Chapter 1. NERVOUS SYSTEM

Objectives

After completing this unit, you should be able to:

1. List the basic functions of the nervous system and explain the structural and functional divisions of the central and peripheral nervous system. Identify the structure of a neuron and describe the structural and functional classification of neurons. Describe the function of astrocytes, ependymal cells, oligodendrocytes, and Schwann cells. Explain the significance of white matter. Define a nerve and ganglion.
2. Explain the significance of the cerebral cortex. List the major lobes of the cerebrum and identify the location and function of the major motor, sensory, and association areas.
3. Describe the function of the cerebellum. Identify the thalamus, hypothalamus, pineal gland and the major areas of the brain stem. Describe the general function of each area. Explain the role of the limbic system and the reticular formation.
4. Define an electroencephalogram and describe the relation of the four types of brain waves (alpha, beta, theta, and delta) to brain activity. Indicate the importance of non-REM sleep and REM sleep. Identify the categories of memory.
5. Describe the gross and microscopic anatomy of the spinal cord. Describe how the skull, meninges, cerebrospinal fluid, and blood-brain barrier protect the central nervous system. Explain the significance of dural sinuses, the epidural space, and the subarachnoid space.
6. Explain the difference between the somatic and autonomic divisions of the peripheral nervous system. Describe the function of the dorsal and ventral roots of a spinal nerve. List the nerve plexuses and identify the major peripheral nerves of the body.
7. Define reflex and explain the components of a reflex arc. Explain the difference between somatic and autonomic reflexes.
8. Define an electrical impulse and identify the ion movements during depolarization and repolarization. Explain saltatory conduction of an electrical impulse along a myelinated axon.

Nervous Tissue

Neuroglial Cells

Neuroglial cells are small cells of the nervous system that function to support, nourish, and protect neurons. Astrocytes are star shaped cells found in the central nervous system. They form an important part of the blood-brain barrier and help to pass materials back and forth between capillaries and neurons. Ependymal cells are found lining the ventricles of the brain and central canal of the spinal cord. They help to circulate cerebrospinal fluid and regulate the chemical composition of tissue fluid in the central nervous system. Oligodendrocytes and Schwann cells form myelin sheaths around the axons of neurons. Oligodendrocytes do this for cells in the central nervous system (CNS), while Schwann cells are found in the peripheral nervous system (PNS). Myelin sheaths help to electrically insulate the axons and allow axons to conduct electrical impulses faster.

Neuron

Neurons are responsible for sensing changes inside and outside of the body, processing information, and stimulating responses. Their structure is specialized for collecting and sending impulses. The cell body of a neuron contains the nucleus and most of the cytoplasm of the cell. It also contains many mitochondria for carrying out aerobic metabolism to provide the neuron with energy. Dendrites respond to stimuli in their environment and carry signals toward the cell body. The axon carries electrical impulses away from the cell body toward the next cell. Axons in the peripheral nervous system may be surrounded by myelin sheaths produced by Schwann cells. There are small gaps between adjacent Schwann cells along the length of the axon called nodes of Ranvier. Ions move in and out of the axon at the nodes of Ranvier when the axon is conducting an electrical impulse in a process called saltatory conduction. The axon ends at the axon terminal where neurotransmitters are released to influence the next cell. Each axon has multiple axon terminals to impact many cells simultaneously.

Neuron Types

Neurons can be classified by their general structure and how many processes extend from the cell body. Bipolar neurons are neurons with one dendrite and one axon extending from the cell body. Neurons found in the eye and olfactory areas are bipolar neurons. Unipolar neurons have only one process, an axon, extending from the cell body. Many sensory neurons responsible for monitoring skin sensations and muscle length are unipolar. Multipolar neurons have one axon and more than one dendrite extending from the cell body. Motor neurons controlling activity of other tissues are typically multipolar. There are two types of motor neurons. Those that influence skeletal muscle tissue are under voluntary control and called somatic motor neurons. Autonomic motor neurons are controlled involuntarily, and stimulate smooth muscle, cardiac muscle, and glands. Most of the neurons in the CNS are multipolar. Neurons in the CNS are called interneurons and are responsible for processing information and creating perceptions.

