Principles of the nervous system I (Anatomy)

Question 1

Describe the basic structure and function of a neuron

Answer 1

Structure of a Neurone

A neuron is a specialised cell designed to transmit electrical signals, allowing communication within the nervous system. Its structure includes:

1) Dendrites:

• Dendrites are branching structures that emerge from the neuron’s cell body
• They receive incoming signals (often neurotransmitters from other neurones) and convert them into electrical impulses. These impulses are then conducted towards the cell body.
• Dendrites act as the input regions of the neuron, helping collect and integrate signals from multiple sources

2) Soma (Cell Body):

• The soma is the central part of the neuron that contains the nucleus and other organelles (ER, mitochondria).
• It integrates incoming signals from the dendrites. If the combined signal is strong enough, the soma generates an electrical impulse (an action potential) that will travel along the axon
• Also the site where essential proteins and neurotransmitters are synthesised

3) Axon:

• A long, thin projection that extends from the soma
• Primary role is to transmit electrical signals (action potential) away from the cell body to other neurons, muscles or glands.
• This transmission occurs via the axon in a process known as axon conduction.
• In longer neurons, axons are often covered by a myelin sheath, which facilitates faster signal transmission through saltatory conduction.

4) Myelin Sheath:

• A fatty insulating layer made up of Schwann cells in the PNS and oligodendrocytes in the CNS. These cells wrap around the axon, forming the myelin sheath
• Increases the speed of electrical transmission by allowing the action potential to jump between gaps in the sheath called Nodes of Ranvier. This process, known as saltatory conduction, is much faster than continuous conduction along unmyelinated axons

5) Nodes of Ranvier:

• Small, uncovered gaps between the myelin sheath segments along the axon
• The action potential jumps from node to node, increasing the efficiency and speed of signal transmission along the axon

6) Axon Terminal (Synaptic Bouton):

• End part of the axon that makes contact with the target cell, another neuron, muscle or gland
• Stores and releases neurotransmitters into the synaptic cleft during synaptic transmission. When an action potential reaches the axon terminal, it triggers the release of neurotransmitters into the synaptic cleft, allowing chemical communication between neurons or other cells

7) Synapse:

• The synapse is the junction between the axon terminal of one neuron and the dendrite (or cell body) of another neuron or target cell
• It facilitates communication between neurons via neurotransmitter release. The synapse includes the presynaptic neuron, the synaptic cleft, and the postsynaptic neuron.

Function of a Neuron

1) Electrical Signalling (Action Potential):

• Neurons communicate with each other via electrical impulses. This electrical signal called an action potential, is generated when the neuron becomes excited (through inputs from other neurons or sensory stimuli)
• Action potentials are caused by the movement of ions (particularly sodium and potassium) across the neuron’s membrane, leading to a change in membrane potential. WHen a neuron reaches a certain threshold of stimulation, an action potential is initiated, which travels down the axon to the axon terminal

2) Neurotransmission:

• When the action potential reaches the axon terminal, it triggers the release of neurotransmitters stored in vesicles. These chemical messengers cross the synaptic clef and bind to receptors on the postsynaptic neuron (target cell)

3) Integration of Signals:

• A signal neuron can receive thousands of inputs from other neurons through its dendrites. These inputs can be excitatory (increasing the likelihood of an AP) or inhibitory (decreasing the likelihood). The cell body integrates these signals to determine whether or not to generate an action potential

4) Neuronal Communication and Network:

• Neurons do not work in isolation but are part of larger networks that process and transmit information throughout the body. Neurons connect with each other to form complex circuits that regulate every aspect of human function, from basic reflexes to higher cognitive processes

Neuron Types

• Sensory Neurons: Transmit sensory information from receptors to the CNS
• Motor Neurons: Send signals from the CNS to muscles or glands to control their actions.
• Interneurons: Found mostly in the CNS, these neurons connect other neurons within the brain and spinal cord to integrate information and coordinate responses

Neuroplasticity

• Through neurons are generally non-dividing (post-mitotic), they can undergo changes in their structure and function uin response to learning and injury, this ability to adapt is called neuroplasticity

Question 2

Describe the role of various supporting cell types within the nervous system

Answer 2

Supporting cells in the nervous system, known collectively as glial cells or neuroglia, are non-neuronal cells that provide crucial support, protection and nourishment to neurons.

