The Nervous System

By: Sarah Klapka, Isabel Hugh, and

Sofia Da Silva

8.1 The Divisions of the Nervous System

The Nervous System is divided into two sections : the central nervous system and the peripheral nervous system

The Central Nervous System: (CNS) Lies in the midline of the body, made up of the brain and spinal cord.

The Peripheral Nervous System: This system connects the central nervous system to the rest of the body. It is divided into afferent (sensory) and efferent (motor) sections. Since, the peripheral nervous system is in-sync with the central nervous system it includes the cranial and spinal nerves as well. These nerves have a peripheral route, meaning they extend outward from the peripheral nervous system.

The Nervous System: Three Specific

FunctionsSensory Input: There are sensory receptors present in the skin and organs, which are responsive to external and internal stimuli by generating nerve signals that travel to the brain and spinal cord. For example, temperature sensors in the skin may send a signal to the brain alerting a drop in the temperature.

Integration: The brain and spinal cord read the information received from the sensory receptors all over the body, and then signal the corresponding nerve responses. For example, with the colder temperature of the body, sensory information from the temperature receptors is sent to the hypothalamus, the brain center that controls temperature.

Motor Input: The nerve impulses from the brain and spinal cord are sent to the effectors, which are muscles, glands, and organs. The muscle contractions, gland secretions, and changes in the organ functions are responses to the stimuli received by sensory receptors. For example, to adjust the body temperature, the hypothalamus triggers shivering (skeletal muscles contract rhythmically) which warms the body.

*Through this process, the body is able to maintain homeostasis by receiving sensory information, integrating that information, and making an appropriate response.

Nervous Tissue

The seemingly complex nervous tissue is made up of two types of cells:

1. Neurons: (Nerve cells) Transmit nerve impulses.

2. Neuroglia: Support and nourish neurons.

Neuron Structure

Every neuron has the same three parts.

Cell body: Contains the nucleus of the neuron, as well as other organelles.

Dendrite: Shorter, branched extensions that receive signals from sensory receptors or other neurons.

Axon: Portion of a neuron that conducts nerve signals. Axons grouped into a bundle of parallel axons in the peripheral nervous system are called a nerve. Axons bundled in the central nervous system are called a tract.

Some axons in nerves or tracts are covered in a myelin sheath, a lipid coating that insulates the nerve. In the PNS, the myelin covering of axons is formed by neurological cells called Schwann cells or neurolemmocytes. In the CNS, oligodendrocytes, another type of neurological cell perform a similar function. Gaps in the myelin sheath are called nodes of Ranvier (neurofibril nodes). These gaps speed up the transmission of a nerve signal.

Types of Neurons

Neurons can be classified based on their function and structure.

Sensory neurons: Transmit nerve impulses from the sensory receptors to the CNS. These neurons can be simple or complex and are considered to be unipolar. They are located at the end of the long axon of a sensory neuron.

Interneurons: Also called association neurons, these neurons occur entirely within the CNS. They are deemed multipolar. Interneurons work between the sensory and motor neurons, they process the sensory information and send it to the motor neurons. Interneurons also form complex pathways where processes regarding thinking, memory, and language occur.

Motor neurons: Deliver nerve impulses from the CNS to muscles, glands and organs. They are thought to be multipolar because they have multiple dendrites and one axon. As a result of motor neurons, muscles are able to contract, organs modify, and glands secrete.

Nerve Signal Conduction

The function of neurons is to conduct nerve signals; thus, to transmit information. A nerve cell is also called an action potential.

Resting potential: Is a neuron which has potential energy. The cell's membrane is polarized; it is positively charged on the outside and negatively charged on the inside. Positive sodium ions (Na+) on the outside and the continuous flow of positive potassium ions (K+) from the cell; thus make the outside of the neuron positive.

Sodium-Potassium Pump: Pumps sodium (Na+) out of the cell and pumps potassium (K+) into the cell. This process keeps the neuron charged, so that it can perform work.

