Nervous System Overview

Subdivisions of Nervous System

Subdivisions of Nervous System

Two major subdivisions• Central nervous system (CNS)

– brain and spinal cord – Is protected by the cranium and vertebral column– Nucleus = a group of cell bodies in the CNS

• Peripheral nervous system (PNS)– Consist of the rest of the nervous system.– 31 Spinal nerves and 12 cranial nerves are bundled

extend of both the brain and spinal cord.– ganglion = groups of cell bodies in a nerve that are

located outside the (CNS)

Gray and White Matter

• Gray matter = neuron cell bodies, dendrites, and synapses

– forms cortex over cerebrum and cerebellum

– forms nuclei deep within brain

• White matter = bundles of axons

– forms tracts that connect parts of brain

Meninges

• Dura mater: outermost, tough membrane– outer periosteal layer

against bone• Arachnoid layer:

– subarachnoid space were cerebral spinal fluid flows

• pia mater:– is thin layer in direct

contact with brain tissue

Meningitis• Inflammation of the meninges

• Disease of infancy and childhood– between 3 months and 2 years of age

• Bacterial and virus invasion of the CNS by way of the nose and throat

• Signs include high fever, stiff neck, drowsiness and intense headache and may progress to coma

• Diagnose by examining the CSF– lumbar puncture (spinal tap)

Cerebrospinal Fluid• Choroid plexus creates CSF by filtration of blood which

then fills ventricles and subarachnoid space• Functions

– Creates a buoyant environment for the brain and spinal. • Blood-brain barrier is endothelium that is permeable to lipid-

soluble materials

• alcohol, O2, CO2, nicotine and anesthetics

• 3rd and 4th ventricles are breaks in the barrier where blood has direct access

– monitors glucose, pH, osmolarity

Flow of Cerebrospinal Fluid

Cerebral CortexNewest part of brain consisting of 4 lobes

– frontal, parietal, temporal and occipital– The cortex is involved in sensory perception of all 5 senses,

motor control of skeletal muscles, memory, emotion and the ability to learn learning

• Frontal – voluntary motor functions – planning, mood, smell and social judgement

• Parietal – receives and integrates sensory information

• Occipital – visual center of brain

• Temporal – areas for hearing, smell, learning, memory,

emotional behavior

Functions of Cerebrum - Lobes

Functional Regions of Cerebral Cortex

• Functions in regulating many primal regulatory functions. – hormone secretion – Regulation of Pituitary gland– autonomic NS control– thermoregulation– food and water intake (hunger

and satiety)

Hypothalamus

Cerebellum

• Cerebellum– Important for coordinating

various sensory and motor inputs.

– Involved in coordinating – Thoughts and planning

events– maintains posture

influences volitional movement

– Aids in speech and distinguishing pitch and similar sounding words

Brain Stem

Medulla OblongataCardiac center – Acceleratory center: increases

blood pressure, heart rate and force of heart contraction by stimulating the sympathetic nervous system

– Inhibitory center: decreases heart rate and force by stimulating parasympathetic nervous system

• Respiratory centers – Works with Pons in controlling

rate and depth of breathing

• Pons– concerned with posture, sleep,

hearing, balance, taste, eye movements, facial expression, facial sensation, respiration

Cranial Nerves

• 12 pairs of cranial nerves originate from the base of the brain • They control a variety of functions relating to the head, neck, and

internal organs• They can be sensory ,motor or both

Overview of Spinal Cord• Information highway between brain and body

• Cylinder of nerve tissue within the vertebral canal (thick as a finger)

• The spinal cord has 31 pairs of spinal nerves. – sensory component which enters the back of the cord – motor component that exits the front of the spinal cord

to their specific muscles or glands.

