Nervous System Overview

The knowledge of the structure and functioning of the nervous system can be very relevant to a forensic examination that seeks to determine the cause of an illness or death. For example, in cases where suspected drugs or toxins may have been used, a forensic scientist may be able to determine what compounds were used by the symptoms produced. Drugs including barbiturates can slow or cripple the transmission of nerve impulses, while amphetamines stimulate the nervous system by causing the excessive release of norepinephrine, which is involved in the transmission of the nerve impulses. The toxin produced by the bacterium Clostridium botulinum inhibits nerve transmission by binding to sites at the junction between adjacent nerves.

The nervous system is responsible for short-term immediate control of the human body and for communication between various body systems. Although the endocrine system achieves long-term communication and control via chemical (hormonal) mechanisms, the nervous system relies on a faster method of alternating chemical and electrical transmission of signals and commands through a network of specialized neural cells (neurons).

There are three differing types of neurons, including sensory neurons, neurons associated with transmission of impulses, and effecter neurons such as motor neurons that transmit nerve impulses to specialized tissues (e.g., motor neurons to muscle tissue) and glands. In addition to neurons, there are a number of cell types that play a supportive role in the nervous system. Principal among these neuron-supporting cells are Schwann cells, which are associated with an insulating myelin sheath that wraps around specific types of neural fibers or tracts.

Neurons contain key common components. At one end, the dendrite end, specialized cell processes and molecular receptor sites bind neurotransmitters released by other neurons and sensory organs across a gap known as the neural synapse. At the dendrite, the nerve impulse within a particular neuron is generated by a series of chemical and electrical events associated with the binding of specific neurotransmitters. The nerve impulse then travels down the neuron cell body, the axon, via an electrical action potential that results from rapid ion movements across the neuron's outer cell membrane. Ultimately, the action potential reaches the presynaptic terminus region where the electrical action potential causes the release of cell specific neurotransmitters that diffuse across the synapse (the gap between neurons) to start the impulse generation and conduction sequence in the next neuron in the neural pathway. The major chemical neurotransmitters include acetylcholine, norepinephrine, dopamine, and serotonin.

Neural transmission and the diffusion of neurotransmitters across the synapse do not always produce a subsequent action potential without the combined input of other neurons in a process termed summation. Depending on the specific neurotransmitters, receptor binding can produce either excitation or inhibition of action potential production. Subject to a refractory period, during which a neuron returns to its normal state following the production of an earlier action potential, once the neuron reaches a properly timed threshold stimulus, it will produce an action potential. The production of action potentials is an "all or none" process and once produced the axon potential (nerve impulse) sweeps down the axon.

The nervous system is organized along morphological (structural) and functional lines. Structurally, the nervous system can be divided into the central nervous system (CNS) that includes the brain and spinal cord, and the peripheral nervous system (PNS) that contains all other nerves (e.g., sensory and motor neurons), ganglia, and associated cells.

The CNS is protected by a tri-fold layer of specialized membranes, termed the meninges. The brain and spine are organizationally reversed. The spinal cord contains gray matter tracts surrounded by white matter. In contrast, the brain contains centralized white matter.

Functionally, the nervous system can be divided into the somatic or voluntary nervous system (VNS), which coordinates voluntary muscles and reflexes, and the autonomic nervous system (ANS), which is associated with the regulation of viscera, smooth muscle, and cardiac muscle. The autonomic nervous system is further subdivided into sympathetic and parasympathetic systems.

The sympathetic nervous system (SNS), when related to the classic "fight or flight" response, heightens activity in bodily organs or systems (e.g., the respiratory system) and the metabolic rate (the rate at which energy is consumed by bodily processes such as respiration). In contrast, the parasympathetic nervous system (PNS) lowers response and decreases the metabolic rate. The sympathetic and parasympathetic systems work in opposition to control bodily systems.

The brain is divided into various areas or lobes. The large left and right anterior lobes represent the convoluted (wrinkled) cerebral cortex or cerebrum. Posterior lobes represent the cerebellum. At the top of the spinal cord lie the pons and medulla. The cerebellum, pons, and medulla together are referred to as the hindbrain and are associated with many basic process involved in body maintenance, metabolism (e.g., breathing and heart rate), and homeostasis. In general, the forebrain (the cerebrum and some related areas) is the area responsible for higher intellectual functions involved in sensory interpretations, memory, language, and learning. The midbrain tract acts as a switching system that directs, coordinates, and integrates impulses among various regions of the brain.

Within the peripheral nervous system, mechanoreceptors, most of which are located in the skin (integumentary system), respond to physical stimuli such as pressure and motion. Thermoreceptors are specialized to respond to changes in temperature. Chemoreceptors associated with taste and smell senses respond to specific molecules. Highly developed complex sensory structure such as the eyes and ears respond to light (electromagnetic radiation) and sound.

In addition to a complex network of nerves throughout the body that act as a transmission system, the PNS contains specialized nerve cells to interface and transmit signals to muscles and glands.

Nerves usually contain neuron cell bodies that lie in tracts or fibers. Unmyelinated axons form gray matter. When Schwann cells wrap around the axon they create a myelin sheath around neurons (in the peripheral nervous system) that in tracts or fibers are termed white matter. Because the myelin sheath disrupts the normal transmission of the electrical action potential down the neuron, a specialized form of conduction of the nerve impulse or action potential occurs between spaces in the myelin sheath termed the nodes of Ranvier. Accordingly, diseases that disrupt or destroy the myelin sheath (demyelinating diseases) can impair or destroy normal nerve function.

Schwann cells are only one form of neuroglia or glial cells that are required to support normal neural function. Other glial cells include astrocytes, microglia, ependymal cells, oligodendrocytes, and satellite cells. Astrocytes are necessary for the proper vascularization of nerve cells and for the transport of nutrients and the removal of cellular waste products across the blood brain barrier. Microglia cells engage in phagcytosis and are capable of helping defend neural cells from attacks by a range of pathogenic agents. Ependymal cells line brain and spinal ventricles (fluid filled cavities in the brain and spine) and produce and maintain cerebrospinal fluid. Oligodendrocytes are responsible for the production of the myelin sheath in the CNS. Satellite cells protect neurons in ganglia.

see also Amphetamines; Barbiturates; Botulinum toxin; Epilepsy; Neurotransmitters; Psychotropic drugs; Toxicology.