HP- Lec 2 Intro to Nervous System

Nervous System Terminology- CNS,PNS, Inter, snesory, motor neuron

CNS: Brain and Spinal Cord

PNS: Nerves, ganglia, and nerve plexuses (outisde of CNS)

Interneuron: multipolar neuron located entirely within the CNS
Sensory Neuron (afferent Neuron): neuron that transmits impulses from a asensory receptor into the CNS

Motor Neuron (efferent neuron): neuron that transmits impulses from the CNS to an effector organ (ex: muscle)

Nervous system Division & Tissues

Divided into:

1. Central nervous system (CNS): Organs- brain and spinal cord

2. Peripheral nervous system(PNS): Organs- cranial and spinal nerves, and ganglia

Tissue is composed of two types of cells

1. Neurons: conduct electrical activity (impulses), but, in adults, typically lack the ability to divide

2. Glial cells (neuroglia): support neurons, do not generate electrical impulses, but retain the ability to divide

Neurons

Structural & Functional units of the nervous system

Features & General Functions

1. generates and conducts electrical activity

2. release neurotransmitters, which are chemical regulators used for neuronal communication through chemical synapses

3. depending on their role, neurons can sense external sensory info (sensory neuron), send motor inputs (motor neuron) or be an interneuron

   • neurons enable perception of sensory stimuli from both the external environment and the internal body as well as memory and control of muscles and glands (IMAGE)

Neurons Features

there are approx 100 billion neurons in the human brain, they vary in size and shape

1. Dendrites (input): receive signals and conducts a graded impulse towards the cell body

2. Cell Body: contains the nucleus and other organelles; after integrating all the graded impulses from the dendrites, it may generate action potential.

   Cluster in groups= Nuclei in CNS; ganglia in PNS

3. Axon (output): conducts action potentials AWAY from the cell body

4. Synapse: place at which the axon of one neuron comes in close contact to the dendrite of another neuron

Functional Classification of Neurons

Neurons can be classified based on the direction in which they conduct electrical impulses

Interneurons: located entirely within the CNS, these neurons integrate the functions of the nervous system

Sensory Neurons: conduct impulses from sensory receptors to the central nervous system (afferent)

Motor neurons: conduct impulses from the CNS to target organs (efferent)

• Somatic motor neurons: control voluntary movements

• Autonomic motor neurons: regulate involuntary functions

Structural Classification of Neurons (3)

based on their morphology

1. Pseudounipolar: single short process that branches like a T to form 2 longer processes; sensory neurons

2. Bipolar Neurons: have two processes, one on either end, found in retina of eye

3. Multipolar neurons: several dendrites and one axon; most common type

Axons(output)

• conducts action potentials AWAY from the cell body

• vary in length from a few millimeters to a meter

• connected to the cell body by the axon hillock, where action potentials are generated at the initial segment of the axon

• can form many branches called axon collaterals

• covered in myelin with open spots called nodes of ranvier

Classification of bundle of axons

• Nerves are bundle of axons located in the PNS

• Tracts are bundles of axons located in the CNS

• most are composed of both sensory an motor neurons and are called mixed nerves

• some of the cranial nerves have sensory fibers only

Neuroglial cells and their functions

• Schwann Cells: PNS; produce the myelin sheaths around the MYELINATED axons of the PNS; surrounded all PNS (myelinated/nonmyelinated) to form a neurilemmal sheath

• Oligodendrocytes: CNS; form myelin sheaths around CENTRAL axons, producing “white matter” of the CNS

Myelin Sheath

• in the CNS: the myelin sheath is produced by oligodendrocytes

• in the PNS: the myelin sheath is produced by Schwann cells

• One oligodendrocyte sends extensions to several axons and each wraps around a section of an axon (like insulation)

• axon is like a power cord, wrapped by insulation cord

• can have unmyelinated fiber

Demyelinating Diseases

• are those in which the myelin sheaths are specifically attacked

  1. Guillain-Barre syndrome: the T cells of the immune system attack the myelin sheaths of the PNS, this produces rapid onset of symptoms that include muscle weakness

  2. Multiple Sclerosis: produced by an autoimmune attack by T lymphotcytes causing lymphocytes and monocyte-derived macrophages to enter the brain and target the myelin sheaths of the CNS causing demyelination

Neuroregeneration in the PNS

• when an axon in the PNS is cut, the severed part degenerates, and a regeneration tube is formed by Schwann cells

