Nervous System Functions and Divisions of the nervous

Nervous System

Functions and Divisions of the nervous system § The nervous system has three basic functions: § Sensory input: gathering information. § Integration: interpreting sensory input and what to do with it. § Motor output: causes a response by activating effector organs (muscles and glands).

Dividing the nervous system § The nervous system can be subdivided into two parts: § The central nervous system: § The brain and spinal cord § The peripheral nervous system: § All other neural units outside the brain and spinal cord

Further subdividing the PNS § The peripheral nervous system is further subdivided in these two parts: § Sensory/afferent division: § Consists of nerve fibers called axons that send impulses to the CNS from receptors throughout the body. § Sensory fibers sending impulses from muscles are called somatic afferent fibers. § Fibers sending impulses from visceral organs are called the visceral afferent fibers.

§ Motor/efferent division: transmits information from CNS to effectors like muscles and glands that allow them to respond.

Subdividing the motor division § The motor division of the PNS is also subdivided into two parts: § Somatic nervous system: voluntary, allows conscious control of muscles. § Autonomic nervous system: Involuntary, responsible for control muscles and other organs that we cannot voluntarily control, like the heart.

Subdividing the ANS § Sympathetic division: “fight or flight” reactions. (Decrease digestion speed, increase heart rate, etc. ) § Parasympathetic division: involves organ stimulation when body is at rest. (slow heart beat, normal digestive rates, normal breathing, etc. )

Sympathetic v. s. parasympathetic response § Sympathetic Structure § Rate increased § Force increased Heart § Broncial muscle relaxed Lungs § § § Pupil dilation Eye Motility reduced Intestine Sphincter closed Bladder § Decreased urine secretion Kidneys Parasympathetic Rate decreased Force decreased Broncial muscle contracted Pupil constriction Digestion increased Sphincter relaxed Increased urine secretion

Histology of Nervous Tissue § Neurolgia (also called glial cells) are smaller cells that are closely associated with the nervous system. § There are six types of glial cells in the nervous system, four in the CNS and two the PNS.

CNS Neuroglia § The glial cells of the CNS include: § Astrocytes: most abundant, looks star shaped in the cell. Braces neuron and anchors it to its food and oxygen supply line (capillaries) § Microglia: oval shaped cells with a thorny branches. Can become a type of macrophage to fight foreign invaders.

§ Ependymal cells: squamous shaped with many cilia. Helps move ceuribrospinal fluid (which acts as a cushion) around the brain and spinal cord. § Oligodendrocytes: Branched like astrocytes, but with less branches. They form myelin sheathes, which are used for insulation.

Neuroglia in PNS § The PNS contains two types of glial cells: § Satellite cells: Serves most of the same functions of astrocytes in the CNS. § Schwann cells: bundles and forms myelin sheathes around large nerve bundles, similar to oligodendrocytes.

Neurons § Structural unit of the nervous system. § Stats: § § Conducts nerve impulses Incredibly long life: can last 100 years or more with good nutrition. § They are amitotic (can’t divide). Very few places where neurons can regenerate (must come from stem cells) § High metabolic rate. Needs a lot of oxygen and nutrients.

Neuron: Cell Body § Contains a spherical nucleus, and a perikaryon (the body of the neuron). § The neuron’s protein making machinery comes from ribosomes and rough ER. § The rough ER in a neuron is called a Nissl bodies. § Neurofilaments: give structural integrity to the cell body.

§ Neuron clusters located in the CNS are called nuclei. § Neuron clusters located in the PNS are called ganglia.

§ Neurofibrils (neurofilament) and microtubules are found in the neuron to help hold the cells shape.

The Process (tail part) § Extends from the body of the neuron. § The processes are called tracts in the CNS. § The processes are called nerves in the PNS.

Dendrites § Dendrites are branching extensions that are found on the neuron and are designed to pick up chemical messages. The chemical messages are sent towards the cell body (in other words, dendrites are receivers)

The axon § The axon is designed to generate an action potential within the neuron. This is where the “impulse” part of a nerve impulse comes from.

§ Impulses are carried down the “tail” called a nerve fiber until it reaches an axon terminal which releases chemicals that signal the next neuron.

Neuron Classification § Most neurons are classified by either structure or function.