Neural Activity

Resting Membrane Potential

When a neuron is at rest, there is an unequal distribution of ions on the outside of its membrane compared to the inside. The inside of the neuron has a higher concentration of potassium ions (K⁺), and a lower concentration of sodium ions (Na⁺) than the extracellular fluid. There are sodium/potassium pumps on the membrane of the neuron that actively shuttle sodium ions outside of the neuron and potassium inside in order to maintain this difference in ion distribution. The difference in ion concentrations across the membrane of a neuron allows the neuron to generate a negative membrane potential when at rest as K⁺ constantly leaves.

Electrical Impulse

The electrical impulse that can be measured when a neuron “fires” is a switch in the membrane potential of the neuron. When at rest, the neuron has a negative membrane potential. When it sends a signal, the charge along the membrane of the axon switches to a positive charge and then back to negative. This change in charge is a result of the movement of sodium and potassium ions across the membrane of the neuron. The first part of the electrical impulse is the switch from negative to positive charge. This is called depolarization, and is a result of the rapid movement of sodium ions into the neuron. The second step of the electrical impulse returns the neuron to a negative charge. This stage, called repolarization, is caused by the rapid diffusion of potassium ions out of the neuron.

Chemical Synapse

When the electrical impulse reaches the axon terminal, the neuron releases chemical messengers known as neurotransmitters. The neurotransmitter diffuses across a small space between the neuron that released it (presynaptic neuron), and the next neuron (postsynaptic neuron). This space is called the synaptic cleft. Once it crosses the synaptic cleft, the neurotransmitter will bind to receptors on the postsynaptic neuron. The binding of neurotransmitter to receptors on the postsynaptic neuron influences the membrane potential of the postsynaptic neuron. Changes in the membrane potential of the postsynaptic neuron that make it more likely to send its own electrical impulse are excitatory; changes in the membrane potential that make it less likely to send its own signal are inhibitory. Several important neurotransmitters and their characteristics are listed in the table below.

Nervous System Organization

Peripheral Nervous System (PNS)

The peripheral nervous system consists of all of the sensory and motor neurons that carry information to and from the brain and spinal cord. A nerve is a collection of axons from many neurons that are bundled together by layers of connective tissue that carry information between the same body tissues and the brain or spinal cord. There are twelve pairs of cranial nerves that connect body tissues directly with the brain. The characteristics of the twelve cranial nerves are listed in the table below. There are 31 pairs of spinal nerves that connect body tissues with the spinal cord. Ganglia are collections of the cell body of neurons whose axons travel through the peripheral nerves.

Central Nervous System (CNS)

The central nervous system consists of the brain and spinal cord. The main function of the CNS is the integration of sensory information, and initiation of motor impulses. The cerebrum is the largest part of the brain and is responsible for memories, thinking, problem solving, sensations, and initiating skeletal muscle movements. The cerebellum aids in coordinating muscle movements. The thalamus plays an important role in sorting sensory information to the correct part of the brain. The hypothalamus is an important regulator of homeostasis. The pineal gland produces melatonin which influences the body’s biological clock. The brain stem connects the brain with the spinal cord and plays important roles in regulating breathing, heart rate, and blood pressure. The spinal cord travels through the vertebral canal, controls spinal reflexes, and connects the brain with much of the peripheral nervous system.

Cranial Nerves (12 pairs)

The cranial nerves are nerves that connect directly to the brain. The table below summarizes the major functions of the cranial nerves.

