Nervous System - Human Bio

What is a neuron?

A nerve cell

What is a nerve fibre?

Any long extension of cytoplasm of a nerve cell body (the axon)

What is a nerve?

A bundle of nerve fibres held together by connective tissue.

Sensory Neuron

Carry messages from receptors in sense organs, or in the skin, to the CNS (brain and spinal cord)

Motor Neurons

Carry messages from the CNS to muscles and glands (the effectors)

Interneurons

The link between the sensory and motor neurons/

Multipolar Neurons

Has one axon and multiple dendrites. Found in brain, spinal cord and motor neurons.

Bipolar neurons

have one axon and one dendrite, which has many branches on its end. Found in eye, ear and nose.

Unipolar Neurons

Have one extension, an axon. Found in insects.

Pseudounipolar Neurons

Have properties of Unipolar + biopolar. Has single axon from the cell body which separates into two extensions.

Neuron Structural type of the functional neurons.

Sensory = pseudounipolar

Motor = Multipolar

Interneuron = multipolar

Location of the functional types of Neurons.

Sensory = Cell body/dendrite outside spinal cord.

Motor = cell body/dendrites in spinal cord. Axon outside spinal cord.

Interneuron = Entirely within CNS.

Dendrite Function

Carries nerve impulses (messages) in the cell body

Nucleus

Found in the cell body. The nucleus control’s the cell’s activities, it contains the cell’s genetic material (DNA).

Myelin Sheath

-Acts as an insulator
-Protects the axon from damage
-Speeds up the movement of nerve impulses along the axon.

Axon

Carries nerve impulses (messages) away from the cell body.

Schwann Cell

Supports and insulates the axon. Regenerates nerve fibres and produces myelin sheath.

Lateral Branch

Side branch that extends from the axon allowing it to connect with multiple target cells.

Node of Ranvier

gaps in the myelin sheath that allow the chemical part of nerve impulse to occur. They allow for ions to diffuse in and out of the neuron, propagating an electrical signal down the axon.

Motor End Plate (Axon Terminal)

Point at which an impulse passes to a muscle causing contractions.

Neurilemma

Helps in the repair of injured nerve fibres.

Cell Body

Contains the nucleus and is responsible for controlling the functioning of the cell.

Filament

Transports impulses to the motor end plate.

Steps to an action impulse.

1. Resting Potential

2. Threshold

3. Depolarization

4. Repolarization

5. Hyperpolarization

What is an action potential?

an electrochemical event created by a sudden concentration of ions on oppose sides of a neurons cell membrane.

Describe resting potential

• Not transmitting a nerve impulse.

• Outside of neuron is positively charged while inside is negatively charged; membrane is polarized

• Resting potential voltage = -70mv

• Sodium - Potassium Pump = 3 Na+ out for every 2 K+ it moves in.

• Gated ion channels remain closed until membrane is stimulated,

• Proteins inside cell = negatively charged and cannot move.

Describe the Threshold

• Stimulus triggers action potential

• Stimulus —» gated Na+ channels open. Na+ rushes into cell along concentration gradient.

• Membrane potential rises to -55mv

• This threshold voltage causes nearby voltage gated Na+ channels to open.

Describe depolarization

• Threshold reached —» voltage gated Na+ channels open

• Allows more Na+ ions to enter the cell.

• Large influx causes a strong depolarization called an action potential.

• Membrane potential rises to +40mv

• Action potential triggers depolarization in each area along the neuron like a wave.

Describe Repolarisation

• Action potential moves down axon —» membrane repolarizes

• All Na+ channels close

• K+ channels open, causing the K+ ions to exit the neuron.

• Large outflux of (+) ions = move towards -70mv

• Entire depolarization/repolarization occurs in less than 1ms.

Describe Hyperpolarization

• K+ channels stay open longer than needed, resulting in hyperpolarization

• Membrane briefly falls to -75mv

• Sodium potassium pump must move more Na+ ions out of the cell and more K+ ions into the cell —» return to resting potential.

• Once resting potential is reached again, neuron can fire again.

Refractory Period

time after an action potential has occurred when a neuron is unable to fire.

Absolute refractory period

• Neuron cannot trigger an action potential

• Due to Na+ channels closed

• Lasts for 1ms

Relative Refractory Period

• Neuron can trigger an action potential from a larger stimulus.

• Due to K+ channels being open

• Can last up to 3ms

What is the “all or nothing” rule?

An action potential will not occur if the stimuli is not large enough to go over the threshold of -55mv

How are large and small stimuli coded by ions?

A larger stimulus does not result in a large action potential (over +40mv). Rather the frequency of the action potential determines how a stimulus in perceived by the CNS, Small stimuli = lower frequency of AP. Large stimuli = higher frequency of AP.

Unmyelinated Neurons

• Continuous conduction → continues smoothly down the axon

• Not covered in Myelin sheath

• Is slower = 5m/s

Myelinated Neurons

• Saltatory conduction → jumps node to node across the axon.

• Covered in myelin sheath.

• Is faster, 150m/s

Steps to synaptic transmission

1. Neurotransmitters are stored in synaptic vesicles in the pre-synaptic neuron.

2. Impulse triggers release of neurotransmitters, which merge with axon terminal membrane.

3. Neurotransmitters rapidly cross synaptic cleft

4. Neurotransmitters bind to receptors on post-synaptic neuron; trigger opening of Na+ channels.

5. Influx of Na+ ions stimulates an impulse in the next neuron.

Common Neurotransmitters

• Acetylcholine (Brain, muscles, ANS)

• Noradrenaline (Adrenal Glands)

• Adrenaline (Adrenal glands)

• Dopamine (Brain)

• Serotonin (Brain & spinal cord)

Central Nervous System

made up brain and spinal cord. (Cranial nerves) Where incoming messages are processed, and outgoing messages are initiated.

