Nervous System

General Nerve Functions

\-Monitors the internal and external environments

\-Integrates sensory information

\-Coordinates voluntary and involuntary responses of many organ systems

Central Nervous System (CNS)

\-brain & spinal cord

\-Relay neurons

Peripheral Nervous System (PNS)

\-outside of CNS, spinal nerves & cranial nerves, peripheral nerves

\-Sensory neurons, motor neurons

Afferent (sensory) division

\-Nerve fibers that send impulses to the CNS from sensory receptors (somatic & visceral)

\-Touch, taste, sound, sight, smell

\-Message starts in PNS, goes to CNS

Efferent (motor) division

\-nerve fibers that send impulses from the CNS to the effector organs causing a motor response

\-Message starts in CNS, goes to PNS

Somatic Nervous System (SNS)

conscious voluntary control of skeletal muscle

Autonomic Nervous System (ANS)

control over automatic or involuntary responses (smooth & cardiac muscle)

Sympathetic Nervous System

\-Fight or Flight part of ANS

\-Emergency response

Parasympathetic Nervous System

\-Craniosacral part of ANS

\-We are in this most of the time

Stimuli

a change in the environment causing a response by the body

Sensory input

information gathered by the body from the stimuli

Integration

the processing and interpreting of sensory input and deciding what should be done at each moment

Motor output

activation of effectors (muscles or glands) by the nervous system

Neuroglia

regulate environment around neurons

Neurons

Nerve Cells

Cell body (soma)

\-metabolic center of cell, no centrioles (no mitosis), contains large round nucleus w/ prominent nucleolus

\-Lacks centrioles (no cell division)

Nissl substance

clusters of rough ER and free ribosomes, function in protein synthesis

Processes (fibers)

vary in length microscopic 🡪3-4 ft

Dendrites

carry impulses to the cell body

Axons

\-generate impulses and send them away from the cell body

\-There is only ever 1 axon per neuron

\-Axons are long because they extend from the CNS out to the PNS

Axon hillock

\-cone like region of the cell body that leads to the axon

\-Where action potential is generated

Synaptic (Axon) terminals

\-branching end of the axon furthest from the cell body, contain vesicles

\-Bulb-like structure

Neurotransmitters

chemicals released from the vesicles (send message)

Synaptic cleft/synapse

Tiny gap that separates one neuron from another or a neuron from the cell it stimulates

Myelin sheath

\-fatty whitish material that covers and protects the axon and speeds up impulse transmission

\-(Schwann cells – outside CNS)

\-Like the rubber part on a phone charger (insulator)

Multipolar

2+ dendrites and single axon (all motor & association – common in CNS)

Bipolar

1 axon and 1 dendrite, rare in adults, special sense organs (eye, nose), act as receptor cell

Unipolar

\-have single process, short, but divides into proximal (central) & distal (peripheral) processes

\-only small branches are dendrites

\-rest of peripheral & central are axons and serve to both carry away and take in impulses

Sensory (Afferent) neurons

in PNS, carry impulses from sensory receptors to the CNS

Interneurons (association neurons)

\-connect motor and sensory neurons, cell bodies are in CNS

\-Interpret sensory data and determine what to do with it

Motor (Efferent) Neurons

\-In PNS

\-Carries impulses from CNS to viscera, muscles, or glands

Somatic Motor Neurons

Affect skeletal muscles

Visceral Motor Neurons

Affect cardiac or smooth muscle, or glands

Nodes of Ranvier

\-Exposed parts of the axon

Receptors

\-found in dendrite endings

\-activated by specific changes nearby

Somatic sensory receptors

detect information about the outside world or our physical position in it

External (cutaneous) receptors

touch, temperature, pressure, sight, smell, hearing

Proprioceptors

Monitor position and movement of skeletal muscles and joints

Visceral receptors or internal receptors

\-Monitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive)

\-Internal senses (taste, deep pressure, pain)

Astrocytes

\-in CNS

\-star-shaped

\-function in exchanges between capillaries & neurons

\-protect from harmful substances in the blood

\-control chemical environment of brain (maintain blood-brain barrier)

Microglia

\-in CNS

\-spider-like phagocytes

\-get rid of dead brain cells & bacteria/viruses

\-rare

Ependymal cells

\-In CNS

\-line cavities of brain & spinal cord

\-cilia help circulate cerebrospinal fluid (CSF)

Oligodendrocytes

\-In CNS

\-wrap flat extensions around nerve fibers creating insulating myelin sheaths

Schwann Cells

\-In PNS

\-form myelin sheaths around axons

\-outer surface of cells called the neurilemma

Satellite Cells

\-In PNS

\-protective cushioning cells

Ion

\-An atom or molecule in which the total number of electrons is not equal to the total number of protons

\-Has a net positive or net negative charge

\-Important to neurons because all plasma (cell) membranes produce electrical signals by ion movements

Membrane Potential (transmembrane potential or membrane voltage)

difference in electrical charge between the inside and outside of a cell

Factors Responsible for Membrane Potential

\-Concentration Gradient of Ions (Na+ outside, K+ inside)

\-Channel Proteins, which selectively allow ions to cross cell membrane

Passive (leak) Channels

Always allow ions through

Active (gated) Channels

Only open when stimulated

Resting Potential

\-membrane potential of a resting cell

\-Outside of cell (+) charge, inside of cell (-) charge

\-Cell in state of polarization with fewer K+ inside than Na+ outside

Graded Potential

\-Temporary

\-localized change from resting potential caused by a stimulus where the charges are slightly more positive on the inside of the cell

\-if it reaches a specific threshold, action potential occurs

Action Potential

\-shortlasting event where the electrical membrane potential of a cell rapidly changes to (+) charge inside cell and (-) charge outside cell

\-All or none response down the axon

Conductivity

ability to transmit an electrical impulse

Irritability (generation and propagation of an action potential)

the ability to respond to a stimulus & convert it into a nerve impulse

STEP 1: Depolarization

\-when cell membrane at resting potential allows Na+ to diffuse slowly into the cell which changes the polarity of the neuron’s membrane at that site (inside becomes more +)creating a graded potential to the threshold (-70mv to -60mv)

\-Can go back or forward at this point

STEP 2: Rapid depolarization

\-Sodium gates open and Na+ rushes into the cell

\-Inside of cell becomes positive, outside of cell becomes negative

\-Action potential is generated

\-inside of the cell, excess of positive ions are attracted to the negative charges.

