Unit 4: The Nervous System & Sense Organs

nervous system

brain, spinal cord, and nerves

coordinate the body’s systems by receiving and sending information through electrical signals; maintains homeostasis

functions of the nervous system

sensory input: receives internal and external bodily information through sensory receptors

integration: processes and interprets sensory input to determine where information is sent

motor output: activates effector organs to respond to signals and produce a response

central nervous system (CNS)

brain and spinal cord; body’s command center for processing information

peripheral nervous system (PNS)

31 pairs of spinal nerves, 12 pairs of cranial nerves

nerves that branch out from the CNS and connect to other body parts to relay sensory input and motor commands

somatic nervous system (SNS)

division of the PNS; facilitates voluntary movement of skeletal muscles, transmits sensory information to the CNS

autonomic nervous system (ANS)

division of the PNS; regulates involuntary physiological processes

parasympathetic nervous system

division of the ANS; network of nerves that relaxes the body after periods of stress and danger (“rest and digest”)

lowers heart rate, boosts metabolism, calms the body

sympathetic nervous system

divison of the ANS; network of nerves that stimulate and prepare the body for stress, danger, or exercise (“fight or flight”)

increases heart rate, dilates pupils, widens airways, redirects blood flow to muscles

parasympathetic and sympathetic divisions act

antagonistically, one is excitatory, other inhibits

neurons

nerve cells that transmit information; fundamental unit of the brain and nervous system

cell body (soma)

contains the nucleus and other cell organelles

dendrites

short, branched extensions of a neuron, receives information and transmits to the cell body

axons

single long fibers of a neuron, conducts information away from the neuron

myelin

fatty insulating sheath surrounding axons; increases neural signal conduction speed

produced by oligodendrocytes or Schwann cells

nodes of ranvier

gaps in the myelin sheath along the axon

neurofibrils

fibers and structural proteins within the axons of neurons, provide structural support to the neuron

neuroglial cells

non-neuronal cells in the CNS and PNS that outnumber neurons and provide support, protection, and maintenance of neurons

microglial cells

immune function (neuroprotection), digest debris, kills bacteria, synaptic pruning

oligodendrocytes

make myelin sheath around neurons in the CNS

schwann cells

make myelin sheath around neurons in the PNS

ependymal cells

forms membranes around simple epithelium tissue, filters blood to make and regulate cerebrospinal fluid (CSF)

astrocytes

physically connect blood vessels to neurons to regulate energy and oxygen supply to neurons; most abundant glial cell in the CNS

white matter

myelinated axon that transmits signals rapidly, appears white due to the fat component of myelin sheath

grey matter

unmyelinated axon that primarily processes information

lesions

evidence of nerve cell damage in the brain or spinal cord

symptoms of lesions

varies depending on the location of the lesion;

spinal cord lesions may cause motor problems

back of brain lesions may cause balance problems

reflex arc

the shorter neural pathway that bypasses the brain to mediate an involuntary and rapid response to stimuli, provides protection and facilitates survival

components of a reflex arc

stimulus, receptor, sensory neuron, CNS (spinal cord), interneuron, motor neuron, effector pathway, response

brain function in a reflex arc

message gets sent to the brain later, can choose to override the initial response or act further

simple reflex

reflex arc designed for protection or homeostasis without requiring conscious brain involvement

conditioned reflex

learned physiological, motor, or emotional responses to a certain stimulus 

sensory (afferent) neurons

transmit impulses from receptors to the CNS to perceive stimuli

motor (efferent) neurons

transmit impulses from the CNS to muscles and glands to enable action 

relay neurons (interneurons)

connect sensory neurons to motor neurons to transmit signals between neurons to facilitate quick communication such as in reflex arcs

action potential (nerve impulse)

a rapid electrical signal traveling along a neuron’s membrane to enable communication throughout the nervous system

action potentials originate from

cell body

direction of action potential

dendrites receive signal, signal passes down axon, crosses to the dendrites of a neighboring neuron

resting membrane potential

the difference in electrical charge (voltage) between the inside and outside of a cell membrane when the cell is at rest and not actively signaling, typically -70 mV

steps of an action potential

resting state (1),  threshold, depolarization, repolarization, hyperpolarization, resting state (2)

resting state (1)

stable negative electrical charge of a neuron’s interior (-90 to -70 mV relative to outside), NA+ and K+ channels are closed, neuron is ready but not actively sending a message

threshold

critical membrane potential needed for an action potential to fire (-55 mV), stimulus causes ligand-gated NA+ channels to open, some NA+ begins to diffuse into the cell from outside the cell

depolarization

membrane potential rises from -55 mV to 30 mV, when threshold of -55 mV is met NA+ channels open and causes rapid diffusion of positive NA+ ions into the cell, action potential is elicited

repolarization

membrane potential decreases from 30 mV to a negative value after depolarization peak, caused by NA+ channels closing after depolarization peak, K+ channels open to allow K+ ions to diffuse out of the cell to restore resting membrane potential

hyperpolarization

membrane potential becomes more negative than the resting potential (less than -70 mV) as K+ ions diffuse out of cell, K+ channels close, cell resets, action potentials are temporarily inhibited during refractory period

resting state (2)

sodium-potassium pump uses ATP to pump 3 NA+ out of the cell and 2 K+ into the cell, re-establishes the original ionic gradient, returns the neuron to its initial resting state

