WEEKS 13 AND 14 HANDOUT, VERTEBRATE ZOOLOGY: NERVOUS SYSTEM 4/20/00
Text (7th ed.): Ch. 11. Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)
possible web site: http://www.exploratorium.edu/learning_studio/cow_eye/index.html
(step-by-step dissection of cow eye: a good introduction this major vertebrate sensory organ)
neuron (nerve cell): functional unit of the nervous system. Made up of:
cell body: with large nucleus
dendrites: multiple extensions off of the cell body that conduct signals toward it
axon: long single process that conducts signals away from the cell body
All neurons work by sending signals in the form of impulses. The cell membrane of a neuron is the key to the propagation of this signal. At rest, the plasma membrane is polarized; that is, a charge or resting membrane potential is across it. In particular, the sodium-potassium pump (which uses ATP) sets up differing concentrations of Na+ and K+ on the outside vs. the inside of the cell membrane. Combined with the K+ permeability of the membrane (so that K+ leaks out), this creates the membrane potential, or a situation where just inside the cell membrane it's slightly more negative than just outside of the cell membrane. A stimulus changes this resting membrane potential into an action potential via depolarization. Myelin or a sheath made of lipid around the axon helps to conduct the nerve impulse more quickly.
What happens when an action potential reaches the next cell? There is a junction or synapse to the dendrites of the next nerve cell. At a synapse, a nerve impulse will cause the release of a neurotransmitter from the presynaptic cell. This travels across the synaptic cleft, binds with receptors at the postsynaptic site, and starts the depolarization routine again. An enzyme then wipes out the neurotransmitter (or the neurotransmitter is reuptaken by the neuron) for the next time.
The vertebrate nervous system has two main divisions:
central nervous system of brain and spinal cord
peripheral nervous system of everything else, consisting of
sensory (afferent) nerves bringing information toward the central nervous system
motor (efferent) nerves sending out commands from the CNS
We can divide the (involuntary) autonomic nervous system into two categories:
sympathetic nervous system for"fight or flight" responses
parasympathetic nervous system for digestion and other"vegetative" reactions and activities.
The brain and spinal cord are hollow tubes (remember"dorsal, hollow nerve cord" as a chordate feature), which allows for more gray matter (neuron cell bodies) to be packed in and nourished by surrounding CSF (cerebrospinal fluid) . The hollow parts inside the brain are called ventricles.
Brain: the forebrain has two main parts of telencephalon and diencephalon. The telencephalon we are more familiar with as in the cerebrum with its cerebral hemispheres. In particular, the cerebral cortex becomes larger and more folded in on itself as the animal groups become more dependent on learning, e.g., the mammals.
The diencephalon is almost completely surrrounded by the cerebral hemispheres. There, the thalamus is a relay center for all sensory impulses (except smell) to the cerebral cortex. The hypothalamus secretes hormones to control the pituitary and regulates many functions, e.g., temperature, hunger, thirst. The pituitary gland, inferior and attached to the hypothalamus, secretes hormones such as TSH (thyroid-stimulating hormone) and GH (growth hormone), when stimulated by the hypothalamus. Posterior to the hypothalamus is the midbrain, which contains many switching centers, e.g., for visual, auditory systems (your head turning in response to a sudden loud noise is such an example of this visual/auditory interplay at work).
The hindbrain contains the cerebellum and the medulla oblongata. The cerebellum is the second largest part of the human brain, occupying an inferior, posterior position. Basically, the cerebellum assists in coordinating skeletal muscle movements. Finally, the medulla oblongata is elongated and joins with the spinal cord (at the level of the foramen magnum). This is the brainstem that controls many autonomic, involuntary features, e.g., breathing (as we discussed in the control of respiration). Different parts of the vertebrate brain are enlarged depending on the group. The overall trend in the vertebrates is to see more cerebrum and cerebellum, and less of separate lobes, e.g., optic lobe.
Arising out of the brain come 12 pairs of cranial nerves that have important functions ranging from sight to hearing to tongue innervation. The spinal cord connects the brain to the body and is the site for many spinal reflex actions. The spinal cord and brain are covered with membranes called meninges, including the dura, arachnoid, and pia mater. Spinal nerves arise from the spinal cord between successive vertebrae, to innervate skeletal muscle and the overlying skin, as discussed in week 3 and the"typical vertebrate segment."
Reflex arc: There are many situations where an organism does not want to spend too much time thinking about the response to a particular situation, e.g., hand in fire. The spinal cord can handle this situation alone via a speedy spinal reflex. In particular, a reflex arc connects a sensory receptor with an effector such as a muscle. The sensory neuron sends the painful stimulus message to the spinal cord. In the spinal cord, an interneuron synapses between the sensory neuron and a motor neuron, which then stimulates a contraction in an appropriate muscle. Other interneurons send the message up to the brain, so that after the fact one can utter the appropriate response:"ouch."
We are familiar with the five senses: touch, hearing, smell, taste and sight? How are perceived in humans? How are they perceived differently in other vertebrates? In general, what one is trying to achieve with sensory reception is to take the external information and convert it into a nerve impulse. If all action impulses are alike, how do the senses and sensations vary? Sensory receptors, regardless of the sensation, contain cells or neurons that respond to a particular stimulus and turn it into a receptor potential (a local electrical potential, which if it is great enough, will start the action potential routine). This sensory impulse will travel towards the central nervous system for interpretation.
Touch: The lateral line system of fish may sense slight changes in pressure to distinguish the presence of nearby fish. Similarly, in land vertebrates there can be a difference between touch and other similar sensory experiences. For example, in mammals, there are sensors for light vs. deep pressure. Many mammals may also have vibrissae (whiskers) for touch. All mammals will have pain receptors for more noxious stimuli. Temperature or themoreceptors can be modified, as in the pit organ of the rattlesnake.
Hearing: Our hearing organ or ear has two functions, hearing and balance (equilibrium). Sound is the energy of a pressure wave through air, water, etc. Water is denser than air, so that fish do not need the large external ear of say, a fox. Hearing for land vertebrates is more of a problem since air is less dense than water; hence the large eardrum (tympanic membrane) seen in frogs, humans, etc. The sound energy hitting this large surface is then passed on by a series of bones (middle ear ossicles) into the inner ear, where nerve pulses are sent to the brain to interpret the sounds. Balance in all vertebrates is handled similarly, with semicircular canals (for rotation) of the inner ear in X - Y - Z planes connect with the saccule and utricle (for linear acceleration) to sense movement
Smell: Olfactory receptor cells in the roof of the nasal cavity pick up smell, which is well developed in some vertebrates (e.g., many mammals), but poorly developed in others (e.g., most birds). Snakes can also smell the air with their tongue, with the help of an accessory mouth organ (Jacobson's organ).
Taste: Taste buds are chemoreceptors that recognize four taste sensations of sweet, sour, bitter, and salty. While mammalian taste buds are on the tongue, other vertebrates can have their receptors elsewhere in the mouth.
Sight: Sight is very useful for most vertebrates (except cave dwellers). The vertebrate eye is sophisticated, with light traveling through the clear cornea, through the lens (which can focus) to the retina on the back of the eye. The photoreceptors are rod and cone cells. Rod cells pick up dim light, but cone cells pick up bright light and are the ones involved in color.
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