Results/Conclusion -- Nervous Tissue

General Function: Nervous tissue forms a network for communication in the body by conducting signals from one part of the body to another. This involves three processes; sensory input, the conversion of signals from sensory receptors all over the body to electrical signals that are sent to integration centers; integration, the interpretation of those signals and the production of a plan of action; and motor output, the relay of integrated data back to the parts of the body that carry them out. These processes allow the body to sense and respond to stimuli and function as a coordinated whole.

General Characteristics: Nervous tissue consists of two types of cells; neurons, which actually carry the signals around the body, and the far more numerous supporting cells that insulate and protect the neurons. Each of these cells falls into either the Central Nervous System (CNS), which is responsible for integration and in vertebrates consists of the brain and spinal cord, or Peripheral Nervous System (PNS), which conveys signals in and out of the CNS and around the body. Conveying signals from sensory receptors in the PNS to the CNS are sensory neurons; integrating that information are interneurons entirely within the CNS. Conveying those signals back out from CNS to effectors are motor neurons. In structure, a typical neuron has three main parts. The cell body houses the nucleus and most organelles. The dendrite, an extension of the cell, conveys signals from another cell, usually a sensory neuron or interneuron, inward to the cell body. Most cells have many dendrites, each one short and branched. The axon, another type of extension, conveys signals from the cell body outwards. Usually a neuron has only one very long axon, insulated by many supporting Schwann cells, together forming a myelin sheath. The sheath covers the axon along its length with small spaces between each Schwann cell (called nodes of Ranvier) where signals are transmitted. When transmitted, the electrical nerve signal jumps over each Schwann cell, allowing it to travel up to 30 times faster than if it had to travel the length of the entire neuron. The axon then branches out into several synaptic knobs, where nerve signals can be relayed to another neuron or effector.


Spinal Cord
Function: The nervous tissue of the spinal cord receives information from skin and muscle sensory receptors and sends out movement instructions. It governs the autonomic nervous system, and generally is responsible for sending out sympathetic signals (signals that prime the body for intense, energy-consuming activities), though some parasympathetic neurons (neurons that work in preparing the body for digesting food and rest) originate in the lower part of the spinal cord.

Characteristics: The spinal cord, about 43-45 cm long (Neuroscience for Kids, accessed 4/7/02, http://faculty.washington.edu/chudler/spinal.html), is hollow; the channel going through its center is called the central canal. Like the ventricles of the brain, cerebrospinal fluid circulates through the central canal, supplying the spinal cord with nutrients, leukocytes and hormones. In addition, the cerebrospinal fluid and surrounding layers of connective tissue cushion the spinal cord. Numerous spinal nerves take information to and from it.

Location: Inside the vertebral column


Motor Neuron
Function: The motor neuron conducts signals from the brain or spinal cord to effectors.

Characteristics: The motor neuron is made of one large, dark cell body, with one axon, and many dendrites. All over the slide, the dendrites are receiving dozens of signals from many other neurons via their synaptic knobs. The point where these knobs meet the dendrites is called a synapse (the many dark dots on the slide). Synapses can be either chemical or electrical; chemical ones involve the sending of a chemical signala neurotransmitteracross a small cleft between the sending synaptic knob and the receiving dendrite.

Location: The PNS, attached to every effector.


Spinal Nerve
Function: Spinal nerves carry signals to and from the spinal cord. They are part of the PNS. They form reflex arcs that are responsible for our reflexes, autonomic reactions to a stimulus without first having conscious sensation (e.g. withdrawing your hand from a hot pot before you realize how hot it is).

Characteristics: Spinal nerves are made up of pairs of motor and sensory neurons that connect to the spinal cord.

Location: Branches of the spinal nerves are in the muscles and skin of the arms and legs


Monkey Nerve
Function: Nerves carry signals to and from the brain and spinal cord, and make up the majority of the PNS.

Characteristics: A nerve, in a monkey or in any other animal, is a bundle of neuron extensions wrapped in connective tissue. (Biology Concepts and Connections; Campbell, Mitchell and Reece, Third Edition; Addison Wesley Longman, Inc., San Francisco; 2000. pg. 564)


Brain
Function: The brain integrates information coming from sensory receptors and formulates a response; maintains homeostasis; is responsible for memory and learning, as well as emotion and perception.

Characteristics: The neurons in the brain are organized with white matter made up of mainly neuron extensions and gray matter made up of mainly cell bodies. The gray matter is the cerebral cortex, and thus is used for more complex brain functions, such as language, reasoning, and mathematical ability. Cranial nerves, made up of both sensory and motor neurons, take signals in or out of the brain.

Location: Inside the skull.