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Transcript of Nervous System
The basic units of the nervous system
Perform all communication, information processing and control functions of the nervous system
Cell Body: Contains organelles that provide energy and synthesize organic compounds
Dendrites: Receive incoming signals
Axon: Carries outgoing signals
Synaptic Terminals: The neuron
communicates with other neurons
Parts of the Neuron
Neurons do not have centrioles and cannot undergo mitosis.
CNS neurons cannot divide, so when an injury or disease occurs, the cells cannot regenerate.
The only exception to this is in the olfactory receptors and in the hippocampus where adult stem cells can be found.
The neurons have numerous mitochondria, free and fixed ribosomes, and membranes of the rough endoplasmic reticulum (RER).
This makes the cytoplasm a grainy appearance.
Nissl bodies: Clusters of rough ER and free ribosomes. The nissl bodies give a gray color to the areas containing neuron cell bodies. This is the "Gray Matter" found in the brain and spinal cord.
Classification of Neurons
The most common type of neuron
Controls skeletal muscles
The dendrites and axons are continous and the cell body lies off to the side.
The action potential begins at the base of the dendrites.
Make up most sensory neurons in the PNS
Made up of two processes: one dendrite and one axon.
The cell body lies between the axon and dendrite
Responsible to relay information about sight, smell or hearing from receptor cells to other neurons.
10 million neurons form the afferent division in the PNS
Receive information from sensory receptors
Relays the information from the sensory receptors to the CNS
Somatic Sensory Receptors
External Receptors: Provide information about the external environment.
Touch, temperature, pressure, sight, hearing, smell
Proprioceptors: Monitor the position and movement of the skeletal muscles and joints
Visceral and Internal Receptors: Monitor the activities of the digestive, respiratory, cardiovascular, urinary, reproductive systems and provide sensations of taste, deep pressure and pain
Carry instructions from the CNS to other tissues, organs or organ systems.
The peripheral targets are called effectors because they respond by doing something.
Somatic Motor Neurons: Part of the somatic nervous system that innervate skeletal muscles
Visceral Motor Neurons: Part of the autonomic nervous system that innervate all other effectors.
ex. cardiac muscle, smooth muscle, glands and adipose
20 billion interneurons are located in the brain and spinal cord.
Interconnect other neurons
Responsible for distribution of the sensory information
Coordinate motor activity.
The more complex the response to a given stimulus, the greater the number of interneurons involved.
Play a role in higher functions.
Memory, planning and learning
Found in the CNS and PNS
Regulate the environment around neurons
Provide supporting framework for neural tissue
Act as phagocytes
More numerous than neurons
Most can still divide
Four Types of Neuroglia in the CNS:
1. Astrocytes 2. Oligodendrocytes
3. Microglia 4. Ependymal Cells
Largest and most numerous
Secrete chemicals vital to the blood-brain barrier. These secretions cause the capillaries of the CNS to become impermeable to compounds that could interfer with neuron function.
Create the structural framework for CNS neurons and perform repairs in damaged neural tissues
Have fewer processes (cytoplasmic extensions) than astrocytes
Wrap around axons to create a membrane sheath of insulation called myelin
The oligodendrocyte myelinates short segments of several axons. Several oligodendrocytes are needed to wrap one neuron.
Once an axon is covered with myelin, it is called myelinated. Axons not covered with myelin are unmyelinated.
Myelin increases the speed that an action potential travels along the axon.
Areas of the axon covered in myelin are called internodes.
The gaps between adjacent cell processes are called nodes of Ranvier.
Myelin is rich in lipids and gives the
CNS a white appearance. These areas
make up the white matter of the CNS.
Smallest and rarest
Phagocytic cells derived from white blood cells that migrated into the CNS as the nervous system formed.
Perform protective functions
Engulfing cellular waste and pathogens
Line both the central canal of the spinal cord and the ventricles of the brain
The lining of the epithelial cells is called the ependyma.
In some regions, the ependyma produces cerebrospinal fluid (CSF).
In other regions, the cilia on the cells help to circulate the CSF within the CNS.
Two Types of Neuroglia in PNS:
1. Satellite Cells: Surround and support neuron cell bodies. Similiar to how astrocytes act in the CNS.
2. Schwann Cells: Cover every axon outside the CNS.
Outer surface is called the neurilemma
Can only myelinate one segment of a single axon
Can enclose portions of several different unmyelinated axons
Organization of Neurons in the PNS
Neuron cell bodies (gray matter) are located in ganglia
The white matter of the PNS contains axons bundled together in nerves.
