neurones topic 8 gray matter

`Topic 8 Grey MatterThe Nervous System

Central nervous system (CNS)= brain + spinal cord

Peripheral nervous system= nerves running to and from to and from the CNS to all parts of the body

• All organisms show sensitivity (or response); this is the ability to detect changes in their environment (i.e. stimuli) and make appropriate adaptive responses (i.e. changes) to them.

• All animals (Kingdom Animalia) have a nervous system. It is a very rapid and highly specific control and coordination system which matches specific responses to a particular stimuli to ensure “the right thing happens in the right place at the right time to the right degree

• Coordination and control is achieved by the transmission of electrical signals (called nerve impulses) along specialised cells (called neurones).

receptor sensory neurones motor neurones effector

Basic plan of a nervous system

The functional unit of the nervous system is the neurone.

stimulus

response

relay neurones

incentral nervous system

muscle or gland

change in the environment of the receptor

generates nerve impulses

Neurones in the brain

Axon

Cell body

Dendrites

All neurones have:• A cell body

– contains all the organelles so is the metabolic unit which supports the neurone

– e.g. mitochondria, RER, Golgi apparatus etc• Long extensions of the cell body

– fine dendrites • transmit nerve impulses towards the cell body

– axon – very long single process • transmits nerve impulses away from the cell body

• Synaptic bulbs at the end of the axons

Types of neurones3 main types of neurone• Sensory neurones• transmit nerve impulses from sensory receptors

to the CNS• Motor neurones• transmit nerve impulses from the CNS to the

effectors• Relay neurones• found within the CNS to connect other neurones

to form neurone circuits; in the spinal cord they connect sensory and motor neurones.

Many neurones also have a myelin sheath around the axon – called myelinated neurones.It is formed by specialised cells called Schwann cells which wrap around the axon

Myelin sheath

Bundle of nerve fibres

Connective tissue coat of nerve

TS Nerve

The cell membranes are compressed together to form a tightly packed layer of myelin which is rich in a particular type of membrane lipid.

It acts as an insulator to prevent the movement of ions across the cell membrane and so acts as an insulator which speeds up the speed of transmission of nerve impulses

nucleus of Schwann cell

cytoplasm of axon

Saltatory conduction animationhttp://www.blackwellpublishing.com/matthews/actionp.html

• Checkpoint 8.1Compare the structure and location of motor, sensory and relay neurones

Checkpoint 8.1Compare the structure and location of motor, sensory and relay

neuronesMotor Relay Sensory

Structure Cell body, short dendrites, long axon

Cell body, short dendrites, short axon

Cell body, long dendrites, short axon

Location of cell body

Cell body + dendrites in CNSAxon outside CNS

Cell body etc inside CNC

Cell body and dendrites outside CNS, axon inside CNS

Dendrites Dendrites synapse with effectors

Dendrites synapse with other neurones

Dendrites synapse with receptors

Axons End in effector Axons synapse with other neurones

Axons synapse with relay and other neurones in CNS

Function Transmit impulses from CNS to effector

Connect sensory and motor neurones to form nerve circuits

Transmit impulses from receptors to CNS

Nerve circuitsThe nervous system consists of the• Central nervous system• = Brain + spinal cord• concerned with integrating and coordinating

inputs from receptors with outputs to effectors to generate the appropriate response to a particular stimulus

• Peripheral nervous system• nerves (contain large numbers of sensory and motor

neurones) passing to and from the CNS and the receptors and effectors in all parts of the body.

• The functional organisation of the nervous system is based on nerve circuits comprising of a series of neurones linked together at synapses which transmit nerve impulses.

• In terms of coordination and control functions the nervous system is organised into:

• The somatic (or voluntary) nervous system• controls skeletal muscles rapid responses

involving movement • The autonomic nervous system• coordinated by the hypothalamus and the medulla

of the brain, controls smooth muscle and glands coordination and control of internal systems

• Although the routing of the various neurones involved differs between the different functional divisions of the nervous system they all depend on nerve circuits called reflex arcs.

• The simplest type of behaviour controlled by the nervous system is a reflex action

• = a simple, fixed, rapid specific response to a particular stimulus.

