Chapter 13 - Neuronal Communication

Role of neurones

To transmit electrical impulses rapidly around the body to allow the organism to respond to changes in internal and external environment

Parts of a general neurone

Cell body, Dendron, axon,

content and role of cell body

contains the nucleus surrounded by cytoplasm. large amounts of endoplasmic reticulum and mitochondria which are involved in the production of neurotransmitters

What are neurotransmitters?

chemicals used to pass signals from one neurone to the next

Function and structure of dendrons

-short extensions which come from the cell body. divide into smaller branches AKA denrites.
-To transmit electrical impulses towards the cell body.

Structure and functions of axons

\-Singular elongated nerve fibres. can be v long e.g. those that transmit impulses from the tips of toes and finger to the spinal cord

\-cylindrical in shape, consisting of a very narrow region of cytoplasm surrounded by plasma membrane -To transmit electrical impulses away from the cell body

Function of sensory neurones

To transmit impulses from a sensory receptor cell to a relay neurone, motor neurone or the brain.

Has one dendron, which carries the impulse to the cell body, one axon which carries the impulse away from the cell body

Function of relay neurones

To transmit impulses between neurones. e.g. sensory neurones and motor neurones.
short axons and dendrons

Function of motor neurones

To transmit impulses from a relay or sensory neurone to an effector, e.g. muscle/gland.
one long axon and many short dendrites

Travel of electrical impulses

receptor --> sensory neurone --> relay neurone --> motor neurone --> effector cell

Myelinated neurones

Neurones that have axons covered in myelin sheaths
the myelin sheath acts as an insulating layer and allows these myelinated neurones to conduct the electrical impulse at a much faster speed than unmyelinated neurones

What makes the myelin sheath in myelinated neurones?

Schwann cells grow around the axon multiple times, surrounding the axon with layers of membrane

Name for gap between Schwann cells

Node of Ranvier

Why nodes of Ranvier are useful?

Cause signal to jump which allows faster rate of transmission

Why is the rate of transmission slower in non-myelinated neurones?

No nodes of Ranvier so no jumping, continuous transmission is much slower

Types of sensory receptors

Mechano, chemo, thermo, photo

Stimulus mechanoreceptors respond to

Pressure, movment e.g. pacinian corpuscle receptor (detects pressure)

Example of chemoreceptor

Olfactory receptor

Example of thermoreceptor

End bulbs of Krause

Where do you find end bulbs of Krause?

Tongue

Shared features of sensory receptors

Specific to a single type of stimulus, transducers

Role of sensory receptors as transducers

Sensory receptors convert stimulus into a nerve impulse (Generator potential)

Structure of Pacinian Corpuscle

End of neurone surrounded by layers of connective tissue separated by layers of gel, sodium ion channels in membranes, stretch-mediated sodium channels

How Pacinian Corpuscles do transducing

Sodium ion channels too narrow in a normal state, resting potential present, corpuscle changes shape when pressure applied to the corpuscle, membranes stretch, channels widen, sodium ions diffuse in, membrane depolarises, generates generator potential, generator potential creates action potential

What is a resting potential

The potential difference across a neurone's membrane when it isn't transmitting an impulse

When there is a resting potential, where is there a more positive charge?

Outside the membrane

How resting potential develops

3 sodium ions actively transported out of the axon and 2 potassium ions actively transported in by sodium potassium pump, more sodium ions outside the membrane and more potassium ions inside the cytoplasm, sodium ions try to diffuse in and potassium ions try to diffuse out, gated sodium ion channels closed so sodium ions can't diffuse, potassium ions can move freely, more positive ions outside than inside

General value for resting potential

-70mV

Depolarisation

Change in potential difference across a membrane from negative to positive

How generator potential develops

Receptor cells respond to stimuli, gated sodium ion channels open, larger stimuli will open more channels, sodium ions diffuse into the axon, inside of neurone is less negative, change in potential difference across the membrane is a generator potential

How action potential develops

Generator potential reaches threshold

Voltage gated Na+ channels open

Lots of Na+ diffuse into the axon (Positive feedback)

Membrane depolarised, voltage gated Na+ channels close

Voltage gated K+ channels open

K+ diffuse out of membrane and become depolarised

Potential difference overshoots

Membrane becomes hyper polarised

Resting potential restored by sodium potassium pump

Refractory period

Where is there positive feedback in action potentials?

