Neural Control and Coordination

The brain is the central information processing organ of the body. It is protected by the skull and covered by three cranial meninges: dura mater (outer), arachnoid (middle), and pia mater (inner). It has three major parts: forebrain, midbrain, and hindbrain. The forebrain includes the cerebrum, thalamus, and hypothalamus; the midbrain lies between the forebrain and hindbrain; and the hindbrain includes the pons, cerebellum, and medulla oblongata.

The CNS consists of the brain and spinal cord and is the site of information processing and control. The PNS consists of all the nerves associated with the CNS.

Differences:
- CNS: brain and spinal cord; controls and coordinates body activities.
- PNS: nerves of the body connected to CNS; carries impulses to and from the CNS.
- CNS processes information; PNS acts as the communication network.
- The PNS has afferent fibres and efferent fibres; the CNS does not have this division.

Resting potential is the electrical potential difference across the membrane of a neuron when it is not conducting an impulse. The outer surface is positive and the inner surface is negative.

Action potential is the electrical potential difference developed across the membrane at the stimulated site during an impulse. Here the polarity is reversed: the outer surface becomes negative and the inner surface becomes positive.

Difference:
- Resting potential: membrane is polarised; no impulse is being conducted.
- Action potential: membrane is depolarised at the excited site; this is the nerve impulse.

When a neuron is at rest, its membrane is in a polarised state. This happens because:
- the membrane is more permeable to **K$^{+}$ than to Na$^{+}$,
- the membrane is almost impermeable to Na$^{+}$ and negatively charged proteins,
- the sodium-potassium pump actively transports 3 Na$^{+}$ out of the cell and 2 K$^{+}$ into the cell.

As a result, the outer surface becomes positively charged and the inner surface becomes negatively charged. The electrical potential difference so produced is called the resting potential**.

When a stimulus is applied to a polarised membrane, the membrane at that site becomes freely permeable to **Na$^{+}$. This causes a rapid influx of Na$^{+}$ into the axon.

Because of this:
- the inner side becomes positively charged,
- the outer side becomes negatively charged,
- the polarity is reversed.

This reversal of polarity is called depolarisation, and the electrical potential difference at that site is the action potential** or nerve impulse.

At a chemical synapse, the membranes of the pre-synaptic and post-synaptic neurons are separated by a synaptic cleft. When an impulse reaches the axon terminal:
1. Synaptic vesicles move to the terminal membrane.
2. They fuse with the membrane and release neurotransmitters into the synaptic cleft.
3. The neurotransmitters diffuse across the cleft.
4. They bind to specific receptors on the post-synaptic membrane.
5. This opens ion channels and produces a new potential in the post-synaptic neuron.

The new potential may be excitatory or inhibitory.

Neural coordination is the coordination and integration of body functions by the neural system. It provides a fast, point-to-point network of communication through neurons.

It helps coordinate the activities of different organs so they work in a synchronised manner. For example, during physical exercise, the neural system helps coordinate increased respiration, heart rate, blood flow, and later their return to normal. The neural and endocrine systems together maintain homeostasis.

The forebrain consists of the cerebrum, thalamus, and hypothalamus.

- The cerebrum is the major part of the brain and has two cerebral hemispheres connected by the corpus callosum.
- Its outer layer, the cerebral cortex, is the grey matter and contains motor areas, sensory areas, and association areas.
- The thalamus is a major coordinating centre for sensory and motor signalling.
- The hypothalamus controls body temperature, hunger, thirst, and contains neurosecretory cells that secrete hypothalamic hormones.

The forebrain is also involved in memory, intelligence, emotions, and complex behaviour.

The midbrain lies between the thalamus/hypothalamus of the forebrain and the pons of the hindbrain. A canal called the cerebral aqueduct passes through it. Its dorsal part has four rounded swellings called the corpora quadrigemina.

The hindbrain comprises the pons, cerebellum, and medulla oblongata.

- The pons contains fibre tracts that connect different regions of the brain.
- The cerebellum has a highly folded surface and provides space for many neurons.
- The medulla connects to the spinal cord and contains centres controlling respiration, cardiovascular reflexes, and gastric secretions.

Together with the midbrain, it forms the brain stem.