SV

SBI4U - Unit Four - Homeostasis - Nervous and Endocrine System

Nervous System

  • Communication system

  • 100 billion nerve cells in your brain alone

  • 2 main divisions to a vertebrate nervous system

  1. Central Nervous System (CNS)

    • Brain and spinal cord

    • Coordinating centre

  1. Peripheral Nervous System (PNS)

    • Nerves that carry info between the CNS and the rest of the organ systems

    • Somatic Nerve System:

      • voluntary (nerves you can control)

      • connected to skeletal muscles and skin

    • Autonomic Nerve System:

      • involuntary (nerves you can’t control)

      • connected to other organ systems (e.g. circulatory, digestive, respiratory)

Autonomic System

  • Sympathetic System

    • “Fight or flight” mode

      • involves cortisol and adrenaline

    • Increased heart rate, breathing rate, blood flow, blood pressure

    • Reduced digestion

    • Much harder on the body

  • Parasympathetic System

    • “Rest and digest” mode

    • Increased digestion

    • Resting HR, breathing rate, blood flow, BP

Stress

  • Activates the Sympathetic System (fight or flight)

  • Releases cortisol and epinephrine (adrenaline) - both hormones

  • Cortisol increases HR, BP, and breathing rate

  • Chronic stress leads to overactivation of the sympathetic system, wearing the body down over time

Neurons

  • Nerve cells that send electrochemical signals to each other and other parts of the body

  • A nerve is a bundle of many neurons

Reflex Arc

  • The simplest nerve pathway is called a reflex arc

  • Typically occurs in the spinal cord

  • Contains 5 essential components

    • receptor

    • sensory neuron (afferent)

    • interneuron

    • motor neuron (efferent)

    • effector

Sensory Neurons (Afferent Neurons)

  • unipolar

  • carry impulses from sensory receptors to CNS

    • e.g. photoreceptors in eyes (light), thermoreceptors in skin

Motor Neurons (Efferent Neurons)

  • multipolar

  • carry impulses from the CNS to effectors (muscles, organs, glands… AKA things that produce a response)

Interneurons

  • bipolar

  • connects sensory and motor neurons (found mostly in the CNS)

Nervous System - Communication

Nerve Cell Anatomy

  • Cell body

    • nucleus and majority of cytoplasm

  • Dendrites

    • projections of cytoplam that carry impulses toward the cell body

  • Axon

    • extension of cytoplasm that carries nerve impulses away from the cell body

  • Axon Terminal

    • impulses end and chemical is released

  • Myelin Sheath

    • insulated covering (fatty protein) over the axon of some nerves, “myelinated”

      • prevents loss of charge

  • Nodes of Ranvier

    • regularly occuring gaps between sections of the Myelin Sheath

Signal Transmission

  • Nerve impulses jump from one node to another - increases speed of the impulse

    • non-myelinated nerves carry impulses at a slower rate

    • axon diameter also effects speed

      • narrower = faster

  • Neurons in the brain have less myelination on the axons than those located in the spinal cord

    • axons in spinal cord are longer and therefore the signal needs to travel faster

  • Mass of less-myelinated neurons - gray matter - found mostly in the brain and inner section of the spinal cord

  • Mass of highly-myelinated neurons - white matter - found mostly in the peripheral NS (nerve system) and outer section of the spinal cord

Action Potential

  • Firing an action potential is an “all-or-none” response to a stimulus that has reached the THRESHOLD POTENTIAL

  • Threshold examples:

    • “just enough” pressure change on the skin

    • “just enough” temperature change to notice

  • If the threshold is not reached, the neuron does not fire an action potenital; even if there is a stimulus, it’s just not enough

Sodium Potassium Pump

  • Neurons maintain a resting potential by constantly moving Na+ out and K+ across the concentration gradient

  • When the threshold is reached, the cell opens Na+ channels and as those ions rush in, the cell is depolarizing

Neuron Communication

  • Once the electrical imulse reaches the axon terminal and hits the threshold, it releases chemicals called neurotransmitters

  • Neurotransmitter moves across the synapse

    • gap between the axon terminal of one neuron and the dendrite of another neuron

  • Neurotransmitters will bind to receptors of dendrites - when enough bind to receptors, it starts the action potential

Neurotransmitters

  • Acetylcholine

    • sent from motor neurons to the muscle and tissue

  • Glutamate

  • GABA

  • Dopamine

    • responsible for motor function, learning and memory, addiction

  • Serotonin

    • associated with happiness and pleasure

  • Epinephrine

  • Norepinephrine

SSRIs

  • Selective Serotonin Reuptake Inhibitors

  • Antidepressant and anit-anxiety medication

  • Prevents serotonin from going back to the axon terminal from the synapse

  • Allows serotonin to bind to receptor multiple times, eliciting more pleasure and happy feeling

The Brain

  • Composed of three main parts

    • cerebrum (forebrain)

    • cerebellum (hindbrain)