The diffusion of sodium ions into the axon when doing a generator potential will open voltage-gated sodium ion channels so more sodium ions diffuse in

Threshold voltage value

-50mV

Potential difference across membrane when depolarised

+40mV

Name for phase after repolarisation

Refractory period

Role of refractory period

To allow cell to recover, to only allow action potentials to be transmitted in one direction

How an action potential is transmitted down a myelinated neurone

Depolarisation happens at the nodes of Ranvier, sodium ions pass through protein channels at the nodes, localised circuits between nodes, action potential jumps from one node to another

Technical name for transmitting an action potential down a myelinated neurone

Saltatory conduction

Benefits of saltatory conduction

Faster as fewer places where channels have to open, more energy efficient as less repolarisation so less ATP required

All-or-nothing principle

If a stimulus crosses a threshold value, a response will always be triggered. If it doesn't, no action potential will be triggered. Size of action potential not affected by the size of the stimulus

How does size of the stimulus affect action potentials?

Larger stimuli cause more action potentials to be generated in a given time, increasing frequency, increasing degree of response.

Parts of a synapse

Synaptic cleft, presynaptic neurone, postsynaptic neurone, synaptic knob, synaptic vesicles, neurotransmitter receptors

Approximate size of the synaptic cleft

20-30 nm

Organelles the synaptic knob contains

Mitochondria, large amounts of endoplasmic reticulum

Types of neurotransmitter

Excitatory, inhibitory

Excitatory neurotransmitters

Neurotransmitters that result in the depolarisation of the postsynaptic membrane

Inhibitory neurotransmitters

Neurotransmitters that result in the hyperpolarisation of the postsynaptic membrane

Example of excitatory neurotransmitter

Acetylcholine

Example of inhibitory neurotransmitter

GABA

How impulses are transmitted across a synapse

Action potential reaches end of presynaptic neurone, depolarisation causes calcium ion channels to open, calcium ions diffuse to knob, vesicles containing neurotransmitters fuse with membrane, released by exocytosis, diffuse over, bind with receptor on the membrane, sodium ion channels open, sodium ions diffuse into neurone, triggers action potential, propagated along the neurone

Why neurotransmitter must be removed

Prevents response from happening again, neurotransmitter can be recycled

Specifics of the structure of cholinergic synapses

Acetylcholine is the neurotransmitter, hydrolysed by acetylcholinesterase, breaks down to choline and ethanoic acid, reformation requires ATP

Role of synapses

Ensuring impulses are unidirectional, allow impulse from one neurone to be transmitted to a number of neurones, allow an impulse from a number of neurones to feed into one

Summation

When the amount of neurotransmitter builds up to reach the threshold to trigger an action potential

Types of summation

Spatial, temporal

Spatial summation

When a number of presynaptic neurones are connected to one postsynaptic neurone

Temporal summation

When a single presynaptic neurone releases neurotransmitter several times over a short period as a result of several action potentials

Central Nervous System

Brain and spinal cord

Peripheral nervous system

The neurones that connect the CNS to the body,

How is the mammalian nervous system organised functionally?

Somatic and autonomic nervous systems

Somatic nervous system

System under conscious control, carries impulses to the muscles

Autonomic nervous system

System under subconscious control, carries impulses to glands and smooth muscle and cardiac muscle

How is the autonomic nervous system organised?

Sympathetic and parasympathetic nervous systems

Gross structure of the brain

Protected by the skull, surrounded by meninges, five main areas

Main areas of the brain

Cerebrum, cerebellum, medulla oblongata, hypothalamus, pituitary gland

Functions of the cerebrum

To receive sensory information and interpret it with respect to previous experiences, to send impulses along motor neurones to act on the information, used to control both voluntary and involuntary responses

Structure of the cerebrum

Highly convoluted, split into two halves, has discrete areas for different functions, outer layer called cerebral cortex

How is the brain able to judge distance and perspective?

