Nervous System & Sensory Receptors
I. Nervous System
Central Nervous System
Limbic System and Higher Mental Functions
Peripheral Nervous System
Drug Abuse
II. Sense and Sensation
Sensory Receptors and Sensations
Proprioceptors and Cutaneous Receptors
Senses of Taste and Smell
Sense of Vision
Sense of Hearing
Sense of Equilibrium
I. THE NERVOUS SYSTEM
The nervous system recieves sensory input by stimuli, processes it through the brain and spinal cord, producing an output of muscle movement or glandular secretion. There are two systems, the central nervous system (CNS) and the peripheral nervous system (PNS). The brain and spinal cord make up the central nervous system and the nerves outside the central nervous system make up the peripheral nervous system.
Nervous tissue consists of two types of cells. Neurons carry "messages" through an electrochemical process known as action potential. And neuroglia support and nourish the neurons. Neurons come in different shapes and sizes. There are three types of neurons, that perform different functions. Sensory neurons receive external stimuli by the sensory receptors which relay impulses going to the CNS. Within the CNS an interneuron integrates all nerve impulses. The third type motor neurons carry impulses from the CNS to an effector, muscle or gland that actualizes the response.
Neurons have a cell body consisting of a nucleus containing genes, cytoplasm, mitochondria and other organelles. A neuron differs from other cells because they have extensions called dendrites and axons. Dendrites bring information from sensory receptors and the axoms conducts nerve impulses away from the cell body.
The myelin sheath is a covering protecting the axom made of Schwann cells a kind of neuroglia.The spaces in sheathing is called nodes of Ranvier. The sheathing is found along long axoms and not on short axoms. The myelin serves as an protector and insulator since it has poor conductivity. Nerve areas that are not covered appear gray and areas that are covered with the sheathing appear white.
Loss of the myelin sheathing can result in the disease of multiple sclerosis an immune system impairment and luekodystrophies caused by a genetic defect.
Nerve impulses carry information within the nervous system by way of an electrochemical process. This is how neurons communicate with each other. The impulse can be measured by a voltmeter and the reading is called potential. There is a resting potential and a action potential. As it's name suggests, resting potential is when there is no impulse and the inside is more negative than the outside. The concentration of sodium ions is greater on the outside of the axon, and the concentration of potassium is greater on the inside. This unequal distribution of ions is maintained by the sodium-potassium pump. The pump works to transport sodium out and potassium into the axon. The polarity is also helped by negatively charged organic ions in the axoplasm. The rapid transmission of info is known as action potential. As this occurs there is a quick change in polarity. During an action potential depolarization is followed by repolarization. There are two types of gated channel proteins that allow an action potential to occur. First the sodium gates open to allow sodium to enter the axon. it depolarizes since the charge on the inside changes from negative to positive. Next the potassium gates open allowing Potassium to leave the axon. As the inside resumes it's negative charge it repolarizes. Nerve impulse can jump from one node to another it is known as saltatory conduction. This happens very quickly and with thick myelinated axons the rate is more than 100m/sec. Because an action potential generates another it is self propagating.
The amount of impulses generated within a fixed time determines it's intensity. A refractory period assures that the impulse passes along its intended path and does not move backwards.
The ends of the axon branch into endings called axon terminals. It is in close distance to a dendrite or cell body. The distance between the two is called a synapse and the gap is called a synaptic cleft. The signal cannot make it across, so neurotransmitters are needed to carry the message across. These molecules diffuse across through the receiving membrane and bind with receptor proteins. Different neurotransmitters produce different responses. The life of neurotransmitters at a synapse is short. This lets cells respond to new signals.
Some neurotransmitters are acetycholine, norepinephrine, dopamine, serotonin, glutamate, and GABA.
Other molecules called neuromodulators block the release of neurotransmitters or modify it's response. Substance P and endorphins are neuromodulators. They are natural painkillers.
The result of incoming signals to produce potential change are summed up in the process called synaptic integration. If the excitatory impulses surpass the inhibitory impulses threshold, action potential can occur. And if the inhibitory outweigh the excitatory the synaptic integration might prohibit the axon from firing.
Central Nervous System
Enclosed in the skull and vertabrae are the brain and spinal cord, the main components of the CNS where information is received. This is also where motor control is initiated. specific functions are performed in different parts of the brain. The spinal cord woks like a communication channel, sending and receiving info from the brain to the body. The brain and spinal cord is protected by a membrane called meninges with spacings of cerebrospinal fluid. Cerebrofluid is found in the center of the spinal cord and in the four ventricles of the brain. Fluid normally drains into the cardiovascular system. If accumulation occurs it can cause pressure.
Two types of nervous tissue are found in the CNS. Cell bodies that are nonmyelinated are known as gray matter and myelinated axons are known and white matter.
The spinal cord is a bundle of nerves that starts at the base of the brain and extends into the vertebral canal. It serves as the main pathway for information from the brain to the PNS. It is protected by the vertebral column. The nerves extend from the spinal cord and out beyond the vertebrae. The spinal cord has gray matter, white matter and a central canal. The central canal and the meninges contain cerebrospinal fluid. Sensory and motor neurons are also found here, and interneurons which communicate with the other two. There are sensory fibers in the dorsal root of the spinal nerve and the ventral root contains motor fibers. The two are joined as a mixed nerve as it leaves the vertebral canal. Spinal nerves are part of the peripheral nervous system.
Within the white matter of the spinal cord information travels to and from the brain. The left side of the brain controls the right side of the body and vise versa.
