unit 4 : Nervous system and brain

3 functions of the nervous system

1) monitors the external and internal environments

2) Processes and integrates information (specifically the CNS )

3) Coordinates a response to the incoming information and sends commands to effectors of the body

Major divisions of the nervous system

1) central nervous system

2) Peripheral nervous system

What is PNS the link to?

Between our CNS and the outside world

Central nervous system

Consists of brain and spinal cord and is housed in dorsal cavity of the body

Peripheral Nervous System

this division consists of all neural tissue outside of the CNS. It is composed of ganglia, nerves and receptors

2 subdivisions of PNS

1. Sensory (afferent) division

2. Motor (efferent) division

Motor (efferent) division

delivers motor commands from CNS to the muscle and glands of the body, which are called effectors

Sensory (afferent) division

consists of specialized structures called receptors that detect sensory information and send it to the CNS

Subdivisions of the Sensory Division of the PNS

1)The somatic sensory division

2)The visceral sensory division

somatic sensory division

detects information from regions of the body that are external to the ventral cavity, which includes the skin, skeletal musculature and bones

visceral sensory division

includes sensory receptors that detect stimuli in the viscera of the body, including the digestive, urinary and reproductive organs

Subdivisions of the Motor Division of the PNS

1) somatic motor division

2) visceral motor division (autonomic nervous system)

Subdivisions of the Autonomic Nervous System (ANS)

1) sympathetic division

2) parasympathetic division

somatic motor division

The effector targets are the skeletal muscles of the body

visceral motor division

has effector targets that include cardiac and smooth muscle as well as glands of the body

sympathetic division

the ‘fight or flight’ system

parasympathetic division

the ‘rest and digest’ system

What are 2 Cell Types in Nervous Tissue

1. Neurons

2. Neuroglia (glial cells)

Neurons

1. The basic unit of the nervous system. The neurons carry out the functions of the nervous system. To carry out these functions, neurons communicate with one another and with other types of cells (such as muscle cells or glands).
2. Neurons are not capable of mitotic division; if neurons are damaged they are not replaced. They also have a very high demand for oxygen and glucose

Neuroglia (glial cells):

1. Support the neurons

2. The glial cells actually outnumber the neurons (for everyone 1 neuron there are about 10 glial cells) but they are much smaller than the neurons

3.Also, unlike the neurons, the glial cells can mitotically divide.

5 basic features of neurons

1) dendrites

2) soma (cell body)

3) axon hillock

4) axon

6) axon (synaptic) terminals

Soma (cell body)

Houses most of the organelles

Dendrites

short cytoplasmic extensions. They receive incoming information

Axon hillock

transition region from soma to axon, site of action potential initiation

Axon

a single, long projection that carries
electrical signals away from the soma, towards the
distal ends of the axons

Axon (synaptic) terminals

very distal, enlarged region of the axon which will communicate with the target of the neuron

Synaptic End Bulb

Dilated knob-like end

Structural classifications of neurons

1) multipolar neuron

2) unipolar neuron

3) bipolar neuron

Multipolar neurons

2 or more dendrites and 1 axon. These are the most common types of neurons

Unipolar neurons

there is a single extension from the soma, and dendrite and the axon are continuous

Bipolar neurons

there is one dendrite and one axon that extend from the cell body. These are the most rare
type of neuron

Functional classifications of neurons

1) afferent neuron (sensory neuron)

2) efferent neuron (motor neuron)

3) interneuron

afferent neuron (sensory neuron)

-sends signals toward the CNS

-It generates action potentials from sensory receptors at Its peripheral end

-It has a long axon and is found mainly in the PNS

efferent neuron (motor neuron)

-sends signals away from the CNS to an effector organ

-It has a long peripheral axon in the PNS

Interneuron

• is found entirely within the CNS.

