Nervous System

Functions

  • monitors and controls most body processes from autonomic functions to activities that require fine motor coordination, learning and thought

  • maintains homeostasis despite fluctuations in internal and external environment

  • homeostasis - self regulating process used by biological systems to maintain stability while adjusting to conditions required for survival

Process of Carrying Out Its Role

  • messages are relayed throughout the body via nerve impulses and hormones

  • sensory input is brought to central nervous system for processing

  • integration involves sorting, interpreting and determining responses

  • motor output is delivered to effectors to carry out tmay bhe response

Neurons and Glial Cells

  • the two types of cells in the nervous system

  • neurons transmit messages

  • glial cells nourish the neuron, remove their wastes, defend against infection, also provide structural support by making myelin

Types of Neurons

  1. Sensory - relays info from environment to central nervous system (CNS), connect to receptors

  2. Interneurons/Association - linking neurons found in brain and spinal cord, integrate information, non-myelinated

  3. Motor - relay information to effectors such as glands and muscles, end at the end plate attached to the muscle

Neuron Anatomy

  • dendrites - receive information either from receptor cells or other nerve cells, conduct towards the cell body (around 200 per cell body)

  • cell body (soma) - location of the nucleus, high metabolic rate (contains mitochondria)

    • neuron cell bodies are bundled together into ganglia in the PNS

  • axon - may be up to 1 m long, very thin, conducts the impulse towards other neurons or effectors, starts at axon hillock, the smaller the neural diameter, the faster the neuronal transmission

    • neuron axons are bundled together into nerves, nerves are macroscopic and neurons are microscopic

  • myelin - lipid-based insulator surrounding axons, insulates against ion flow speeding up neuronal transmission (around 200 m/s for myelinated neurons vs 1-5 m/s for non-myelinated)

  • nodes of Ranvier - the unmyelinated sections of a myelinated neuron, impulses jump between nodes of Ranvier

  • Schwann cell nucleus - responsible for myelin synthesis, a type of glial cell (supporting and nourishing the neuron)

  • axon terminal/terminal branches - either at a synaptic bulb or end plate to muscle, contains neurotransmitter

  • neurilemma - a thin layer encompassing neurons in the peripheral nervous system, promoting regeneration

Neurons are generally comprised of many neurons together.

Myelinated neurons in the brain are termed white matter (myelin makes them look white) and these will regenerate after injury, whereas grey matter (unprotected) will not regenerate.

Neural Circuits

  • two types of neural circuits

  1. Reflex arcs - without brain coordination

  2. Complicated neural circuit - involving the brain and conscious thought

Reflex Arc

  • when the response is made at the spinal cord level (information does not have to go to the brain to be processed) the response is quick and always correct

  • reflexes protect body from injury

Electrochemical Impulse/Action Potential

  • nerve transmission is not like electrical transmission along a wire

  • Julius Bernstein (1900) suggested electrochemical transmission via ion movement

  • Cole and Curtis (1939) measured resting membrane potential to be -70mV, when excited it changed to +40mV —> action potential

  • once the transmission passed, the membrane repolarized in milliseconds back to -70mV

  • the changes in the transmembrane potential of the axon are a result of sodium ions flowing into the axon and potassium ions flowing out

Phases of the Action Potential

  1. Membrane Polarization (Resting membrane potential)

    1. to establish the -70mV potential in the cell Na+ is actively pumped out an K+ is actively pumped into the cell, this is maintained by the Na/K pump

    2. Na+ and K+ diffuse down the concentration gradient but K+ ions have higher permeability due to an increased number of ion channels (gates) open to K+ ions

    3. since there is a net loss of positive ions to the outside of the cell, -70mV is established

  2. Membrane Depolarization

    1. when the nerve cell is excited, the membrane depolarizes

    2. membranes polarity changes - Na+ channels open, Na+ rushes in and K+ gates close

    3. the positive ions flowing in causes a reversal to +40mV

  3. Membrane Repolarization

    1. once the charge inside the neuron becomes positive, the Na+ gates close and the K+ gates open, eventually restoring charge to -70mV, but the ions are in opposite sides of the membrane

    2. the charge inside can overshoot to -90mV - hyperpolarization

    3. during repolarization the nerve cannot be reactivated - refractory period (1-10ms), this is the recovery time for the neuron

  4. Restoring Membrane Polarization

    1. the Na/K pump restores the ion concentrations inside and outside the cell

    2. pump required ATP to operate

    3. the Na/K exchange pump actively transports three sodium ions outside the cell for every two potassium ions moved inside the cell

    4. small amounts of Na+ and K+ diffuse slowly across the cell membrane following their concentration gradient