Action Potential

Action potential: The process of conduction, occurs in the axons of neurons. A stimulus sets the action potential off. The stimulus causes protein channels for sodium to open, and sodium ions enter the cell. The addition of positively charged sodium ions causes the inside of the cell to turn positive and the outside negative. This change is labeled depolarization.

After depolarization, the pathways for sodium close and a different pathway for potassium opens. As positively charged potassium exits the cell, the cell once again becomes negative. The change in polarity is called repolarization.

Conduction of Action Potentials

If an axon is unmyelinated, then an action potential at one point stimulates the adjacent point on the axon to create an action potential. This process is slower compared to a myelinated process.

In a myelinated fiber, an action potential at one node of Ranvier causes an action potential at the next node, it then jumps over the entire myelin sheath covered portion of the axon. This type of conduction is called saltatory conduction, and is much quicker.

The conduction of an action potential is an all-or-none process. After the action potential, the ions are returned to their proper place by the sodium-potassium pump. The cell is again at resting potential.

As the action potential moves down the axon, those portions enter a short refractory period during which it is unable to conduct an action potential. This period ensured the one way direction of the impulse down the cell body through the axon.

Transmission Across a Synapse

Every axon branches into multiple small swellings called an axon terminal.

The axon terminals are in close proximity to either a dendrite or cell body of another neuron. This region is called a synapse.

At the synapse sight, the first neuron is called the presynaptic membrane, and the membrane of the next neuron is called the postsynaptic membrane. The small gap between is the synaptic cleft.

Neurotransmitters: Stored in synaptic vesicles in the axon terminal, carries out transmissions across the synapse.

When the nerve impulse reaches the axon terminal, the pathway for calcium ions (Ca2+) opens, and calcium enters the cell. This intake of calcium stimulates synaptic vesicles to merge with the presynaptic membrane, and the neurotransmitter molecules are released into the synaptic cleft. They diffuse across the cleft to the postsynaptic membrane, where they bind with specific receptor proteins.

The response will either be excitation or inhibition, depending on the neurotransmitter.

Graded Potentials and Synaptic Integration

Each of the small signals from a synapse is called a graded potential.

In an excitatory neurotransmitter, the neuron’s sodium channels open which produce a graded potential that drives the polarity closer to an action potential.

In an inhibitory neurotransmitter, the neuron’s potassium channels open and allow potassium ions to leave the cell; thus creating a graded potential that makes it harder for an action potential to take place.

Integration: The adding up of excitatory and inhibitory signals that will determine if the neuron fires or not.

Receptor potentials: Sensory receptors what have special graded potentials.

Neurotransmitter Molecules

There are at least 50 different neurotransmitters which have been identified.

Two very well known ones are acetylcholine and norepinephrine.

After a neurotransmitter has been released into the synaptic cleft and has initiated a response, it is removed from the cleft.

In certain synapses, the postsynaptic membrane contains enzymes which rapidly inactivate the neurotransmitter.

*Example: the enzyme acetylcholinesterase breaks down acetylcholine.

However, in other synapses, the presynaptic membrane rapidly reabsorbs the neurotransmitter.

The short existence of neurotransmitters at a synapse prevents continuous stimulation of postsynaptic membranes.

Alzheimer DiseaseAlzheimer Disease is a disorder characterized by a gradual loss of reason that begins with memory lapses and ends with the inability to perform any type of daily activity.

Researchers have discovered that in some families whose members have a 50% chance of AD, a genetic defect exists on chromosome 21. Down syndrome results from three copies of this chromosome and people with Down syndrome tend to develop AD.

AD is characterized by the presence of abnormally structured neurons and a reduced amount of acetylcholine (ACh).

Two different drugs are used to slow the process of AD: Cholinesterase and Mematine.

Cholinesterase inhibitors work at neuron synapses in the brain, slowing the activity of the enzyme that breaks down acetylcholine. This allows memory pathways in the brain to function for a longer amount of time.

Mematine is another drug used to block the tendency of diseased neurons to self-destruct. This is usually used on patients with a more severe case of AD.