\-causes a local current inside the cell.

\-local current depolarizes adjacent portions of the membrane like a chain reaction. This is called continuous propagation.

\-action potential can ONLY move forward, NOT backward

STEP 3: Repolarization

\-sodium channels close and potassium channels open

\-This starts repolarization - potassium ions diffuse out, restoring the negative charge on the inside of the membrane and the positive charge on the outside surface.

STEP 4: The return to normal

permeability (Na+ diffuses outside, K+ diffuses inside) and resting potential (+ outside, - inside)

Refractory Period

From the moment the voltage-gated sodium channels open at threshold until repolarization (Steps 2-3) is complete, the membrane cannot respond normally to further stimulation

Myelinated axon

contains sections of myelin around axon

Nodes of Ranvier

gaps between sections of myelinated axon

Internodes

Areas of axon covered in myelin

Unmyelinated Axon

Axons without myelin

White matter

white areas of CNS containing myelinated axons

Gray matter

darker areas of CNS consisting mostly of neuron cell bodies (little myelination)

Continuous Propagation

\-occurs along unmyelinated axons

\-travels along entire axon

Saltatory Propagation

faster type of impulse conduction that occurs in myelinated fibers where the impulse jumps from node of ranvier to node of ranvier

Postsynaptic

\-After synaptic cleft

\-binds neurotransmitters

\-either excites or inhibits the neuron/cell

Presynaptic

\-before synaptic cleft

\-neurotransmitters released

Meninges

3 protective tissue coverings (membranes) that cover & protect the CNS from physical impacts and blood-borne pathogens/compounds

Dura mater

\-outermost layer

\-tough, double-layered membrane

\-surrounds entire brain

Periosteal layer/periosteum

\-attached to inner surface of skull

Meningeal Layer

\-forms outermost covering of brain and continues as dura mater of spinal cord

Dural Folds

help hold brain in place

Arachnoid Mater

middle meningeal layer separated from Dura mater by subdural space

Subarachnoid space

deep to arachnoid mater; filled with cerebrospinal fluid (CSF) (circulates)

Arachnoid villi

\-projections of arachnoid membrane;

\-place where CSF is absorbed into venous blood

Pia mater

-innermost membrane

\-extremely vascularized to provide oxygen for very high rate of metabolism (3 lb. brain at rest = 61 lbs. skeletal muscle in O2 usage)

Cerebrospinal fluid (CSF)

\-fluid containing less protein & more vitamin C

\-formed from blood by choroid plexuses (capillaries)

\-Protects and cushions brain and spinal cord from trauma (circulates)

\-Forms and drains at a constant rate

\-Presence of blood cells or change in composition 🡪 meningitis, brain tumor, multiple sclerosis

Blood-Brain Barrier

\-keeps neurons separate from bloodborne substances

\-least permeable capillaries in entire body - only glucose, water, & essential amino acids through, while metabolic wastes (urea, toxins, proteins, & most drugs) are prevented from entering brain tissue

\-Forms after 2 years

Cerebrum

Largest part of the brain

Gyri (Gyrus)

Ridges

Sulci (sulcus)

Grooves

Central Sulcus

separates frontal and parietal lobes

Fissures

deep grooves

Longitudinal fissure

Separates the two hemispheres

Left hemisphere

Dominant for speech and motor activity

Right hemisphere

dominant for spatial (recognition of shape and form) and temporal (timing, music) activities

Adult Human Brain

* 750-2100 CC
* Roughly 97% of neural tissue in the body
* Average weight 3 lb

Parietal Lobe

\-The somatic sensory area is located in the parietal lobe posterior to the central sulcus

\-impulses traveling from body’s sense receptors (pain, cold, touch), except special senses, are localized and interpreted here

Occipital Lobe

Visual area

Temporal lobe

Auditory area

Frontal lobe

The primary motor area (allows us to consciously move our skeletal muscles) is anterior to the central sulcus in the frontal lobe

Diencephalon/Interbrain

\-sits atop brain stem, linking it to the cerebrum, and is enclosed by cerebral hemispheres

Thalamus

\-Encloses third ventricle

\-a complex relay station for sensory impulses passing upward to the sensory cortex (except smell)

\-get crude recognition of whether sensation we’re about to experience is pleasant or unpleasant; actual interpretation is done in sensory cortex

\-regulates states of sleep and wakefulness

\-plays a major role in regulating arousal, levels of awareness and activity

\-damage to area can cause permanent coma

Hypothalamus

\-makes up floor of diencephalon

\-important autonomic nervous system center b/c it plays a role in regulation of body temp., water balance and metabolism

\-hormone production

\-deals with sleep/wake cycles

Limbic system

\-“emotional-visceral” brain made up of many different brain areas

\-deals with emotion, motivation, and emotions associated with memory

\-influences formation of memory by integrating emotional states with stored memories of physical sensations

\-Amygdala and Hippocampus

Amygdala

Aggression, jealousy, fear

Hippocampus

Formation of long-term memories