Nervous System

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Nervous System

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the master controlling and communication system of body.

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functions of the nervous system

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*responsible for all behaviors
*system uses neurons as a form of communication with other cells via the electrical signals.
*uses sensory receptors to detect change occuring both in/outside body.
*processes/interprets sensory inputs (called integration) reaches brain.
*causes a response by the effector oregions (called motor output). moves muscle- action to pick up glass.

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Two major divisions of the nervous system

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1. central nervous system(CNS) -brain and spinal cord (integrative and control center).
2. peripheral nervous system(PNS) -12 pairs cranial nerves, 31 pairs spinal nerves. communication lines between the CNS and the rest of the body.

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Two functional divisions of the PNS

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1. sensory (afferent) division-keeps the CNS informed of changes occurring inside and outside of body. somatic and visceral sensory nerve fibers. conducts impulses from receptors to the CNS.
2. motor (efferent) division-activates muscles and glands of body. motor nerve fibers - conduct impulses from the CNS of effectors.

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Subdivisions of motor division

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1. somatic nervous system- somatic motor nerve fibers- voluntary (somatic motor) -conducts impulses from CNS to skeletal muscle.
2. autonomic nervous system (ANS)- visceral motor nerve fibers -involuntary(visceral motor). -conducts impulses from CNS to cardiac muscle, smooth muscle, and glands. (stomach muscle-no huger)

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Two Divisions of ANS

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1. Sympathetic division- mobilizes body systems during emergency situations (fight or flight division). *only gets turned on in extreme conditions. fright or horror speeds up everything (heart rate).
2. Parasympathetic division -conserves energy. -promotes nonemergency functions (resting/digesting division)-slows things down after being sped up.

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Major cell types

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1. neurons- excitable cells that transmit electrical signals throughout body. extreme longevity (100yrs or more) -amitotic (constant supply of oxygen and sugar) -high metabolic rate
2. neuroglia or glial cells. -supportive cells not capable of transmitting a signal -small in size -wrap around neurons -nonexcitable -protect/insulate neurons -highly mitotic (regenerate at a high rate).

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Neuroglia of CNS

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1. Astrocytes -star shaped -most abundant -brace and anchor neurons to blood capillaries -control chemical environment around neuron -mop up excess potassium ions -recapture/released neurotransmitters.
2. Microglia -small and oval shaped -monitor the health of nearby neurons -can transform into macrophages (white blood cells) to phagocytize/eat microbes or dead neurons

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Neuroglia of CNS

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3. Ependymal -squamous or columnar shaped -ciliated -line the central cavities of brain and spinal cord -form barrier between cerebrospinal fluid/tissue fluid -help circular CSF
4. Oligodendrocytes -branching processes -form myelin sheaths (insulating covering) around nerve processes in CNS. -effects mobility

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Neuroglia of PNS:
satellite cells-

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wrap around cell bodies of neurons found in ganglia.

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Neuroglia of PNS:
schwann cells-

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form myelin sheaths around nerve fibers in PNS.

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Nerve cell structure:
cell body-

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-the biosynthetic center of neuron (biosynthesis).
-contain -nucleus -nucleolus -cytoplasm -ribosomes -rough ER (nussel bodies) -mitochondria -microtubules/neurofibrils (maintain shape/integrity of cell) -cell bodies in the CNS called nuclei -cell bodies in PNS called ganglia

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processes that are arm-like extension-

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1. axon
2. dendrites
3. bundles of neuron processes in CNS called tracts
4. bundles of processes in PNS called nerves

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Dendrites

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-provide large surface area for collecting and transmitting signals -specialized in collecting information -transmit incoming messages (called graded potentials) to cell body -same organelles as cell body - highly brached extensions -useful only short distance communication

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Axon

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-long slender process (3 to 4 ft) -same organelles as dendrites except no nissel bodies -can split (axon collatoral_ -one neuron -other neurons may have short or no axons -axon can divide into branches called axon collaterals

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telodendria

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end of axon segment

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axonal terminal or bouton

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bulb-like ending of telodendria that stores neurotransmitters; also known as the secretory component of neuron.

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axolemma

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plasma membrane of axon -function to transmit nerve impulses away from cell body.

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myelin sheath

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*electrical insulators- increase speed of impulses. -white matter- fatty substance -covers nerve fibers -myelinated fibers in CNS make up white matter -unmyelinated fibers in CNS make up gray matter -dendrites always unmyelinated (conduct impulses at a slower rate).

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myelin sheaths in PNS

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form from Schwann cells which wrap around axons.

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Classification of neurons-
structural classification- based on # of processes:

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1. multipolar neurons- have 3 or more processes (numerous dendrites and one axon). most common type (> 99%)
2. bipolar neurons -have 2 processes (one axon and one dendrite) -processes extend to opposite sides of cell body -rare -found in retina of eye and olfactory mucosa (smell receptors) of nasal cavity.
3. unipolar neurons -have one process that divide into peripheral and central branches -peripheral branch associated with sensory receptor and has dendrites -central branch enters the CNS to function as an axon -originate as bipolar neurons -found mainly in ganglia of PNS -function more as sensory neurons

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Functional classification

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*based upon direction of nerve impulse sensory (afferent) neurons
-impulse travel from sensory receptors to CNS
*most unipolar except bipolar neurons of eyes and nose.
*cell bodies located in ganglia of PNS.
*peripheral process long (>3ft).