Spinal nerves are connected to the spinal cord
Cranial nerves are connected to the brain.
Both sensory and motor axons may be present in the same nerve
Organization of Neurons in the CNS
A collection of neuron bodies with a common function is called a center.
A center with a discrete boundary is called a nucleus
Portions of the brain are covered by a thick layer of gray matter called neural cortex
The white matter of the CNS contains bundles of axons that share common origins, destinations and functions. These bundles are called tracts.
Tracts form larger groups called columns
Pathways link the centers of the brain with the rest of the body.
Sensory pathways distribute information from sensory receptors to processing centers in the brain.
Motor pathways begin at CNS centers concerned with motor activity and end at the skeletal muscle they control.
Polarized cell membranes - Separates the excess positive charges on the outside and the excess negative charges in the inside.
Because the charges are separated by the cell membrane, this potential is called membrane
Factors that Affect Resting Potential
1. Extracellular fluid contains high concentrations of sodium ions (Na+) and chloride ions (Cl-)
2. Intracellular fluid contains high concentrations of potassium ions (K+) and negatively charged proteins (Pr-)
3. Selective permeability of the cell membrane
4. Positively charged Na+ are attracted to the negatively charged inner membrane. K+ is repelled by the overall positive charge on the outside of the cell membrane
4. Sodium-Potassium Exchange Pump - Allows for stable resting potential. This ion pump exchanges three extracellular sodium ions for two intracellular potassium ions. As soon as the sodium ions enter the cell, they are kicked back out. The cell undergoes a net loss of positive charges. The interior of the cell membrane contains an excess negative charge from the proteins
Changes in Membrane Potential
Any stimulus that alters membrane permeability to sodium or potassium or that alters the activity of the exchange pump will disturb the resting potential of the cell.
Chemicals, stimuli, change in temperature and mechanical pressure can all affect membrane potential.
Depolorization: A stimulus that opens gated sodium channels accelerates the entry of Na+ into the cell. As the number of positively charged ions in the cell increases, the membrane potential shifts toward 0 mV.
Hyperpolarization: A stimulus that opens gated K+ channels will shift the membrane potential away from 0 mV because potassium ions will leave the cell. This may take the membrane potential from -70 mV to -80 mV.
The membrane potential of an undisturbed cell.
The resting potential of a neuron is -70 millivolts (mV)
Changes in the membrane potential that cannot spread far from the site of stimulation.
Only affects a limited portion of the cell membrane.
- For example, stimulation can cause a gland to begin secretion of hormone
Occurs in the membranes of all cells in response to environmental stimuli
Affects too small an area to have effect on the activities of large cells like skeletal muscle cells.
A propagated change in the membrane potential for the entire cell membrane (spreads throughout the entire membrane).
Only in skeletal muscle fibers and axons of neurons are able to conduct action potentials because they have excitable membranes.
Generated by the opening and closing of gated sodium and potassium channels in response to local depolarization.
Action potential will only appear when the membrane depolarizes to a level called the threshold.
All or None Principle: A given stimulus either triggers a typical action potential or it does not produce one at all. Every stimulus that brings the membrane to threshold will genrate an identical action potential.
Generation of an Action Potential
1. Depolarization of threshold.
2. Activation of voltage-regulated sodium channels and rapid depolarization.
3. Repolarization: Inactivation of sodium channels and activation of voltage-regulated potassium channels.
4. The return to normal permeability and resting state.
The process continues in a chain reaction that soon reaches the most distant portions of the cell membrane.
Occurs along unmyelinated axons at a speed of about 1 meter per second (2mph)
Saltare - means "to leap" in Latin
Action potential jumps from node to node
Carries nerve impulses along a myleniated axon at speeds ranging from 18 - 140 meters per second (40 - 300 mph).
Information moves from one location to another in the form of action potentials along the axons.
These action potentials are called nerve impulses.
Information is transferred from one neuron to another or effector cell by neurotransmitters
Chemicals released from the synaptic terminal.
Transfer information from neuron to neuron or neuron to effector cell
Synapses between a neuron and another cell type.