Impulses from right side of the field of vision sent to left hemisphere, impulses from left side sent to right hemisphere, integration gives distance and perspective

Function of the cerebrum

Controls voluntary actions, such as learning, memory, personality and conscious thought.

How does the cerebrum work?

Receives sensory information, interprets it with respect to previous experiences
Sends impulses along motor neurones to effectors to produce an appropriate response.

Function of the medulla oblongata

To control reflex activities as part of the autonomic nervous system, such as ventilation and heart rate

Function of the hypothalamus

Main controlling region for the autonomic nervous system, one centre for parasympathetic system, one centre for sympathetic system, controls complex behaviour patterns, monitors composition of blood plasma, produces hormones

Function of the pituitary gland

To control most of the glands in the body

Structure of the pituitary gland

Divided into anterior and posterior pituitary gland

Function of anterior pituitary gland

To produce hormones such as FSH

Function of posterior pituitary gland

To store and release hormones made by the hypothalamus such as ADH

Reflex arc

Receptor detects stimulus, creates action potential for sensory neurone, sensory neurone carries impulse to motor neurone within the spinal cord via a relay neurone, motor neurone carries impulse to effector

What type of reflex is the knee jerk reflex?

Spinal reflex

Spinal reflex

When the neural circuit only goes up to the spinal cord, not the brain

How does the knee jerk reflex work?

Leg tapped just below the patella, stretches the patellar tendon, initiates the reflex arc, extensor muscle on top of the thigh contracts, relay neurone inhibits motor neurone of the flexor muscle so it relaxes, contraction of the extensor muscle causes the leg to kick

What is the knee jerk reflex used for?

Maintaining posture and balance

Examples of reflex actions

Knee jerk, blinking

What type of reflex is the blinking reflex?

Cranial reflex

Optical reflex

Blinking as a reaction to overly bright light

Cranial reflex

A reflex that occurs in the brain

How does the blinking reflex work?

Irritation of the cornea triggers an impulse along the fifth cranial nerve, passes through a relay neurone in the lower brain stem, impulses then sent along branches of the seventh cranial nerve, results in the eyelids closing

Consensual response

Both things respond in the same way to a stimulus

Example of a consensual response

Blinking reflex

How do reflexes increase your chances of survival?

Involuntary responses so the brain can deal with more complex responses, not learnt so provide immediate protection, fast

Stressor

Stimulus that causes the stress response which causes wear and tear on the body's physical or mental resources

Fight or flight response

Full range of coordinated responses of animals to situations of perceived danger

What is the cause of the fight or flight response?

Shift in the balance of stimulation to increase activity of the sympathetic nervous system and a decrease in activity of the parasympathetic nervous system

How is the fight or flight response coordinated?

Hypothalamus activates sympathetic nervous system and the adrenal-cortical system by releasing CRF, sympathetic nervous system activates the adrenal medulla which releases adrenaline and noradrenaline, sympathetic nervous system leads to impulses that activate glands and smooth muscles, anterior pituitary gland releases ACTH which leads to the adrenal cortex which releases hormones

Hormones that are released by the adrenal cortex in the fight-or-flight response

Cortisol, corticosterone

Role of cortisol

To regulate metabolism and blood pressure responses to stress

Role of corticosterone

Regulates immune response and suppresses inflammatory reactions

How does adrenaline use cell signalling?

Binds to its receptor, activates inactive adenyl cyclase to make active adenyl cyclase which is an enzyme, ATP is then converted into cAMP, cAMP is the second messenger

How is the nervous system involved in increasing heart rate?

Centre in the medulla oblongata which increases heart rate sends impulses through the sympathetic nervous system in the accelerator nerve to the SAN

How is the nervous system involved in decreasing heart rate?

Centre in the medulla oblongata which decreases heart rate sends impulses through the parasympathetic nervous system in the vagus nerve to the SAN

Types of receptors involved in changing heart rate

Chemoreceptors, baroreceptors