• It lies between afferent and efferent
  neurons

Neuroglia in the CNS

• Astrocytes

• Oligodendrocytes

• Microglia

• Ependymal cells

Astrocytes

• Has many functions, overall will support the
  neurons

• Help to maintain the blood brain barrier (a protective structure of the brain)

Oligodendrocytes

These cells form the myelin in the CNS

Myelin

• is a lipid sheath that wraps around the axons of some neurons in the nervous system

• Myelin increases the speed of action potential movement down an axon

myelinated axons

Axons that are wrapped in myelin

unmyelinated axons

Axons that are not wrapped in myelin

Microglia

• They are phagocytes

• They phagocytize pathogens and wastes
  in the CNS

Ependymal cells

• These cells line the central canal of the spinal cord and chambers in the brain called ventricles, both of which are filled with a fluid called cerebrospinal fluid.

• The ependymal cells help to produce and circulate the cerebrospinal fluid

Neuroglia in the PNS

• satellite cells

• schwann cells
  
  

satellite cells

small cells which surround the neuron cell body
in the PNS (their name comes from the resemblance to satellites around a planet)

Schwann cells

• create the myelin in the PNS

• (another name they are called is neurolemmocytes)

The myelinated axons of PNS

are wrapped in concentric layers of the Schwann cell plasma membrane, which
creates a lipid sheath around the cell

nodes of Ranvier

The gaps between the myelinated segments

Glossy shiny matter in nervous system

Is due to the lipid layers that create the myelin, myelinated axons are a shiny, glossy white

White matter in the nervous system

In the CNS where there are myelinated axons, the nervous tissue has a white appearance

Gray matter in the nervous system

Areas of the CNS where there are unmyelinated axons or cells body and dendrites appear gray

Ion channels

is a transmembrane protein in the plasma membrane that allows specific ions to move into or out of the cell

2 categories of ion channels

1) leaked

2) gated

leak channel

is always opens and always allowing current to pass through

Classes of Gated Ion Channels

1. Chemically (ligand)-gated

2. Voltage-gated

3. Mechanically-gated

Mechanically-gated

• opened when a mechanical stimulus is present, such as stretch, pressure

• Think “ketchup bottle” you must squeeze the bottle to force the ketchup through the gate

Voltage-gated

• opened when there is a depolarization of the membrane potential

Chemically (ligand)-gated

• open when a ligand, or extracellular chemical
  messenger, binds to the receptor region
  of the channel

• • Channels will only open when a
  specific chemical is compatible with
  that channel.

• • Neurotransmitter or chemical does
  NOT pass through channel. It only
  opens the ‘door’.

• • Only the ion the channel is permeable
  to will pass through the ‘door

Polarized

• Neurons, like all other cells in the body, are polarized

• This means the cell has an electric charge that is created by a difference in distribution of positive and negative charges across the plasma membrane.

Membrane potential

separation of the positive and negative charges creates a potential difference

(This is analogous to the potential of a battery)

Voltage

• is also a term that is used for potential difference

• The unit for voltage is Volts (V)

• In the cells, the voltage is small and so is measured in mV.
  

potential difference in a resting cell

• is known as resting membrane potential (RMP)

• Typically, the RMP of a neuron is -70mV

Average concentration in ICF

• 150mM K+

• 15mM of Na+

Average concentration of ECF

• 5mM K+

• 150mM Na+

Major factors of RMP

1) unequal distribution of ions (mostly Na+ and K+,but other ions as well) across the plasma membrane. The membrane is selectively permeable to these ions. Meaning more K+ is allowed to leave than Na+ to come in due to leaky channel differences

2) the large proteins are negatively charged anions that cannot leave the cell

3) Na+/K+ pump (3Na+ pumped out and 2K+ pumped in

The central nervous system is composed of what?

The brain and spinal cord

The brain

Is an extremely complex organ responsible for an individuals behavior, personality, and intellect.

