The Nervous System

: a system that controls all the activities of the body

→ NERVES are what allows you to feel, touch, and understand your feelings and your surroundings

homeostasis/equilibrium: balance. state of balance. keeping a constant internal environment

the main things we use our nervous system for:

  • maintaining blood glucose levels

  • maintaining temperature

  • systolic blood pressure

  • maintaining blood pH

the nervous system is made up of:

  • the brain

  • the nerves- talk to the spinal chord

  • the spinal chord- transmits info to the brain (like the highway)

  • senses- taste, sight, hearing

the main process of the nervous system:

  1. sensory input

  2. integration

  3. motor output

sensory input: using senses/sensory receptors we receive information about our internal/external environments.

integration: interpreting what that information means, and what needs to happen consequentially. the information is often later integrated with stored information.

motor output: in situations where it is necessary, effector organs are told to respond and fix the issue.

  • effector organs: the part of the body that carries out the response. example: legs run, eyelids will blink, pancreas will produce insulin, so on.

the organization of the nervous system:

there are two main divisions:the central nervous system & the peripheral nervous system

CNS: the brain and spinal chord

  • coordinates the incoming and outgoing information

  • spinal chord acts as the message highway between the brain and the body

protecting the CNS:

  • bone coverings: the skull and the vertebrae

  • protective membranes surrounding brain: meninges

    • outer layer: dura mater

    • middle layer: arachnoid

    • inner layer: pia meter

  • shock absorber between the pia meter (inner) and arachnoid (middle layer) of the meninges and the central canal of the spinal chord: cerebral spinal fluid

the spinal chord: carries messages from receptor to brain and then back again from the brain to effector organs → receptor → up spine→ brain → down spine →effector organs

  • forum magnum: opening in skull for spinal chord

    • gray vs white:

      • gray: unmyelinated (no fat) neurons/axons (increased processing power)

      • white: myelinated (yes fat) inter neurons that connect spinal chord to brain (speed, message sending/receiving)

        • remember: smart think a lot people seem more grey. white counter tops fast fashion millennial- speedy

    • dorsal vs. ventral nerve:

      • dorsal: brings sensory info in (usually sensory neuron) (on the top)

      • ventral: carries motor info out to the effectors (usually motor neuron) (on the bottom)

the brain: the brain has three regions

  • the forebrain: reason, intellect, memory, personality, language

  • the midbrain: relay center for eyes and ears

  • the hindbrain: muscles, balance, autonomic control

the forebrain deep into it:

  • contains:

    • the cerebrum: what you think of when you think brain. the cerebral cortex is REALLY what you think of when you think brain. (gray matter- unmyelinated, processing power)

      • coordinating center for motor actions, speech, memory, personality

      • 2 distinct hemispheres: right brain (visual/spatial awareness) and left brain (verbal skills and speech)

        • these two distinct hemispheres are linked by a communication bridge called the corpus callosum→ remember man with no corpus callosum

        • each hemisphere of the brain has 4 lobes: frontal, temporal, parietal, occipital

      • thalamus: interprets sensory information

      • hypothalamus: interprets internal environment and instructs the pituitary to produce hormones or the medulla Omblongota to send a nerve signal (autonomic)

      • olfactory bulbs: detect smell

how the cerebrum interprets speech:

  • DOES broca’s area: coordinates speech muscles and translates thoughts into speech (does the speech)

  • INTERPRETS wernickce’s area: language storage and comprehension (interprets the speech)

the four lobes of the forebrain:

  • frontal: voluntary movement (walking, speech), personality, memory, intellect, and conscious thought

  • temporal: interpreting sensory information (hearing, smell, vision)- receives auditory information → ear stuff sends it here

  • parietal: touch, pain, taste (temperature) awareness, interpreting speech, emotions

  • occipital: receives visual information

the mid brain

  • 4 spheres of grey matter

  • relay center for eyes and ears reflexes

hindbrain:

  • contains:

    • cerebellum: controls limb movement, balance, and muscle tone

    • pons: relay station between cerebellum and medulla (like a bridge)

    • medulla oblongata: joins spinal chord to the cerebellum, the sight of autonomic nerve control

PNS: nerve and senses

  • carries information between the effectors/organs and the CNS

within the PNS there are two divisions: the somatic NS and the autonomic NS

→ the somatic (body) NS: consists of the nerves connected to sensory receptors and skeletal muscles, and is what permits voluntary action

  • controls all the nerves involved in body movement: eyes, mouth, ears muscles, and the nerves that create their actions: blink now, move your finger here- exception: reflexes

  • composed of:

    • 12 paired cranial nerves: controls vision, hearing, balance, taste, smell, facial, tongue, head and neck movement

    • 31 paired spinal nerves: controls skeletal muscles

→ the autonomic NS: permits the involuntary function of the organs inside of our body. additionally, the autonomic NS consists of the sympathetic and parasympathetic systems.

