Nervous System

Invertebrate nervous systems are highly diverse

• Flatworms have a simple “brain” containing many large interneurons that coordinate most nervous functions. Two or more nerve trunks travel posteriorly in a ladderlike system with transverse nerves connecting the main trunks.
• Annelids and arthropods have a well-defined ventral nerve cord and a prominent brain. Often contain ganglia in each body segment to coordinate actions of that segment.
• Cephalopods have the most sophisticated invertebrate nervous system containing a large brain and giant axons.

• The Peripheral Nervous System (PNS)

• Sensory (afferent) nervous system which brings information from sensory receptors to the CNS.
• Motor (efferent) nervous system which carries signals from the CNS to effector cells.

• Control responses to external environment.
• Maintain homeostasis by coordinating internal organ functions.

• The somatic nervous system’s neurons carry signals to skeletal muscles in response to external stimuli; includes reflexes (automatic responses to stimuli) and is often considered “voluntary” since it is subject to conscious control.
• The autonomic nervous system controls primarily “involuntary”, automatic, visceral functions of smooth and cardiac muscles and organs of the gastrointestinal, excretory, cardiovascular and endocrine systems.

• Divided into a parasympathetic division that enhances activities that gain and conserve energy, and a usually antagonistic sympathetic division that increases energy expenditures.

• The Central Nervous System (CNS)

• Consists of the spinal cord, which is located inside the vertebral column and receives information from the skin and muscles and sends out motor commands for movement; and the brain, which carries out complex integration for homeostasis, perception, movement, intellect and emotions.
• Covered with meninges, three protective layers of connective tissue.
• In the brain, white (myelinated) matter is in the inner and gray matter is in the outer regions. This orientation is reversed in the spinal cord.
• Cerebrospinal fluid fills the ventricles in the brain and the central canal of the spinal cord; it functions in circulation of hormones, nutrients and white blood cells and in absorption of shock.

• The patellar (knee-jerk) reflex is one of the simplest and involves only two neurons. A stretch receptor in the quadriceps muscle is timulated by stretching of the patellar tendon; this activates a sensory neuron that carries the information to the spinal cord where it synapses with a motor neuron; if an action potential is generated in the motor neuron, it travels back to the quadriceps which contracts and causes the forward knee jerk.
• Larger-scale, more complex responses result when branches of the reflex pathway carry signals to other parts of the spinal cord or to the brain.

• More complex brains have further subdivisions

• Relative brain size increases in certain evolutionary lineages.
• Increased compartmentalization of function with certain areas of the brain assuming specific responsibilities.
• Increaseing complexity and sophistication of the forebrain; increased complexity of behaviors parallels an increase in growth of the cerebrum.

• It develops at the anterior end of the spinal cord from three primary bulges that later differentiate into distinct structures with specific functions.

• The medulla oblongata and pons control visceral functions like breathing, heart and blood vessel activity, swallowing, vomiting and digestion; also coordinates large-scale body movements like walking.
• The cerebellum functions in balance and coordination of movement.

• The superior and inferior colluculi are areas of the midbrain that function in the visual and auditory systems.
• The reticular formation regulates states of arousal.

• Lower diencephalon that contains two integrating centers, the thalamus and the hypothalamus.
• Upper telencephalon that consists of the cerebrum-complex integrating center in the CNS.

• Contains many different nuclei, each one dedicated to one type of sensory information.
• Sorts incoming sensory information and sends it to appropriate higher brain centers for further interpretation and integration.
• Receives input from the cerebrum and from parts of the brain that regulate emotion and arousal.

• Is the source of releasing hormones of the anterior pituitary and two posterior pituitary hormones.
• Contains the body’s thermostat and centers for regulating hunger and thirst.
• Plays a role in sexual response and mating behavior, the fight-or-flight response, and pleasure.
• Contains the suprachiasmiatic nucleus which uses visual information to synchronize certain bodily functions with the natural cycles of day length and darkness. This biological clock maintains daily bio-rhythms such as when:
• Sleep occurs
• Blood pressure is highest
• Sex drive peaks

• Outer covering of gray matter, the cerebral cortex
• Internal white matter
• Cluster of nuclei deep within the white matter, the basal ganglia which:
• Are centers for motor coordination, relaying impulses from other motor systems.
• Send motor impulses to the muscles
• Degeneration of cells entering basal ganglia occurs in Parkinson’s disease.

