Nervous System part i

What is the nervous system?

The nervous system The master controlling and communicating system of the body Every thought, action, and emotion shows the nervous system’s activity

How does the nervous system Work?

How does the nervous system Work? Electrical signals are how the brain communicates with the nerves Do you think these signals are fast or slow?

Organization of the Nervous system

Organization The nervous system can be broken down into structural and functional classification

Structural Classification (What’s it made up of) 2 subdivisions: CNS PNS

1. CNS Central nervous system made of the brain and spinal cord Interprets information and responds

2. PNS Peripheral nervous system is made up of the nerves outside the CNS Links all parts of the body together through electrical impulses

Functional Classification (What does it do) Only deals with PNS structures (nerves) 2 major divisions: Sensory division (afferent) Motor division (efferent)

Sensory division (afferent) Convey information TOWARD the CNS from sensory receptors

Motor division (Efferent) Carry impulses AWAY from the CNS to muscles for movement and glands for hormone secretion There are 2 types of motor nerves:

2 types of motor nerves Somatic: VOLUNTARY impulses Ex) Skeletal muscles Autonomic: INVOLUNTARY impulses Ex) Cardiac and smooth muscle Ex) Glands 2 types of autonomic nerves

2 types of autonomic responses Sympathetic: “fight or flight” Parasympathetic “rest and digest”

The 3 steps to Normal activity 1. Sensory input 2. Integration 3. Motor output

1. Sensory input The nerves contain sensory receptors to monitor changes in and out of the body (stimuli) to send to the CNS The gathered information is called sensory input

2. Integration The brain/spinal cord processes the sensory input and makes a decision about what must be done. This decision is called integration

3. Motor output A response is then created by activating muscles or glands (away from CNS) The response is called motor output

Reflex Arc

Nervous Tissue Complex nervous tissue can be separated into two overall types of cells: Supporting cells Neurons

Supporting cells in the CNS Called neuroglial cells, or simply called glia Each type of glial cell has a unique shape and function

1. Astrocytes Star-shaped (hence the name) Account for nearly half of neural tissue Attached to blood capillaries and neurons

Functions of astrocytes: 1. Protect neurons from harmful substances in blood (BBB) 2. Help control chemicals in the brain (Na+, K+, CO2) Creating a 3-d framework for the CNS Performing repairs in damaged neural tissue Guiding neuron development Controlling the interstitial environment

2. Microglia Look like spiders Function to dispose of debris in the brain Start off as phagocytic white blood cells that have migrated across capillary walls in the neural tissue of the CNS

3. Ependymal cells Thin coat with cilia line cavities of CNS tissue and move fluid around brain and spinal cord Forms a protective cushion around the CNS Distributes dissolved gases, nutrients and wastes Cuboidal columnar Cilia or microvili Some can secrete CSF

4. Oligodendrocytes Made up mostly of myelin Wrap tightly around nerve fibers to produce myelin sheaths

Supporting cells of the PNS 2 types: 1. Schwann cells that help form the myelin sheaths around nerve fibers 2. Satellite cells that act as protective cushioning around cell bodies

Neurons AKA nerve cells Transmit messages sent as electrical signals (nerve impulses)

General neuron

Cell Body (soma) Contains the nucleus; is the metabolic center of the cell

Dendrites Conduct electrical currents TOWARD the cell body, and to RECEIVE information This is where sensory input occurs Highly branched Fine processes called dendritic spines

Axons Generate nerve impulses and send them AWAY from the cell body Axon hillock- where the axon attaches to the soma

Collateral branch A branch of the same axon Allow a single neuron to communicate with several other cells

Schwann cell (PNS only) Form the myelin sheath

Myelin Sheath Fatty, protective coating around the axon

Nodes of Ranvier Gaps or indentations in between Schwann cells Occur at regular intervals

Axon terminals Branching ends of axons

Synapse End of the axon terminal releases the neurotransmitter

Synaptic cleft Where an axon terminal interacts with another neuron

General Neuron

Organization of the nervous system PNS: nerves CNS: Brain and spinal cord Motor/efferent Sensory/afferent Somatic Autonomic Sympathetic Parasympathetic

