NERVES

what is a nerve made up of

a bundle of neurones

sensory, relay and motor neurones structures

sensory, relay and motor neurones comparison

cell body

• contains nucleus

• carries genetic code for production of neurotransmitters

• dense group of ribosomes and ER

• NISSL granules

• site of protein synthesis to make neurotransmitter

• sensory neuron cell bodies in dorsal root ganglia

• motor neuron cell bodies in spinal cord or brain

axon

• transmit action potential away from the cell body

• can be over 1 m in length

• 10 µm diameter

• allows for rapid transmission of impulse

• reduces the number of synapses required which are the area of slower transmission

• contains axoplasm and usual cell organelles

dendrite

transmits action potential towards cell body

dendron

allow communication with other neurones

plasma membrane

phospholipid bilayer with many protein ion channels

schwann cells

• thin cells which have wrapped themselves around the neurone

• have a higher than usual phospholipid content in their membranes and fewer ion channels, increasing electrical insulation of the neurone

myelin sheath

the enclosing layer created by schwann cells

nodes of ranvier

regions of uninsulated membrane where ion movement occurs to create action potential

synaptic knobs

• Point at which neurotransmitter is released from neurone to transfer the action potential to another neurone

motor end plate

• Point at which neurotransmitter is released from neurone to transfer the action potential to a muscle

sensory neurone

• cell body positioned in a ganglia just outside of the CNS

• transmit nerve impulse from sensory receptor to the CNS.

• at the CNS it may sign up with a relay or motor nuerone

motor neurone

• transmit nerve impulses from the CNS to an effect (muscle or a gland)

• cell body in the CNS

relay neurone

• connect sensory and motor neurones

• totally within the CNS

myelinated neurones

• covered by myelin sheaths

• happens when schwann cells wrapped around neurone creating myelin sheath

• schwann cell plasma membranes have a higher than usual phospholipid content with few ion channels, therefore iron movement can only occur at the nodes of ravier

  • this electrically insulate the neurone

what are sensory receptors

• Specialised cells

• Can detect changes in our surroundings [stimulus]

• Initiate a nerve impulse.

• Are transducers.

• Are specific to a stimulus

what are transducers

• a cell that converts on store of energy to another

• stimulus converts to nerve impulse → electrical energy

receptors and the energy changes they detect

pacinian corpuscle

• detects pressure changes on skin

• changes deform the layers of connective tissue

  • pushes against the nerve ending

  • initiates a nerve impulse.

• is sensitive to changes in pressure, so if the pressure becomes constant it will stop initiating nerve impulses

  • explains why you stop feeling clothes soon after you put them on

polarised

• membrane which has a potential difference across it, this is the resting potential.

• created by moving Na + and K +

depolarised

• loss of polarisation across the membrane

• Na + entering cell making inside less negative

resting potential

• potential difference across the membrane while the neurone is at rest

• approximately -70 MV

action potential

• depolarisation of the cell membrane

• fleeting reversal of resting potential

• approximately +40 MV

hyperpolarisation

• potential difference overshoots slightly and becomes more negative than resting potential

• approximately -80 mV

repolarisation

• time after an action potential has passed when it is impossible to stimulate the cell membrane because NA + voltage voltage channels will not reopen

• ensures that action potentials move in one direction and keeps each impulse separate

threshold potential

• approximately -50 mV

• if depolarisation of the membrane does not reach this value then an action potential is not generated

resting potential (explanation and diagram)

at rest:

• neurone membrane kept polarised

• some sodium/potassium gated some open.

• there are more open K + channels so K + can move back at its concentration gradient

• resting potential is due to the Na+/K+ pumps in the membrane

• membrane more permeable to K + (Na+ can’t move across) therefore higher concentration of anions inside the cell

role of sodium potassium ion pump in maintaining resting potential

• for every three Na+ pumped out, two K+ are pumped in, maintaining the more negative charge inside the membrane

• concentration gradient created so K + diffuses out

role of ion leakage channels in maintaining resting potential

they are more open to K +, so it can move back into the cell, maintaining the electrochemical gradient and the resting potential

why does inhibiting respiration/metabolic poison prevent resting potential

• inhibits ATP production (due to no respiration)

• ATP required for Na+/K+ pumps to function so resting potential can’t develop

• instead there is an equilibrium on either side of the membrane

how is action potential generated in a pacinian corpuscle

• when pressure is applied, stretch mediated Na+ channels open and allow Na+ to enter.

