Nervous System Functions

Nervous System Overview

• Plays a crucial role in multicellular animals
• Helps monitor and maintain a constant internal environment
• Coordinates with the endocrine system
• Both systems gather and analyze to maintain homeostasis in relation to external conditions

Functions of the Nervous System

• Gathers information about the outside world and from inside the body (sensory function)
• Transmits information to the brain and spinal cord for processing
• Processes information to determine the best response (integrative function) i.e. memory
• Sends information to muscles, glands, and organs (effectors) enabling them to respond correctly through muscular contraction or glandular secretions (motor function)

Sensory Input

• Relies on receptors sensing internal and external environmental changes
• Sensory input comes in many forms, including pressure, taste, sound, light, blood pH, or hormone levels
• These are converted into a signal and sent to the brain or spinal cord

Integration and Output

• It occurs in the sensory centers of the brain or spinal cord
• Input is integrated and a response is generated
• The response of motor output is a signal transmitted to organs
• Organs convert the signal into some form of action (movement, changes in heart rate, release of hormones, etc.).

Endocrine System

• An auxiliary control system found in some animals
• Rapid responses to external stimuli are coordinated by the nervous system
• The endocrine system controls slower, longer-lasting responses to internal stimuli
• The activity of the nervous and endocrine systems is integrated

General Construction of the Nervous System

• It has two major divisions
  • Central Nervous System
  • Peripheral Nervous System

Central Nervous System (CNS)

• Consists of the brain and the spinal cord
• The spinal cord transmits messages from the body to the brain for analysis and interpretation
• Response messages are passed from the brain through the spinal cord to the rest of the body.

Peripheral Nervous System (PNS)

• Consists of neurons NOT included in the brain and spinal cord
• Afferent neurons collect information from the body and transmit it toward the CNS.
• Efferent neurons transmit information away from the CNS
• Responsible for body functions not under conscious control (heartbeat, digestive system)
• Smooth operation is achieved by the sympathetic and parasympathetic systems, which have opposing actions to provide a balance

Neuron Types

• Neurons and neuroglia are the main cell types in nervous tissue
• Neurons transmit nerve messages.
• The neuron is the functional unit of the nervous system
• The human brain has about 100 billion neurons
• All neurons have three key parts: dendrites, cell body, and axon

Neuron Components

• Dendrites receive information from other cells and transmit the message to the cell body
• The cell body contains the nucleus, mitochondria, and other organelles typical of eukaryotic cells
• Neurons lack centrioles, so they cannot divide
• Soma contains clusters of rough ER and free ribosomes known as Nissl bodies, which give grey color to areas containing neuron cell bodies.

Neuron Types

• Sensory neurons have a long dendrite and short axon, carrying messages from sensory receptors to the central nervous system
• Motor neurons have a long axon and short dendrites, transmitting messages from the central nervous system to muscles or glands
• Interneurons are found only in the central nervous system, connecting neuron to neuron

Myelin Sheath

• Some axons are wrapped in a lipoid material forms from the plasma membranes of specialized glial cells called Schwann cells
• Schwann cells support, nourish, and service neurons
• The myelin sheath is interrupted at intervals of about a millimeter by small gaps called the nodes of Ranvier
• These gaps are points along the neuron for generating a signal
• Signals jump from node to node to travel hundreds of times faster than signals traveling along the surface of the axon
• Some myelinated axons conduct impulses as rapidly as 200 meters per second

Diseases of the Myelin Sheath

• Destruction of large myelin patches characterizes multiple sclerosis (MS)
• In MS, small, hard plaques appear throughout the myelin
• Normal nerve function is impaired, causing symptoms such as double vision, muscular weakness, memory loss, and paralysis

Glial Cells

• Commonly called neuroglia or glia, are non-neuronal cells that provide support and nutrition for neurons
• They maintain homeostasis, form myelin, and participate in signal transmission in the nervous system
• Glia outnumber neurons by as much as 50 to 1 in the human brain

Nerves and Ganglia

• Nerves are enclosed, cable-like bundles of nerve fibers or axons, including the glia that cover the axons in myelin
• Ganglia are aggregations of nerve cells bound together by connective tissue, forming small irregular bodies
• Nerve cells are gathered into a long central column forming the brain and spinal cord (cerebro-spinal axis, e.g., CNS)