• Spinal cord is a part of the central nervous system • Spinal nerves are part of the peripheral nervous

system

Gross Anatomy of Lower Spinal Cord

Meninges of Vertebra and Spinal Cord

Cross-Sectional Anatomy of the Spinal Cord

• Central area of gray matter shaped like a butterfly and surrounded by white matter in 3 columns

• Gray matter = neuron cell bodies with little myelin• White matter = myelinated axons

Gray Matter in the Spinal Cord• Pair of dorsal or posterior horns

– Sensory input returns to the spinal cord via the dorsal root of spinal nerve– Here it may synapse with an interneuron, motor neuron or ascend up the

spinal cord• Pair of ventral or anterior horns

– Motor fibers descending from the brain leave the spinal cord via ventral root of spinal nerve.

• Central cavity :location of cerebrospinal fluid (CSF)

Sensory Receptors• Specialized structures that detect change in the environment.

Stimulation of sensory receptors results stimulates afferent impulse to the CNS The brain or spinal cord will interpret the afferent impulse and respond accordingly.

Sensory Receptors

• Mechanoreceptors : respond to physical deformation such as vibration, pressure, stretch and tension

• Thermoreceptors : respond to heat and cold• Photoreceptors: Located in the eyes respond to light• Chemoreceptors : respond to chemical change in the

body such a taste smell and body fluid composition• Nociceptors : respond to pain• Proprioceptors: gives your body a sense of position

and movements – Located in joint capsules, muscles and tendons.

Sensory Perception• Specific receptors for touch, pressure, stretch,

temperature, and pain transmit afferent impulses to cortex• Somatosensory area in postcentral gyrus

Sensory Homunculus

• Area of cortex dedicated to sensations of body parts is proportional to the sensitivity of that body part (# of receptors)

Somesthetic Projection

(Ascending) Pathways

• Pain or temperature ascends via the anterolateral pathway• Proprioception( awareness of where one’s body is ), touch and

pressure ascends via the dorsal column pathway.• Sensation is projected to the other side of the brain.

Motor Control

• Intention to contract a muscle begins in motor association (premotor) area of frontal lobes

• Precentral gyrus (primary motor area) relays signals to spinal cord to stimulate muscles on the opposite side of the body.

• Motor homunculus - proportional to number of muscle motor units in a region

• Skeletal muscle control has a 2 neuron pathway– A neuron from the cerebral

cortex ( precentral gyrus)descends downward.

– The axons cross in the medulla before the terminate at the alpha motor neuron (upper motor neuron)

– The upper motor neuron synapses with the alpha motor neuron in the ventral horn of the spinal cord.

– The axons of the alpha motor neurons project to specific muscles.

(lower motor neuron)

Corticospinal Tract

Spina Bifida

• Congenital defect in 1 baby out of 1000• Failure of vertebral arch to close covering spinal cord• Folic acid (B vitamin) as part of a healthy diet for all

women of childbearing age reduces risk

• Sensory (afferent) divisions (receptors to CNS)– visceral sensory( messages from organs) allow our CNS

to interpret internal environments. – somatic sensory division ( messages from skin, joints,

muscles) allow our CNS to interpret both our external environments.

• Motor (efferent) division (CNS to effectors) response to the environment through excitation of: – visceral motor division (ANS) Involuntary

effectors: cardiac, smooth muscle, glands• sympathetic division (fight or flight)• parasympathetic division (rest and digestion)

– somatic motor division (voluntary) effectors: skeletal muscle

Functional Divisions of PNS

Anatomy of a Nerve

• A nerve is a bundle of nerve fibers (axons)• Epineurium covers nerves, perineurium surrounds a

fascicle and endoneurium separates individual nerve fibers

Anatomy of Ganglia in the PNS

• Cluster of neuron cell bodies in nerve in PNS is the dorsal root ganglion

Autonomic Nervous System• Motor nervous system controls glands, cardiac

and smooth muscle – also called visceral motor system

• Regulates unconscious processes that maintain homeostasis– BP, body temperature, respiratory airflow

• ANS actions are automatic– biofeedback techniques such as controlling

breathing rate can dramatically alter the bodies internal state.