• Example: finger touches something hot or sharp and the hand is pulled away rapidly (the withdrawal reflex); this is a somatic reflex (involving skeletal muscles) coordinated by the voluntary nervous system.

Nerve circuit of reflex arc

1. Finger touches hot plate heat damage to skin2. Sensory receptors in skin generator

potentials action potentials in sensory neurones nerve impulses transmitted towards spinal cord

3. Sensory neurones enter spinal cord via dorsal root synapse in grey matter

4. Neurotransmitter released which initiates nerve impulses in relay neurone

5. Neurotransmitter released from synapse of relay neurone nerve impulses initiated in motor neurone

6. Nerve impulses transmitted out of spinal cord along motor neurone to effector

7. Neurotransmitter released at synapse stimulates muscles in arm to contract to pull finger away from pin

Reflex arc animation

• http://www.sumanasinc.com/webcontent/anisamples/neurobiology/reflexarcs.html

Examples of reflex actions• OHT

The pupil response

The pupil response• See p185 - 8.• Describe in detail what happens in

the pupil reflex (receptors, effectors, neurone pathways etc)

• Explain the significance of the pupil reflex.

• In bright light, light enters eye, absorbed by photoreceptor cells in retina

• Nerve impulses in neurones in optic nerve• Impulses visual cortex vision• Some impulses midbrain coordinating centre impulses along parasympathetic nerve

(oculomotor nerve) to circular muscles contraction

• No impulses along sympathetic nerve relaxation of radial muscles

• stretch back to full length by antagonistic contraction of circular muscles.

• Function: • To reduce the amount of light entering the eye in

bright light to prevent damage to the retina and to avoid producing an over-exposed image.

• In dim light nerve impulses along the sympathetic nerve contraction of radial muscles to widen iris

• No impulses along parasympathetic nerve relaxation of circular muscles which are stretched back to their full length by the action of the antagonistic radial muscles

• Function• To let more light in to provide sufficient stimulation

of the retina to produce a clear image.

• Red eye reduction

Significance of reflex arcs.

• Under normal circumstances mild stimuli will result in nerve impulses being initiate by the sensory receptors in the sensory neurones and these are transmitted along the sensory neurones to the spinal cord and then across synapses with neurones which will transmit them to the sensory area of the brain so we are aware of what is being touched. However there are too few impulses to exceed the threshold to initiate nerve impulses in the relay neurone.

• The motor area of the brain can generate nerve impulses which are transmitted down neurones in the spinal cord to synapses with the motor neurones which control the arm muscles so we could move our hand independently of any other stimulus (a voluntary action).

• However if the stimulus is strong enough) as a result of damage (i.e. lots of nerve impulses are generated) the route to the brain is ‘short circuited’ by the relay neurone which links the sensory neurones transmitting the sensory information with the motor neurone (because now the number of impulses arriving at the synapse is sufficient to exceed its threshold and initiate nerve impulses) This will initiate the response to remove the finger from the damaging stimulus rapidly without the need to delay the action by routing it through the brain (because the response is obvious – get the finger out of the way – and no decision about what to do is necessary!).

Importance of reflexes

• these control very simple and predictable fixed forms of behaviour; the same stimulus will always initiate the same response

• the ‘short circuit’ provided by the relay neurone will link specific receptors with specific effectors by the shortest – and hence quickest – neurone pathway to produce a rapid and specific response to a particular stimulus; this will be automatic and requires no conscious thought

• it allows for very rapid responses– few neurones are involved, and no routing through

brain, so short transmission distances involved– neurones are myelinated so transmission speeds

high– few neurones are involved, so few synapses, so

little synaptic delay to slow transmission down (there are many synapses in brain to link the sensory and motor areas of the brain and elsewhere which are involved in ‘making decisions’ which would slow impulse transmission down).

Describe the nerve pathways involved if the hand was picking up a hot dinner plate but the person did not want to drop the plate.

