U6 - Nervous System

Nervous system divided into

• CNS (brain and spinal cord _{^{(includes interneurons)}})

• Peripheral PNS (all nerves including cranial/spinal nerves)

Peripheral nervous system divisions

PERIPHERAL NERVOUS SYSTEM (all nerves)

• sensory (afferent) (send impulses from the senses to the CNS)

• motor (efferent) (sends impulses from CNS to muscles/glands)

  • Somatic nervous system (voluntary muscle control)

  • Autonomic nervous system (involuntary muscle control)

    • Sympathetic (fight or flight, emergencies)

    • Parasympathetic (calms down sympathetic response, provides resting function e.g. digestion, urination)

!!! Nervous system = master control center for body, works w/ endocrine system

Neurons carry impulses, which pass thru processes and into cell body

action potential moves down the length of axon, when reaches axon terminals causes release of neurotransmitters to travel across synaptic cleft

cutaneous sensations - impulses produced in the skin (ex: touch, pressure, hot/cold, pain) - not equally distributed, some parts more sensitive

light passes thru eye, hits retinal pigment epithelium (RPE), light causes molecular changes that begin an impulse, impulse travels thru rods/cones → bipolar neurons → ganglion cells → optic nerve

How nervous system does its job

• Sensory input - detects changes (stimuli) from inside/outside body w/ PNS

• Integration - process/interpret info in CNS

• Response - activation of muscles (motor output) or glands thru PNS

Types of nerve cells

• Neurons - conduct impulses around the body (10%)

• Neuroglia - ‘nerve glue,’ support/protect/insulate neurons (90%)

Structure of neuron

• cell body (nucleus, cytoplasm, organelles)

• Dendrites (bring impulses toward cell body, has receptors)

• Axon (send impulses away from cell body)

  • end in axon terminals which release neurotransmitters to pass the impulse to the next neuron

• Myelin sheath

• Nodes of Ranvier

• Neurilemma (outside layer of myelin)

How to classify neurons

By # of processes that extend outwards

• unipolar - 1 process

• bipolar - 2

• multipolar - many

By function

• Afferent (sensory) carry impulses toward CNS

• Efferent (motor) carry impulses away from CNS

• Interneurons connect afferent & efferent neurons (part of CNS)

Myelin sheath

• axons wrapped in myelin sheath (acts as waxy insulation & helps nerve impulses travel more quickly)

• Schwann cell (type of neuroglia) wraps itself around axon to make myelin

  • many Schwann cells = myelin sheath

  • outer layer = neurilemma

  • gaps btwn myelin = Nodes of Ranvier

Synapse

• where 2 neurons meet (not touch)

• space = synaptic cleft

• at axon terminal, impulse stimulates vesicles to release neurotransmitters

• neurotransmitters open receptors to continue action potential from 1 neuron to next

Meninges

• 3 layers of connective tissue that protect CNS _{^{(superficial to deep listed below:)}}

• Dura mater - thick, tough layer

• Arachnoid membrane - thin, cobweb-like layer

• Pia mater - thin layer containing lots of blood vessels

Cerebrospinal fluid

• btwn arachnoid layer and pia mater (subarachnoid space)

• protects brain by preventing contact w/ the skull

• maintains blood-brain barrier to prevent infection, provide cushioning, & control brain homeostasis

• constantly produced/circulated/reabsorbed in ventricles (4)

  • lateral ventricles (2) (connected to the 3rd ventricle by thin interventricular foramen (foramen of Monro)

  • 3rd & 4th ventricle

• flows around ventricles and absorbed by arachnoid granulations (thru dural venous sinuses) into the blood

_{^{*if CSF is blocked by tumor/injury, can build up and cause hydrocephalus and lead to brain damage (infant → enlarged head)}}

Choroid plexuses

clusters of capillaries in the ventricles that produce CSF

Cerebrum

• largest part of brain

• left and right hemispheres, connected by corpus callosum

• surface covered in ridges (gyri), grooves (sulci), and deeper grooves (fissures) _{^{(see pic)}}

• 4 lobes (frontal, parietal, temporal, occipital) _{^{(names from parts of skull protecting them)}}

3 major layers of the brain (superficial to deep)

• Cerebral cortex - ‘gray matter,’ made of dendrites/cell bodies

• Cerebral medulla - ‘white matter,’ made of myelinated axons

• Basal nuclei - islands of gray matter

Frontal lobe controls:

• 🏃🏻‍♀️voluntary movements

• 🤔reasoning/decision-making

• 🗣️verbal communication (Broca’s area)