  • if enough enters this will depolarise the membrane.

• if the initial depolarisation passes the threshold potential (~50 mV) and action potential will occur

• membrane begins to depolarise, causing Na+ voltage gated channels to open

  • causes more depolarisation, so more Na+ voltage gated channels open (positive feedback)

• inside is now positive compared with the outside – action potential has been created

• once depolarised to ~+40 mV the Na+ voltage gated channels shut and K + voltage gated channels open (K + diffuses out of neurone causing repolarisation)

• when repolarisation has occurred, K + voltage channels stay open too long – causing hyperpolarisation, and K + channels shut

• resting potential is re-established by the action of the Na+/K+ pump

what happens during depolarisation?

• threshold potential of around -50 mV is reached and voltage gated Na+ ion channels open

• Na+ rapidly diffuses in, causing the potential difference to raise to +40 mV

  • inside of cell is now more positive than the outside

what happens during repolarisation

• voltage gated K+ channels open and Na+ channels close

• K+ ions quickly diffuse out, repolarising membrane

• restores resting potential, inside negative again

action potential generation (with graph)

1. The membrane starts in its resting state (polarised) and the inside of the cell is -70 MV compared with the outside.

2. A stimulus causes Na+ ion channels to open and some Na+ ions diffuse into the cell.

3. The membrane depolarises – it becomes less negative with respect to the outside and reaches the threshold value of -40 mV

4. Positive feedback occurs causing nearby voltage gated Na+ channels to open and many Na+ ions diffuse in, as more enter the cell becomes positively charged inside compared with the outside.

5. The potential difference across the membrane reaches +40 mV, inside of the cell is positive compared with the outside

6. Na+ ion channels close and K+ channels open

7. K+ ions diffuse out of the cell bringing the potential difference back to negative inside compared with the outside (repolarisation)

8. Potential difference overshoot slightly making the cell hyperpolarised.

9. The original potential difference is restored so that the cell returns to its resting state

propagation of action potential in a non-myelinated neurone

• Na+ ions enter the neurone and a local flow of electrical current occurs due to Na+ ions diffusing sideways down the electrochemical gradient (local circuits)

• With the arrival of some Na+ ions in the next part of the neurone, the membrane is depolarised.

• This change in potential difference causes Na+ voltage gated channels in the next part of the membrane to open.

• Na+ ions rapidly diffuse into the neurone, and the action potential has moved along.

• Each region of the membrane stimulates the next region to undergo an action potential.

• Behind the action potential re-polarisation occurs

what is the refractory period

• a short period of time when the neurone cannot be depolarised again

• Na + voltage gated channels cannot reopen even with the raised potential difference.

  • ensures that the action potential go in one direction only (do not go backwards) so that they don’t combine

propagation of action potential in a myelinated neurone

• schwann cells wrap tightly around the neurone

  • high phospholipid content with few ion channels.

  • so Na+ and K+ ions are not present along the outside of the neurone where the myelin sheath is

• depolarisation of the membrane can only occur at the nodes of ranvier, and much longer local circuits are created

  • faster than conduction in non-myelinated neurones, uses less ATP

• action potential then jumps between the nodes in a process called saltatory conduction

  • reduces the amount of repolarisation required

factors that will increase the speed of transmission of an action potential

• myelination

• temperature increase

  • more kinetic energy = faster ion diffusion

• axon diameter

• bigger = faster (less resistance due to less flow of the ions in the cytoplasm)

what is the all or nothing principle

• the threshold value will always trigger a response

• no matter the size of stimulus, the action potential is the same size

  • the size of the stimulus can be transmitted by the frequency of the action potentials

diagram of a cholinergic synapse

synaptic transmission (with diagram)

excitatory synapse

• eg: acetylcholine

• depolarises synaptic membrane

• if threshold is reached then action potential is initiated

inhibitory synapse

• eg: GABA

• causes hyperpolarisation (involves K+ channels) of post synaptic membrane

  • makes it much less likely that the threshold will be reached

• prevents action potential from starting

four roles of synapses

1. to ensure action potentials travel in one direction only (vesicles containing the neurotransmitter are only in the synaptic knob and receptor molecules for neurotransmitter are only in the postsynaptic membrane)

2. to allow impulses from one neurone to be spread to many neurones

3. to allow many neurones to feed into one synapse so only one neurone transmits the action potential any further