• train people to control hypertension, stress , anxiety, and migraine headaches

Divisions of ANS• Two divisions innervate same target organs

– may have cooperative or contrasting effects

1. Sympathetic division – prepares body for physical activity or any kind of stress!

• increases heart rate, BP, airflow to organs vital for dealing with the stress( skeletal muscles, heart and brain), blood glucose, fatty acids levels all increase– If you are chased by a tiger he won’t be very sympathetic

2. Parasympathetic division – calms many body functions and assists in rest and

restoration functions such as digestion and waste elimination.

• Decreased heart rate, BP and blood will be shunted away from skeletal muscles and towards vital organs i.e. intestinal tract ,kidneys and reproductive organs

What part of the ANS is this person activating.

Efferent Sympathetic vs. Parasympathetic

Dual Innervations of the Iris• ANS has membrane receptors to both

ACh and NE• Depending on which neurotransmitter

is released– PNS causes the release of ACH

from its postganglionic neuron ACH then binds to its receptor on the will cause the effector to respond in one way ( pupils constrict)

– The SNS causes the release of NE from the postganglionic neuron. That will bind to its receptor causing the effector to respond in the opposite manor ( pupils dilate)

• Some organs have receptors for both. The heart :– NE increases both force and rate

of heart contraction– ACh decreases both force and

rate heart contraction.

Blood Flow in Response to Needs

The Stretch (Myotatic) Reflex• When a muscle is stretched, it contracts and

maintains increased tonus (stretch reflex)– helps maintain equilibrium and posture

• head starts to tip forward as you fall asleep• muscles contract to raise the head

– stabilize joints by balancing tension in extensors and flexors smoothing muscle actions

• Very sudden muscle stretch causes tendon reflex– knee-jerk (patellar) reflex is monosynaptic reflex– testing somatic reflexes helps diagnose many diseases

• Reciprocal inhibition prevents muscles from working against each other– When the biceps contracts which muscle is inhibited?

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The Patellar Tendon Reflex Arc

Golgi Tendon Reflex• Proprioceptors in a

tendon near its junction with a muscle -- 1mm long, encapsulated nerve bundle

• Excessive tension on tendon inhibits motor neuron– muscle contraction

decreased

The Eye

• Humans have binocular vision (use both eyes)

• Sclera is the white of the eye provides protection and site for muscular attachment.

• The iris contracts and dilates controlling the amount of light that enters the eye

• Ciliary muscle change the shape of the lens enabling the eyes to focus at different distances.

• Extra ocular eye muscles and nerves

Retina• Light gets focused here. Photoreceptors are a specialized receptors

that convert light waves into electrical impulses. – Rods allow for tracking of objects, night and peripheral vision .

Most abundant receptor of the retina.– Cones : color vision and acuity. In highest concentration of in

the fovea. ( turn heads towards objects)– Optic disc is where the blood vessels enter and optic nerve exits

the retina to project to the cortex. (blind spot)

The Retina

Visual Cortex

• Images from the retina are projected to the cortex.

• The optic nerves join at the optic chiasm. Half of the neurons will cross here before they project to the cortex via the optic tracts.

• Left optic tract :• L lateral retina medial vision for

the Left eye• R medial retina for lateral vision

of the right eye • Optic tracts synapse in the thalamus

and then project to the visual cortex of the occipital lobe.

• What effect will cutting the optic chiasm in the sagittal plane have on vision?

.

What Do You See?

• Ambiguous images will appear the same on the retina however can often be interpreted differently in the visual cortex.

• The association areas are influenced by previous exposure, mood and environment.