It carries out these and many other functions via the interactions of neurons

Rostral

Towards the nose / forehead

(Synonym with anterior )

Caudal

Toward the tail (cord)

(synonymous with posterior)

Cortex

The outer gray matter located at the surface of the brain

Nuclei

Groups of neuronal cell bodies

Corpus callous

Nerve fibers that connect the left and right hemispheres

Internal capsule

Nerve fibers that connect the brain stem and the cerebral cortex

Septum pellucid

Membrane connecting the corpus callous and fornix

4 regions of the brain

1) the brain stem 2) cerebellum 3) diencephalon 4) cerebrum

Cerebrum

• The most rostral part of brain

• Covers the diencephalon and the brain stem

• Divided into 2 cerebral hemispheres which make up 80% of total brain mass

• Wrinkled surface

Fissures

Deep groves that are associated with the cerebrum. They separate different regions of the brain

Transverse cerebral fissure

Separates cerebral hemisphere from the cerebellum

Longitudinal fissure

Separates the right and left hemispheres

Gyri

Elevated ridges on the cerebrum

Sulci

Shallow grooves adjacent to the gyri

Purpose of gyri and sulci

To increase the surface area of the brain

3 main regions of the cerebral hemispheres

1) outer gray matter (cerebral cortex)

2) white matter internal to the cerebral cortex

3) gray matter within the white matter

What is the ratio for k+ channels to Na+ channels

There are 25 k+ channels for every 1 Na + channel

Depolarization

• Changes in membrane potential that cause the potential to become more positive ( or less negative)

• Probability of producing impulses increases

• Membrane potential moves towards 0mV

Hyper polarization

• Changes that cause the membrane potential to become more negative

• Probability of producing impulse decreases

• Away from zero

2 types of electrical signals created by ionic movement

1. Graded potential

2. Action potentials (AP’s)

Graded potential

• Mediated by ligand-gated ion channels or mechanically gated ion channels

• Initiated by dendrites (or soma) in response to synaptic input from another neuron

• Local changes in membrane potential that decay rapidly as the current (carried by the ions) moves from the initial site of entry into the cell

• Are called graded potentials because their magnitude varies directly with the strength of the stimulus (the stranger the stimulus the larger the voltage change and the further the current goes )

Action potentials (AP’s)

• Mediated by voltage gated or leaky ion channels

• All or none (either happen completely if threshold is reached or not at all)

• Do not vary in amplitude or strength (always the same magnitude)

• Very rapid ,very large change in membrane potential

• Polarity of membrane reverses during action potential

• Used for long-distance communication

• Do not decay

• Do not decrease in strength with distance they are same magnitude at the synaptic terminals as they are when they are initiated at the axon hillock

What are 2 voltage gated channels responsible for the action potential?

Voltage gated Na+ and voltage gated K+ channel

3 different states voltage - gated Na+ can be in

1) Closed state: closed but capable of opening

2) Open (Activated ) state: open with Na+ ions passing through

3) Inactivated state: closed,not capable of opening

States in which voltage gated k+ channel can be in

1) closed state

2) open (activated) state

Voltage gated Na+ channels kinematic

Fast to open and close (Like a sports car)

Voltage gated K+ channel kinematic

Slow to open and close ( like a semitruck)

Tetrodotoxin (TTX)

• blocks vgNa+ channels only

• Found naturally produced in some species like pufferfish

• 1-2mg is lethal. LD of 50. 5.0-8.0 microgram/kg. Less than the tip of a pinhead can unalive a human. No antidote exists

TEA (Tetraethylammonoium)

• man made for laboratory research to block vgK+ channels only

• No antidote. No reported cases of unalivement

What is needed for action potential to be initiated by the opening of the voltage-gated
channel?

the membrane potential has to depolarize to the threshold potential

What is the threshold potential in mV?

-55 mV

What causes membrane potential to depolarizes to the threshold from RMP?

graded potentials in the neuron caused by synaptic activity

What happens during depolarization?

Once threshold (-55mV) is reached, voltage-gated Na+ channels open and Na+ moves
into the cell (Na+ influx) causing a rapid depolarization to +30mV