Movement of Action Potential

  • the depolarization of the neuron adjacent to an area of resting membrane causes that area to depolarize, moving the action potential along due to attraction of opposite charges

  • since the area from which the action potential came is still in recovery (refractory period) the action potential will only move in one direction

When the impulse travels on myelinated neurons it is termed “saltatory” conduction which jumps between the node of Ranvier

Threshold Potential

  • all neurons provide an all-or-none response - in response to a stimulus, they either activate and provide a certain level of response or don’t fire at all

  • to enable a neuron to fire, it must be stimulated with an intensity of at least threshold level

  • two ways that brain is informed of intensity of a stimulus

    • the frequency of the neuronal firing is increased (not speed, which is constant for each neuron)

    • the number of neurons that respond to that level stimulus (neurons may have different thresholds)

Synaptic Transmission

  • method used to carry a nerve impulse between neurons or neurons and effectors

  1. the impulse travels to the synaptic terminal

  2. synaptic vesicles move toward and fuse with presynaptic membrane

  3. neurotransmitters are released into the synaptic cleft

  4. neurotransmitters bond to receptor proteins and affect the postsynaptic neuron, afterward an enzyme will break up the neurotransmitter, and its components will be reabsorbed by the presynaptic neuron

  • the junction between neurons or neurons and effectors is called the synapse

  • neurotransmitters (typically acetylcholine) are stored in the presynaptic neuron at the end plate

  • enzymes (typically acetylcholinesterase) found in the synaptic cleft deactivated the neurotransmitter

  • if two neurons are connected by the synapse, the postsynaptic neuron will be depolarized at the dendrites

  • the action potential arrives at the terminal branch of of the motor neuron, depolarization causes the release of the neurotransmitter stored in synaptic vesicles via exocytosis at the presynaptic membrane

  • the neurotransmitter diffuses across the synaptic cleft attaching to the receptors on postsynaptic neuron or effector

  • the neurotransmitter causes either an excitatory response (leading to an action potential by depolarizing the post-synaptic membrane) or an inhibitory response (leading to hyperpolarization in the post-synaptic membrane) making it more difficult to generate an action potential

  • an enzyme is released that breaks down the neurotransmitter, allowing for its uptake by the presynaptic neuron. The postsynaptic membrane is now in the recovery phase and will repolarize

  • many mitochondria are found in the postsynaptic bulb suggesting that the synthesis, storage and release of neurotransmitters requires a lot of energy

  • most synapses involve more than just 2 neurons/effectors

  • synapses are on average 20nm apart, however neurotransmitters move only by diffusion so synaptic transmission is much slower than axonal transmission

Ion movement is faster than molecular movement.

Drugs and Neurotransmitters

  • endorphins and enkephalin are natural painkillers produced in CNS, blocking the pain transmitter that usually attaches to the injured organ allowing the perception of pain, opiates block production of pain transmitter, since they act to decrease the production of natural painkillers the amount of opiate taken must be increased or at least maintained to maintain the same effect

  • valium and other depressants are believed to enhance the action of inhibitory synapses

  • alcohol acts to increase hyperpolarization of the membrane, increasing the threshold required to generate an action potential

  • hallucinogenic drugs such as ketamine and psilocybin increase dopamine, serotonin, glutamate and GABA extracellular levels in the frontal cortex

  • LSD chemically resembles serotonin and binds to serotonin receptors, LSD interacts with particular serotonin receptors but not always in the same way, this is why LSD has complex sensory effects

  • insecticides interfere with enzymes that break down neurotransmitters causing their hearts to remain contracted

  • lidocaine, an anesthetic , works by stabilizing the neuronal membrane so it can depolarize

Summation

  • since many neurons will connect to a postsynaptic neuron, it is the summation of the effects of the presynaptic neurons that determine whether or not the postsynaptic neuron or effector will depolarize

Neurotransmitters

  • other neurotransmitters include serotonin, dopamine, GABA and glutamic acid and norepinephrine (noradrenaline)

Nervous System

  1. Central Nervous System

    1. Brain - forebrain, midbrain, hindbrain

    2. Spinal Cord

  2. Peripheral Nervous System

    1. Sensory Division (afferent)

      1. Somatic Sensory

      2. Visceral Sensory

    2. Motor Division (efferent)

      1. Autonomic Nervous System

        1. Sympathetic Nervous System

        2. Parasympathetic Nervous System

      2. Somatic Motor

Central Nervous System (CNS)

  • brain and spinal cord

  • responsible for coordinating incoming and outgoing information

  • integrative and control centers

Peripheral Nervous System (PNS)