This disease is impossible to cure but scientists are researching different ways to prevent the disease. For example, it has been found that a lifestyle tailored for good cardiovascular health can prevent AD.

8.2 The Central Nervous System

The CNS consists of the spinal cord and brain, and it is made out of white and gray matter.

The white matter is white because it consists of myelinated axons that run together in a bundle which are called tracts.

The gray matter is gray because it consists of cell bodies and short nonmyelinated fibers.

The white get color because of the myelin that covers the axons which also give them a shiny appearance.

The Meninges and Cerebrospinal Fluid

Meninges (sing and menix) are what covers the brain and spinal cord and what is used to protect them.

On the outer shell of the meninges they have what is called the dura mater.

This dura mater is a tough, white, fibrous connective tissue, that lies near the skull and the vertebrae.

The dura mater has two different membrane layers that are fused, which help form the dural venous sinuses.

The dural venous sinuses get venous blood and excess cerebrospinal fluid back to the cardiovascular system.

The MeningesThree layers: the dura mater, the

arachnoid, and the pia mater.

The Meninges and Cerebrospinal Fluid

Deep into the dura mater is the arachnoid mater, which has a sort of spider-web like look, it is a connective tissue.

It’s thin strands attach themselves to what is called a pia mater, a pia mater is deep in the meninges, it is the deepest meninx.

The pia mater is very thin like the arachnoid mater and closely follows the brain and spinal cord.

The pia mater is like a subarachnoid space filled with cerebrospinal fluids.

Cerebrospinal fluid, is a clear tissue fluid, it creates a protective cushion around and within the CNS.

It is created by the ependymal cell, a type of neuroglial cell, they are found in hollow, interconnecting cavities of the brain called ventricles.

The CSF fills ventricles and the central canal of the spinal cord, it drains into the dural venous sinuses and returns to the cardiovascular system.

Blockades can occur in this process where the CSF does not return to the cardiovascular system and the brain becomes large because the amounts of CFS that accumulates, this condition is called hydrocephalus and it occurs in babies.

The Spinal CordThe spinal cord is a cylinder of the nervous tissue that starts at the base of the brain and extends through a large opening called the foreman magnum.

The spinal cord is protected by the vertebrae column, the cord passes through the vertebrae canal and ends at the first lumbar veterbra.

The structure of the spinal cord:

The spinal nerves extend from the cord between the vertebrae.

Intervertebral disks are composed of tough fibrocartilage vertebra. If this disk is broken open, this herniated disk may compress spinal nerves. This would cause a large amount of pain and a lost of function.

A cross section of spinal cords is in the central canal, which has gray and white matter. The central canal contains cerebrospinal fluid.

Spinal Cord StructureThe gray matter is located in the center and some sensory neurons and motor neurons are found there as interneurons that communicate with the two types of neurons.

The posterior or dorsal root of the spinal nerve contains sensory fibers entering the gray matter, and the anterior or ventral root of a spinal nerve contains motor fibers exiting the gray matter.

The posterior and anterior roots join, forming a spinal nerve that leaves the vertebral canal.

Spinal nerves are part of the PNS.

The white matter of the spinal cord contains ascending tracts, which take sensory information to the spinal cord and brain, and descending tracts which take motor information from the brain.

Ascending tracts are generally located in the posterior white matter and descending tracts are found in the anterior white matter.

The tacts cross just after they enter and exit the brain. The left half of the brain controls the right side of the body and the right half of the brain controls the left side of the body.

The Functions of the Spinal Cord

The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord.

Sensory receptors generate action potentials that travel by way of sensory nerve axons to the spinal cord.

One of the several ascending tracts next carries the information to the sensory area of the brain.

Action potentials come from the motor control area of the brain and pass down one of several descending tracts to the spinal cord and out to your muscle by the way of motor nerve axons.

The spinal cord has thousands of reflex arcs.

A stimulus causes sensory receptors to generate action potentials that travel in sensory neurons to the spinal cord.

Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract.