Nerve Message - Membrane Potential

• The plasma membrane of neurons, like all other cells, has an unequal distribution of ions and electrical charges between the two sides of the membrane
• The outside of the membrane has a positive charge, and the inside has a negative charge
• This charge difference is the resting potential and is measured in millivolts
• Most cells have a negative resting potential value, indicating an excess of negative charge inside compared to outside
• Resting potential is -65 mV (millivolts) at rest
• Resting potential results from differences between sodium and potassium charged ions and charged ions in the cytoplasm
• Sodium ions are more concentrated outside the membrane
• Potassium ions are more concentrated inside the membrane
• The Na+/K+ pump maintains potential difference by actively transporting ions against their concentration gradients
• Pumps 3 Na+ ions out of the cell and 2 K+ into the cell
• K+ channels typically have greater permeability than Na+ channels, although there are Na+ and K+ channels in the membrane
• Another source of potential difference is the presence of negatively charged proteins inside the cell

Action Potential

• It is a temporary reversal of the electrical potential along the neural membrane, lasting a few milliseconds

Steps in an Action Potential

• At rest, the outside of the membrane is more positive than the inside
• A stimulus causes sodium channels to open, allowing sodium to move inside the cell, causing depolarization
• Influx of positive sodium ions makes the inside of the membrane more positive than the outside
• If depolarization reaches the threshold, an action potential happens
• Potassium ions flow out of the cell, restoring the resting potential
• This phase of the action potential is called repolarization
• Sodium ions are pumped out of the cell, and potassium ions are pumped into the cell, restoring the original distribution of ions

Threshold

• A sufficiently strong stimulus causes a neuron to conduct an impulse
• Impulse strength is constant: either there is an impulse in response to a stimulus, or there is not
• The minimum stimulus level required to activate a neuron is called the threshold
• Any stimulus weaker than the threshold will not produce an impulse
• Stimuli stronger than the threshold will still only produce an impulse; this is called the all-or-none principle of nerve stimulation

Nerve Stimulation

• Action potential begins at one spot on the membrane but spreads to adjacent areas, propagating the message along the length of the cell membrane
• After the action potential, there is a brief refractory period during which the membrane cannot be stimulated
• This prevents the message from being transmitted backward along the membrane
• Conduction of action potential differs in myelinated and nonmyelinated axons

Axon Types and Action Potential

• Depolarization can only happen in the nodes of Ranvier of myelinated axons (other parts of the axon are insulated with lipoid myelin sheath)
• Propagation of action potential is much faster in myelinated than in nonmyelinated axons

Synapse

• The axon ends with many small swellings called axon terminals
• At these terminals, the neuron may contact another neuron's dendrites, a receptor, or an effector
• Receptors are special sensory neurons in sense organs that receive stimuli from the external environment
• Effectors are muscles or glands that cause a coordinated response
• Neurons transmitting impulses to other neurons do not actually touch
• The small gap between the axon of one neuron and the dendrites or cell body on the next neuron is called the synaptic cleft or synapse
• The presence of synapses ensures one-way transmission of impulses

Synaptic Events

• Action potential arrives at the synaptic knob (axon terminal) and depolarizes the presynaptic membrane
• Voltage-gated Ca2+ channels open, and Ca2+ rushes into the cells
• The intracellular Ca2+ level increase causes secretory vesicles to move to the cell membrane and release their content (neurotransmitters) into the synaptic cleft
• Neurotransmitters diffuse through the synaptic cleft and selectively bind to their receptors
• Receptor binding causes the opening of membrane channels, allowing Na+ ions to rush into the cells
• If signals reach the threshold, an action potential is generated in the postsynaptic cell
• Neurotransmitter molecules detach and are rapidly destroyed
• Neurotransmitter molecules may be broken down by enzymes, taken back by the axon terminal and recycled, or simply diffuse away

Synapses

• The slowest part of the nervous system
• The advantage of having many neurons with gaps is that we can control and receive information from different body parts at different times
• Synapses also ensure one-way transmission of impulses in a living person.