Spinal Nerves and Nerve Plexus

There are eight pairs of cervical spinal nerves that connect with the spinal cord between the cervical vertebrae. The cervical plexus is formed from parts of the first four cervical nerves. Nerves in this area transmit sensory impulses from the skin of the neck and back of the head. The phrenic nerve originates here and stimulates the diaphragm for breathing. The brachial plexus is formed from parts of the fifth through eighth cervical nerves, and the first thoracic nerve. The brachial plexus carries sensory information from skin of the arm, and motor impulses to the muscles of the arm. There are twelve pairs of thoracic spinal nerves that connect with the spinal column between the thoracic vertebrae. Generally, thoracic spinal nerves do not form a nerve plexus. Instead they travel along the ribs as intercostal nerves to the muscles and skin of the chest and abdominal wall. Lumbar spinal nerves connect with the spinal cord through openings between the lumbar vertebrae. The lumbar plexus is formed from parts of the first four lumbar nerves. Nerves from the lumbar plexus innervate muscles of the abdominal wall and thigh. The five sacral spinal nerves travel through the sacral foramen to connect with the spinal cord. The sacral plexus is formed from branches of the fourth lumbar nerve through the fourth sacral nerve. Nerves from this plexus travel all the way down the leg and also innervate the pelvic structures and perineum. There is one pair of coccygeal nerves that connect with the spinal cord near the coccyx.

Dermatomes are areas of the skin that are innervated by a single spinal nerve. Physicians can use dermatomes to determine where damage to the spinal cord occurred based upon which regions of the skin are impacted. This is particularly useful when nerves that go out to the upper or lower limbs are involved as there is very little overlap in the innervation of dermatomes in the limbs compared to the torso region.

Phantom pain is the feeling of pain coming from a limb or portion of the limb that is absent. Referred pain occurs when pain is felt in a body part other than the one that is actually being stimulated. It is caused because pain signals from your organs or viscera often travel along the same pathways as somatic pain. Our perception of the visceral pain signals can cause us to feel like the pain was coming from the associated body region. A good example of this is the pain that is often associated with a heart attack. Impulses from the heart travel along the same spinal segments as the left chest and arm. The brain may interpret pain signals from the heart as coming from the left chest or medial side of the left arm.

Peripheral Nerves

The brachial plexus is formed from parts of the fifth through eighth cervical nerves, and the first thoracic nerve. The brachial plexus carries sensory information from skin of the arm, and motor impulses to the muscles of the arm. The axillary nerve stimulates the deltoid muscle. The musculocutaneous nerve stimulates the biceps brachii. The ulnar nerve stimulates pronator teres. The median nerve stimulates muscles of the hand and the triceps brachii. The radial nerve stimulates the supinator and brachioradialis.

The lumbar plexus is formed from parts of the first four lumbar nerves, and gives rise to two important nerves. The femoral nerve stimulates the quadriceps femoris muscles, and the obturator nerve stimulates the medial muscles of the thigh such as gracilis.

The sacral plexus is formed from branches of the fourth lumbar nerve through the fourth sacral nerves. The sciatic nerve originates from the sacral plexus. It is the thickest and longest nerve in the body, and stimulates the hamstrings among other muscles. The common fibular nerve extends from the sciatic nerve anteriorly to stimulate the tibialis anterior muscle. The tibial nerve continues from the sciatic nerve to stimulate calf muscles such as gastrocnemius.

Nervous System Organization

Spinal Cord

The spinal cord is a cylindrical mass of nervous tissue that extends from the brain stem through the vertebral canal to the second lumbar vertebrae. It is organized into regions of white matter surrounding central portions of grey matter. The white matter is made up of axons that allow for communication between different regions of the spinal cord and the brain. Ascending tracts carry sensory information toward the brain, while descending tracts carry motor impulses from the brain.

The gray matter is a butterfly shaped region of neural cell bodies located in the center of the spinal cord. Dorsal horns are the projections of the gray matter that extend towards the back. The dorsal horns contain mainly interneurons. The dorsal root ganglion is a cluster of the cell bodies of sensory neurons from the spinal nerves of the peripheral nervous system. Axons from these neurons form the dorsal root to meet the spinal cord. Axons in the dorsal root carry sensory impulses into the spinal cord. The ventral horn contains the cell bodies of motor neurons. The axons from these neurons exit the spinal cord forming the ventral roots of the spinal nerves. The central canal is a small cavity filled with cerebrospinal fluid located in the center of the spinal cord. The spinal cord extends from the brain stem through the vertebral canal. It ends in a cone shaped structure around the second lumbar vertebra. Nerves continue inferiorly through the lower lumbar vertebrae and sacrum forming the cauda equina.