4. summation (when the effects of many generator potentials are added together)

spatial summation

when the combined effect of neurotransmitter released from several neurones reaches threshold level in the post synaptic neurone (neurotransmitters → several neurones)

temporal summation

when frequent impulses from one neurone result in enough neurotransmitter being released to reach threshold level in postsynaptic neurone (one neurone → neurotransmitters)

organisation of the nervous system diagram

ganglion definition

a cluster of cell bodies (therefore has million of synapses)

structural differences between autonomic and somatic systems

somatic

• one neurone

• no ganglion

• from CNS to skeletal muscle/effector

autonomic

• two neurones (preganglionic and motor/postganglionic)

• ganglion

• from CNS to smooth muscle/effector

structural differences between parasympathetic and sympathetic systems

parasympathetic

• long preganglionic fibre

• short postganglionic fibre

• ganglion in the effector (more specific)

• acetylcholine released at synapse

• acetylcholine released at junction with effector

• few nerves leading out of CNS

sympathetic

• short preganglionic fibre

• long postganglionic fibre

• ganglion in the adrenal medulla

• acetylcholine released at synapse

• noradreanline released at junction with effector

• many nerves leading out of CNS

what is the difference between white and grey matter

• white → myelin sheath

• grey → no myelin sheath

  • mainly synapses and cell bodies

  • relay neurones make up most of the grey matter (mostly un myelinated)

effect of sympathetic and parasympathetic systems on tissues

antagonistic actions

gross structure of the human brain (diagram)

skull

• bones called cranium

• protect delicate nervous system

meninges

• membranes, surround CNS

• secrete cerebral spinal fluid

• offer protection

cerebral spinal fluid

• secreted by meninges

• provides protection

  • absorbs mechanical shock

• provides nutrients and oxygen to brain cells

ventricles

• spaces within the brain filled with cerebral spinal fluid

corpus callosum

• tissue

• connects left and right cerebral hemispheres

  • allows the two sides to communicate

  • each controls opposite sides of the bosy

• ascending and descending nerve tracts cross over in the medulla oblongata

grey matter and white matter in the brain

• grey

  • outer 2mm

  • site of cell bodies and synapses

  • highly folded, communication happens here, lots of connections due to large surface area

• white

  • connects different parts of the cortex together

cerebrum

• controls higher brain function (conscious thought, conscious actions, emotion, reasoning, memory)

• sensory organs → processes → initiates impulses

• sensory areas

  • receives impulse/sensory information directly from receptors

• association areas

  • compares sensory information receives with previous experiences and other association areas in order to interpret what the input means and judge an appropriate response

• motor areas

  • initiate nerve impulses to voluntary muscles/effectors

cerebellum

• controls unconscious functions (eg: posture, balance, non-voluntary movement)

• contains over half the neurones in the brain

• many of it’s processes require learning before becoming automatic

• damage results in jerky/uncoordinated movement

medulla oblongata

• used in automatic control (eg: heart, breathing rate)

• autonomic control over non skeletal muscles

• controls swallowing, vomiting, coughing

hypothalamus

• regulatory centre for temperature and water balance

• has two centres → sympathetic and parasympathetic

• controls homeostatic mechanisms by negative feeback

• temperature regulation, osmoregulation, produces hormones, feeding, sleeping, aggression

pituitary gland

• stores and releases hormones that regulate many body functions

• has two lobes

  • posterior pituitary → stores and releases hormones from hypothalamus

  • anterior pituitary → produces it’s own hormones, moved into the blood via releasing factors

basic reflex arc

1. receptor

2. sensory neurone

3. relay neurone

4. motor neurone

what is a reflex action?

• response to changes in the environment, no brain processing to coordinate movement

• short pathway/rapid

how do reflex actions increase survival?

• immediate → removes from danger

• innate → not learned, gives protection from birth

• involuntary & invariable → same response every time, brain freed up for more complex decisions

knee jerk reflex

• used to help maintain posture/balance/help if you trip

• consists of only two neurones (sensory & motor)

• spinal reflex → neural circuit only goes up to spinal cord

• stimulus starts reflex arc, causes extensor muscles on top of the thigh to contract

  • at the same time, a relay neurone inhibits the motor neurone of the flexor muscle, causing to to relax

• contraction coordinated with relaxation of antagonistic flexor hamstring muscles, causing leg to kick

• absence of reflex → can indicate nervous problems

• overreaction → multiple oscillation can indicate cerebellar disease