• (planning)

• (memory)

• (ability to predict consequences of actions)

Parietal lobe

• separated from frontal lobe by central sulcus

• sensations (pain, temp, touch/pressure)

• body position

• visual-spatial processing

Occipital lobe controls:

visual processing (vision/memory of objects)

Temporal lobes

• separated from frontal lobe by lateral sulcus

• memory _{^{(emotional association of memories)}}

• comprehension/pronunciation of words

• (sensations of smell and sound/hearing)

Areas of the Brain

• frontal lobe

• parietal lobe

• temporal lobes

• occipital lobe

• central sulcus

• lateral sulcus

• primary somatic sensory area (parietal, near central sulcus)

• primary motor area (frontal, near central sulcus)

• Broca’s area (frontal, near lateral sulcus)

How senses react to stimuli

5 major types of sensory receptors

• Mechano- (touch)

• Thermo- (temp. variations)

• Pain (aka nociceptors)

• Chemo- (chemical)

• Photo- (light)

Types of touch receptors

• Free nerve endings (sense pain, temp, touch, pressure - cutaneous sensations)

• Meissner’s corpuscles (_{^{encapsulated nerve endings, found in hairless skin that}} detect light touch)

• Merkel’s disks (detect light touch & pressure w/in epidermis)

• Hair follicle receptors (detect movement of hair)

• Ruffini’s corpuscles (detect deep pressure and stretching of skin)

• Pacinian corpuscles (_{^{encapsulated nerve endings that}} detect deep pressure & vibrations)

Layers of the eye

• Sclera (white of the eye; made of fibrous connective tissue; protects/shapes the eye)

• Choroid (pigmented, vascular membrane that includes the iris & pupil)

• Retina (contains photoreceptors that turn light energy into nerve impulses)

How light enters the eye

• cornea (made of thick transparent tissue) allows light into eye

• iris (colored part of eye located behind cornea) works w/ pupil to regulate light entering

• pupil (opening in the center of the iris) is where light enters

  • low light = open

  • high light = closed

Lens

• semi-solid disc that directs light waves towards the retina

• controlled by ciliary muscles and suspensory ligaments (bend/flatten lens based on distance of image being viewed)

• in front of lens is aqueous humor (thick, jelly like fluid that refracts light and fills the space btwn the lens and cornea)

_{^{as one ages, the lens becomes less elastic, and need corrective lenses}}

Eye anatomy

• sclera

• choroid

• retina

• vitreous humor (thick, gel-like substance btwn lens and retina)

• aqueous humor

• cornea

• iris

• pupil

• lens

• fovea centralis

• optic nerve

Types of photoreceptors in the RPE

• Rods

  • all over retina

  • very sensitive

  • for low light vision

• Cones

  • center of retina

  • less sensitive

  • color/detail

Fovea centralis

only has cones, point w/ the sharpest image

center focal point of the retina

Blind spot

no photoreceptors where the optic nerve meets the eye

_{^{*brain ‘fills in’ the missing images}}

Section of the retina

• RPE _{^{(retinal pigment epithelium)}}

• rods

• cones

• bipolar cells

• ganglion cells

What causes near-sightedness and far-sightedness?

• eye too long → myopia (short-sightedness) _{^{(focuses in front of the retina in the vitreous humor)}}

• eye too short → hyperopia (far-sightedness) _{^{(focuses behind the retina)}}

• Corrective lenses help refract light so it accurately converges on the retina

Structure of the Brain

• Scalp

• Skull

• dura mater

• arachnoid

• pia mater

• subarachnoid space (think arachnoid layer) (btwn arachnoid layer and pia mater)

• subdural space (think dura mater) (btwn dura mater and arachnoid layer)

• choroid plexus

• Lateral ventricle

• Foramen of Monro

• Third ventricle

• Fourth ventricle

!!! Where cerebellum/brainstem is

!!!
aqueous humor (maintains eye pressure/shape)

vitreous humor (keeps eye shape, keeps lens/retina in place)

drugs addicting -> dopamine, glutamate (reward)

Bell's palsy caused by inflammation/swelling of facial nerve (7th)

^symptoms: weakness/paralysis of 1 side of face, droopy eye/mouth/brow, hard to smile, drool, headache (+loss of taste, more sensitive to sound)

diminished prefrontal cortex in adolescence

-pros: more creative/explore, more sensitive to rewards, social learning/connect, neuroplasticity ('use it or lose it')

-cons: poor impulse control/judgment/planning, difficulty regulating emotions, more risk-taking