CHAPTER OUTLINE
17.1 Nervous Tissue
The nervous system helps coordinate and regulate the functioning of the body’s other systems. It is divided into the central nervous system (CNS), which consists of the brain and spinal cord, and the peripheral nervous system (PNS), which consists of nerves that carry messages to the CNS and from the CNS to the muscles and glands. The nervous system contains two types of cells: neurons and neuroglia.
Types of Neurons and Neuron Structure
There are three classes of neurons: sensory neurons take messages to the CNS, interneurons are in the CNS and can receive input, and motor neurons take messages away from the CNS. Most neurons have three parts: a cell body, dendrites, and an axon.
Myelin Sheath
Some axons are covered by a protective myelin sheath. The myelin sheath plays an important role in nerve regeneration in the PNS and gives nerve fibers a white appearance.
17.2 Transmission of Nerve Impulses
The nervous system uses the nerve impulse to convey information. The nature of a nerve impulse can be characterized by voltage changes.
Resting Potential
When the axon is not conducting an impulse, the inside of the axon is negatively charged compared to the outside, giving it a resting potential of -70 mV difference across the membrane. This charge difference is due to the ion distribution on either side of the membrane, which is a result of the action of the sodium-potassium pump that actively transports sodium out of and potassium into the axon.
Action Potential
An action potential is a rapid change in polarity across an axonal membrane as the nerve impulse occurs. It is an all-or-none phenomenon.
Sodium Gates Open
When the action potential begins, the gates of the sodium channels open and sodium flows into the axon. The membrane potential changes from –70 mV to +40 mV.
Potassium Gates Open
Second, the gates of the potassium channels open, and potassium flows to outside the axon. This repolarizes the axon as the inside resumes a negative charge.
Conduction of an Action Potential
The action potential travels down an axon toward its terminals. It travels faster in myelinated than nonmyelinated axons.
Transmission Across a Synapse
Every axon branches into many fine endings, each tipped by an axon terminal. Each terminal lies very close to either the dendrite or cell body of another neuron. This region is called a synapse, the two neurons are separated by the synaptic cleft. Communication between the two neurons is carried out by molecules called neurotransmitters, which are stored in synaptic vesicles in the axon terminals and released when nerve impulses reach the axon terminal.
Synaptic Integration
A single neuron may receive many excitatory and inhibitory signals, which have a depolarizing or hyperpolarizing effect, respectively. Integration is the summing up of excitatory and inhibitory signals by a neuron.
Neurotransmitters
At least 25 different neurotransmitters have been identified. Once a neurotransmitter has been released into a synaptic cleft and has initiated a response, it is removed from the cleft.
17.3 The Central Nervous System
The spinal cord and the brain make up the central nervous system (CNS), where sensory information is received and motor control is initiated.
The Spinal Cord
The spinal cord extends from the base of the brain through a large opening in the skull and into the vertebral canal.
Structure of the Spinal Cord
The spinal nerves project from the cord between the vertebrae. Fluid-filled intervertebral disks cushion and separate the vertebrae. A cross section of the spinal cord shows a central canal, gray matter, and white matter.
Functions of the Spinal Cord
The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. The spinal cord is also the center for thousands of reflex arcs, which allow the nerves and muscles to respond very quickly.
The Brain
The four major parts of the brain are the cerebrum, the diencephalon, the cerebellum, and the brain stem.
The Cerebrum
The cerebrum is the largest portion of the brain. It is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain.
The Cerebral Hemispheres
The cerebrum is divided by a deep groove, called the longitudinal fissure, into the left and right cerebral hemispheres. The two halves communicate via the corpus callosum, an extensive bridge of nerve tracts. Shallow grooves divide each hemisphere into lobes: frontal, parietal, occipital and temporal lobes. The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres, it accounts for sensation, voluntary movement, and all the thought processes associated with consciousness.
Primary Motor and Sensory Areas of the Cortex
The primary motor area is located in the frontal lobe, voluntary commands to skeletal muscles begin here. The primary somatosensory area is located in the parietal lobe and is where sensory information from the skin and skeletal muscles arrives.
Association Areas
Association areas are places where integration occurs and memories are stored.
Processing Centers
Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions.
Central White Matter
Most of the rest of the cerebrum beneath the cerebral cortex is composed of white matter. Tracts within the cerebrum take information between the different sensory, motor, and association areas.
Basal Nuclei
Masses of gray matter located deep within the white matter are called basal nuclei, they integrate motor commands, ensuring that proper muscle groups are activated or inhibited.
The Diencephalon
The hypothalamus and the thalamus are in the diencephalon. The hypothalamus is an integrating center that helps maintain homeostasis by regulating hunger, sleep, thirst, body temperature, and water balance. The thalamus integrates sensory input from the visual, auditory, taste, and somatosensory systems. The pineal gland is located in the diencephalon and secretes a hormone that maintains our normal sleep-wake cycle.
The Cerebellum
The cerebellum receives sensory input from the joints, muscles, and other sensory pathways about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. The cerebellum maintains balance and posture by integrating this information.
The Brain Stem
The brain stem contains the midbrain, the pons, and the medulla oblongata. The midbrain relays messages between the cerebrum and the spinal cord or cerebellum. The pons contains bundles of axons traveling between the cerebellum and the rest of the CNS. The medulla oblongata regulates vital functions like heartbeat, breathing, and blood pressure.
Electroencephalograms
The electrical activity of the brain can be recorded in the form of an electroencephalogram (EEG).