  • autonomic nervus system is involuntary internal homeostatic control

  • two types of ANS nerves:

    • preganglionic: CNS-ganglion (sends message from brain)

    • post ganglionic: ganglion-target organs (delivers to effectors)

  • autonomic nervous system halves:

    • the parasympathetic system (the off switch, rest and digest)

      • restores balance

      • long preganglionic nerve

      • preganglionic and post ganglionic release acetylcholine

      • master “off nerve”: vagus nerve (has control over heart, bronchi, liver, and digestive tract)

    • the sympathetic system (on switch, fight or flight)

      • prepares the body for stress

      • SHORT (fast) preganglionic nerve

      • pre ganglionic nerves release acetylcholine

      • post ganglionic nerves release norepinephrine (adrenaline)

cells of the nervous system: glial cells and neurons

  • glial cells are meant for support

    • neurons are to simple and basic to keep themselves alive, so glial cells act as a mother and hold them in place, provide them with nutrients, defend against infection and clean up after them if a cell dies

  • neurons (3 types)

    • sensory neurons (afferent, 90% are found only in PNS)

      • they relay information about the bodies internal/external environment to the CNS (example: from the hand to the spinal chord)

      • receive the first information in the process of the nervous system

      • a comes first in the alphabet- so afferent comes first here

        → afferent means that the cells carry information towards the CNS

    • inter neurons (100% are found in CNS)

      • links neurons in the spinal chord to neurons in the brain

    • motor neurons (efferent, everywhere)

      • carries impulses from the CNS to the effectors

      • efferent which means away from the CNS

        → efferent heads towards effectors

motor neuron intern neuron sensory neuron

the anatomy of a nerve cell:

  • neurons: vary in size, shape, and appearance. work three main jobs: input (sensory neurons) integration (inter neurons), and output (motor neurons).

cell body: functional portion of a cell

→ when inside CNS called cell body, when inside PNS called ganglion

dendrites: looks like they are the arms of the cell body, but actually they are short extensions off the cell body that receive signals from other neurons or the environment

→ can connect to another cell body

axon: long extension off of the cell body that transmits impulses away to other neurons or effectors

the neurilemma: the membrane surrounding the axon that promotes cell regeneration

myelinated cells: cells with myelin sheath

  • Schwann cells are responsible for making myelin sheath → myelin sheath is essentially FAT. technically speaking: myelin sheath is the white insulation that surrounds the axon

    • myelin sheath acts as an insulator which helps prevent the loss of charged ions as they move through the axon. additionally myelin sheath allows for the nerve impulses to transmit across the axon MUCH faster.

    • people with MS end up with damaged myelin sheath- which makes it difficult and a very long process to transmit nerve impulses. the result of this is that eventually everything will slow down/degenerate.

  • the areas between myelin sheath are known as: nodes of Ranvier (essentially these are just gaps in the fat along the axon)

reflexes: involuntary actions that initially surpass interpretation by the brain

  • the message still transmits through all three neurons before there is a reaction, however the brain does not process what has happened (no thought)

    → this is called reflex arc and was developed in humans evolutionarily

    →reflex arc: autonomic response controlled primarily by the spinal chord. the process of a reflex:

    1. stimulus

    2. receptors

    3. sensory neuron

    4. inter neuron

    5. motor neuron

    6. effector organ

      = a response (leg has kicked or whatever)

nerve impulses: nerves have two states

  1. resting potential

    • nerves are polarized: they are off and just resting

    • internal charge is -70 mV

  2. action potential

    • nerves are depolarized: turned on

    • charge jumps to +40 mV

how do we keep it -70 mV inside a neuron?

  • three main methods

    1. sodium potassium pumps: uses ATP to pump 3 sodium ions in and 2 potassium ions out. this is an unequal distribution and leads to a polarized membrane

    2. the presence of negatively charged plasma proteins and ions that never move: example chlorine (Cl-)

    3. k+ (potassium) channels that allow potassium to leak out of the inside of the axon

action potential:

  1. depolarization: stimulus bypasses threshold, causing -70mV to jump to +40mV

    • you cant feel something just a little bit- you either feel it or you don’t. which explains the all or nothing response

  2. repolarization: Na+ channels close, and K+ gates open causing potassium to diffuse out of the cell which restores the original level of polarization (-70mV)

  3. refractory period: the recovery time required before a neuron can return to its resting potential

saltatory conduction (in myelinated axons): HOP SCOTCH→ action potential jumps over myelin sheath and from node or Ranvier to node of Ranvier