The Ear

Outer, Middle and Inner Ear• Outer Ear

– auricle • focuses sound into the auditory canal towards the tympanic

membrane (ear drum)• Middle Ear

– auditory (Eustachian) tube connects to nasopharynx• equalizes air pressure on tympanic membrane

– ear ossicles• malleus • incus • Stapes

• Inner Ear– cochlea

• organ of sound reception– vestibular apparatus

• semicircular ducts, utricle and saccule– organs of equilibrium and balance

Stimulation of Cochlear Hair Cells

• Vibration of ossicles causes vibration of basilar membrane under hair cells.– as often as 20,000 times/second

Spiral Organ

• Stereocilia of hair cells attach to gelatinous tectorial membrane

• Inner hair cells

– hearing

• Outer hair cells

– adjust cochlear responses to different frequencies

– increase precision

Sensory Coding of Sound

• Sensory neurons associated with the hair cells exit the cochlea via the cochlear nerve and ultimately project to the primary auditory cortex

• Vibrations of the basilar membrane excite more inner hair cells over a larger area

• louder sound – triggers higher frequency of action potentials• Pitch depends on which part of basilar membrane

vibrates– at proximal end, membrane narrow and stiff

• brain interprets signals as high-pitched– at distal end, membrane wider and more flexible

• brain interprets signals as low-pitched

Equilibrium• Control of coordination and balance

• Receptors in vestibular apparatus

• Static equilibrium – perceived by macula– perception of head orientation

• Dynamic equilibrium– perception of motion or acceleration

• linear acceleration perceived by macula ( moving in a car)

• angular acceleration perceived by crista ( turning head)

Macula

• Made up of the saccule and utricle

• Require gravity to work.• Static equilibrium - when

head is tilted, weight of membrane bends the stereocilia

• Moving in a car the linear acceleration is detected as otoliths lag behind.

• This bends the stereocilia and one kinocilium resulting in a specific direction would result in stimulation of the vestibulocochlear nerve

Saccule and Utricle

• Referred to as the gravitational receptors. – hair cells with stereocilia

and one kinocilium buried in a gelatinous otolithic membrane

– Otoliths embedded in the membrane (CaC03 add to the density which increases inertia and enhance the sense of gravity and motion

• Utricular hairs respond to horizontal movement( driving)

• Saccular hairs respond to vertical movement ( elevator)

Semicircular Channels• Crista ampullaris :consists

of hair cells buried in a mound of gelatinous membrane (one in each duct)

• Orientation causes ducts to be stimulated by rotation in different planes

• The combination of 6 channels can provide sensory feedback in all planes of motions.– Each channel signals

2 direction. (Forward and backwards)

– This gives us information regarding acceleration and deceleration.

Crista Ampullaris - Head Rotation

• As head turns, endolymph lags behind, pushes cupula, stimulates hair cells

• This results in stimulation of the vestibular division on the vestibulocochlear nerve

Psychological Disorders• Anxiety: involves the

hippocampus (memory) and amygdala (fear) of the limbic system.– Characterized as an intense

fear, apprehension, or worrying.

– Neural connections to the hypothalamus result in a sympathetic response.

• Increases in – Heart rate (palpations)– Blood pressure (Headache)– Excessive sweating

(diaphoreses)

The Red Flags For Systemic Pathology

The ANS is talking to you! 1.Diaphoresis / Night sweats2.Nausea3.Diarrhea4.Pallor5.Dizziness / Syncope6.Fever

How can the above symptoms be related to the ANS?

Common Medication Side Effects and the ANS

GI motility & distress :( Almost all medications taken orally)

• Bradycardia/Tachycardia ( Beta Blockers vs Pseudoephedrine)

• Pupils dilated /constricted (Amphetamine vs Alcohol)

• Difficulty breathing /Bronchoconstriction (Non specific Beta Blockers)

• Dizziness Syncope/ Orthostatic hypotension /(Blood pressure meds)

• (Nicotine) ?

• Pain stimuli arising from the viscera are perceived as somatic in origin

• This may be due to the fact that visceral pain afferents travel along the same pathways as somatic pain fibers

CNS Modulation of Pain

• CNS activity can suppress painful sensory information by inhibiting the spinothalamic pathway through the secretion of opiods. – enkephalins, endorphins and dynorphins