Interneurons send signals to several other interneurons in addition to motor neurons.

The Brain

The brain is divided into 4 ventricles: the two lateral, 3rd and the 4th.

The two lateral ventricles would be associated with the cerebrum. The diencephalon with the 3rd ventricle and the brain stem and cerebellum with the 4th.

The electroencephalogram (EEG)- is the form in with the electrical activity of the brain is recorded.

The Cerebrum

The cerebrum is the largest portion of the human brain.

It is the last center to receive sensory input and carry out integration before commanding voluntary motor responses.

The cerebrum communicates and coordinates activities of the other parts of the brain.

The cerebrum also carries out the higher process required for learning, memory, language and speech.

Cerebral Hemispheres- the cerebrum has two halves, the right and left hemisphere. A deep groove, longitudinal fissure, divides both sides but are still connected by a bridge of white matter called corpus callosum.

Cerebral Cortex- Thin but highly convoluted outer layer of grey matter that covers the cerebral hemisphere. This part contains over one billion bodies and is the region that accounts for all the thought process with consciousness including sensation, and voluntary movement.

Motor and Sensory Areas of the Cortex

Primary Motor Area- lies in the frontal lobe just anterior to the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area and each part of the body is controlled by a certain section.

Primary Somatosensory Area- Posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here. The face and hands are some of the areas with the greatest voluntary control, having the largest area of motor cortex in the brain dedicated to them. Primary taste is located within adjacent areas of the parietal lobe. Primary visual area is located in the occipital lobe and the primary auditory in the temporal lobe.

Association Areas and Processing Centers

Association Areas- place where integration occurs and memories are stored. The somatosensory area processes and analyzes sensory information from the skin and muscles. Both the visual and auditory associate new information with previous memories to help you recognize a face or sound.

Cerebral palsy- a condition characterized by a spastic weakness of the arms and legs. It is developed due to a momentary lack of oxygen at birth, which can damage the motor areas of the cerebral cortex.

Processing Centers- of few areas where the cortex receives information from other association areas and perform higher-level analytical functions.

Prefrontal Area- processing in the frontal lobe, receives information from other association areas and uses this information to reason and plan out our actions.

Motor Speech Area- ability to speak is partially dependent on this.

Wernicke's Area- (general interpretive area) receives information from all other sensory association areas. Damage to this hinders a person’s ability to interpret written and spoken messages, even if the words are understood.

Central White Matter, Basal Nuclei, and the

Limbic SystemCentral White Matter- Much of the rest of the cerebrum beneath the cerebral cortex is composed of white matter. Tracts within the cerebrum take information between different sensory, motor, and association areas. The corpus callosum contains tracts that join the two cerebral hemispheres.

Basal Nuclei- While large parts of the cerebrum is made up of tracts, there are masses of gray matter located deep within the white matter. The basal nuclei integrates motor commands, ensuring proper muscle groups are activated or inhibited.

Limbic System- stimulation in this areas causes rage, pain, pleasure or sorrow. By causing pleasant and not pleasant feelings about experiences, it helps guide individuals into behavior likely to increase chance of survival

Hippocampus- vital in processing short-term memory to become long-term. Learning requires memory and memory is stored in sensory regions. What permits memory development is not known. Emotionally charged events result in most vivid memories.

The Diencephalon

Diencephalon- Both the thalamus and hypothalamus located here, in eh 3rd ventricle.

Hypothalamus- forms the floor of the 3rd ventricle. It is an integrating center that aids in the maintaining of homeostasis by regulating hunger, thirst, sleep body temp. and body balance. Produces hormones secreted by posterior pituitary gland and secretes hormones that control anterior pituitary. This serves as the link between the nervous and endocrine system.

Thalamus- consists of two masses of grey matter located in sides and roof of the 3rd ventricle. It is on the receiving end for all sensory input except smell, it functions as a “relay center”. Visual, auditory, and somatosensory information arrive at the thalamus via the cranial nerves and tracts from the spinal cord. It integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus in involved in arousal and also participates in higher mental functions such as memory and emotions.