Nervous System Organization

Reflex Activity

A reflex is an automatic, involuntary response to a stimulus. These rapid and predictable responses help to protect us from harm, as in the case of the withdrawal reflex. The withdrawal reflex causes a rapid and forceful movement of a body part away from a painful stimulus, such as moving your hand quickly away after touching a hot stove. The quick response of the reflex removes your finger before any further damage can occur even before you become consciously aware that you have touched the stove. Other reflexes help to regulate organ activity without conscious thought, such as the gastrocolic reflex. The gastrocolic reflex simulates the movement of waste or feces through the colon in response to stretching of the stomach when you consume more food.

A reflex arc refers to the pathway that the signal travels to stimulate a reflex. The sensory receptor detects the stimulus. Then the signal travels along a sensory neuron to the central nervous system. In a reflex arc the integration center can be the synapse between a sensory neuron and a motor neuron, or can involve additional synapses between neurons within the CNS. From the central nervous system, the signal travels along a motor neuron toward an effector. An effector is the organ or structure that is responsible for carrying out the action that is the response. Reflexes that activate skeletal muscle are somatic reflexes. Reflexes that activate smooth muscle, cardiac muscle, or glands are autonomic reflexes.

Nervous System Organization

Central Nervous System Protection

The structures of the central nervous system are protected by the bones of the skeleton and by three layers of dense connective tissue called the meninges. The skull protects the brain and the vertebral column protects the spinal cord. The outer most layer of the meninges is called the dura mater. Superficial to the dura mater in the vertebral cavity is the epidural space. The epidural space contains adipose tissue and larger blood vessels. It is the site where epidural anesthetics are injected. The dural sinuses are found in the dura mater of the brain. They collect blood from the brain and drain into the internal jugular veins of the neck. The middle layer of the meninges is the arachnoid mater. Internal to the arachnoid mater is the subarachnoid space that contains cerebrospinal fluid. The subarachnoid space is the site of injections of spinal anesthetics. The innermost layer of the meninges laying directly on the brain and spinal cord is the pia mater.

Cerebrospinal Fluid (CSF)

Cerebrospinal Fluid (CSF) Cerebrospinal fluid (CSF) is found in the subarachnoid space of the meninges surrounding the brain and the spinal cord. It can also be found in the fluid filled ventricles of the brain and central canal of the spinal cord. The main functions of CSF are to support the CNS, cushion it from shock, and transport nutrients and metabolic wastes.

Cerebrum

The cerebrum is the largest, most superior portion of the brain. It is divided into left and right halves or cerebral hemispheres at the top by the longitudinal fissure. Each hemisphere of the cerebrum has the ability to specialize to carry out tasks that are not done by the opposite hemisphere. As a result, the left side of the brain has greater control over language, math and logic. The right hemisphere is dominant in tasks relating to artistic expression. This is known as lateralization, and the corpus callosum connects the two halves of the cerebrum allowing them to communicate with each other. The transverse fissure separates the cerebrum from the cerebellum. The central sulcus is a major groove found in the cerebrum that separates the frontal and parietal lobes. The lateral sulcus is the groove that separates the temporal lobe of the cerebrum.

Cerebral Gray Matter

The cerebral cortex is the outer most layer of the cerebrum. It consists of gray matter and is the site of most of the neural activity in the cerebrum. The cerebral cortex is important in conscious thought, memory, reasoning, judging consequences, planning, and initiating voluntary movements. Other areas of gray matter embedded deep within the cerebrum are called basal ganglia. Parkinson’s disease is a degenerative brain disorder caused by the death of dopamine producing neurons in the basal ganglia.

