The nervous system is a single integrated system composed of distinct regions.

A GANGLION is a bead-like swelling along a nerve, and it contains the cell bodies of certain nerve cells or NEURONS.

The autonomic nerve fibers which radiate from the central nervous system to the ganglia of the autonomic nervous system are called preganglionic nerves.

The autonomic nerve fibers that continue on from the ganglia to the organs which they innervate are called postganglionic fibers.

The ganglia of the sympathetic nervous system are mostly located under the vertebral column so that their preganglionic nerves are SHORT and their postganglionic nerves are LONG.

The ganglia of the parasympathetic nervous system are mostly located close to or within the innervated organs so that their preganglionic fibers are LONG and their postganglionic fibers are SHORT.

The peripheral nervous system innervates muscle and skin while the autonomic nervous system innervates glands and viscera.


This image shows a side view of a pig brain, with the skull split longitudinally. The dark area was caused by the placement of an electrode in an experiment undertaken to find ways to improve animal slaughter methods.Notice the cerebellum at the bottom right corner. It is darker in color and like a small version of the cerebellum.

The surface of the brain is covered by a delicate membrane (pia mater) that carries a network of small blood vessels supplying the brain. The pia mater is covered by another thin membrane called the arachnoid membrane. On top of this membrane is a layer of tough tissue, the dura mater, that adheres to the inner surface of the skull. In the image above, you can see that these membranes are being peeled off the surface of the brain towards the bottom right corner. In the image below, the dark ruler is showing the dura mater still adhering to the inside of the cranial cavity.

The surface of the brain in meat animals is increased in area by folds and grooves called, respectively, (gyri) and (sulci).

The cerebrum is composed of left and right cerebral hemispheres separated by a deep fissure. If the brain is exposed by cutting through the skull with a band saw, the outer layers of the brain appear gray in color while the inner parts appear white. The gray areas are dominated by nerve cell bodies while the white areas are dominated by axons. Axons are cable-like extensions of the nerve cell body or (perikaryon), and they are electrically insulated by a sheath of myelin.

The gyri and sulci on the surface of the brain allow large numbers of nerve cell bodies to connect with the bundles of axons which carry information within the central nervous system.

The function of the cerebrum is to regulate higher forms of nervous activity such as recognition, learning, communication and behavior.

The cerebellum is posterior to the cerebrum, and is formed from a middle lobe called the vermis and two lateral hemispheres. The cerebellum coordinates muscle movements during locomotion and in the maintenance of posture. Its internal architecture shown below matches up to the gyri on its surface.

At higher magnification, some of the largest neurons are visible.

The thalamus is the region of the brain located ventrally to the cerebrum. It links the cerebrum to the rest of the central nervous system. The hypothalamus is located ventrally to the thalamus and connects the major regulatory gland of the endocrine system, the pituitary, to the brain.

The most posterior region of the brain, where it tapers down to the diameter of the spinal cord, is called the medulla oblongata. This region controls the heart rate via elements of the autonomic nervous system described earlier.

Cavities or ventricles filled with cerebrospinal fluid run through the brain and extend down the spinal cord as a small canal.

Spinal cord

The spinal cord is an extension of the brain. It emerges from the skull through the foramen magnum, and extends posteriorly to the first (cattle and sheep) or third (pigs) sacral vertebrae in the sirloin region.

At regular intervals, pairs of dorsal and ventral roots enter and leave the spinal cord. Dorsal and ventral roots unite outside the spinal cord to form the nerves of the peripheral nervous system. The nerves that are visible in meat are composed of very large numbers of microscopic axons.

Sensory neurons with their cell bodies in the dorsal root ganglia at the side of the spinal cord carry incoming sensory information from the skin, muscles and tendons. Sensory axons terminate on a variety of different types of neurons in the spinal cord. These neurons may relay information up the spinal cord to the brain, or to other regions of the spinal cord, or to nearby motor neurons.

Motor neurons have their cell bodies in the ventral horns of the central gray region of the spinal cord. In a simple spinal reflex, motor neurons are activated via the dorsal root by axons from sensory neurons). Motor axons leave the spinal cord in ventral roots and innervate muscle fibers in the carcass muscles. Motor axons do not branch much before they reach their muscles but, once inside a muscle, they branch extensively to innervate a large group of muscle fibers called a motor unit. The synapse at which a terminal branch of an axon connects to a muscle fiber is called a MOTOR END PLATE.

Myelinated axons which ascend and descend the spinal cord are located outside the central gray areas so that, unlike the situation in the brain, the white matter is placed outside the gray matter.

The spinal cord is located within the vertebral column. It is separated from the inner bony surfaces of the vertebrae by an epidural space.


Many different types of neurons contribute to the complex circuitry of the central nervous system. Two relatively simple types of neurons are sensory neurons, with their cell bodies located part-way along their axons outside the spinal cord, and motor neurons, with their cell bodies located within the spinal cord. Dendrites are small root-like branches that provide the input to motor neuron cell bodies and to the axons of sensory neurons.

Information is communicated along axons by waves of ionic activity called action potentials. Information is communicated between neurons by the release and reception of chemical transmitters. A synapse is a junction between two neurons, or between a neuron and a muscle fiber.

Action potentials

The cell bodies, dendrites and axons of a neuron are bounded by a cell membrane that is able to pump sodium ions outwards. This allows the concentration of potassium ions to build up inside the neuron. Because of the unequal distribution of these and other ions, the neuronal membrane carries an electrical charge of 50 to 70 millivolts, with the negative charge on the inner face of the membrane. If the membrane is briefly short circuited by a change in its ionic permeability, sodium ions rush inwards and potassium ions rush outwards for a brief instant. This rapid movement of ions short circuits an adjacent region of the membrane so that the cycle is propogated along the membrane. This self-propogating ionic and electrical change is called an action potential. Once an action potential has passed a region of a membrane, an equilibrium is restored ready for the next action potential. During this brief restoration period, called the refractory period, the membrane does not respond to any further stimuli. Action potentials normally are carried in only one direction, away from the origin of the action potential. All action potentials are identical once they are underway, and information is coded by their number and frequency pattern.