Structural Classification § Unipolar-have a single short process that forms the cell body that will divide into a T-shape to form a proximal and distal process. § Usually associated with sensory receptors.

§ Bipolar- have two processes. Has an axon and dendrite extending from different parts of the neuron. § Found only in special sensory organs like the retina and the olfactory canal.

§ Multipolar- have three or more processes: One axon, and the rest dendrites. More than 99% of neurons are multipolar. § Major neuron type of the CNS.

Classification Based on Function § Sensory (afferent) neurons- Transmits nerve impulses toward the CNS. Mostly unipolar neurons.

§ Motor (efferent) neurons- carries impulses away from the CNS towards the effector organs. § Usually multipolar. § Usually Located in the CNS.

§ Interneurons (association neurons)- act as shuttle and pathways for impulses to travel. § Most all are multipolar.

Membrane Potentials § Neurons are highly irritable. Their response to a stimulus is an action potential (a. k. a. nerve impulse).

Voltage, Resistance, and Current § The measure of potential energy generated is called voltage. § The difference in voltage between two points is called potential difference.

§ The flow of electrical charge is called current. § Resistance is the hindrance of electrical flow.

Ohm’s Law § Ohm’s law shows the relationship between current, voltage and resistance. § Current = Voltage/Resistance § Less resistance equals greater current. § Greater voltage equals greater current.

Roles of membrane ion channels § Membrane ion channels are large proteins found in nerve cells. They are designed to regulate the flow of ions within a neuron.

Channel types: § Nongated channels- always open. § Gated channels- change shape in response to a certain stimulus. § Chemical gated channels- respond to a certain chemical stimulus (neurotransmitters). § Voltage gated channels- change in response to membrane potential. § Mechanically gated channels- opens in response to a physical deformation in the protein.

Resting membrane potential § Resting potential in a neuron represents the standing charge in a neuron when at rest. § RP in a neuron- -40 m. V to -90 m. V. § Most common ion pumps in neurons: the sodium-potassium pump. § The negative sign means that there is a negative charge within the cell.

Sodium-potassium pumps § Sodium-potassium pumps help stabilize membrane potential by maintaining the proper levels of sodium and potassium within the cell.

Membrane potential acts as a signal § Neurons use change in their membrane potential to receive, integrate, and send information. § Changes in membrane potential can be caused by: § 1. anything that alters the ion concentration on either side of the cell membrane. § 2. anything that changes membrane permeability.

Depolarization and hyperpolarization § These terms are relative to the resting membrane potential within the neuron. § Depolarization- becoming less negative. Moving closer to zero. i. e. - -70 m. V to -65 m. V. (think of it as becoming more positive) § Hyperpolarization- Becoming more negative. i. e. moving from -70 m. V to -75 m. V.

Graded potentials § Graded potentials- short lived localized changes in membrane potential. Caused by either hyperpolarization or depolarization.

Action Potentials § A. k. a. nerve impulse § A brief depolarization of the neuron membrane followed by a quick hyperpolarization. § Unlike a graded potential, Action potentials don’t change in strength over a distance.

Generation of an action potential § Generating an action potential occurs in four stages: § 1. Resting State: all Na+ and K+ gates closed. § 2. Depolarization: Na+ gates open. § 3. Repolarization: Na+ channels begin to inactivate; K+ channels open. § 4. Some K+ channels stay open and Na Channels reset.

§ Action potentials must be propagated (sent or transmitted) down the entire length of the neuron. § When A Na+ channel closes during AP generation, no new Na+ is coming in, because it stays depolarized, the neuron will always send the AP away from its origin.

§ An AP is self propagating, so when it starts, it continues without any outside assistance.

All or Nothing Phenomenon § In order for an AP to occur a certain threshold charge (around 15 to 20 m. V or depolarization) must occur. If it is less than that an AP will not occur. § There is no “almost” AP. It either completely happens, or does not happen at all.

§ Once stimulated, an AP will not be affected by a stimulus strength. Once the AP is fired, there is no change in intensity.

Absolute Refractory Period § Simply put, it is the period in an AP where Na+ channels reset. § This guarantees that an AP is all or nothing.

Relative refractory period § This period follows the absolute refractory period. § Na channels return to original states § Some K channels still open § Repolarization occurs. § Stimulus for a new AP at this time is much higher due to the neuron trying to rebalance itself.