Cerebral Lobes

The different regions or lobes of the cerebrum carry out specialized functions. The frontal lobe is the most anterior region of the cerebrum. The primary motor cortex is responsible for initiating skeletal muscle movements. The premotor cortex controls learned, complex movements of skeletal muscle such as how to play the piano. Broca’s area is the part of the brain that is responsible for producing speech. The prefrontal cortex is the area of the brain that functions in planning complex behaviors, personality expression, and moderating correct social behavior. The parietal lobe of the cerebrum is located under the parietal bones of the skull. It contains the primary somatosensory cortex which interprets sensory information from the skin such as touch and temperature. The temporal lobe contains the auditory cortex which receives and interprets impulses from the ear. Wernicke’s area is important in interpreting the meaning of speech by recognizing words and translating thoughts into words. The occipital lobe is located in the back of the cerebrum. It contains the visual cortex which receives impulses from the eye.

Cerebellum

The cerebellum is located inferior to the cerebrum. The cerebellum helps to coordinate voluntary muscle movements, maintain balance and muscle tone. It receives information about muscle tension, balance, joint position, and movement from sensory receptors and determines the best way to coordinate muscle activity to carry out desired movements. It compares the intended movements to what is actually occurring and makes necessary adjustments to correct and smooth muscle movements. More recent studies of the cerebellum indicate that it may also function in other types of sensory perception such as comparing the textures of objects, spatial relationships, and the amount of time that has passed. Finally, the cerebellum may play a role in language and emotions.

Nervous System Organization

Thalamus

The thalamus is located near the center of the brain. One of its primary functions is to route sensory information to the correct part of the cerebrum. The thalamus is part of the limbic system which determines our emotions. As sensory impulses travel through the thalamus, we begin to perceive them as pleasant or unpleasant, even before we are aware of what the sensation is. The thalamus also aides muscle coordination by relaying messages between the cerebrum and the cerebellum.

Hypothalamus

The hypothalamus is located inferior to the thalamus. It is the main control center for the autonomic nervous system. The hypothalamus connects directly to the pituitary gland, and influences the activity of the pituitary gland and the endocrine system. As a result, the hypothalamus plays a central role in regulating homeostasis of body systems. The hypothalamus plays an important role in regulating body temperature, hunger, and thirst. The hypothalamus is also an important part of the limbic system which influences emotions such as pleasure, pain, fear, and rage.

Pineal Gland

The pineal gland is located posterior to the thalamus. It produces melatonin which influences the body’s internal clock and sleep/wake cycles. Seasonal affective disorder is a mood disorder where people experience the winter blues. Because the pineal gland secretes more melatonin in dim light or dark conditions, it may contribute to seasonal affective disorder.

Brain Stem

The brain stem contains three major parts: the midbrain, pons and medulla oblongata. The midbrain is the most superior portion of the brain stem and controls reflexes of the eyes and ears such as tracking objects as they move or turning toward a sudden sound. The pons is located inferior to the midbrain. It contains a respiratory center which helps regulate the rate and depth of breathing. The most inferior portion of the brain stem is the medulla oblongata. The medulla oblongata contains the main respiratory center that influences breathing, a cardiac center that influences heart rate, and a vasomotor center that influences blood pressure and flow. The medulla oblongata is also involved in reflexes such as coughing, sneezing, vomiting and swallowing.

Nervous System Organization

Electroencephalogram (EEG)

An electroencephalogram (EEG) records the brain’s electrical activity. Alpha waves indicate relaxation and closing of the eyes. Beta waves are the busiest, and indicate activities such as concentration, anxiety, or analyzing the importance of biology in everyday life. Theta waves indicate drowsiness. They can also be seen when waking from sleep. Delta waves are produced during deep sleep.

Nervous System Organization

Functional Brain Regions

The reticular formation is found in the brain stem. It works with the cerebrum and cerebellum to coordinate muscle movements and maintain balance. The reticular activating system is a part of the reticular formation and filters sensory input from all the sensory receptors. This allows us to focus on important and meaningful information and disregard background chatter and noise. It is also important in sleep and arousal from sleep. There are two general categories of sleep. Non-REM sleep is a deep, restful sleep where the body is relaxed and there is little cerebral activity. REM sleep occurs when a person is dreaming, and is characterized by increased brain activity and rapid eye movements. During REM sleep the body is less responsive to outside stimuli.