  • this leads to faster conduction of action potential than on an unmyelinated axon

threshold level: the minimum amount of stimulus required to get a response (example >2mv)

  • all or nothing response: nerves and muscles respond completely or not at all

  • a charge that is greater than the minimum amount of stimulus required will not create an increased response

    • an increased response: more neurons are being activated

message priority is determined by:

  1. more frequency of impulses

  2. some neurons have higher threshold levels only set of with increased stimulus

    • therefore: the greater the number of impulses reaching the brain the greater the intensity of the response

THE SYNAPSE: the space between

  • a synapse is what divides neurons

    • presynaptic neurons: releases neurotransmitters into the synapse

    • postsynaptic neurons: receives neurotransmitters from the synapse

    • neurotransmitter: chemical messenger for neurons

      • IN ORDER FOR action potential to bridge the gab between two cells (the synapse) it must be converted into chemical energy

        • → here is how it is done

        1. the presynaptic membrane is depolarized (with the aid of calcium- Ca2+)

        2. synaptic vesicles release neurotransmitters (ACETYLCHOLINE) from the axon bulbs/end plate

        3. neurotransmitters diffuse and bind to receptors on post synaptic dendrites

        4. post synaptic membrane opens either ion channel

          • Na+ flows in= excitatory

          • K+ flows out = inhibitory

        5. - neurotransmitters are broken down by enzyme (cholinesterase) and ion gates close

          - neurotransmitters are absorbed by pre-synaptic neurons for rebuilding

  • the role of neurotransmitters:

    • common neurotransmitters: dopamine, serotonin, endorphins

    • excitatory (when Na+ flows in): triggers receptors in post synaptic cleft to allow positive ions (sodium) in→ this leads to depolarization

    • inhibitory (when potassium goes out): triggers potassium channels to open→ leads to hyperpolarization

summation: the effect of the accumulation of neurotransmitters from two or more neurons, can be inhibitory or excitatory

  • neurotransmitter disorders:

    • parkinson’s disease: inadequate dopamine (inhibitory) causes involuntary muscle contraction

    • alzheimer’s disease: inadequate acetylcholine (excitatory) deterioration of memory and mental capacity

brain imaging technology

  • PET scan: track activity and usage- radioactive glucose is consumed in certain parts of the brain and when that part of the brain is used it will light up with different colors on the scan.

  • MRI: giant magnets that detect changed in H+ that emit radio signals

the structure of the eye:

  • composed of three separate layers: sclera, choroid layer, retina

    1. sclera: outer most layer of the eye- protection. “whites”- no blood vessels, gets O2 from dissolved tears

      • covered by the cornea (transparent tissue that refracts light towards pupil)

      • gets nutrients from the aqueous humor- clear liquidly fluid in the front of the eye

      • lack of nutrients and O2: glaucoma

    2. choroid layer: middle layer that has pigments that prevent scattering by absorbing stray light- has many blood vessels

      • iris: colored muscle that regulates the amount of light that enters the eye

      • pupil: opening for light to go in

      • lens: focuses image on the retina by action of dorsal and ventral ciliary muscles

      • vitreous humor: cloudy, jellylike material that maintains eye shape and lets light through

    3. retina: innermost layer, composed of three layers of cells:

      • light sensitive rods and cones: c=color

      • bipolar cells: pass messages from rods and cones to cells of the optic nerve

      • optic nerve cells: ganglion cells (cell in a cluster of cells found in the PNS)

      HOW IT WORKS:

      1. light depolarizes bipolar cells

      2. bipolar cells deliver messages to rod and cones

      3. bipolar cells send color shade/information to optic nerve

rods and cones:

  • rods: used in dim light

  • cones: used for identifying colors- red, blue, and green cones

    1. retina continued:

      • fovea centralis: closely packed with cones at the center of the retina; most sensitive area of the eye

      • blind spot: lack of rods and cones, where the optic nerve comes in contact with the retina

focusing an image: light entering the eye is bent/focused by cornea towards the pupil→ the lens further bends (refracts) light towards a focal point (fovea centralis). The image is inverted but the brain corrects it.

