Integration and Control: Nervous Systems

Nervous Systems detect, interpret, and issue calls for response to stimuli on the an animal’s external and internal environments.  They can be as simple as a nerve net or a complex system.  On vertebrates, the brain and spinal cord are the main organs in controlling the body.

 

1. Invertebrate Nervous Systems

A.     Life-Styles and Nervous Systems

1. Nearly all animals have nervous system, form a communication line for detecting stimuli; responding in suitable ways.
2. More complex animal life-style: more elaborate modes of receiving, integrating, and responding.

1. A nervous system includes nerves—sensory axons, motor axons, or both bundled together inside connective tissue.
2. Primitive systems: nerve net—a loose mesh of nerve cells, information flow is not directional.

B.     Regarding the Nerve Net

1. The nerve net in the cnidarians reflects their radially symmetrical bodies.
2. Reflex pathways : simple movements; provide the basic operating machinery of nervous systems such as the nerve net.

C.     On the Importance of Having a Head

1. Flatworms: the simplest animals with bilateral symmetry The ladderlike nervous system includes two longitudinal nerve cords, associated ganglia, and nerves.

1. Some flatworms have a brainlike clump of nervous tissue at the head end of the nerve cords. (cephalization).

2. Perhaps evolved from the nerve net of cnidarian planula larvae. 
3. Cephalization (formation of a head) result of the layering of more and more nervous tissue over reflex pathways of ancient origin.

 

2. 
   

2. Vertebrate Nervous Systems—An Overview

1. Evolutionary High Points
1. Early forms of vertebrates relied on the interaction of notochord and muscles to move.

1. Above notochord, a hollow, tubular nerve cord was evolving also.
2. In early vertebrates, simple reflex pathways predominated.
3. The oldest parts of brain: reflex coordination, other parts: storage of information.  most recent: memory, learning, and reasoning.

2. Neural tube in vertebrate embryos undergoes expansion to form brain and spinal cord and associated nerves; the spinal cord enclosed within vertebral column.
1. Functional Divisions of the Vertebrate Nervous System
1. The central nervous system includes the brain and spinal cord.

1. Communication lines within the brain and spinal cord: tracts; in white matter contain axons with glistening myelin sheaths and specialize in rapid transmission of impulses.
2. Gray matter: unmyelinated axons, dendrites, nerve cell bodies, and neuroglia cells, that protect and support neurons.

2. Peripheral nervous system includes all of the nerves carrying signals to and from the brain and spinal cord.

 

 

 

 

3. The Major Expressways

1. Peripheral Nervous System
1. Somatic and Autonomic Subdivisions

1. The human peripheral system has two types of nerves

1. Spinal nerves (31 pairs) connect with the spinal cord, innervate most areas of the body.
2. Cranial nerves (12 pairs) connect vital organs directly to the brain.

2. Spinal and cranial nerves can be classified on function:

1. Somatic nerves relay sensory information from receptors in the skin and muscles and motor commands to skeletal muscles (voluntary control).
2. The autonomic nerves sends signals to and from smooth muscles, cardiac muscle, and glands (involuntary control).
1. Sympathetic and Parasympathetic Nerves

1. Parasympathetic nerves slow down body activity when body is not under stress.
2. Sympathetic nerves increase overall body activity during times of stress, excitement, or danger; call on the hormone epinephrine to increase the fight-flight response.

2. The Spinal Cord
1. Spinal cord: pathway for signal travel between the peripheral nervous system and the brain.

1. The cord is the center for controlling some reflex actions.
2. The spinal cord covered with tough membranes–the meninges–and within the protection of the stacked vertebrae.

2. Signals move up and down the spinal cord in bundles of sheathed axons.

 

 

 

IV.  Functional Divisions of the Vertebrate Brain

1. The brain, the master control panel, is a continuation of the anterior end of the spinal cord, and is also protected by meninges and bones.
1. Forebrain, midbrain, and hindbrain form from three successive portions of the neural tube.
2. Most primitive tissue is brain stem, contains simple, basic reflex centers.
2. Hindbrain
1. Medulla oblongata: respiration, blood circulation, motor response coordination, and sleep/wake responses.
2. Cerebellum : maintaining posture, coordinating limbs.
3. Pons ("bridge") possesses bands of axons that pass between brain centers.
3. Midbrain
1. Midbrain originally coordinated reflex responses to visual input; the tectum still integrates visual and auditory signals in vertebrates (amphibians and reptiles)
2. In mammals it is now mostly a pathway switching center.
4. Forebrain
1. Large olfactory lobes dominated early vertebrate forebrains.
2. Cerebrum integrates sensory input and selected motor responses.
3. Thalamus (below cerebrum) relays and coordinates sensory signals.
4. Hypothalamus monitors internal organs, influences responses to thirst, hunger, sex.
5. The Reticular Formation
1. Reticular formation: mesh of interneurons from the uppermost part of the spinal cord , through the brain stem, and into the cerebral cortex.
2. Serves as a pathway and activates centers in the cerebral cortex.
6. Brain Cavities and Canals
1. Brain, spinal cord are in cerebrospinal fluid in cavities and canals.
2. Cushions vital nervous tissue from sudden movements.
3. Blood-brain barrier operates at the plasma membranes of cells forming the capillaries that service the brain.

1. Tight junctions fuse capillary cells together forcing substances to move through the cells to reach the brain.
2. Membrane transport proteins allow essential nutrients (glucose) to move through but bar wastes (urea) and certain toxins.

5. A Closer Look at the Human Cerebrum

1. Organization of the Cerebral Hemispheres
1. The human cerebrum is divided into left and right cerebral hemispheres.

1. Left hemisphere: speech, math, and analytical skills; the right half: nonverbal skills, such as music.
2. Two halves communicate by nerve tracts (corpus callosum).

2. Cerebral cortex, thin layer of gray matter with the cell bodies of interneurons above the axons, is divided into four lobes:

1. Occipital lobe, which is located in the rear, has centers for vision.
2. Temporal lobe, near each temple: processing center for hearing, emotional behavior.
3. Parietal lobe: somatosensory cortex–the main receiving area for signals from the skin and joints.
4. Frontal lobe includes the motor cortex: motor responses.

2. Connections With the Limbic System
1. Limbic system, (mid. of the cerebral hemispheres), emotions and memory.
2. Distantly related to olfactory lobes, still deals smell related to memory.
3. Connections from cerebral cortex pass through the limbic system allowing correlation of organ activities of self-gratifying behavior, eating and sex.

 

6. Memory

1. Memory is the storage and retrieval of information about previous experiences.
1. Association: linkage of information into larger packages that can be sent to other brain regions for storage.
2. Information stored memory traces–chemical and structural changes in brain regions.

1. Short-term memory: seconds to hours; limited to seven to eight bits of information.
2. Long-term memory is more permanent and seems to be limitless.

3. Retrograde amnesia: lose short-term memory, long-term memory remains.
1. Information is moved into long-term storage with epinephrine, which increases a person’s state of arousal.

7. States of Consciousness

1. States of consciousness (sleeping and aroused) are governed by the central nervous system, can be monitored by electroencephalograms (EEGs).
2. Reticular formation governs changing levels of consciousness via inhibitory and excitatory signals.
3. One sleep centers secretes serotonin, which causes drowsiness and sleep.