Unit 6: Nervous System

Nervous System

• Detects environmental changes that impact body, then works in tandem with endocrine system to respond to events

• Responsible for all behaviours, memories, and movement

Excitable Characteristics

Allows for generation of nerve impulses

Two Divisions

• Central nervous system

• Peripheral nervous system

CNS

Central nervous system

PNS

Peripheral nervous system

Peripheral Nervous System

• Consists of cranial and spinal nerves that contain both sensory and motor fibers

• Connects CNS to muscles, glands, and all sensory receptors

Central Nervous System

Consists of brain and spinal cord

Subdivisions of PNS

• Somatic nervous system

• Autonomic nervous system

SNS

Somatic nervous system

Somatic Nervous System

• Voluntary

• Neurons from cutaneous and special sensory receptors to CNS

• Motor neurons > skeletal muscle tissue

Autonomic Nervous Systems

• Motor neurons to smooth and cardiac muscle and glands

• Sensory neurons from visceral organs > CNS

Autonomic Nervous Systems Divisions

• Sympathetic

• Parasympathetic

Sympathetic Division

• Tachycardia

• Fight or flight

Parasympathetic Division

• Bradycardia

• Rest and digest

3 Steps of Nervous System

• Sensory function

• Integrative function

• Motor function

Sensory Function

Sense changes in internal and external environment through sensory receptors

Sensory Neurons

Serve sensory function

Integrative Function

Analyze sensory information, store some aspects, and make decisions regarding appropriate behaviours

Interneurons

• Association neurons

• Serve integrative function

Motor Function

Respond to stimuli by initiating action

Motor Neurons

Serve motor function

Neurons

• Highly specialized

• Intricate connections with other cells

• Cannot divide

• Functional unit of nervous system

• Capacity to produce action potentials

Action Potentials

Electrical excitability

Neuroglia

• Less specialized

• Support, nourish, and protect neurons

• Can divide

Cell Body

• Single nucleus with prominent nucleolus

• Nissl bodies

Nissl Bodies

Rough ER and free ribosomes for protein synthesis

Neurofibrils

Give cell shape and support

Microtubules

Move material inside cell

Dendrites

• Processed look like tree branches

• Receive and transmit stimuli towards cell body

Axon

• Transmits info away from cell body

• Forms branches > axon terminals store neurotransmitter

• Many have myelin sheath

Synaptic Knobs

Axon terminals

Cell Processes

Dendrites and axons

Multipolar Neurons

• Many dendrites and only one axon

• Located throughout brain and spinal cord

• 99% neurons

Bipolar Neurons

• One main dendrite and one axon

• Convey special senses of sight, smell, hearing, and balance

• Retina of eye, inner ear, olfactory

Unipolar Neurons

• One process which extends from body and divides into a central branch that functions as an axon and a dendritic root

• For sensory neurons that convey touch and stretching information from extremities

Pseudounipolar Neurons

Unipolar neurons

Afferent Neurons

• Carry impulses from peripheral sense receptor to CNS

• Skin, sense organs, visceral organs

Efferent Neurons

• Carry impulses away from CNS to effectors

• Muscles, glands in PNS

Association Neurons

• Receive input for sensory neurons, communicate with one another or with motor neurons

• Entirely in CNS

Glia

• Support, nourish, and protect neurons cells

• Do not conduct nerve impulses

CNS Neuroglia

• Astrocytes

• Oligodendrocytes

• Microglia

• Ependymal cells

PNS Neuroglia

• Schwann cells

• Satellite cells

Astrocytes Types

• Protoplasmic

• Fibrous

Protoplasmic

Grey matter

Fibrous

White matter

Astrocytes Functions

• Support neurons

• Create a blood-brain barrier

• Secrete chemicals that regulate growth, migration, interconnection among brain neurons

• Role in learning and memory

Oligodendrocytes

Form and maintain myelin sheath around CNS axons

Microglia

Function as macrophages

Ependymal Cells

• Line ventricles of brain and central canal of spinal cord

• Produce, monitor, assist in CSF circulation

Schwann Cells

• Encircle PNS axons

• Form myelin sheath around PNS axons

Satellite Cells

• Surround cell bodies of neurons and PNS ganglia

• Regulate exchanges of material b/w neuronal cells and interstitial fluid

Ganglia

• Small masses of neuronal cell bodies located outside brain and spinal cord, usually associated with cranial and spinal nerves

• Somatic, autonomic, and enteric

Myelination

Process of forming myelin sheath which insulates and increases nerve impulse speed

Myelin Sheath

• Multilayered lipid and protein that covers some axons

• Increases from birth to maturity

• Increases speed of nerve conduction

Myelin Sheath Formation

Formed by oligodendrocytes in CNS and schwann cells in PNS

Nodes of Ranvier

• Are gaps in myelin sheath

• Each schwann cell wraps around one axon segment b/w two nodes of raniver

Nodes of Ranvier Length

Myelinated nodes are about 1mm in length and up to 100 layers

M.S.