Classification of Neurons 2 ways to classify neurons: Function Structure

Functional Classification Groups neurons based on what direction the electrical impulse is travelling related to the CNS 3 general types

1. Afferent Neurons AKA sensory neurons Carry impulses from the sensory receptors to the CNS Sensory neurons keep us informed about what is happening inside and outside the body

2. Efferent neurons AKA motor neurons Carry impulses from the CNS to muscles and glands The cell bodies of these neurons are always located in the CNS

3. Interneurons Connect the motor and sensory neurons in neural pathways Cell bodies are always located in the CNS

Structural classification In structural classification, neurons are classified based on the number and type of projections off the cell body

1. Anaxonic Small, have no anatomical clues to distinguish dendrites from axons Found in special sense organs

2. Multipolar Most common type 1 axon Many dendrites CNS & motor neurons

3. Bipolar Most rare type 2 processes 1 axon 1 dendrite Sight, smell, & hearing

4. Unipolar (pseudounipolar) The dendritic and axonal processes are continuous, cell body lies off to the side Common in sensory neurons

Nerve impulses Neurons have 2 major functional properties: Irritability Conductivity

Irritability Ability to respond to a stimulus and convert it to a nerve impulse So in order to function, the neuron must first receive a stimulus, then be able to send the message out Let’s then take a look at possible stimuli, then observe how an impulse is sent out

Stimuli Most neurons are stimulated by neurotransmitters (found in the synaptic cleft) However, different types of neurons are stimulated by different stimuli Ex) light excites eye receptors Ex) pressure excites cutaneous receptors

Electrical impulses (How it works) The following 4 steps are going to explain exactly how an electrical signal is created in the neurons This process is similar to the process in muscles and follows the same concept

Step 1: Resting state (resting potential) At rest, the neuron has a negative charge inside (where K+ ions are) the neuron and a positive charge outside (where there are more Na+ ions) -70v

Step 2. Generation of action potential The nerve is stimulated , which opens up “sodium gates” to allow Na+ into the neuron This “charges” up the neuron, generating electricity (action potential) The charge inside the cell is now more positive because both K+ ions and Na+ ions are both inside +30v

Step 3. Propagation of action potential When the neuron is fully charged, the charge is released This sends an electrical impulse through the neuron

Step 4: Repolarization Once the charge is released, K+ leaves the neuron, returning the neuron to a negative charge Until repolarization is finished, a neuron cannot conduct another impulse

Action potential video https://www.youtube.com/watch?v=OZG8M_ldA1M

Conductivity The ability to transmit an impulse to other neurons, muscles, or glands

reflexes When conductivity occurs across a muscle or gland, it is called a reflex A series of reflexes across many neurons is called a reflex arc

Autonomic reflexes Involuntary reflexes Stimulate smooth muscles, heart, and glands

Somatic reflexes Voluntary reflexes Stimulate skeletal muscles

Nerve fiber classification Just like neurons, nerves are classified the same way

Nerve Fiber Classification Mixed nerves carry BOTH sensory and motor fibers Sensory nerves carry information TOWARD the CNS Motor nerves carry information AWAY from the CNS to muscles and glands etc

Somatic nervous system Voluntary impulses Effects skeletal muscles

1. Sympathetic division Often referred to as the “fight or flight” system Signs of activity of this division are: Pounding heartbeat Rapid breathing Cold, sweaty skin Dilated eye pupils Increased adrenaline

How long do these signs last? Effects usually last several minutes, until the liver dissolves released hormones

2. Para-sympathetic division Most active when the body is at rest Promotes normal digestion, elimination of urine and feces, and conserving body energy (especially concerning the cardiovascular system)

Lack of oxygen Since the nervous system has the highest metabolic rate in the body, lack of oxygen for even just a few minutes leads to death of neurons

Older age Neurons cannot reproduce themselves, so we lose neurons as we age, but cannot replace them Loss of neurons and lack of oxygen lead to senility; symptoms: Forgetfulness Irritability Difficulty concentrating