17.4 The Limbic System and Higher Mental Functions
Emotions and higher mental functions are associated with the limbic system in the brain. The limbic system blends primitive emotions and higher mental functions into a united whole.
Anatomy of the Limbic System
The limbic system is a complex network of tracts and nuclei that incorporates portions of the cerebral lobes, the basal nuclei, and the diencephalon. The hippocampus communicates with the prefrontal area of the brain, which is involved in learning and memory. The amygdala allows us to respond to and display anger, avoidance, defensiveness, and fear; it prompts release of adrenaline and other hormones.
Higher Mental Functions
The human cerebrum is responsible for higher mental functions such as memory and learning, as well as language and speech.
Memory and Learning
Memory is the ability to hold a thought in mind or to recall events from the past. Learning takes place when we retain and utilize past memories.
Types of Memory
Short-term memories are stored in the prefrontal area. Long-term memory is typically a mixture of semantic and episodic memory. Skill memory is another type of memory involved in performing motor activities.
Long-Term Memory Storage and Retrieval
Our long-term memories are stored in bits and pieces throughout the sensory association areas of the cerebral cortex. The hippocampus serves as a bridge between the sensory association areas and the prefrontal area. Long-term potentiation is an enhanced response at synapses within the hippocampus.
Language and Speech
Language is dependent upon semantic memory and involves the motor speech (Broca’s) area and sensory speech (Wernicke’s) area. The left and right hemispheres have different functions in relation to language and speech, recent studies suggest that the hemisphere process the same information differently.
17.5 The Peripheral Nervous System
The peripheral nervous system (PNS) is composed of nerves, which are bundles of axons, and ganglia, which contain collections of cell bodies.
Somatic System
The PNS is subdivided into the somatic system and the autonomic system. The somatic system serves the skin, skeletal muscles, and tendons. Some actions in the somatic system are due to reflex actions, which are automatic responses to a stimulus.
The Reflex Arc
Reflexes are programmed, built-in circuits that allow for protection and survival. They require no conscious thought. Nerve impulses travel from the sensory neuron to the spinal cord and back to the motor neuron.
Autonomic System
The autonomic system regulates the activity of cardiac and smooth muscle and glands. The system is composed of the sympathetic and parasympathetic divisions. These divisions function automatically and usually in an involuntary manner, they innervate all internal organs, and utilize two motor neurons that synapse at a ganglion.
Sympathetic Division
The sympathetic division is especially important during emergency situations when a “fight or flight” response is required.
Parasympathetic Division
The parasympathetic division is sometimes called the housekeeper division because it promotes all the internal responses we associate with “rest and digest.”
17.6 Drug Abuse
Most illicit drugs affect the action of a particular neurotransmitter at synapses in the brain. Stimulants are drugs that increase the likelihood of neuron excitation, while depressants decrease it.
Some Specific Drugs of Abuse
Nicotine
Nicotine causes a release of epinephrine, increasing blood sugar levels and causing an initial feeling of stimulation. It induces both physiological and psychological dependence.
Alcohol (Ethanol)
Ethanol influences the action of GABA, an inhibitory neurotransmitter. As it is metabolized it disrupts the normal workings of the liver and may eventually damage it.
Marijuana
Marijuana is rich in tetrahydrocannabinol (THC), which binds a receptor for a neurotransmitter in the brain that is important for short-term memory processing.
Cocaine and Crack
Cocaine is sold in powder form and as crack, a more potent extract. It prevents the synaptic uptake of dopamine, causing the user to experience a rush sensation and a state of arousal that last for several minutes afterward. Cocaine causes extreme physical dependence.
Heroin
Heroin binds to receptors meant for endorphins, natural neurotransmitters that kill pain and produce a feeling of tranquility.
“Club” Drugs
Ecstasy, Rohypnol, and ketamine are drugs that are abused by teens and young adults who attend night-long dances called raves or trances.
Methamphetamine
“Meth” or “crank” is a powerful CNS stimulant, the most immediate effect of which is an initial “rush” of euphoria.
“Bath Salts”
“Bath salts” are synthetic powders that contain synthetic amphetamine-like chemicals and inhibit the reuptake of several neurotransmitters, producing a euphoric sensation.
17.7 Disorders of the Nervous System
A myriad of abnormal conditions can affect the nervous system.
Disorders of the Brain
Alzheimer disease is the most common cause of dementia, an impairment of brain function that interferes with a patient’s ability to carry on daily activities. Parkinson disease is characterized by a gradual loss of motor control. Multiple sclerosis is the most common neurological disease that afflicts young adults, it is an autoimmune disease in which the patient’s own white blood cells attack the myelin of the nervous system. A stroke results in disruption of the blood supply to the brain. Meningitis is an infection of the meninges that surround the brain and spinal cord. Several brain diseases are caused by prions, infectious agents that are believed to be composed of only protein, examples are kuru, Creutzfeldt-Jakob disease, fatal familial insomnia and mad cow disease.
Disorders of the Spinal Cord
Spinal cord injuries may result from trauma. Because little or no nerve regeneration is possible in the CNS, any resulting disability is usually permanent. The location and extent of the damage produce a variety of effects. Amyotrophic lateral sclerosis is a rare, but devastating condition that affects the motor nerve cells of the spinal cord.
Disorders of the Peripheral Nerves
Guillain-Barré syndrome is an inflammatory disease that causes demyelination of peripheral nerve axons. Myasthenia gravis is an autoimmune disorder in which antibodies are formed that react against the acetylcholine receptor at the neuromuscular junction of the skeletal muscles, preventing muscle stimulation.