• growth factors (neurotrophic factors) are leased that stimulate growth of axon sprouts within the tube

• new axon eventually connects to the undamaged axon or effector

Neurotrophic (growth) Factors or Neurotrophins

• neurotrophins are secreted proteins that promote the survival, differentiation and growth of neurons

• promote neuronal growth in the fetal brain both in CNS and PNS

  • Nerve Growth Factor (NGF)

  • Brain-derived neurotrophic factor (BDNF)

  • Glial-derived neurotrophic factor (GDNF)

  • Neurotrophin-3, neurotropin-4/5

• In adults, neurotrophins aid in the maintenance of sympathetic ganglia (PNS) and the regeneration of sensory neurons

CNS Regeneration

• injury in the mature (adult) CNS triggers limited regeneration in central axons compared to peripheral axons

• Nogo: proteins produced predominantly by oligodendrocytes, inhibit axon regeneration in the mature CNS

• Glial scars form from astrocytes also prevent regeneration

Electrical Activity in Neurons

Resting Membrane Potential

• Neurons have a resting potential of -70mV

• why? bc of the imbalance of charged ions across the membrane, the inside of the resting neuron is negative relative to the outside

  Mechanism responsible?

• established by large negative molecules inside the cell

• Na+/K+ pumps (moves 3 Na+ positive charges OUT and 2K+ in)

  At Rest, we do have:

• electrical gradient: more negative inside

• concentration gradient: K+ inside the cell, Na+ outside the cell

Altering Membrane Potential

• neurons and muscle cells can change their membrane potentials

• excitability: the property of a neuron to produce electrical activity (change in membrane potential)

• caused by changes in the permeability to certain ions

• ions will follow their electrochemical gradient: combination of concentration gradient and attraction to opposite charges

• ion currents: flow of ions which occur where ion channels are located

Two main types of electrical activity in neurons

1. Graded Potential

2. Action Potential

Graded Potential

Graded Potential (ex: Depolarization)

• Graded potential: is a small local change in the membrane voltage

• when a “stimulus” (ex:ligand) reaches the neuron membrane certain channels (like Na) open, positively charged ions (sodium) flow into the cell

• this makes the inside of the neuron slightly less negative (a small depolarization, also called excitatory postsynaptic potential (EPSP)

• EPSPs are additive: if enough occur close together, they can bring the neuron to threshold for an action potential

Graded Potential-How do they generate? (ex: Hyperpolarization)

Graded Potential: small local change in membrane voltage

• when a stimulus (ligand) reaches the neuron membrane certain channels (Cl channel) open, negatively charged ions (Chloride) flow into the cell

• making the inside of the neuron slightly more negative ( a small hyper polarization, also called inhibitory postsynaptic potential or IPSP)

• IPSPs are additive- if enough occur close together, they can bring the neuron to FAR AWAY from the threshold, REDUCING the chance for an action potential

EPSP, IPSP

• EPSPs are additive: if enough occur close together, they can bring the neuron to threshold for an action potential

• IPSPs are additive: if enough occur close together, they can bring the neuron to FAR AWAY from the threshold, REDUCING the chance for an action potential

Action Potential Generation

• Excitatory Post Synaptic Potential (EPSP): as graded stimulus, depolarize the membrane

• if the depolarization reaches the threshold (-55mV) an action potential is generated

  All or None Law:

  1. once the threshold has been reached an action potential will happen

  2. stimulus will not affect the size of the action potential; it will always reach +30mV

  3. the size of the stimulus (EPSP) will not affect action potential duration

All or None Law

1. Once threshold has been reached, an action potential WILL happen

2. The size of the stimulus (EPSP) will NOT effect the size of the Action Potential; it will always reach +30mV

3. The size of the stimulus (EPSP) will NOT affect the Action potential duration

Generation of Action Potential Cont

• changes in membrane potentials and the generation of action potentials are controlled by changes in the flow of ions (Na+ and K+) through channels

Na+ Channels

• Na+ has voltage-gated channels that are closed at rest (they have an inactivation and an activation “gate”)

• the membrane is less permeable to Na+ at rest, normally they open at -55mV (threshold)

K+ Channels

has 2 types:

1. Not gated (always open): sometimes called K+ leakage channels

2. Voltage gated K+ channels: open when a particular membrane potential is reached; closed at resting potential (open at +30mV to -80mV)