• Highly folded with a surface area of about 0.5 square meters.
• Bilaterally symmetrical with two hemispheres connected by a thick band of fibers (white matter) known as the corpus callosum. Each hemisphere is divided into four lobes (frontal, parietal, occipital, temporal); some functional areas within each lobe have been identified.

• In response to sensory stimuli, the motor cortex sends appropriate commands to skeletal muscles.
• The somatosensory cortex receives and partially integrates signals from the body’s touch, pain, pressure, and temperature receptors.

• For example, more brain surface area is committed to sensory and motor communication with the hands than with the entire torso.
• Impulses transmitted from receptors to specific areas of somatosensory cortex enable us to associate pain, touch, pressure, heat or cold with specific parts of the body receiving those stimuli.

• The special senses- vision, hearing, smell and taste- are integrated by cortical regions other than the somatosensory cortex. (Example: vision is processed in a cortical region in the occipital lobes)
• Each of these functional regions, as well as the somatosensory cortex, cooperate with an adjacent association area.

• Arousal and Sleep
• Controlled by several centers in the cerebrum and brainstem; the most important is the reticular formation (group of 90 separate brain nuclei which extend from the medulla to the thalamus).
• Almost all neuron processes which reach the cerebral cortex pass through the reticular formation; serves as a filter that selects what sensory information will reach the cortex.
• Right Brain/Left Brain
• Lateralization (right/left brain) refers to the fact that the association areas of the cerebral cortex are not bilaterally symmetrical; each side of the brain controls different functions.
• The left hemisphere controls speech, lanuguage and calculation.
• The right hemisphere controls artistic ability and spatial perception.
• The corpus callosum transfers information between left & right hemispheres. Severing the corpus callosum will not alter perception, but will dissociate sensory input from spoken response.
• Language and Speech
• Controlled by two areas on the left hemisphere of the cerebral cortex.
• Wernicke’s area stores information required for speech content (arrangement of learned words in a grammatical order).
• Broca’s area contains necessary information for speech production (programs the motor cortex to move tongue, lips and speech muscles)
• Emotions
• Depend on interactions between the cerebral cortex and the limbic system, a group of giant nuclei and interconnecting axon tracts in the forebrain (Figure 48.23 Campbell text).
• Limbic system includes parts of thalamus, hypothalamus, and inner portions of the cerebral cortex, including two nuclei called the amygdala and the hippocampus.
• Cerebral cortex components of the limbic system are linked to the prefrontal cortex (in frontal lobe), which is involved in complex learning, reasoning, and personality, and ability to have “moral” judgment, so there is a close relationship between emotion and thought.
• Frontal lobotomy, used to treat mental illness, is the surgical destruction of the limbic cortex or its connection with the prefrontal cortex.
• Memory
• Ability to store and retrieve information related to previous experiences.
• Occurs in two stages: short-term memory and long-term memory
• Short-term memory reflects immediate sensory perceptions of an object or idea and occurs before the image is stored.
• Long-term memory is stored information that can be recalled at a later time.
• Transfer of information from short-term to long-term memory is enhanced by rehearsal, favorable emotional state, and association of new information with previously learned and stored information.
• Fact memory differs from skill memory.
• Fact memory involves conscious and specific retrieval of data from long-term memory. (Examples: remembering biological term definitions, phone numbers, locker combinations, people’s names, etc.) Fact memory involves a pathway in which sensory information is transmitted from the cerebral cortex to the hippocampus and amygdala which are two parts of the limbic system. Thus, memory is filtered by these parts of the limbic system, labeling information to be saved by tying it to an event or emotion.
• Skill memory usually involves motor activities learned by repetition which are recalled without consciously remembering specific details. (Examples: driving, walking, riding a bicycle, etc.)
• There is no highly localized memory trace in the nervous system; instead, memories are stored in certain association areas of the cortex.