Located in the diencephalon the pineal gland secretes the hormone melatonin ad regulates our body’s daily rhythms.

The Cerebellum

Cerebellum- Separated from the brain stem by the 4th ventricle. Has to hemispheres which are joined by a narrow median portion. Each portion is primarily composed of white matter and has a treelike pattern, forming a series of complex folds is the overlying thin layer of grey matter on the white.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It receives motor output from the cerebral cortex about where these parts should be located.

It sends motor impulses through the brain stem to the skeletal muscles. By doing this posture and balance can be maintained. This also ensures that all muscles work together to produce smooth, coordinated voluntary movements.

The cerebellum also assists in the learning of new motor skills like learning piano or basketball.

The Brain Stem

Brain Stem- contains the midbrain, pons, medulla, oblongata and the reticular formation.

Midbrain- acts as a relay station or tracts passing between the cerebrum and spinal cord or cerebellum. Has reflects centers for visual, auditory, and tactile responses.

Pons, meaning bridge in Latin, contains bundles of axons traveling between the cerebellum and rest of the CNS. The pons functions with the medulla oblongata to regulate breathing rate and has reflex centers concerned with head movements in response to visual and auditory stimuli.

Medulla Oblongata- contains number of reflex centers for regulating heartbeat, breathing, and vasoconstriction. Contains reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing.

Reticular Formation- assists the cerebellum in maintaining muscle tone as well as helping the pons and medulla in regulating respiration, heart rate, and blood pressure. The sensory component processes sensory stimuli-sounds, sights, touch- and uses these signals to keep us mentally alert. This also helps us rouse a sleeping person.

8.3 The Peripheral Nervous System

The peripheral nervous system (PNS) lies outside the central nervous system and is composed of nerves and ganglia.

Ganglion(ganglia plural) are swellings associated with nerves that contain collections of cell bodies.

The PNS is divided into the afferent or sensory system and the efferent or motor system.

There are two types of sensory systems and two types of motor systems : The somatic sensory system and the visual sensory system and then the somatic motor system and the autonomic motor system.

The somatic sensory system serves the skin, skeletal muscles, joints, and tendons. The special senses such as vision, hearing, taste, and smell are also a part of this system.

The visual sensory system supplies the internal organs. Nerves from both sensory systems take information from the peripheral sensory receptors to the central nervous system.

The somatic motor system carries commands away from the CNS to the skeletal muscles. The autonomic motor system, with a couple exceptions, regulates the activity of cardiac and smooth muscles and glands.

Types of Nerves: Cranial and Spinal

Cranial Nerves and Spinal Nerves

Cranial nerves are attached to the cranium. Humans have 12 pairs of cranial nerves. Some of the cranial nerves are sensory nerves, motor nerves, or mixed nerves. Mixed nerves contain both sensory and motor fibers.

The cranial nerves are mainly concerned with the head, neck, and facial regions of the body. The vagus nerve (cranial nerve X) has sensory and motor branches to the face and many internal organs. This nerve contains both types of sensory nerves and motor nerves.

Humans have 31 pairs of spinal nerves. One of each pair is on either side of the spinal cord. The spinal nerves are grouped together by what region of the vertebral column they are in, the cervical, thoracic, lumbar, or sacral region.

Spinal nerves are mixed nerves because they have both sensory fibers that conduct impulses to the spinal cord and motor fibers that conduct impulses away from the cord to effectors.

Sensory fibers enter the cord via the posterior root and motor fibers exit through the anterior root.

The cell body of a sensory neuron is in a posterior (dorsal)-root ganglion.

Table 8.1

Table 8.2

Somatic Motor Nervous System and

ReflexesMost of the actions in the Somatic Motor Nervous System are voluntary. And example would be when we decide to move a limb. Actions like these originate in the motor cortex. The motor cortex is located in the posterior part of the frontal lobe.

Other actions are due to reflexes which are automatic involuntary responses occurring inside or outside the body. Reflexes are protective mechanisms needed for homeostasis.