Neurotransmitters

• Axon terminals at a synapse contain tiny vesicles, or sacs, filled with chemicals known as neurotransmitters
• A neurotransmitter is a chemical substance used for signaling from one neuron to another
• The impulse changes from an electrical to a chemical signal in a synapse
• Each neurotransmitter has a special receptor
• Some neurotransmitters cause an action potential; others are inhibitory
• Neurotransmitters are typically small molecules, even hormones
• Time for neurotransmitter action is between 0.5 and 1 millisecond
• More than 30 organic molecules act as neurotransmitters
• Acetylcholine and norepinephrine (NE) are examples, though each acts in different responses
• Both acetylcholine and NE are excitatory neurotransmitters that cause action potential on the postsynaptic membrane

Neurotransmission and Associated Illness

• Gamma-Amino Butyric Acid (GABA) carries an inhibitory message
• Binding to the receptor of the postsynaptic membrane causes hyperpolarization to prevent an action potential from happening
• Once in the cleft, neurotransmitters are active for only a short time
• Enzymes in the cleft inactivate the neurotransmitters
• Inactivated neurotransmitters are taken back into the axon and recycled
• Diseases affecting signal transmission function can have serious consequences
• Parkinson's disease involves a deficiency of the neurotransmitter dopamine
• Progressive death of brain cells increases this deficit, causing tremors, rigidity, and unstable posture
• L-dopa is a chemical related to dopamine that eases some symptoms (by acting as a substitute neurotransmitter) but can't reverse disease progression

Diseases Linked to Neurotransmission

• Clostridium tetani produces a toxin preventing GABA release, where GABA is important in the control of skeletal muscles
• A loss of control chemical unregulates muscle contraction, and may be fatal when it impacts muscles used in breathing
• Clostridium botulinum produces a toxin found in improperly canned foods, causing muscle relaxation and may be fatal
• Several drugs also operate in the synapses like cocaine, LSD, caffeine and insecticides

Effects of Drugs

• Some neurotransmitters are excitatory (acetylcholine, norepinephrine)
• GABA is an inhibitory neurotransmitter
• Dopamine release relates to sensations of pleasure
• Endorphins are natural opioids, eliciting elation and reducing pain
• Artificial chemicals (opium, heroin) have the same effect
• Drugs are stimulants or depressants that block or enhance certain neurotransmitters
• Alcohol causes euphoria, followed by depression

Drugs

• Marijuana (from the Indian hemp plant, Cannabis sativa) has potent chemical THC (tetrahydracannibinol)
• Low THC concentrations cause euphoria if inhaled (most common action form is smoke inhalation)
• High dosages cause hallucinations, anxiety, depression, and psychotic symptoms
• Cocaine (from the plant Erthoxylon coca) can be inhaled, smoked, or injected
• Cocaine users report a "rush" of euphoria
• The rush is followed by a short (5-30 minute) period of arousal and then a depression
• Repeated use cycles terminate in a "crash" once the cocaine is gone
• Prolonged use decreases dopamine production, causing the user to need more of the drug
• Cocaine interferes with uptake of dopamine from the synaptic cleft
• Heroin is a morphine derivative, obtained from opium (milky secretions from the opium poppy, Papaver somniferum)
• Usually injected intravenously, can be snorted or smoked
• Heroin binds to opioid receptors in the brain, where natural chemical endorphins are involved in the cessation of pain
• Physically addictive
• Prolonged use causes less endorphin production
• Euphoria is no longer felt, only dependence and delay of withdrawal symptoms

Central Nervous System (CNS)

• It is composed of the brain and spinal cord
• The CNS is surrounded by bone (skull and vertebrae), as well as fluid and tissue
• These provide insulation for the brain and spinal cord
• The brain has four major parts: the cerebrum, diencephalon, cerebellum, and brain stem
• The latter two are part of the "unconscious brain"

Brain Stem and Midbrain

• In the evolutionary viewpoint the brain stem is the oldest and most primitive part of the brain
• The brain stem is continuous with the spinal cord composed of the medulla oblongata, pons and midbrain
• Medulla oblongata and pons control heart rate, constriction of blood vessels, digestion and respiration

