The Nervous System

What is the Nervous System?

• The nervous system is the vast network of cells specialized to carry information (in the form of nerve impulses) to and from all parts of the body in order to bring about bodily activity.

Nerve Nets

• The nerve net is the name applied to the cnidarians nervous system.

• Cnidarians are symmetrical and have the simplest nervous system.

• Nerve network conducts signals from sensory cells to

muscle cells. • There is no centralization of the nervous system.

Flatworms

• Some flatworms have a nerve net like Cnidarians but others show more organization.

• Bilateral symmetry has led to paired structures (nerves, muscles, sense organs, brain).

• The nervous system of planarians resembles a ladder.1. two nerve cords w/ganglia ("brain") at end

2. sensory receptors in auricles

3. photoreceptors in eyespots

4. transverse nerves keep movements of 2 sides coordinated

5. rudimentary CNS (brain and nerve cords)

6. rudimentary PNS (transverse nerves)

Segmentation

• Annelids and arthropods have repeating segments and an anterior brain.

• Each segment contains a ganglion; the nerve cord extends through all of the segments.

• The ganglion in each segment controls the muscles of that segment. The brain exerts overall control to coordinate the animal.

Vertebrates

• Vertebrates have complex sense organs and exhibit complex behaviors. These require a complex nervous system. The vertebrate nervous system therefore is complex and is extremely cephalized.

• Vertebrates are segmented and this segmented arrangement can be seen in their nervous systems.

Divisions of the Vertebrate Nervous System

• The central nervous system (CNS) is the brain and spinal cord.

• The peripheral nervous system (PNS) is the nerves and ganglia. (Ganglia are clusters of nerve cell bodies outside the CNS.)

Peripheral Nervous System

NERVES• Nerves are bundles of neurons; either long dendrites

and/or long axons.• There are no cell bodies in nerves. The cell bodies are in

the ganglia (PNS) or nuclei (in gray matter of the CNS).• Most nerves contain both kinds of neurons (sensory and

motor). The sensory neurons conduct information to the CNS, the motor neurons conduct away from the CNS.

• All of the neurons in some nerves conduct in the same direction.   These nerves contain either sensory or motor neurons.

Cranial Nerves and Spinal Nerves

• Humans have 12 pairs of cranial nerves and 31 pairs of spinal nerves.

• Cranial nerves are sensory, motor, or mixed, and all but the vagus are involved with the head and neck region;

the vagus nerve manages the internal organs.• Spinal nerves are connected to the spinal cord by two

branches called roots.

Somatic and Autonomic Nervous System

Somatic• The somatic nervous system provides conscious, voluntary control.• It includes all of the nerves that serve the skeletal muscles and the

exterior sense organs.

Autonomic• This part of the nervous system sends signals to the heart, smooth

muscle, glands, and all internal organs.• It is generally without conscious control.• The autonomic nervous system uses two or more motor neurons:

The cell body of one of the motor neurons is in the CNS. The cell body of the other one is in a ganglion.

Sympathetic and Parasympathetic Nervous System

• The sympathetic nervous system prepares the body to deal with emergency situations. This is often called the "fight or flight" response. Stimulation from sympathetic nerves dilates the pupils, accelerates the heartbeat, increases the breathing rate, and inhibits the digestive tract. The neurotransmitter is norepinephrine (similar to epinephrine [adrenaline], the heart stimulant).

• When there is little stress, the parasympathetic system tends to slow down the overall activity of the body. It causes the pupils to contract, it promotes digestion, and it slows the rate of heartbeat. The neurotransmitter is acetylcholine. The actual rate of stimulus to each organ is determined by the sum of opposing signals from the sympathetic and parasympathetic systems.

Central Nervous System

• The central nervous system is the brain and spinal cord.• It is wrapped in 3 layers of membranes called

meninges. Meningitis is an infection of these coverings.• The brain contains fluid-filled ventricles that are

continuous with the central canal of the cord.