Zoology Notes: 017 Chapter 13

Chapter 13. The Nervous System The nervous system is a rapid communication system that interacts continuously with the endocrine system in the control and coordination of the body functions. It is essential to sight and hearing, our perception of pain and pleasure, control of movements, regulation of body functions such as digestion and breathing and the development of thought, language, memory and decision making. The Neuron. The neuron, the basic structure of the nervous system, is a specialized cell designed to conduct self-propagating impulses to other cells. They can be: 1. Afferent or sensory - transmit impulses received by receptors 2. Efferent or motor - carry outgoing signals from the brain or spinal cord to muscles or glands. 3. Interneurons - neither sensory nor motor, but it connects neurons with other neurons- it received signals from the sensory then send it to either the motor or to other interneurons. Neurons have three parts: dendrites, a body and an axon. Dendrites are the nerve cell’s receptive apparatus. It receives information from several sources at the same time. It’s either excitatory or inhibitory. The body connects the dendrites to the axon and contains the nucleus. The axon carries impulses away from the cell.

Fig. 13.1. The neuron.

A bundle of elongated axon belonging to hundreds and thousand of neurons forms the nerves. Nerves are connected to receptors (in the skin and other sense organs) whose function to convert some environmental stimuli into nerve impulses. These impulses travel along the nerves to the brain where information is analyzed and reactions are determined. The brain then sends impulses (through motor neurons) to muscles, organs, glands, etc. so that these organs respond to the environmental stimuli, all of these happening within an instant. Ganglions are groups of neuronal cell bodies found outside the brain and spinal cord. Glial cells are non-neural cells whose function is to hold the neuron in place. Division of the Nervous System The central nervous system, or CNS, includes all neurons in the brain enclosed in the cranium and the spinal cord enclosed in the spinal column. The peripheral nervous system, or PNS, consists of nerves connecting the brain and spinal cord to other part of the body. The PNS includes 12 pairs of cranial nerves, 31 pairs of spinal nerves, and the somatic system which carries messages to and from the sense receptors, muscles and the body surface. The autonomic nervous system, or ANS, connects with the internal organs and glands. It is divided into the parasympathetic system, which consists of motor neurons and active during resting conditions, and the sympathetic system, which is active during physical activity and stressful conditions. The Central Nervous System The Brain. The three developmental divisions of the brain are the: 64

1. 2. 3.

Forebrain (prosencephalon) - located in the front or anterior part of the brain; includes the cerebrum and the diencephalon (thalamus and hypothalamus) Midbrain (mesencephalon) - located at the middle Hindbrain (rhombencephalon) - includes all structures or part located in the hind or posterior; closest to the spinal cord; includes the pons, medulla oblongata, and cerebellum

Fig. 13.2. The different parts of the brain.

The Cerebrum. The cerebrum accounts for seven-eights of the brain’s weight. It is responsible for discriminatory identification of and integration of sensory information, memory, reasoning, use of language, emotional behavior, and initiation of movement. The surface layer of gray matter (cerebral cortex) is greatly expanded by convolutions called gyri. A longitudinal fissure divides the cerebrum into two hemispheres, each divided for convenience into four major lobes bearing names of overlying bones of the skull: frontal, parietal, occipital, and temporal. The frontal lobe contains areas for initiating movement. The parietal lobe contains areas for perception of somesthetic sensation (tactile, temperature, pain, and kinesthesia). The temporal lobe, areas for the perception of sound, and the occipital lobe, areas for the perception of visual sensations. Association areas adjacent to sensory areas and spread throughout the cortex correlate data and relate past and present data to give it significance. Thalamus. The thalamus is composed of two eggs-shaped masses of gray matter just above the midbrain. It acts as a relay station for sensory perception (except olfactory). It controls sleep and wakefulness. It is also responsible for crude awareness of sensation (protopathic sensibility) and acts as a relay and integration center for emotional behavior. Hypothalamus. The hypothalamus is a small structure located just beneath the thalamus. It regulates eating, drinking, and sexual behaviors. It also regulates endocrine activities and maintains homeostasis (normal level of functioning that a normal organism should have). Midbrain. The midbrain is composed of four rounded masses and includes the tectum, which contain nuclei that serve as centers for visual and auditory reflexes. Pons. The pons lies anterior to the cerebellum between the midbrain and the medulla. It consists of thick bundles of fibers that carry impulses from one side of the cerebellum to other and to higher brain centers.

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Medulla Oblongata. This is continuous with the spinal cord through the foramen magnum. It is the structure that controls breathing, some reflexes including swallowing, vomiting, coughing and sneezing, and upright posture. Cerebellum. The cerebellum is attached at the rear of the brain stem, just above the medulla. Like the cerebrum, the cerebellum has a surface layer of gray matter. It is primarily concerned with the coordination of movement. Spinal Cord. The spinal cord extends from the foramen magnum to the second lumbar vertebrae. It has a central H-shaped core of gray matter surrounded by white matter.

The Peripheral Nervous System Spinal Nerves. The spinal nerves, 31 pairs in all, are distributed along the spinal column in the following manner: 8 pairs in the cervical vertebrae; 12 pairs in the thoracic vertebrae; 5 pairs in the lumbar vertebrae; 5 pairs in the sacral vertebrae; and 1 pair in the coccygeal vertebrae. Cranial Nerves. Cranial Nerve I II III IV V VI

Nerve

Function

Olfactory Optic Oculomotor Trochlear Trigeminal Abducens

Sensory Sensory Motor Motor Mixed Motor

Cranial Nerve Function Nerve VII Facial Mixed VIII Acoustic Mixed IX Glossopharyngeal Mixed X Vagus Mixed XI Spinal Accessory Motor XII Hypoglossal Motor

Table. 13.1. List of cranial nerves and their functions.

Reflexes Reflexes are automatic responses to specific stimuli that are not under conscious control. They provide the body with protective and quick response especially to harmful stimuli. They are important because, being very fast and more/less automatic, they save time and mental energy, when prompt and oftentimes life saving action is required. Muscle Reflexes. Vibrations in the tendons move them. Ex. knee jerks. Orientation Reflexes. These control our speed and our posture. Ex. When we slip on a wet pavement, our body twists to the right and our hands shoot out to stop the fall. Bodily Function Reflexes. These are unconscious control over the muscle but eventually can be controlled. Ex. An infant cannot control urination but as he grows older he can suppress it. Behavioral Reflexes. These reflexes are used to prepare our body’s responses during extreme situations. Ex. “fight or flight” reflexes wherein a pattern of reflex response are produced in answer to a threatening situation. Conditioned Reflexes. These are brought on by a stimulus different from that which first produced it and become attached to a different stimulus. Ex. Ringing a bell each time you feed a dog will produce a similar response in the dog every time you ring a bell.

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