Midbrain

• Consists of connections between the hindbrain and forebrain
• Mammals use this part of the brain only for eye reflexes

Diencephalon

• Composed of the thalamus and hypothalamus
• Provides switching and relay centers integrating conscious and unconscious sensory and motor pathways
• Thalamus acts as a filter incoming sensory information, passing only a small portion of incoming information into the sensory cortex
• Thalamus also plays a role in coordinating voluntary and involuntary movements

Hypothalamus

• Regulates homeostasis
• Has regulatory areas for thirst, hunger, body temperature, water balance, and blood pressure
• Links the nervous system to the endocrine system
• Controls the functioning of the pituitary gland by producing regulatory hormones
• Produces antidiuretic hormone (ADH) and oxytocin, which are stored in the posterior pituitary gland

Cerebellum

• NOT part of the brain stem
• Its functions include fine motor coordination, body movement, posture, and balance
• Enlarged in birds to control muscle action needed for flight

Cerebrum

• Largest part of the human brain
• The cerebrum is divided into left and right hemispheres connected by the corpus callosum
• The hemispheres are covered by a thin layer of gray matter known as the cerebral cortex, the most recently evolved region of the vertebrate brain
• The cortex in each hemisphere is between 1 and 4 mm thick
• Divided into four lobes: occipital, temporal, parietal, and frontal
• No region of the brain functions alone, although major functions of various parts of the lobes are determined
• In reptiles, birds, and mammals, the cerebrum coordinates sensory data and motor functions
• Cerebrum governs intelligence, reasoning, learning and memory

Brain Regions and Functions

• Memory cause isn't definitely known, studies on slugs indicate learning accompanies synapse decrease
• Within the cell, learning involves gene regulation change and increased transmitter secretion ability
• Receives and processes visual information in the occipital lobe (back of the head)
• The temporal lobe receives auditory signals and processes language and the meaning of words
• The parietal lobe associates with the sensory cortex and processes information about touch, taste, pressure, pain, and heat and cold
• The frontal lobe conducts motor activity and integration of muscle activity, speech, and thought processes

Spinal Cord

• The spinal cord runs along the dorsal side of the body to links the brain to the rest of the body
• Vertebrates feature spinal cord encased in a series of body vertebrate
• Gray matter of the spinal cord consists mostly of cell bodies and dendrites
• Surrounding white matter is made up of bundles of interneuronal axons (tracts)
• Some tracts are ascending (carrying messages to the brain); others are descending (carrying messages from the brain)
• The spinal cord also is involved in reflexes that do not immediately involve the brain
• The spinal cord communicates between the brain and the peripheral nervous system
• The spinal cord is continuous with the brain and emerges from a base opening of the skull
• The spinal cord stretches downward for approximately 42 to 45 cm through the vertebral column
• There are 31 pairs of spinal nerves, part of the peripheral nervous system
• Spinal nerves emerge and are named according to respective vertebrae
• Each spinal nerve consists of a dorsal and ventral root

Spinal Cord - Neuron Relationships

• Dorsal roots carry neurons that carry signals to the central nervous system from various sensory neurons
• Ventral roots contain axons of motor neurons, which contact and carry information to the muscles and glands (effectors)
• Within the spinal cord and elsewhere are interneurons, which connect neurons to each other
• In addition to carrying impulses to and from the brain, the spinal cord regulates reflexes

Reflex

• The simplest response to a stimulus.
• Sneezing and blinking are two examples.
• It produces a rapid motor response to a stimulus since the sensory neuron synapses directly with motor neurons in the spinal cord.
• Reflexes are very fast, and most reflexes never reach the brain

Sample Reflex

• Keep your hand over a flame
• Receptors in the stimulated skin
• Sensory neurons carry the impulse to the spinal cord
• Spinal cord neurons automatically activate motor neurons
• Motor neurons cause the muscles (effectors) in your hand to contract to pull your hand away
• This message doesn't go to the brain and is completed in the spinal cord