Impulses will travel along the basic reflex arc…plusReceptor sensory neurone CNS neurone to sensory area of brain brain ‘feels’ hot/pain cerebrum/decision made not to drop plate impulses to motor area impulses from brain to synapse of relay neurone with motor neurone inhibition of synapse biceps does not contract + forearm muscles contract to keep hold of plate

http://w3.uokhsc.edu/human_physiology/presentation/propigate.gif

http://www.blackwellpublishing.com/matthews/channel.html

http://www.lionden.com/nerve_animations.htm

MRI scan of brain

Brain anatomy. Computer artwork of a human brain, seen from the side. The front of the brain is at left. From to top to centre is one of the hemispheres of the cerebrum, which is responsible for conscious thought, emotion and voluntary movement. Branching from the centre of the brain towards the front are the olfactory bulbs. At bottom right is the brainstem, which consists of the medulla oblongata, pons and midbrain. It controls automatic functions, sleep and arousal and relays messages from the brain to the spinal cord. At right is the cerebellum, which controls muscle coordination and balance.

Brain tumour

X-mal Deutchland -- Incubus Succubus II ]

Sensory homunculus: "This model shows what a man's body would look like if each part grew in proportion to the area of the cortex of the brain concerned with its sensory perception."

Motor homunculus: "This model shows what a man's body would look like if each part grew in proportion to the area of the cortex of the brain concerned with its movement."

cornea

iris

pupil

aqueous humour

ciliary body

lens

choroid choroid

retina

vitreous humour

optic nerve

Blood vessel in choroid

choroid

retinal rod and cone cells

optic nerve fibres

synapses

nuclei of retinal cells

nuclei of bipolar cells

nuclei of ganglion cells

LIGHT

Inner segments of rod and cone cells

cell bodies of rod and cone cells

rod cell

cone cell

rod cellcone cell

cell body

bipolar cell

ganglion cell

optic nerve fibres

synapse

fovea

blind spot

fovea

Colour vision deficiency, sometimes inaccurately called colour blindness, occurs when the cells in the retina of the eye which respond to light (cones) are abnormal or not working as well as they should.

The three types of cones are commonly known as red, blue and green, although they actually differ in their ability to recognise high, medium and low wavelength light.When they do not pick up or relay the proper colour signals to the brain, colour deficient vision results. Approximately 8% of men and 1% of women have some form of colour vision deficiency.

Colour vision deficieny or ‘colour blindness’

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There are three main types of colour deficient vision:

•Red-green deficiency (deuteranopia) is the most frequently diagnosed deficiency. Those with this condition cannot distinguish certain shades of red and green.

•Blue deficiency (protanopia) is relatively rare. Blue and yellow are not distinguished by those with this condition, and may be seen as white or grey.

•Total colour blindness (achromatopsia) is extremely rare. In this condition, no colours can be detected and the world is viewed in shades of black, white and grey. People with this condition have poor sight and are extremely sensitive to light.

Color blindness has several forms. Trichromats are the people who have full color vision. Dichromats are the people who can see only two of three primary colors of light (red, green, blue). Dichomacy has several forms. Achromatopsia is the inability to see any colors which may be called the actual color blindness. These people see life in monochrome, or greys.

The world.How the world looks to a

person with a red/green color deficit (deuteranopia).

How the world looks to a person with a blue/yellow color deficit

(tritanopia).

Some colorful hats

As seen by a person with deuteranopia

As seen by a person with protanopia, another form of

red/green deficit

People with color deficiencies may have difficulty distinguishing certain colors (e.g., a red/green color deficiency means that reds and greens are more difficult

to distinguish). But as this photo demonstrates, many other colors are just as distinguishable to a person with a color deficiency as to someone with normal

color vision.

Poppies and cyclamen Protanope Tritanope.

This is an Ishihara plate commonly used to check

for red/green color blindness

This is what a red/green color-blind person might see.

Note that the digit (3) is practically invisible.

normal color vision

Same colors seen by color blind people who have absence of red sensitivity and red weakness

absence of green sensitivity and green weakness

absence of blue sensitivity

•Red-green deficiency (deuteranopia) is the most frequently diagnosed deficiency. Those with this condition cannot distinguish certain shades of red and green.