  • cranial nerves and spinal nerves

  • includes nerves that carry sensory messages to the CNS and nerves that send information from the CNS to muscles and glands

  • communication lines between the CNS and rest of the body

Motor Division

  • motor nerve fibers

  • conducts impulses from the CNS to effectors (muscles and glands)

Somatic Nervous System

  • voluntary (somatic motor)

  • conducts impulses from the CNS to skeletal muscles

Autonomic Nervous System

  • involuntary (visceral motor)

  • conducts impulses from the CNS to cardiac muscles, smooth muscles and glands

Sympathetic Division

  • mobilizes body systems during emergency situations

Parasympathetic Division

  • conserves energy

  • promotes nonemergency functions

The Brain

  • humans compared to mammals have superior brain development

  • share similar structures with other, but our forebrain is more developed

  • brain consumes more oxygen and glucose than any other part of the body

Meninges

  • outer layer of tough elastic tissue directly enclosing the brain and spinal cord

  • protects CNS by preventing the direct circulation of blood through the cells of the brain and spinal cord - blood-brain barrier

  • barrier blocks toxins and infectious agents, but some substances such as oxygen and glucose can still pass through special transport mechanisms, lipid soluble substances can pass through directly

Cerebrospinal fluid - between inner, middle meninges and central canal of spinal cord, carries nutrients, is a shock absorber, relays waste by diffusion and facilitated diffusion, flow within 4 ventricles in the brain

Parts of the Brain

  1. Forebrain - cerebrum, thalamus, hypothalamus

  2. Midbrain

  3. Hindbrain - pons, medulla oblongata cerebellum

Part of the Forebrain

  1. Cerebrum

    1. contains 2 hemispheres (left and right) for coordinating sensory and motor information

    2. speech, reasoning, memory, personality

    3. outer layer of the cerebrum is called the cerebral cortex (~1mm thick), deeply folded into fissures to increase surface area

    4. left and right hemispheres are connected by the corpus callosum, a collection of nerve fibers allowing more information to be shared between the hemispheres

    5. 4 lobes

      1. frontal lobe

        1. voluntary muscle movement, motor and speech (Broca’s area), intelligence, reasoning, critical thinking, personality, memory

        2. primary motor area

        3. premotor area

        4. motor speech (Broca’s area)

        5. prefrontal area

      2. temporal lobe

        1. auditory reception, sensory speech interpretation (Wernicke’s area)

        2. auditory association area

        3. primary auditory area

        4. sensory speech (Wernicke’s) area

      3. parietal lobe

        1. interpreting sensory information from receptors in the skin (taste, pressure, heat)

        2. primary somatosensory area

        3. somatosensory association area

        4. primary taste area

      4. occipital lobe

        1. “vision” lobes

        2. primary visual area

        3. visual association area

  2. Thalamus

    1. below cerebrum at the base of the forebrain, relay information between the sensory system and the cerebellum and between the forebrain and hindbrain

  3. Hypothalamus

    1. below thalamus

    2. helps regulate the body’s internal environment, controls BP, heart rate, body temp and basic drives (thirst, hunger) coordinates actions of the pituitary, connects endocrine to the nervous system

Pituitary gland - influenced by the hypothalamus, part of endocrine system

Pineal gland - part of the endocrine system, melatonin production

Midbrain

  • less developed in humans than the forebrain

  • 4 spheres —> relay center for some eye and ear reflexes (visual and auditory information)

Parts of the Hindbrain

  • located behind the midbrain, connects brain to the spinal cord

  1. Medulla Oblongata

    1. at base of the brain stem, consists automatic, involuntary functions like heart rate, breathing, swallowing, vomiting, BP, digestion

  2. Pons

    1. relay center between neurons on of the left and right halves of the cerebrum, the cerebellum and the rest of the brain

  3. Cerebellum

    1. coordinates movement, balance, muscle tone (hand-eye coordination)

Although the brain must control the entire body, the volume of brain allocated to each art of the body is not proportional to the body part’s size

Spinal Cord

  • carries sensory information to and from the brain

  • 2 types of nerve tissue

    • white matter - myelinated motor and sensory neurons

    • grey matter - contains mostly cell bodies, dendrites and unmyelinated interneurons

  • ventral root (front of the body) carries motor neuron messages to muscles

  • dorsal root (back of the body) carries sensory neuron messages from the body

  • responsible for reflexes

Peripheral Nervous System

  • emerging from the brainstem and spinal cord

  • consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves

  • spinal nerves are named for the region of body where they are located —> cervical (8 pairs), thoracic (12 pairs), lumbar (5 pairs), sacral (5 pairs), coccygeal (1 pair)