Reflex Arc

• Receptor, sensory neuron, motor neuron, and effector make up a quick response
• PNS contains only nerves and connects the brain and spinal cord to the body
• Axons and dendrites are surrounded by the myelin sheath
• Cell bodies are in the CNS or ganglia, which are collections of nerve cell bodies
• Cranial nerves in the PNS take impulses to the brain; spinal nerves take impulses to and from the spinal cord
• Two main components: Sensory (afferent) pathways that provide input from the body to the CNS, and Motor (efferent) pathways that carry signals to muscles and glands (effectors)

Sensory Input

• Most sensory input carried in the PNS remains below the level of conscious awareness
• Input that does reach the conscious level contributes to environmental perception
• PNS motor neuron pathways are of two types: somatic (skeletal) and autonomic (smooth muscle, cardiac muscle, and glands)

Somatic Nervous System (SNS)

• Includes nerves controlling the muscular system and external sensory receptors.
• External sense organs (including skin) are receptors
• Muscle fibers and gland cells are effectors
• The reflex arc is an automatic, involuntary reaction to a stimulus
• Reaction to the stimulus is involuntary, with the CNS informed but not consciously controlling the response
• Reflex arc reflexes include balance, blinking and the stretch reflex
• Sensory input from the PNS is processed by the CNS
• Responses are sent by the PNS organs to organs of the body
• Motor neurons of the somatic system are distinct from those of the autonomic system
• Inhibitory signals cannot be sent through the motor neurons of the somatic system

Autonomic Nervous System (ANS)

• Functions regulate bodily functions and the activity of specific organs
• ANS plays a role in narrowing (constricting) and widening (dilating) blood vessels, increasing heart rate and the force of contraction in the heart's beating action
• ANS controls constriction and dilation of airways (bronchioles) in the lungs
• Digestion, respiration, perspiration, constriction and dilation of the pupils, relaxation and contraction of the bowels and sphincters, erection and ejaculation, parturition (child birth), and tear formation
• Many autonomic functions are beyond conscious control; others are impacted voluntarily such as sphincter in urination
• Part of PNS that controls internal organs

Autonomic System

• Has two subsystems: sympathetic and parasympathetic
• Controls muscles in the heart, smooth muscle in internal organs such as the intestine, bladder, and uterus
• The sympathetic nervous system (SNS) is involved in the fight or flight response
• The parasympathetic nervous system is involved in relaxation
• Each subsystem operates in the reverse of the other (antagonism)
• Both systems innervate the same organs and act in opposition to maintain homeostasis
• When scared the sympathetic system causes your heart to beat faster, the parasympathetic system reverses this effect

Sympathetic Nervous System

• Frequently referred to as "fight or flight" system
• Has a stimulating effect on organs and physiological systems
• SNS narrows the amount of available space inside blood vessels while increasing heart rate and force of the heart's contractions
• Narrowing blood vessels creates a smaller space for blood to flow in, helping to raise the pressure of blood in the body
• Increased heart strength makes blood flow more rapidly to locations in the body distant from the heart and lungs
• Nerves that innervate the lungs widen the bronchioles, rapidly providing more oxygen (oxygenation) to blood flowing in to pick up O2 and nutrients
• SNS gives the body a boost of quick energy by stimulating glycogenolysis, helping the liver with lipolysis in adipose tissue
• Sympathetic nerves that innervate the pupils of the eyes can quickly widen both pupils, allowing more light to enter the eyes

Parasympathetic Nervous System

• The 'rest and digest' response
• Slows and relaxes many organ and body system functions
• Causes blood vessels to widen, while slowing the heart beat and decreasing the force of the heart's contractions

Sensory Activity

• Input to the nervous system is in the form of pain, vision, taste, smell, and hearing through our five senses
• Vision, taste, smell, and hearing input are the special senses; pain, temperature, and pressure are somatic
• Sensory input begins with sensors that react to stimuli in energy form transmitted into an action potential and sent to the CNS
• Receptors in sense organs monitor changes in the external and internal environment
• Each receptor has a low threshold for stimuli (e.g., light, changes in temperature).
• Receptors do not interpret stimuli; they receive stimuli and generate nerve impulses
• The brain, not sensory receptors, interprets the stimulus by where it arrives in the brain
• Sensory receptors are classified according to type of energy they detect and respond to