For example, a neuron
communicates with a muscle cell
at a neuromuscular
Communication between neurons and other cells occur in one direction at they synapse.
Between two neurons, the impulse passes from the synaptic knob of the presynaptic neuron to the postsynaptic neuron.
The cell membranes are separated by a narrow space called the synaptic cleft.
Synaptic Terminal: Contain mitochondria,synaptic vesicles, endoplasmic reticulum.
Synaptic Vesicle: Contain several thousand molecules of neurotransmitter. Upon stimulation, these vesicles release their contents into the synaptic cleft. The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane.
Contains 35 billion neurons organized into neuronal pools.
Contains 98 % of the neural tissue in the body.
Typical adult brain weighs 3 pounds
Regions of the Brain
5. Medulla oblongata
Divided into large, paired cerebral hemispheres
Concious thoughts, sensations, intellectual functions, memory storage and retrieval, complex movements
Hollow portion of the brain
Contains the thalamus which contains relay and processing centers for sensory information.
Narrow stalk connects the hypothalamus to the pituitary gland.
Epithalamus contains the pineal gland
Contains centers involved with emotions, autonomic function, and hormone production.
Pituitary gland is the primary link between the nervous and endocrine system
Contains three major areas of the brain:
3. Medulla Oblongata
Process visual and auditory information and generate involuntary motor responses.
Centers help maintain consciousness
Connects the cerebellum to the brain stem.
Contains nuclei involved in somatic and visceral motor control
Connected to the medulla
Segment of the brain that is attached to the spinal cord
Relays sensory information to the thalamus and other brain stem centers
Contains major centers that regulate autonomic functions. For example, heart rate, blood pressure, respiration, and digestive activities
Almost completely covers the brain stem
Adjusts voluntary and involuntary motor activities on the basis of sensory information and stored memories of previous movements
Produced in the ependymal lining
Surrounds and bathes the exposed surfaces of the CNS
Cushions neural structures
Brain is basically floating in CSF
Transports nutrients, chemical messengers, and waste products
CSF produced in choroid plexus
A network of permeable capillaries that extends into each of the four ventricles.
Covered in large ependymal cells that secrete CSF
The volume of CSF is approx 150 mL and is replaced approx every 8 hours
Series of coverings or layers
Provide support beyond the glial cells
Act as shock absorbers
Blood vessels branch within these layers to deliver oxygen and nutrients
Cranial meninges and spinal meninges
Forms the outermost layer of the CNS
Contains two fibrous layers
Dural folds: Several parts of the dura mater are folded. These hold the brain in position
Dural sinuses - collecting veins - lie between the two layers of the dural fold
Narrow, subdural spaces that separate the inner surface of the dura mater from the second meningeal layer.
Contains a small amount of lymphatic fluid, which reduces friction between opposing surfaces.
Layer of squamous cells
Contains a delicate web of collagen and elastic fibers
Filled with CSF
Acts as a shock absorber and transports dissolved gases, nutrients, chemical messengers and waste products
Separates the arachnoid space from the innermost meningeal layer
Bound firmly to underlying neural tissue
Blood vessels that service the brain and spinal cord are found in this area
Highly vascular area to supply nutrients to the brain
Monitors the internal and external environments
Integrates sensory information
Coordinates voluntary and involuntary responses of many other organ systems
Two major divisions within the nervous system:
Central nervous system (CNS)
Peripheral nervous system (PNS)
Central Nervous System
Consists of the brain and spinal cord
Integrates and coordinates the processing of sensory data and transmission of motor comands
Place where higher functions (ex. intelligence, memory, and emotion) occur
All communication between the CNS and the rest of the body occurs through the Peripheral Nervous System
Peripheral Nervous System (PNS)
Includes all neural tissue outside the CNS
Conducts information received from the receptors to the CNS
PNS is divided into afferent and efferent divisions
Sensory information detected by receptors is transmitted by the afferent division of the PNS to the CNS, where the information is processed
After information is processed, the CNS sends motor commands to effectors (ex. muscles, glands) through the efferent division of the PNS
Efferent Division is subdivided into the somatic nervous system and the autonomic nervous system
Somatic Nervous System
Provides control over skeletal muscle contractions
Autonomic Nervous System
Provides automatic involuntary regulation of smooth muscle, cardiac muscle, glandular secretions and adipose tissue