Cranial reflexes involve the brain such as when we automatically blink our eyes.

A spinal reflex and the path of somatic motor nervous system is shown in Figure 8.13.

The whole reflex series occurs because certain interneurons carry nerve impulses to the brain through tracts in the spinal cord and brain. The brain allows you to then have the reflex and feel the pain once it receives the information and interprets it.

Reflexes can also be used to see if the nervous system is reacting properly.

Types of Reflexes

The knee-jerk reflex (patellar reflex) is initiated by striking the patellar ligament just below the patella. The reaction is the contraction of the quadriceps femoris muscles, which cause the lower leg to extend.

The ankle-jerk reflex is initiated by tapping the Achilles tendon just above its insertion on the calcaneus. The response is plantar flexion due to the contraction of the gastrocnemius and soleus muscles.

These reflexes are important for normal physiological functions such as helping people stand up straight.

Autonomic Motor Nervous System and

Visceral ReflexesThe ANS is composed of sympathetic and parasympathetic divisions. Both of these divisions function automatically and usually in an involuntary manner, supply all internal organs with nerves, and utilize two motor neurons and one ganglion to transmit an action potential.

Visceral reflexes, such as those that regulate blood pressure are very important to the maintenance of homeostasis. These reflexes begin when the sensory neurons in contact with the internal organs send messages via spinal nerves to the CNS. They are completed when motor neurons stimulate smooth muscle, cardiac muscle, or a gland.

Sympathetic Division: “Fight or Flight”

Most preganglionic fibers of the sympathetic division arise from the middle (thoracic- lumbar portion) of the spinal cord and almost immediately terminate in ganglia that lie near the cord.

The preganglionic fiber is short but the postganglionic fiber that makes contact with the organ is long.

The sympathetic system is very important during emergency situations when a person may have to use a fight or flight reaction.

This system accelerates the heartbeat and dilates the bronchi, making active muscles. It also shuts down the digestive tract and the postganglionic axon releases norepinephine, which has a similar effect to adrenaline.

Parasympathetic Division: “Rest and

Digest”The parasympathetic division includes several cranial nerves as well as fibers from the sacral part of the spinal cord. This division is often called the craniosacral portion of the autonomic system.

In this division the preganglionic fiber (axon) is long and the postganglionic fiber is short since the ganglia lie near or within the organ.

The parasympathetic division is often called the “housekeeper” because it promotes all of the internal responses we associate with a relaxed state. It can be referred to as “rest and digest” because the parasympathetic division slows the heart rate, digests food, etc. The neurotransmitter it uses is acetylcholine.

8.4 Effects of Aging

After the age of 60, the brain begins to lose thousands of neurons daily. These cells cannot be replaced, and thus by the age of 80 the brain weighs about 10% less than what it did at young adulthood.

The cerebral cortex shrinks significantly. As a result, mental activities such as, learning, reasoning and memory decline.

Neurotransmitter production decrease as well, slowing down synaptic transmissions.

Mental impairment is not a consequence of getting older.

It is vital to maintain the health of the cardiovascular system in order to retain mental functions.

Avoiding depression at an elderly age is important because depression is a contributor to mental impairment.

Exercise is also necessary for good health and mental health.

8.5 Homeostasis

The nervous system detects, interprets, and responds to changes in internal and external conditions to keep he internal environment relatively constant. Together with the endocrine system , it coordinates and regulates the functioning of the other systems in the body to maintain homeostasis.

Subconscious control is dependent on reflex action that involve both the hypothalamus and medulla oblongata. Both of these act through autonomic motor nervous system to control important parameters like heart rate, blood vessel constriction and breathing rate.

The hypothalamus works closely with the endocrine system producing the hormone ADH, which causes kidneys to absorb water.

The nervous system controls major body movements due to its ability to stimulate skeletal muscles to contract.

When in “fight-or-flight” mode the nervous system stimulates adrenal glands and voluntarily control skeletal muscles to keep us from danger.

The control of voluntary movements help maintain a moderate environment.