Conduction Velocity § How fast does an AP move? § Impulses can typically travel around 100 m/s. § The fastest nerve conduction velocity will be found around neural pathways where speed is essential (i. e. Brain, spinal cord) especially in areas that govern reflexes.

§ Slower conduction velocity is typically found rooted closer to internal organs like the intestines and blood vessels.

Rate of conduction velocity § Depends on two things: § 1. Axon diameter- Large axon= greater speed, less resistance. § 2. degree of myelination- myelin sheaths act as insulators. Insulators prevent charge leakage. Thicker myelin sheaths increase conduction speed.

Nerve Fiber Groups § Group A: Fibers found in skin, skeleton and muscles. Largest axons, and thick myelin sheaths. Velocity: 150 m/s. § Group B: autonomic nervous system. Found in visceral organs and fibers, and smaller somatic fibers. Thin myelin sheaths. Velocity: 15 m/s § Group C: autonomic nervous system. Found in visceral organs and fibers, and smaller somatic fibers. Smallest diameter axon, and are unmyelinated. Velocity: 1 m/s.

The Synapse

The Synapse § Synapses are joints where neurons meet. This is a space that impulses must travel through to reach another neuron. § Axodendritic synapse: Synapse b/w an axon and dendrite of another cell. § Axosomic synapse: Synapses between, two axons (axoaxonic), or two dendrites (dendrodendritic), or a dendrite and a cell body (dendrosomatic).

Electrical synapses § Electrical synapses consists of gap junctions found between cell bodies. § Nerves connected this way are considered to be electrically coupled. § An important feature is that they provide a way to synchronize interconnected neurons.

Chemical synapses § Chemical synapses specialize in the release and reception of chemical neurotransmitters. § A chemical synapse has two parts: § A knoblike axon terminal of the presynaptic neuron that contains synaptic vessels (tiny membrane bound sacs) § A neurotransmitter receptor region on the membrane of the dendtrite or the cell body of the post synaptic neuron.

Synaptic Delay § Impulse can travel at speeds up to 150 m/s. § Impulse across a chemical synapse however is much slower. The delay, called a synaptic delay, last between 0. 3 and 5. 0 ms. This means that the slowest part of neural transmission is the gap between neurons.

Question? § Sex, drugs, and rock ‘n roll, eating till we are fat and happy! Why does it feel so good? ?

Answer § Our brains are wired to reward us whenever we do something that is for the survival of our species. § This reward system consists of Dopamine -releasing neurons in areas of the brain called the ventraltegumental area (VTA for short) which consists of the nucleus accumbens and the amygdala.

§ When people have “relations” they release a flurry of chemicals in the brain including glutamate and norepinephrine, which then cause the release of dopamine, which makes you happy.

§ Drugs like methamphetamine and cocaine will stimulate the receptors in the brain and tell it to release more dopamine! § When the body is given outside neurotransmitters, it will make less of it’s own.

§ Once dopamine producing areas in the brain become “tapped” they will no longer produce dopamine, making those who use it to need it to feel pleasure. § However dopamine is not the only reason that addicts stay addicted.

§ Remember the glutamate we talked about earlier? § Glutamate, a neurotransmitter that is involved in learning, will continue to signal and cause more permanent changes in brain chemistry. § This is why addicts tend to be reoffenders. “Once an addict, always an addict” comes from this information.

Neurotransmitters and their receptors § Neurotransmitters are basically the “language” by which the neurons speak to each other. § Everything from happiness, rage, sadness, lust, thought, and sleep are all controlled by neurotransmitters.

Developmental Aspects of Neurons § Neuroblast- the neuron progenitor. § The nervous system originates from a dorsal neural tube and neural crest. § The neural tube is the progenitor of the CNS.

§ A neuroblast will produce axons and become amitotic and move into the characteristic cell body-dendrite.

§ The area that the axon develops from is called the growth cone. The growth cone branches out with branches called filopodia that guide signals from the surrounding environment to the cell body.

N-CAM § Nerve cell adhesion molecules (N-CAM) are designed to adhere neurons to a single place. § Neurotropins are chemical signals that tell neurons which way to line up.

Neuron Death § When neurons die, either from lack of oxygen or nutrients, new neurons do not replace them. When neural tissue is destroyed, it is gone forever.