Nervous system

The nervous system consists of two closely interacting systems, specialized for the reception, integration and transmission of information. On the one hand, the brain and spinal cord constitute the central nervous system (CNS), and on the other hand, the sensory and motor nerve fibres that enter and leave the CNS or are completely outside the CNS form the peripheral nervous system (PNS). The cells of the nervous system comprise neurons, of which there are 1011 _{that are specialized for the rapid transmission of signals, and glial cells or}

neuroglia of which there are 1012 _{that nourish and protect the neurons. The}

nerve fibres of these cells run in the white matter of the CNS or along peripheral nerves. Groups of these nerve fibres that run in a common direction form a compact bundle (nerve, tract, pathway). Neurons can be classified as either motor, sensory, or interneurons. Information is carried from the CNS to organs, glands, and muscles by motor or efferent neurons. Sensory or afferent neurons transmit information from internal organs or from external stimuli to the CNS. Interneurons relay signals between motor and sensory neurons.

Central nervous system

The CNS is the processing unit of the nervous system that receives information from the PNS, processes this and sends information to the PNS. The CNS consists of the brain, which lies within the skull, and the spinal cord, which lies within the vertebral column. Fig. 1.1 shows the brainstem, the cerebellum, the diencephalon and cerebrum, which all together compose the brain. The brainstem links the spinal cord and the cerebrum and is composed of the medulla oblongata, pons and the midbrain. The cerebellum is attached to the brainstem. The diencephalon consists of the thalamus, subthalamus and hypothalamus. The cerebrum comprises the right and left cerebral hemispheres, which are connected by the corpus callosum.

The spinal cord has a segmental structure with dorsal and ventral roots. The dorsal roots carry information into the spinal cord from peripheral receptors, while the ventral roots carry information out to muscles and glands. In cross- section, the spinal cord has a butterfly-shaped area of grey matter and a surrounding zone of white matter. The grey matter comprises sensory and

PHYSIOLOGICAL BACKGROUND 3

Figure 1.1: A lateral view of the brain. From [84].

motor nuclei. In the ventral horn lie the cell bodies of motor neurons that send efferent signals to muscles and glands; in the dorsal horn lie cell bodies of interneurons dealing with the processing of signals that enter in the dorsal root axons. The white matter of the spinal cord consists of tracts of axons that transmit information to and from the brain. Ascending tracts of white matter carry sensory information from the body to the brain, and descending tracts send efferent signals from the brain to the rest of the body.

Peripheral nervous system

The peripheral nervous system consists of afferent and efferent (motor) neurons. These efferent neurons can further be classified according to their targets; the somatic nervous system (SNS) is responsible for the communication with the environment and controls skeletal muscles, while the autonomic nervous system (ANS) consists of all efferent pathways from the CNS to organs, glands, and

various involuntary muscles, such as cardiac and smooth muscles.

The ANS can further be split into the sympathetic and parasympathetic nervous systems. Most of the tissues are innervated by both systems, and usually have opposing effects. In addition, there is a network of nerves in the digestive tract that is controlled by the ANS but can also act independently of the CNS. This is the enteric system.

Figure 1.2: A schematic overview of the organization of the nervous system. From [170].

efferent fibres passing from the CNS to the ganglia are preganglionic fibres which release acetylcholine, i.e. they are cholinergic. The final motor neurons from the ganglia to the tissues are postganglionic fibres.

PHYSIOLOGICAL BACKGROUND 5

Opposing effects of the sympathetic and parasympathetic nervous system

Fig. 1.3 shows the opposing effects and anatomical differences between the sympathetic and parasympathetic nervous systems. Most sympathetic pathways originate in the thoracic and upper lumbar regions of the spinal cord. Sympathetic ganglia are primarily found in two ganglion chains that course along both sides of the vertebral column. The sympathetic preganglionic neurons are short, while the postganglionic neurons are long. The majority of these postganglionic fibres release the neurotransmitter noradrenaline (norepinephrine) and are referred to as adrenergic fibres.

Most of the parasympathetic pathways originate in the brain stem, and some originate in the sacral region. Parasympathetic ganglia are typically located on or near their target organs, and thus the parasympathetic preganglionic fibres are long and the postganglionic fibres are short and release acetylcholine. The major parasympathetic pathway is the vagus nerve (cranial nerve X), which contains about 70% of all parasympathetic fibres. The vagus nerve carries sensory information from organs to the brain as well as parasympathetic output from the brain to internal organs, such as the heart and lungs. The parasympathetic control via the vagus nerve is therefore also called vagal control. The sympathetic nervous system prepares the body for stress or emergency situations (fight-or-flight response). In these kinds of situations, the sympathetic nervous system causes the heart rate to increase, dilation of the bronchioles of the lungs to increase the oxygen intake, and dilation of the blood vessels to increase the blood supply. The digestion receives a low priority. The sympathetic discharge that occurs in a fight-or-flight situation is mediated by the hypothalamus. However, most sympathetic responses are not the fight-or- flight reflexes to which the whole body responds; often only few sympathetic pathways are activated, without activating all of them.

On the other hand, the parasympathetic nervous system controls the rest-and- digest functions. It reduces blood pressure, breathing and heart rates and stimulates the digestion.