  1. Somatic Nerves

    1. voluntary control of skeletal muscle, bone, skin

    2. contains both sensory (obtaining information from the surroundings) and motor neurons (appropriate muscular response)

    3. control of somatic nerves exists in cerebrum (motor and somatosensory cortex) and cerebellum (coordination)

  2. Autonomic Nerves

    1. controls the internal organs of the body

    2. regulates the involuntary processes of the body (heartbeat, peristalsis)

    3. control exists in the medulla oblongata and hypothalamus

    4. divided into 2 systems with opposing duties

      1. Sympathetic Nervous System

        1. 4 Fs: flight, fight, fright, fuck —> excites

        2. prepares the body for stress

        3. neurons release a neurotransmitter called norepinephrine which has excitatory effects on its target muscles

        4. also triggers the adrenal glands to release the hormones epinephrine and norepinephrine (aka adrenaline and noradrenaline) both which activate the stress response

      2. Parasympathetic Nervous System

        1. activities of rest and recuperation —> calms

        2. activated to restore the body to a calm state and to conserve energy (rest and digest)

        3. uses a neurotransmitter called acetylcholine to control organ responses

The Senses

  • sense organs are equipped with sensory receptors uniquely designed to receive specific types of stimuli

  • once the stimulus is interpreted by the receptor, the message travels to the area of the brain for that type of sensory information along sensory neurons

  • although the receptors are specific to the type of information they receive, all of the stimuli are converted into nerve action potentials

Major Sensory Receptors in the Human Body

  • photoreceptors (vision)

    • rods and cones in the eye stimulated by visible light

  • chemoreceptors (taste, smell, internal senses)

    • taste buds on the tongue stimulated by food particles in saliva

    • olfactory receptors in the nose stimulated by odor molecules

    • osmoreceptors in the hypothalamus stimulated by low blood volume

    • receptors in the carotid artery and aorta stimulated by blood pH

  • mechanoreceptors (touch/pressure/pain, hearing, balance, body position)

    • receptors in the skin stimulated by mechanical pressure

    • hair cells in the inner ear stimulated by sound waves

    • hair cells in the inner ear stimulated by fluid movement

    • proprioceptors in the muscles and tendons, and at the joints stimulated by muscle contraction, stretching and movement

  • thermoreceptors (temperature)

    • heat and cold receptors in the skin stimulated by change in radiant energy

Stimulus acknowledgement is based on survival - ranges of stimuli that are received and their intensity is based on importance for survival

Sensory adaptation - allows for filtering of stimuli that are considered not important and unchanging

Touch

  • variety of specialized receptors located at varying depths of the skin receive stimuli about the surroundings

  • skin contains more than 4 million sensory receptors, but they are not distributed evenly, many are concentrated in genitals, fingers, tongue and lips

Taste

  • taste buds of humans are located on specific areas on the tongue

  • humans can detect 6 tastes —> salty sweet, bitter, sour, umami (savory), starchy and possibly oleogustus (fat)

  • chemical receptors in the taste buds are activated by specific chemical shapes

  • impulses from taste buds travel to areas of the brain stem, to the thalamus, then to the gustatory center of the parietal lobe which is responsible for the perception of taste

Smell

  • human sense of smell can distinguish over 10 000 different odors

  • olfactory receptor cells are located in the nasal cavity and are activated by chemicals binding due to their specific shapes

  • the olfactory bulb of the brain is located at the front of the forebrain

  • to enjoy food, both olfactory and taste senses are involved

The Eye

  • the outside of the eye is designed for protection —> tears, eyebrows, eyelashes, eyelids, recessed in the skull

  • cornea - refracts light towards the pupil

  • iris - controls the amount of light entering the eye (adaptation), allows less light in to accommodate for bright light conditions or to bring near objects into sharper focus

  • aqueous humor - anterior chamber between lens and cornea, provides the nutrients to the cornea and helps refract light

  • lens - focuses the image on the retina, as a person ages, the lens becomes less flexible and can’t change shape enough to allow for focusing on near images

  • ciliary muscles/ligaments - alters shape of the lens to allow near and far focusing (accommodation)

  • vitreous humor - posterior chamber behind the lens, filled with jelly-like fluid, maintains shape of the eyeball and allows transmission of light

  • retina - contains the rods and cones (photoreceptors), forms a thin layer on the inside of the eyeball

  • choroid layer - contains pigments that prevent the scattering of light in the eye by absorbing it, also contains blood vessels

  • arteries and veins - must be present to provide the eye with nutrients and remove waste products

  • fovea centralis - most sensitive area of the retina, contains only cones, is surrounded by a periphery of rods, most of our vision is done in this area (periphery black and white)