      • controls fine motor coordination

    • brainstem

      • connects brain to spinal cord

        • Medulla Oblongata - controls circulatory and respiratory system

Cerebrum

  • Composed of grey matter

  • The folds (fissures) create more surface area

    • allows faster and greater neural activity (more communication between neurons)

  • Most neural activity of the cerebrum occurs on the surface

  • Corpus Callosum

    • highway of axons that allows the left and right hemispheres to communicate

Lobes of the Cerebrum

  • Frontal Lobe

    • contains prefrontal cortex and motor cortex

  • Parietal Lobe

    • contains primary sensory cortex

  • Occipital Lobe

  • Temporal Lobe

Parts of the Cerebrum

  • Prefrontal Cortex

    • in front of frontal lobe

    • in charge of all planning, decision making, goal setting, time management

    • develops during high school

    • fully develops around age 18

  • Motor Cortex

    • coarse motor movement

  • Sensory Cortex

    • tactile (touch)

  • Frontal Lobe

    • thinking, personality, consciousness, inhibition

  • Temporal Lobe

    • long term memory, hearing

  • Parietal Lobe

    • sensory centre, where all sensory input is processed

  • Occipital Lobe

    • vision

Limbic System

  • AKA the Primitive Brain

  • Composed of white matter

  • Thalamus

    • relays sensory signals, regulates consciousness

  • Hypothalamus

    • controls all needs and processes requiring hormones including: thirst, huger, sleep, BP, fight or flight, sugar intake

  • Pituitary Gland

    • receives signal from hypothalamus to release hormones to regulate endocrine system

  • Hippocampus

    • short-term memory processing, spatial memory

  • Amygdala

    • emotional centre

What Protects the Brain?

  • Skull

    • first line of protection, composed of bone

  • Meninges

    • two membranes filled with cerebrospinal fluid

    • hold the brain and spinal cord in place, act as shock absorber

  • Blood-Brain Barrier

    • thin membrane between blood vessels and the brain

    • only water and small molecules like glucose and ions get through

    • keeps out substances that would be harmful to the brain

Concussions and CTE

Concussion

  • Traumatic force causes the brain to hit or bounce against the skull

  • Leads to shearing of the axons in the affected areas

  • Can have long-term effects

Chronic Traumatic Encephalopathy (CTE)

  • Caused by repeated traumatic blows to the head over a long period of time

  • Brain slowly degenerates over a long period of time

  • Changes to personality, mood swings, and memory loss

  • Can lead to dementia and early death

Senses

Vision

Parts of the Eye

  • Sclera

    • tough outer layer

  • Cornea

    • protects the eye and redirects light into the eye

  • Pupil

    • opening for light

  • Lens

    • thickens for “near” focus and flattens for “far” focus (made of protein that becomes cloudy with age - cataracts)

  • Iris

    • controls the amount of light entering the pupil

  • Retina

    • photoreceptor layer (rods and cones)

  • Macula

    • part of the retina that contains most of the cones

  • Optic Disk

    • optic nerve and blood vessels exit eyeball and there are no photoreceptors (blind spot - brain fills in the picture)

Note: the eye uses Rapid Eye Movement to determine changes in picture

The Retina

  • Photoreceptors are found in the retina

  • Rods - detect low levels of light, night vision

  • Cones - detect different wavelengths or colours, fine detail

    • detects Red, Blue, Green light

    • 6 million cone receptors

  • Sends signals along optic nerve

Vision

  • Optic nerve sends sensory input from the eye to the occipital lobe where it is processed

Hearing

Outer Ear

  • Pinna direct sound waves into ear canal

  • Sound waves vibrate the tympanic membrane (eardrum)

Middle Ear

  • Vibrations of sound are amplified by 3 tiny bones called with ossicles which tap on the cochlea

    • Ossicles:

      • hammer

      • anvil

      • stirrup

Inner Ear

  • Cochlea - spiral-shaped organ filled with fluid

    • vibration from the ossicles virbates the fluid inside and activates hair cells

    • the further the vibration goes in the cochlea, the higher the frequency of sound

    • input from the hair cells is sent to the auditory nerve

    • fluid in the cochlea also detects and determines your balance

Hearing

  • The auditory nerve sends sensory input from the cochlea to the auditory cortex in the temporal lobe where it is processed

Tinnitus

  • May be caused from damage to cochlea

  • Neurons will produce this noise if it does not detect a stimulus

Touch

  • Mechanoreceptors pick up touch stimulus

  • It is sent to sensory neurons which send signals to the spinal cord

  • The signal travels up the spinal cord to the thalamus which redirects the signal to the Primary Sensory Cortex

Homunculus

  • Map/model of where the most neurons are located in the body (LOTS in face, lips, HANDS)

Phantom Limb Syndrome

  • When amputees loose limbs, the signal to those dedicated neurons in the cortex disappears

  • To prevent itself from dying the neuron will connect to neurons belonging to another body part

  • Amputees can still feel their missing limb if they touch the other body part