Different parts of the brain perform specific functions. The largest area known as the cerebrum integrates info which then produces motor responses. The cerebrum is divided by the corpus callosum into halves called the right and left cerebral hemispheres. Grooves known as sulci make distinct the lobe areas such as the frontal, parietal, occipital and temporal. Thus there are four lobes per hemisphere. These are the areas where movement, reasoning, somatic, sensing, hearing, and vision take place. The cerebral cortex is the outer layer of the cerebrum. Wernicke's area and Broca's area are two centers important to our ability to speak and are located in the left cerebral cortex. The diencephalon area in the third ventricle contains the hypothalamus and thalamus. The hypothalamus is important to several functions such as homeostasis and controls the pituitary gland. The thalmus is important to functions such as memory and emotions thus it receives visual, auditory, and somasensory information. The cerebellum is important to posture and balance. It integrates and sends impulses to the muscles. The brain stem contains axon bundles known as pon which link the cerebellum to the central nervous system. Within the brain stem the midbrain works as a relay, the medulla oblongata above the spinal cord is a reflex center performing many functions such as the regulation of heartbeat, breathing, and blood pressure. Along the brain stem is a mass of fibers and nuclei called reticular formation which receives and sends sensory signals.
Limbic System and Higher Mental Functions
This system is involved in the process and functioning of emotions, memory, and learning resulting in our behaviors. There are two main structures. Through the amygdala we experience emotions. And the hippocampus is an area important to learning and memory. Learning and speech require memory thus the three functions are interrelated.
Peripheral Nervous System
This system consists of cranial nerves and spinal nerves and carries impulses to and from the central nervous system. Cranial nerves project from the brain and spinal nerves from the spinal cord. Nerves are made up of axons. Cranial nerves come in twelve pairs consisting of sensory, motor, or mixed fibers and are attached to the brain. Spinal nerves come in thirty one pairs containing both sensory and motor fibers.
The peripheral nervous system is comprised of the somatic and autonomic system. The somatic carries impulses to the central nervous system and away form the central nervous system to the skeletal muscles, skin and tendons. Involuntary actions that are produced are called reflexes. A reflex arc describes the path of the spinal reflex. The autonomic system is connected to the activity of the cardiac and smooth muscle glands. Within this system there are two complimentary involuntary pathways called the sympathetic and parasympathetic. The sympathetic produces responses associated with an alert state and the parasympathetic produces responses associated with a relaxed state. Norepinepherine and adrenaline are produced in a sympathetic response and acetycholine is producced in a parasympathetic response.
Drug Abuse
Neurological drugs block or activate the action of neurotransmitteres at a synapse, and can alter the nervous system. Drug abuse can lead to psychological or physical dependency on the drug. once it becomes a dependency it can lead to addiction. The most common forms of drug abuse is alcohol, nicotine, cocaine, methamphetamine, heroine, and marijuana.
II. SENSE
The human body consists of a variety of sensory receptors that are able to detect different stimuli. These impulses are carried to the central nervous system.
Sensory Receptors and Sensations
Sensory receptors carry impulses to the spinal cord and brain. There are four kinds of sensory receptors. They are called chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors. Chemoreceptors respond to chemical substances as pain receptors are a type of chemoreceptor. Photoreceptors respond to light, mechanoreceptors respond to forces such as pressure, and thermoreceptors such as in the skin are sensitive to temperature. When impulses reach the cerebral cortex it registers a sensation. Our interpretation of the sensation creates our perception.
Proprioceptors and Cutaneous Receptors
Proprioceptors are helpful to equilibrium and posture. They are mechanoreceptors important to reflex and muscle tone. Muscle tone is controlled by the action of the muscle spindle which increases contraction and the golgi tendon organ which deceases contraction. Cutaneous receptors are another type found in the dermis layer of the skin. They enable us to feel touch, pressure, pain, and temperature. The skin also contain pain receptors known as nociceptors that are sensitive to chemicals from the release of damaged tissues.
Senses of Taste and Smell
The act of tasting and smelling is made possible by chemoreceptors. They are able to respond to direct and distant stimuli. The receptors are able to bind to molecules. Different areas of the tongue sense various tastes and flavors. our sense of smell is produced by the olfactory cells in the nasal cavity. olfactory cells are neuronsthat transmit this info to the cerebral cortex.
Sense of Vision
The eye, optic nerves and areas of the cerebral cortex are all involved in the visual process. The eyeball has three layers. The sclera, choroid, and retina. The retina is where the sensory receptors cones and rods are located. The lens and the cornea work to focus light on the retina. The image is then carried by the optic nerves to the thalmus and on to the occipital lobe of the brain. Common disorders of the eye are nearsightedness, farsightedness, and astigmatism. These conditions involve the changing shape of the eyeball. Color blindness is normally hereditary and affects the cones ability to perceive light and colors. Other problems that can develop with age are cataracts, clouding of the lens, and glaucoma, fluid pressure in the eyes.
Sense of Hearing
Sensory receptors for the hearing are mechanoreceptors located in the inner ear. The are important to hearing and balance and involve the ear, cochlear nerve, and the cerebral cortex. There are three parts to the ear, The outer, middle, and inner. The structure serves to amplify and direct sound waves to the inner ear. It is within the inner ear where we find the mechanoreceptors which are hair cells. Nerve impulses travel from the receptor to the cochlear nerve on to the cerebral cortex.
Sense of Equilibrium
The ear is also the responsible for maintaining equilibrium. Within the inner ear mechanoreceptors in the semicircular canals help to sense the heads rotational equilibrium and gravitational equiliibrium. also within the inner ear the utricle is sensitive to horizontal and vertical movements.
References:
http://www.sciencedaily.com/articles/s/sensory_neuron.htm
http://en.wikipedia.org/wiki/Spinal_cord
http://faculty.washington.edu/chudler/cells.html
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