The limbic system consists of parts of the hypothalamus, thalamus, brain stem, and cerebrum. It controls emotions and motivations. Because the hypothalamus is a part of the limbic system and it is also the main regulator of the autonomic nervous system, there is a connection between our emotions and our body systems. Psychosomatic illnesses are illnesses caused by emotions such as stress. They can result in a variety of conditions or symptoms such as high blood pressure. The amygdyla is part of the limbic system, and is responsible for our ability to recognize facial expressions such as anger or fear.

The gnostic area of the brain is located in the parietal lobe. Its major function is to correlate sensory impulses with memories to interpret sensations. Memory is the ability to recall ideas, thoughts, and past experiences. There are two types of memory. Short-term memory lasts for only a few seconds to minutes. It is our working memory. Information that is stored in long-term memory can be recalled at a later time. Repetition and alertness aid in transferring information from short-term to long-term memory. Short-term memories are stored in the limbic system as they are being used, long-term memories are stored in the cerebral cortex.

Nervous System Organization

Health Concerns

Cerebral palsy is caused by a temporary loss of oxygen to brain tissues during development or delivery. It results in a decreased ability to control skeletal muscle.

Traumatic brain injuries are caused by physical forces affecting the brain. Concussion is the mildest type, where there is a loss of consciousness and temporary alteration of brain function. More serious traumas can lead to bruising of the brain and contusions where the brain tissue is permanently damaged. Hemorrhage or bleeding of blood vessels in the brain is a potentially life threatening condition. Traumatic brain injuries can also cause cerebral edema or swelling of brain tissue. Coma can be caused by traumatic brain injuries and tumors. A person in a coma is unconscious and unresponsive to sensory stimuli.

Cerebrovascular accidents (CVA) or strokes occur when blood circulation to an area of the brain is blocked. Blockage of blood flow causes death of brain cells in that area. A transient ischemic attack (TIA) occurs when the blockage of blood flow is short lived, and does not result in permanent damage. TIA’s are considered warning signs for a CVA.

Alzheimer’s disease is a degenerative brain disease that results in memory lapses and loss. It primarily affects neurons in the limbic system and frontal lobe, and is caused by a shortage of the neurotransmitter acetylcholine. Parkinson’s disease results in a loss of ability to control skeletal muscle tissue leading to stiff joints and tremors. It is thought to be a result of the death of dopamine producing neurons in the basal ganglia. Huntington’s disease is a genetic disorder where a protein accumulates in brain cells causing them to die. It is characterized by exaggerated muscle movements and eventually mental deterioration and death. Epilepsy causes seizures as a result of excessive brain cell activity. Seizures can be triggered by a variety of different stimuli and last for varying amount of time with varying affects from almost unnoticeable staring to convulsions and loss of consciousness. Multiple sclerosis is an autoimmune disease caused by a loss of myelin surrounding the axons. Lack of myelin decreases the speed that signals travel along axons in the body. Common symptoms of multiple sclerosis include problems with vision, muscle movement, speech, and controlling urination.

Meningitis is caused by an inflammation of the meninges surrounding the brain or spinal cord. Symptoms include headache, fever, stiff neck, nausea and vomiting. Encephalitis is inflammation of the brain, most often caused by a virus. Symptoms include headache, fever, and can lead to hallucinations and problems with speech and memory. Poliomyelitis is an inflammation and destruction of neurons in the ventral horn of the spinal cord. It is caused by the polio virus, and can cause paralysis.

Injuries to the spinal cord can result in paresthesia, a loss of sensory function, or paralysis, a loss of motor function. In paraplegia only the lower limbs are affected. In quadriplegia both the upper and lower limbs are affected.

Peripheral Nervous System Review

Central Nervous System Review

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