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motor (efferent) neurons

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*impulse travel away from CNS to effector organs.
*most are multipolar.
*cell bodies located in CNS.

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interneurons or association neurons

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*most are multipolar in CNS
*lie between motor and sensory neurons
*conduct impulses within CNS
*99% of neurons are of this type

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Neurophysiology

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*nerve impulse that travels down the length of the axon called action potential(AP)
*human body on the whole is electrically neutral
*polarized state of the neuron:
-cell has(+)charges outside cell membrane, but(-)charges inside the membrane.
*potassium highly concentrated within the cell
*sodium highly concentrated outside cell
*charge separation of a polarized cell generates potential energy
*potential energy is measured in voltage either in unites of volts or millivolts
*voltage is always measured between two points and is called the potential difference
*the potential difference between the inside and outside of the nerve cells is -70mV
*-70mV is the resting membrane potential(RMP).

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Two types of membrane potentials:
1. depolarization-

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1. reduction in membrane potential
*inside of cell becomes less negative
*membrane potential moves closer to zero
**this event generates and propagates an action potential.**

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Two types of membrane potentials:
2. hyperpolarization-

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2. an increase in membrane potential
*inside of cell becomes more negative
*membrane potential increases above -70mV
**this event does not produce an action potential.**

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Two signals that produce change in membrane potentials:

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1. graded potentials- short distance signals
-short lived (current flow decreases with distance)
-can produce depolarizations or hyperpolarizations
-vary in magnitude
-transmitted by dendrites
2. action potentials- long distance signals
-main source of communication between cells
-changes the membrane potential from -70mV to +30mV (few milliseconds)
-do not decrease in strength with distance
-function both in short and long distance communication
-generated by axons.

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Ion channels

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-distributes ions in and out of cell
-membrane proteins function as ion channel
-selective (ion specific)
-alter 3-D shape to open or close a channel in response to a stimulus.

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Types of ion channels (2)

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1. passive or leakage channels- always open. -allow ions to cross membrane freely.
2. active or gated channels-respond to a stimulus

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active or gated ion channels

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-respond to a stimulus

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Two types of gated channels:

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1. chemically-gated or transmitter-gated channels -open and close due to presence or absence of neurotransmitters
2. voltage-gated channels - open and close in response to changes in membrane potential.

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Conditions for generating an Action Potential:

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-Resting state of the membrane:
*RMP= -70mV
*all voltage-gated Na+ and K+ channels are closed.
*leakage channels allow only a small amount of ions to cross membrane.
-Depolarization phase-
*rapid opening Na+ channels
*membrane becomes permeable to Na+
*Na diffuses quickly into the cell
*cell interior becomes less negative in charge
*membrane potential decreases to threshold(around -55mV to -50mV)
*action potential(AP) generated and propagated along the membrane
*all Na+ channels are open
*at the peak of the AP, the membrane potential reaches +30mV
*a point of no return for the AP = threshold.

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Conditions for generating an Action Potential:

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-Repolarizing phase
*Na+ channels closing reducing entry of ions
*membrane permeability to Na+ declines
*AP stops and reverses direction
*K+ channels open as repolarization begins
*K+ rushes out of the cell
*the internal negativity of the cell is restored.
*repolarization restores resti g electrical conditions
*activation of the sodium-potassium pump restores the ionic condition.

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Refractory periods-

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time frame when an excitable cell cannot generate another action potential.

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Two types of refractory periods:

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1. absolute- period from the opening to the closing of sodium ion channels.
2. relative- follows absolute period
*time period when sodium ion channels close and potassium ion channels open
*repolaization is occurring.

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The synapse-

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a junction that mediates information from a neuron to an effector organ, gland, or another neuron.

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Types of synapses:

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*axodendritic- between axonal terminal and dendrites.
*axosomatic- between axonal terminal and cell body.
*axoaxonic- between axon and axon.
*dendrodendritic- between dendrites and dendrites.
*dendrosomatic- between dendrites and cell body.

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Synapses based on function:

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**electrical synapses- least common - allow ionic current to spread from one cell into another cell via gap junctions. -transmission of impulse can be unidirectional or bidirectional. -fast transmission rate. -synchronize action potentials between cells. -more abundant in embryonic nervous tissue.
**chemical synapses- ex. neuromuscular junction. -most common. -specialized for the release and reception of neurotransmitters. -allow for a one way transmission of impulses (unidirectional). -consist of 2 parts 1. axonal terminal of a presynaptic neuron containing neurotransmitter. 2. receptor region found on a postsynaptic neuron.

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Neurons of a synapse

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*presynaptic neuron- sender. conduct impulses towards the synapse.
*postsynaptic neuron- receiver. transmit signals away from the synapse.