Peripheral Nervous system

nerves that connect the CNS with receptors, muscles and glands. Nerve fibres in PNS → carry info to and from the CNS and ganglia (nerve cells outside of brain + spinal cord)

Divisions of PNS

Sensory (afferent) and Motor (efferent)

Sensory (afferent) division

carries impulses into the CNS by sensory neurons from receptors in the skin and around muscles and joints (somatic sensory neurons). Visceral sensory neurons → take impulses from internal organ → CNS.

Motor (efferent) division

carries impulses away from the CNS. sub divided into autonomic (involuntary) division and somatic (voluntary) division.

Somatic Division

carries impulses from the CNS to the skeletal muscles. responsible for movement of voluntary muscles.

Autonomic Division

carries impulses from the CNs to the heart, involuntary muscles and glands. responsible for control of the body’s internal environment and is involved in many mechanisms that keep the internal environment constant.

Divisions of autonomic division

Sympathetic and parasympathetic divisions.

Sympathetic Division

involved in changing your internal environment for conditions suited for strenuous physical activity (fight or flight)

Parasympathetic Division

involved in changing your internal environment conditions suited to rest and relaxation.

Cerebrum (aka cerebral cortex/hemisphere)

higher order functions such as thinking, reasoning, memory, learning, conscious awareness of surroundings.

Corpus Callosum

communication between the two cerebral hemispheres (bundle of nerve fibres)

Cerebellum

coordination of fine contractions of muscles resulting in smooth movements and the Maintenace of posture and balance.

hypothalamus

homeostasis; regulation of the heart, digestive system, appetite, thirst, metabolism, body temp., response to fear or anger.

Medulla Oblongata

under the influence of the hypothalamus, regulates the heart (cardiac centre), breathing (respiratory centres) and diameter of blood vessels (vasomotor centre).

Spinal Cord

provides a pathway for communication between muscles and glands and the brain, integration of autonomic, protective reflexes.

Frontal Lobe

Thinking, problem solving, emotions, personality, language and control of movement.

Parietal Lobe

Processing temp., touch, taste, pain, movement and sensation.

Temporal Lobe

Processing memories and linking them with senses; receives auditory info.

Occipital Lobe

Vision

Insula

Recognition of different sense and emotions, addiction and psychiatric disorders.

Longitudinal fissure

a deep cleft that almost separates the cerebrum into two halves, the cerebral hemispheres.

Convolutions or gyri

folds of the surface of the cerebrum

Sulci

shallow downfolds of the surface of the cerebrum.

Grey Matter

the part of the brain and spinal cord up of nerve cells bodies and unmyelinated fibres.

White matter

The part of the brain and spinal cord made of myelinted fibres.

Basal Ganglia

The masses of grey matter inside each cerebral hemisphere → control movement.

How a nerve impulse is propagated along a myelinated nerve fibre.

1. Depolarization occurs at Nodes of Ranvier along the nerve fibre.

2. The impulse jumps from node to node (saltatory conduction)

3. Cell membrane becomes permeable to sodium ions.

4. Sodium ions diffuse across the cell membrane.

5. Inside the cell membrane becomes relative to the outside.

6. Potassium ions diffuse out of the cell across the membrane

7. Inside of the membrane negative relative to the outside

8. A threshold of -55mV needs to be reached

9. A sodium - potassium pump acts. It transports 3 Na+ ions out for every 2 K+ ions into the cell.

10. The neuron returns to a polarised (resting) state.

11. Hyperpolarization occurs.

12. The action potential triggers depolarization in adjacent membrane region.

Neuromuscular junction

gap between axon and skeletal muscle cell.

What is a reflex?

rapid, automatic response to a change in the external or internal environment.

Properties of relfexes

1. A stimulus to trigger a reflex

2. They are involuntary

3. They are rapid

4. They occur the same way each time.

What parts are involved in reflexes?

Coordinated by the spinal cord (spinal reflexes)

Certain reflexes involve unconscious parts of the brain.

Reflex arc

the pathway a nerve impulse follows when travelling from a receptor to an effector.

Parts of a relfex arc

Receptor - reacts to change in external or internal environment → nerve impulse in sensory neuron.

Sensory Neuron - carries impulse from receptor to spinal cord/brain

Synapse - The nerve impulse can be transmitted to a motor neuron or to one or more interneurons, which then relay the impulse to the appropriate motor neuron.

Motor Neuron - carries the nerve impulse to an effector

Effector - muscle cell or gland that receives the impulse and carries out response.

Receptor

a structure that is able to detect a change in the body’s internal and external environment.

Thermoreceptors

detect heat. located in skin (external environment) and hypothalamus (internal environment → regulate body temperature)

Osmoreceptors

detect osmotic pressure in the blood. located in hypothalamus (maintains body’s water content)

Chemoreceptors

detect chemicals. located in nose (odour), mouth (taste) and blood vessels (pH, regulation of heartbeat & breathing)

Touch receptors (aka mechanoreceptors or pressure receptors)

detect touch. Located in skin and hair follicles

Pain Receptors (aka nociceptors)

detect damage to tissues. located in skin and mucous membranes. In most organ but brain.