summarized steps of an action potential

neuron membrane maintains resting potential

threshold stimulus is received

sodium channels open

sodium ions diffuse inward, depolarizing the membrane

potassium channels open

potassium ions diffuse outward, repolarizing the membrane

action potential causes a local bioelectric current that stimulates the membrane

wave of action potentials travel the length of the axon as a nerve impulse

speed of nerve impulses

speed is proportional to the size of the axon, greater diameters produce a faster impulse

myelinated axon conduction

myelinated axons conduct impulses faster than an unmyelinated one; most neurons are myelinated, but not all are

damaged myelin sheath

slows down, disrupts, fails to get the nerve impulse to their destination

synapse

junction between two neurons where signal transmission occurs, converts electrical action potentials into neurotransmitters to pass information

synaptic transmission

the process at synapses when a chemical signal is released across the synaptic cleft in order to communicate

mitochondria

maintains high-energy synaptic transmission

vesicle

stores, transports, and releases neurotransmitters into the synaptic cleft

receptor

specialized proteins that detect neurotransmitters and convert the chemical signals into an electric response

calcium channel

translates electrical signals into chemical messages

transporter (reuptake area)

end neural signals by absorbing neurotransmitters from the synaptic cleft

nerve pathway

nerve impulse travels from neuron to neuron

neurotransmitters

chemical messengers that transmit signals across a synapse between neurons, muscles, or gland cells

role of neurotransmitters

released at the gap of a synapse to signal to the next neuron to complete the signal, land on receptors on the dendrite receive the chemical message

excitatory neurotransmitters

increase membrane permeability, increases the chance for threshold to be achieved to fire an action potential

inhibitory neurotransmitters

decrease membrane permeability, decrease chance for threshold to be achieved to fire an action potential

reuptake

process in which neurotransmitters are returned to the original cell

what happens if the reuptake transporter is blocked?

leaves the neurotransmitter to remain in the synaptic cleft and continues to stimulate receptors

reuptake inhibitors

drugs that increase the amount of neurotransmitter levels by blocking their reabsorption into the presynaptic neuron

selective serotonin reuptake inhibitors (SSRIs)

antidepressants that increase serotonin levels in the brain by blocking its reuptake (reabsorption) into neurons, leaves serotonin active for longer

acetylcholine

stimulates muscle contraction

dopamine

mood, happiness

serotonin

sleepiness, mood

endorphins

pain reduction, mood

agonists

molecule that has the same effect on the neuron as the original neurotransmitter, it mimics the molecule

antagonist

molecule that blocks the effects of a neurotransmitter

neurological disorder

a disorder affecting the brain, spinal cord, and nerves; disrupts the nervous system

multiple sclerosis (MS)

autoimmune disorder when the immune system mistakenly attacks the myelin sheath in the CNS nerve fibers

causes of MS

autoimmune disorder when the immune system mistakenly attacks the myelin sheath in the CNS nerve fibers

risk factors of MS

genetics, viral infections

signs and symptoms of MS

muscle weakness, numbness, tingling in limbs, fatigue, difficulty with coordination and balance, muscle stiffness, cognitive difficulties

difficulty in diagnosing MS

myelin can be attacked at various locations within the CNS which affects different muscles; varies from person to person

meninges

three layers of tissue membrane between bone and soft tissue

dura mater, arachnoid mater, pia mater

layers of brain

skin, periosteum, bone, dura mater, arachnoid, pia mater

cerebrospinal fluid (CSF)

a clear liquid that circulates the brain and spinal cord, protects and cushions the brain by acting as a shock absorber

cerebrum

largest, uppermost part of the brain, consists of left and right hemispheres and outer cerebral cortex

function: higher mental functions, problem solving

cerebellum

hindbrain structure at the back of the skull

function: coordination of voluntary movements, motor learning, and balance and posture

arbor vitae

branching, tree-like white matter within the core of the cerebellum

function: brings sensory and motor information to and from the cerebellum

brain stem

connects the brain and the spinal cord

function: carries signals that regulate autonomic bodily functions (breathing, heart rate, blood pressure, digestion)

cerebral hemispheres

left and right sides of the brain

corpus callosum

bundle of nerve fibers connecting the left and right cerebral hemispheres

brain convolutions

the folds (gyri) and grooves (sulci) on the surface of the cerebral cortex, increases the brain’s surface area to allow more neurons and neural connections

fissures

deep groove

sulcus

shallow groove

central sulcus

divides frontal and parietal lobes

gyrus

raised ridges or bumps

frontal lobe

executive functions, high-level cognitive functions, motor control, decision-making, personality, social behavior

parietal lobe

perception, spatial awareness, navigation, sense-making, math

occipital lobe

processing, integrating, and interpreting visual stimuli

temporal lobe

higher level visual processing, primary auditory cortex, language processing, long-term memory, identifying visual objects and faces

longitudinal fissure

separates right and left cerebral hemispheres, allows each hemisphere to have specialized functions

transverse fissure

 separates cerebrum and cerebellum

ventricles of the brain

network of four connected fluid-filled cavities that produce, circulate, and store cerebrospinal fluid (CSF), cushion and protect the brain

names of ventricles of the brain

lateral ventricles (2), 3rd ventricle, 4th ventricle

diencephalon

between the cerebrum and the brainstem, consists of the thalamus and hypothalamus

function: relaying sensory and motor signals, maintaining homeostasis, managing emotions and autonomic activities (hormone release)