Autoimmune destruction of myelin

White Matter

Lipid part of myelin imparts appearance

Grey Matter

Colour is because it lacks myelin

White Matter Formation

Formed from aggregations of myelinated axons from many neurons

Grey Matter Formation

Formed from neuronal cell bodies and dendrites

Electrical Signals in Neurons

• Electrically excitable due to voltage difference across their membranes

• In living cells, a flow of ions occurs through ion channels in the cell membrane

2 Types of Electron Signals

• Graded potentials

• Action potentials

Graded Potentials

Are used for short distance communication only

Action Potentials

Allow communication over long distances within the body

Neuron Cell Membrane

Separates charged particles on inside of cell from those on outside

Extracellular Fluid

Dominated by Na+ ions and Cl- ions

Intracellular Fluid

Dominated by K+ ions and negatively charged proteins (Pr-)

Resting Cell

• Polarized: ~70millivolts

• Excess of “+” charges on outside of cell membrane and excess of “-” charges on inside

Membrane Potential

Uneven distribution of charges

Depolarization

Sudden change in membrane potential at one spot on surface, an action potential has developed

Special Ion Channels

• In plasma membrane

• Open and allow sudden movement of Na+ ions across cell membrane

• Excess + ions in ICF and - ions in ECF

Reversal

• Polarization to depolarization

• Very short lived because ion channels close immediately however the change causes a chain reaction

Chain Reaction

“Opening and closing” forces the action potential across the entire cell membrane

Factors that Affect Speed of Propagation

• Amount of myelination

• Axon Diameter

• Temperature

Amount of Myelination

• More myelin

• The faster

Axon Diameter

• Larger diameter

• The faster

Temperature

Lower speeds when cooled

Synapse

Communication b/w neurons occurs at a junction called the synapse

Synapse Involves

• Presynaptic neuron

• Synaptic cleft

• Postsynaptic neuron

Presynaptic Neuron

Neuron sending impulse

Synaptic Cleft

Space b/w presynaptic neuron and postsynaptic neuron

Postsynaptic Neuron

Neuron receiving impulse

Step 1 of Signal Transmission

Nerve impulse arrives at synaptic end bulb of presynaptic axon

Step 2 of Signal Transmission

• Depolarizing phase of nerve impulse opens voltage-gated Ca2+ channels

• B/c Ca2+ ions more concentrated in extracellular fluid, Ca2+ flows through open channels

Step 3 of Signal Transmission

• Increase concentration of Ca2+ triggers exocytosis if synaptic vessels

• Neurotransmitters are released into synaptic cleft

Step 4 of Signal Transmission

They diffuse across synaptic cleft and bind to neurotransmitter receptors in postsynaptic neuron

Step 5 of Signal Transmission

Binding of neurotransmitter molecules to receptors on ligand-gated channels opens channels to ion flow

Step 6 of Signal Transmission

This causes voltage across the membrane to change

Step 7 of Signal Transmission

When depolarizing postsynaptic potential reaches threshold, triggers nerve impulse in axon of postsynaptic neuron

Action Potential Crossing Synaptic Cleft

• Action potential arrives at end of axon called synoptic knobs

• Inflow of Ca2+ caused by depolarizing phase of nerve impulse triggers presynaptic neuron to release a neurotransmitter

Neurotransmitter Location

Stored in vesicles in synaptic knob

Exocytosis

Active transport

Neurotransmitter Release

Through exocytosis

Neurotransmitter Crossing Synaptic Cleft

• Diffuse across synaptic cleft until they reach receptors in membrane or receiving cell

• They then bind to these receptors, which open ion channels and allow movement of Na+ ions across postsynaptic cell membrane

Action Potential Triggered

Triggered by the movement of Na+ ions

Neurotransmitter Removal

By diffusion out of synaptic cleft, enzymatic degradation, and re-uptake by cells

Spinal Cord Anatomy

• Protective structures, protect the spinal cord, and provide physical stability

• Vertebral column

• Meninges

Meninges

• Dura mater

• Arachnoid

• Pia mater