Receptors

• Mechanoreceptors detect hearing, balance, and stretching
• Photoreceptors detect light
• Chemoreceptors detect smell and taste mainly, as well as internal sensors in digestive and circulatory systems
• Thermoreceptors detect temperature changes
• Electroreceptors detect electrical currents surrounding the environment
• Mechanoreceptors vary greatly in stimulus and duration of stimulus/action potentials
• Most adaptable vertebrate mechanoreceptor is the hair cell
• Hair cells are in the lateral line of fish
• Human and mammal hair cells are involved with detection of sound and gravity and provide balance

Human Ear

• It has three ear divisions of outer, middle, and inner ear
• The outer ear consists of pinna or auricle and the auditory canal, lined by fine, air-filtering hair, with modified sweat glands secreting earwax
• The middle ear begins at the tympanic membrane, ending in the membrane-covered openings (oval window and round window) in a bony wall that includes ossicles (malleus, incus, stapes)
• The oval window (eustachian which extends from middle ear to pharynx to equalize inside and outside air
• The inner ear includes semicircular canals, vestibule, and cochlea; cochlea resembles a snail shell that spirals
• Hearing process begins with sound waves entering the auditory canal and ossicles vibrate through size differences between the tympanic membrane, is amplified two-fold over the oval window

Hearing

• Stapes strikes oval window membrane to pass pressure waves to fluid in cochlea
• Three canals are located in vestibular, cochlear, and tympanic canal cochlea
• Along basilar membrane hair cells embedded in tectorial membrane, hair cells of the spiral organ (organ of Corti) synapse with nerve fibers
• When stapes strikes membrane from oval to tympanic canal the cochlear or auditory nerve, pressure waves move across basilar membrane and round window bulges

Equilibrium

• Hair cells that bend are embedded in 8 nerves
• Travel to the brain
• This is interpreted as sound
• Orientation and gravity detected at semicircular canals
• Hair cells along three canal planes respond to fluid cochlea shift, providing three dimension equilibrium sense
• Calcium carbonate crystals can shift in response to gravity providing sensory information about gravity and acceleration. by vision on balance

Human Eye

• The human eye is an elongated sphere 2.5 cm in diameter with three layers
• Sclera: outer, white fibrous layer that is transparent, protects and supports the eyeball
• Cornea: is a transparent part of sclera at front of and known as the window layer
• Choroid: is a dark-brown layed that contains many blood vessels and pigments that absorb stray light rays.
• Pupil: is a ring-shaped ciliary body the iris and forms in front of the eye and regulates size of the opening
• Retina inner layer contains photoreceptors.
• Rod cells and cone cells Fovea centralis is a small area of retina that contains only cones which produces acute daylight color vision.

Light Sensitivity

• Cone is barely sensitive at low intensity at night
• Rods are still active Macula is a small area in the light that holds special cells and allows us to see fine details The optic nerve connects the eye carries the impulses by the retina, and brain interprets as images
• Blind spot is the spot the optic nerve passes. It lacks rods and cones

Eye Photoreceptors

• Rods and cones are back on the retina
• Evolved from hair cells, which detect different lights and detect colors
• More common near edge of the eye

Eye Cones

• Located in fovea, activates on detail and color and light
• Three kids contain green,red,blue composed of retinal and opsin
• Opsins can bind in cone cells and make those cells sensitive for the certain type of color
• Three genes code three opsins
• If the genes defect, may cause color blindness and is found in men

Retina

• There are three layers of neurons in the retina. Nearest to the choroid, cones and rods. The middle includes bipolar cells And ganilion cells forms the innermost. Synapse pass through the bipolar to ganglionic when lights hit them.

Taste and Smell

• Chemoreceptors taste, smell and sensitivity for chemicals. mammals have chemo receptors that are thought to be primitive, they have taste and nose. taste buds located on the tounge with different tastes. The brain makes its choice and perceives the particular taste. Smell has olfactory and nasal cavity 1,000 odor for olfactory with many same type. Olfactory buds are connect with limbic system and the smell associates with feelings, memories, taste and each other