  • accommodations: adjustments made to lens and pupil to view near or far objects

focusing for close vision: thickening the lens shape by flexing ciliary muscles allows tendons to relax

focusing for distant vision: lens becomes thin by ciliary muscles relaxing and tendons pulling- pupils may also dilate to allow for as much light in as possible

vision defects:

  • cataracts: lens or cornea becomes clouded

    • solutions: replace cloudy lens with a plastic one or remove lens and use glasses instead

  • astigmatism: abnormal curvature of the cornea or surface of the lens

    • solution: unevenly ground lenses

  • nearsightedness (myopia): image is focused in front of the retina

    • solution: glasses with concave lenses

  • farsightedness (hyperopia): image is focused beyond the retina (the eyeball is too small)

    • solution: glasses with a convex lens

  • color blindness: inherited condition where one lacks certain cones (r. b, or g)

after images: caused by the fatigue of a cone in an area of the retina: can be positive (strobe light) or negative (green and red cross)

the muscles of the eye:

  • extrinsic muscles: OUTSIDE, move the eyeball in the socket

    • rectus muscle: up and down, in and out

    • oblique muscles: rotation, left and right

  • intrinsic muscles: INSIDE, controls lens and iris and ciliary muscle body

    • ciliary body muscles: involuntary muscles that contract, causing the body to move forward and the lens shape to change.

      • ciliary body is a part of the choroid (middle) layer

      • contains ligaments that attach the lens to smooth muscles

the structure of the ear

  • ear is crucial in hearing and balance (equilibrium): both mechanoreceptors

essentially: the inner ear cells have tiny cilia which respond to mechanical stimuli (movement) which causes the nerve cell to generate an impulse

the ear can be divided into three sections: inner, middle, and outer

  1. the outer ear:

    • the pinna: outer flap, acts as a sound funnel, directing it into the auditory canal (what we think of when we think of ear.

      • shape is very intentionally designed for the purpose of directing it into the auditory canal

    • auditory canal: channel that carries sound waves towards the ear drum

      • specialized sweat glands within the auditory canal produce ear wax which is intended to trap invading particles

  2. the middle ear: begins at the ear drum ends at the vestibule

    • tympanic membrane: the eardrum

    • ossicles: amplifies/passes sound waves from the eardrum to the oval window by using three small bones:

      • malleus (hammer)

      • incus (anvil)

      • stapes (stirrup) -smallest bone in the body

    • oval window: smaller than the eardrum, amplifies sound- connects vestibule to middle ear, flap like bone

    • eustachian tube: air filled tube that equalizes pressure between internal and external ear (the reason your ears pop on airplanes)

      • also drains excessive fluid to nasal cavity

  3. the inner ear:

    • vestibule: connected to middle ear by the oval window

      • inside of the vestibule there are two small sacs that are involved in maintaining head balance- the utricle and saccule

    • semi circular canals: three fluid filled canals that help body balance

    • COCHLEA: the snail. coiled snail shell lined with hair cells that respond to different sound waves and frequencies and then convert them into electrical impulses

    • auditory nerve: transmits sound signals to the CNS

hearing and sound HOW IT WORKS:

  • sound energy waves (mechanical energy) get converted into chemical energy (Na+, K+)

  • ossicles move a shorter distance then eardrum =amplification of sound

  • intense noise causes the muscles around the ossicle to contract = restricting movement, and pulling stapes from the oval window which means there will be less sound amplified and transmitted

  • oval window is moved inwards, which moves the round window outwards

  • triggers waves in fluid within the inner ear to the cochlea- this converts the waves to electrical impulses

organ of chorti: primary sound receptor in the cochlea

  • one inner row and three outer rows of hair

  • basilar membrane: anchors the receptor hair cells to the organ of chorti

    • fluid vibrations move the basilar membrane and the hair cells bend, which stimulates the sensory nerves that send the auditory signal to the brain (temporal lobe)

  • tectorial membrane: non mobile top layer

  • stereocilia: “the ear hairs”,

    • differ in width and lengths for pitch and volume perception

    → sound wave → tympanic membrane → ossicles → oval window →organ of chorti (cochlea) → round window → temporal lobe

hearing loss:

  • conductive (vibration) hearing loss: caused by wax buildup, middle ear infection, punctured ear drum

    • treatment: hearing aid: amplifies sound and transmits to ear drum

  • sensorineural hearing loss: auditory nerve is severed, or cochlear hair cells are damaged

    • treatments: cochlear implants

the ear and balance:

  • the liquid in your inner ear is what is responsible for balance: this liquid moves when we move and sends information to the brain to indicate how we are moving

in the body: balance is maintained by three semi-circular canals

  • in each fluid filled canal there is a pocket (ampulla) that contains hair (cilia) that moves with the gel like (cupulla).

  • cupula is pulled by forces (example gravity)

  • movement causes the cupula to bend the cilia, initiating a nerve impulse that is carried by the CNS

in the head: maintained by two fluid filled sacks in the vestibule: saccule and utricle

  • in each sac there are hair like receptors that are suspended in a jelly like material that contains tiny carbon stones called otoliths

  • when the hair is bent sensory receptors are stimulated and send a message to the brain