The Nervous System Chapter 36

Objectives Analyze how nerve impulses travel within the nervous system. Recognize the functions of the major parts of the nervous system. Compare voluntary responses and involuntary responses.

Neurons Humans have about 100 billion neurons in their brain alone! While variable in size and shape, all neurons have three parts The basic unit of the nervous system is the neurons.

Neurons have 3 parts Dendrites: branchlike extensions that receive information from another cell and transmit the message to the cell body. The cell body contains the nucleus, mitochondria and other organelles typical of eukaryotic cells. The axon is an extension that conducts messages away from the cell body toward other neurons, muscles, or glands.

Sensory neurons typically have a long dendrite and short axon, and carry messages from the body to the spinal cord and brain. Motor neurons have a long axon and short dendrites and transmit messages from the central nervous system to the muscles or glands. Interneurons are found only in the central nervous system where they connect neuron to neuron.

The axons of one neuron lies next to the dendrite of the next. The junction between them is called a synapse. Impulses traveling to and from the brain must pass across this synaptic space.

As the impulse reaches the end of the axon, calcium channels open allowing calcium to enter the end of the axon. The plasma membrane allows the calcium to be released into the synaptic space via exocytosis. These chemicals are called neurotransmitters. When these neurotransmitters reach the dendrites of the next cell, receptors open to allow the calcium to be absorbed; this changes the polarity in the neuron, initiating a new impulse.

Anatomy of the brain The brain can be split into three parts: Cerebrum: divided into two hemispheres that are connected by bundles of nerves. –Memory, intelligence, language, skeletal muscle movements, senses are controlled by the cerebrum Cerebellum: located at the back of the brain. –Controls balance, posture, and coordination Medulla Oblongata: controls involuntary activities such as breathing and heart rate.

The two frontal lobes act as short-term storage sites for ideas, allowing you to consider more than one idea at a time. One section of the frontal lobes helps control voluntary movement, while a place in the left frontal lobe allows thoughts to be transformed into words.

The parietal lobes interpret sensory information, such as taste, temperature and touch; they also help with reading and math.

Occipital lobes process images from the eyes and link that information with images stored in memory.

The temporal lobes translate information from the ears, including music. The underside of the temporal lobe plays a crucial role in memory.

The hemispheres communicate with each other through a thick tract of nerves, called the corpus callosum, at the base of the fissure ystem/cns/brain/brainviews/tutorial.htmlhttp:// ystem/cns/brain/brainviews/tutorial.html

Peripheral nervous system As the central nervous system consists of the brain and spinal column, the peripheral nervous system consists of all the other nerves that carry messages to and from the CNS.

Somatic nervous system Made up of 12 pairs of cranial nerves from the brain, 31 pairs of spinal cord nerves from the spinal cord and all the branches coming off of them – 31 pairs coming out of each side. These nerves are mainly between the skin, the CNS and the muscles. When you touch something hot, they transmit the sense to the CNS and back to move. It is totally voluntary.

Reflex A reflex occurs when you touch something hot, the reflex impulse travels to the spinal cord which then tells the muscle to react. The brain doesn’t know what happened until after it occurred.

Autonomic nervous system When you are surprised by something – like noises in the night, your breathing becomes rapid and your heartbeat begins to pound. This is an involuntary response between your CNS and your internal organs. There are two divisions called the sympathetic and parasympathetic nervous systems

Sympathetic Division: “fight or flight” subdivision – –Prepares the body to cope with some threat –Results in increased heart rate and blood pressure. –Releases hormones Parasympathetic division is the “house- keeping” system –In control most of the time –Maintains homeostasis by seeing that normal digestion and elimination occur and that energy is conserved

Sympathetic nervous system stimulates heartbeat raises blood pressure dilates the pupils dilates the trachea and bronchi stimulates the conversion of liver glycogen into glucose shunts blood away from the skin and viscera to the skeletal muscles, brain, and heart inhibits peristalsis in the gastrointestinal (GI) tract inhibits contraction of the bladder and rectum In short, stimulation of the sympathetic branch of the autonomic nervous system prepares the body for emergencies: for "fight or flight".

Parasympathetic nervous system This determines what your organs do during times of rest Parasympathetic stimulation causes slowing down of the heartbeat lowering of blood pressure constriction of the pupils increased blood flow to the skin and viscera peristalsis of the GI tract

OrganSympathetic SystemParasympathetic System EyeDilates pupilConstricts pupil Tear glandsNo effectStimulates tear secretion Salivary glandsInhibits saliva productionStimulates saliva production LungsDilates bronchiConstricts bronchi HeartSpeeds up heart rateSlows down heart rate GutInhibits peristalsisStimulates peristalsis LiverStimulates glucose productionStimulates bile production BladderInhibits urinationStimulates urination

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