; coordinating centre for sensory and motor signals. Hypothalamus: at the base of the thalamus; controls temperature, hunger, thirst; secretes hypothalamic hormones. (d) Cerebrum: forebrain, largest part, two hemispheres; intelligence, memory, voluntary actions, sensations. Cerebellum: hindbrain, convoluted; balance and coordination of movements.

(a) The cerebrum (forebrain), especially the cerebral cortex. (b) The hypothalamus, which controls the circadian (24-hour) rhythms of the body.

(a) Afferent (sensory): carry impulses from tissues/organs to the CNS. Efferent (motor): carry impulses from the CNS to muscles and glands. (b) Myelinated: impulse jumps from node to node of Ranvier (saltatory), so it is fast and uses less energy. Unmyelinated: impulse travels continuously along the whole membrane, so it is slower. (c) Cranial nerves: arise from the brain (12 pairs in humans); serve the head and some organs. Spinal nerves: arise from the spinal cord (31 pairs); serve the trunk and limbs.