  • optic nerve - collects the information from the rods and cones, sending it to the brain (thalamus and occipital lobe) for processing

  • blind spot - area where the optic nerve adheres to the retina, no receptors in this area, so no visual image may be formed at this location

  • sclera - the outermost layer of the eye, thick, supports and protects the eye

  • pupil - allows light to enter the eye, size is determined by the iris

How the Eye Produces an Image

  • light enters the eye through the cornea and lens bending along the way due to the the change in density of the medium and the shape of the lens

  • the image produced on the retina as it would in a camera

  • focusing —> the lens changes shape to focus the image —> accommodation

  • if object is far away, lens flattens, if object is close, lens becomes more rounded

Rods and Cones

  • retina contains 2 types of light receptors:

  1. Rods

    1. located throughout the retina but have low density at the fovea centralis

    2. contains the pigment rhodopsin (visual purple) which is very sensitive to light —> can produce vision in very low light conditions producing shades of grey

    3. to generate an action potential, a photon must strike the rhodopsin in the rod, breaking it into retinal and opsin

    4. this stops the release of an inhibitory neurotransmitter thus allowing transmission of an action potential to the optic nerve

    5. in very bright light, the photons are breaking down the rhodopsin faster than it is restored, incapacitating the rods and reducing the signal sent to the brain

    6. in darkness, rhodopsin is restored very quickly, making each rod more sensitive to light

  2. Cones

    1. used to detect color, three different types each absorbing a different wavelength (red, green, blue)

    2. most densely packed at the fovea centralis - peripheral vision lacks color

    3. color blindness results from defects in certain cone types

Optic Chiasm

  • allows for binocular vision - some of each of the left and right eyes’ visual field crosses to the brain’s opposite hemisphere

  • primary visual cortex produces the original image

  • the visual association area interprets the information and flips/rotates the image

The Ear

  • serves 2 major sensory functions - hearing and balance (equilibrium)

  • sensory receptors for both functions are located in the innermost part of the ear, the inner ear

The Outer Ear

  1. Pinna

    1. the external part of the ear, funnels the sound to the auditory canal

  2. Auditory Canal

    1. amplifies and directs sound waves to the tympanic membrane, lined with glands that make wax and hairs which prevent foreign material from entering the ear

The Middle Ear

  1. Tympanic Membrane (eardrum)

    1. vibrates with sounds, causing the ossicles to vibrate

  2. Ossicles

    1. 3 small bones - malleus (hammer), incus (anvil), stapes (stirrup)

    2. transmit the vibrations to the oval window (opening wall in the inner ear), magnifying sound

  3. Eustachian Tube

    1. connects middle ear to the throat

    2. air-filled tube allows equalization of pressure in the middle-ear

The Inner Ear

  1. Semicircular Canals

    1. fluid-filled structure providing information about body movement and position

    2. contains receptor cells for position

  2. Vestibule

    1. a chamber at the base of the semicircular canals, important in balance

    2. contains the utricle and saccule - two small sacs that establish head position

  3. Cochlea

    1. coiled tube that identifies sounds and converts them to nerve impulses

    2. contains receptor cells

    3. Cochlear Duct

      1. filled with endolymph (fluid)

    4. Scala Vestibuli and Scala Tympani

      1. filled with perilymph that moves with vibration in the oval window

    5. Organ of Corti

      1. consists of stereocilia extending from hair cells lying on the basilar membrane which will move in response to the movements in fluid

    6. Auditory Nerve

      1. transmits messages to the brain

Hearing

  • sounds vibrate the tympanic membrane, vibrating ossicles, vibrating the oval window

  • sound is amplified by the bones up to 3 times

  • the round window bulges outward as the oval window bulges inward, maintaining the pressure and transmitting the pressure change through the fluid-filled cochlea

  • the movement of the fluid back and forth in the cochlea bend the hair-like receptors (stereocilia) located in the organ of Corti, in the cochlear duct

  • movement of the hair cells stimulate the sensory nerves in the basilar membrane, sending the signal via the auditory nerve to the brain

  • different areas of the cochlea are sensitive to different pitches of sound, allowing the brain to interpret the pitch based on the area of the cochlea that was stimulated

  • loudness is interpreted by the number of sensory neurons that respond to the stimulus

Balance

  • organs of balance - semicircular canals, utricle and saccule

  • each semicircular canal ends in a bulge called an ampulla

  • rotational equilibrium - rotating fluid bends the stereocilia in the cupula and the hair cells send a message through vestibular nerve to the brain

  • gravitational equilibrium - the hair cells of the utricle and saccule bend in response to head position