Neural Control and Coordination

Neural system and coordination

Coordination is the process where two or more organs work together and support each other. When you run, your muscles need more oxygen, so breathing and heart rate go up at the same time. The neural system and the endocrine system together do this. Nerves give fast, point-to-point control; hormones give slower, longer-lasting control.

The neural system is made of special cells called neurons that can sense, pass on and process signals. In very simple animals like Hydra, it is just a net of neurons. Insects have a brain and many ganglia. Vertebrates have the most developed system.

Neuron: structure and types

A neuron is a nerve cell with three main parts:

• Cell body (cyton): has the nucleus, cytoplasm and granules called Nissl's granules.
• Dendrites: short, branched fibres that bring signals toward the cell body. They also have Nissl's granules.
• Axon: one long fibre that carries signals away from the cell body. Its end branches into knobs called synaptic knobs, which hold vesicles of chemicals called neurotransmitters.

Types by number of processes

• Multipolar: one axon, two or more dendrites (cerebral cortex).
• Bipolar: one axon, one dendrite (retina of the eye).
• Unipolar: cell body with one axon only (embryonic stage).

Myelinated and non-myelinated fibres

• Myelinated: the axon is wrapped by Schwann cells forming a myelin sheath, with gaps called nodes of Ranvier. Found in spinal and cranial nerves. Signals jump from node to node, so they travel fast.
• Non-myelinated: a Schwann cell covers the axon but forms no myelin sheath. Common in the autonomic and somatic systems. Slower.

A nerve is a bundle of many axons wrapped in connective tissue.

Central, peripheral and visceral nervous system

• Central nervous system (CNS): the brain and spinal cord. It processes information and gives commands.
• Peripheral nervous system (PNS): all nerves that link the CNS to the body. Two kinds of fibres:
  • Afferent (sensory): carry signals from tissues and organs to the CNS.
  • Efferent (motor): carry commands from the CNS to muscles and glands.

Divisions of the PNS

• Somatic neural system: carries signals from the CNS to skeletal muscles (voluntary actions).
• Autonomic neural system: carries signals from the CNS to involuntary organs and smooth muscles (heart, gut, glands). It has two parts:
  • Sympathetic: prepares the body for action: faster heart, wider pupils, more blood sugar.
  • Parasympathetic: calms the body: slower heart, more digestion.

The visceral nervous system is the part of the PNS made of the nerves, fibres, ganglia and plexuses that carry signals between the CNS and the internal organs (viscera).

Brain: the control centre

The brain sits in the skull, covered by three layers called cranial meninges: outer dura mater, thin middle arachnoid, and inner pia mater touching the brain.

• Forebrain: Cerebrum (two hemispheres joined by the corpus callosum; its grey outer layer, the cerebral cortex, has sensory, motor and association areas for memory, thinking and speech). Thalamus (relay centre for sensory and motor signals). Hypothalamus (controls body temperature, hunger, thirst; makes hormones). The limbic system (with hypothalamus and parts like the amygdala and hippocampus) handles emotions, motivation and sexual behaviour.
• Midbrain: between thalamus and pons; corpora quadrigemina (four lobes) help with sight and hearing reflexes.
• Hindbrain: Pons (links different parts of the brain), cerebellum (balance and smooth movements), medulla (breathing, heart reflexes, gastric secretion). Midbrain + pons + medulla = brain stem, which joins the brain to the spinal cord.

Generation of a nerve impulse

Neurons can be excited because their membrane is polarised, that is, it has different charges on its two sides.

Resting potential

• At rest, the axon membrane lets K⁺ through easily but is almost closed to Na⁺. It is also closed to the negatively charged proteins inside.
• So inside the axon there is a lot of K⁺ and negative protein and little Na⁺; outside there is a lot of Na⁺ and little K⁺.
• The sodium-potassium pump uses ATP to push 3 Na⁺ out for every 2 K⁺ in, keeping this difference.
• Result: outside is positive, inside is negative. This electrical difference at rest is the resting potential (about −70 mV).

Action potential

• When a stimulus reaches a spot, the membrane there becomes freely permeable to Na⁺. Na⁺ rushes in.
• The polarity at that spot reverses: inside becomes positive, outside negative. The membrane is now depolarised. This potential is the action potential or nerve impulse.
• Very soon, Na⁺ gates close and K⁺ gates open. K⁺ moves out and the spot returns to rest (repolarisation). The fibre is then ready for the next impulse.

Conduction of the nerve impulse

At the excited spot (A) the inside is +; just ahead (B) the inside is still −. So a small current flows inside the axon from A to B, and outside from B to A. This current depolarises B, which then excites the spot after it. In this way the impulse runs along the axon in one direction, like a row of falling dominoes.

In myelinated fibres the impulse jumps from one node of Ranvier to the next (saltatory conduction), so it is much faster. Wider axons also conduct faster.

Transmission of impulses across a synapse

A synapse is the junction between a pre-synaptic neuron and a post-synaptic neuron. There may or may not be a gap, the synaptic cleft, between them.

• Electrical synapse: the membranes of the two neurons are very close. Current flows directly across. Very fast, but rare in our body.
• Chemical synapse: a fluid-filled cleft separates the neurons. When an impulse reaches the axon terminal, vesicles move to the membrane and release a neurotransmitter (like acetylcholine) into the cleft. It binds to receptors on the post-synaptic membrane, opening ion channels. This can create a new action potential (excitatory) or make one harder (inhibitory).

Chemical synapses let a signal pass in one direction only, from the axon of one neuron to the dendrite or cell body of the next.

Key formulas and definitions

• Resting potential ≈ −70 mV (inside negative)
• Na⁺/K⁺ pump: 3 Na⁺ out, 2 K⁺ in per ATP
• Time for impulse = distance ÷ speed
• Signal path: dendrite → cell body → axon → synapse → next neuron
• CNS = brain + spinal cord; PNS = somatic + autonomic (sympathetic + parasympathetic)

Worked examples

Common mistakes

• Saying the inside of a resting axon is positive. At rest, inside is negative and outside is positive.
• Thinking K⁺ rushing in causes the action potential. It is Na⁺ rushing in; K⁺ moving out brings the membrane back to rest.
• Believing signals can cross a chemical synapse in both directions. They go only from the pre-synaptic to the post-synaptic neuron.
• Calling the autonomic system part of the CNS. It belongs to the PNS.