Supporting Cells in the CNS

1) Astrocytes:

• Star-shaped cells found abundantly in the CNS, forming a significant part of the brain’s cellular network
• Structural Support: help form the physical structure of the CNS, ensuring that neurons stay in place
• Blood-brain barrier (BBB): astrocytes form the BBB, a protective barrier that prevents harmful substances (e.g. pathogens) in the bloodstream from entering the brain, while allowing essential nutrients to pass through
• Regulation of the extracellular environment: Astrocytes regulate the ion concentration in the CNS by removing excess potassium (K+) and neurotransmitters from the synaptic cleft, maintaining optimal environment for neurons to function properly
• Nutrient supply: They supply neurons with nutrients, e.g. glucose and help remove waste products
• Neurotransmitter recycling: Astrocytes play a role in the uptake and recycling of neurotransmitters, ensuring that synpatic transmissions remain efficient
• Repair and Scarring: When neurons are damaged, astrocytes contribute to repairing the CNS by forming glial scars, which can limit the spread of damage
• Recycle excess neurotransmitters

2) Oligodendrocytes:

• These cells have a few branching extensions (hence the prefix ‘oligo-‘) and are only found in the CNS
• Myelination: Oligodendrocytes produce the myelin sheath that insulates axons in the CNS. Each oligodendrocyte can myelinate multiple axons at once, wrapping its extensions around them to create segments of the sheath
• The myelin sheath allows for saltatory conduction, where the action potential jumps from on Node of Ranvier to the next, increasing the speed of signal transmission along the axon
• Myelin mostly consists of Myelin Basic protein (MBP) and Proteolipid Protein (PLP)

3) Microglia

• Small cells with many fine processes that can change depending on the activation state
• immune defense: Microglia serve as the primary immune cells of the CNS. They constantly survey the environment for signs of infection or damage and can bcome activated in response to pathogens or injury
• When activated, act like macrophages, engulfing and digesting debris, pathogen, and dead or damaged neurons
• Play a role in synaptic pruning during development and in adulthood, removing weak or unnecessary synapses to fine-tune neural circuits
• Release factors that promote repair and regeneration after injury
• 5% of all cells in the brain, most numerous, 10:1 neuron

4) Ependymal Cells

• Cells line the ventricles of the brain and the central canal of the spinal cord
• CSF production: Produce and circulate CSF, which cushions the brain and spinal cord, provides nutrients and remove waste
• Have cilia that help circulate the CSF, ensuring that it moves properly through the ventricular system

Supporting Cells in the PNS

1) Schwann Cells:

• Main glial cells of the PNS, unlike oligodendrocytes, each Schwann cell myelinates only a single axon
• Schwann cells spiral wraps around axons to produce the myelin sheath in the PNS, which facilitates saltatory conduction, speeding up the transmission of electrical signals
• Axonal regeneration: Schwann cells play a critical role in the regeneration of damageed axons in the PNS. After injury, they guide the regrowth of axons by providing a substrate for the axon to grow along, promoting repair and functional recovery
• Nutritional support, provide nutrients and support to unmyelinated axons in the PNS

2) Satellite cells:

• Satellite cells are found in peripheral ganglia, surrounding neuronal cell bodies
• Structural support: provide physical support to neuron cell bodies in the ganglia
• Regulation of the microenvironment: satellite cells regulate the extracellular environment around neurons in the PNS by controlling the exchange of ions and nutrients, similar to astrocytes in the CNS
• Protection: They protect neurons from harmful substances and help maintain proper cellular function
• similar to astrocytes, wrapping around cell body however we do not know much about these cells so knowledge is not required

Glial cell dysfunction is linked to several neurodegenerative diseases. For example, in multiple sclerosis, oligodendrocytes are damaged, leading to the breakdown of the myelin sheath, which disrupts nerve signal transmission. Similarly, abnormal activation of microglia has been implicated in Alzheimer’s disease and other neuroinflammatory conditions

Question 3

Outline the basic structure of a synapse

Answer 3

Components

1) Presynaptic Neuron (Sending Neuron):

• The neuron that sends the signal
• Axon Terminal (Synaptic Bouton): The endpoint of the presynaptic neuron where neurotransmitters are stored in vesicles. When an electrical signal (action potential) reaches the axon terminal, it triggers the release of neurotransmitters into the synaptic cleft.
• Synaptic Vesicles: Small membrane-bound sacs containing neurotransmitters (e.g., dopamine, serotonin). These vesicles are docked near the membrane of the presynaptic neuron, ready to release their contents when signalled.

2) Synaptic Cleft:

• A small gap (approximately 20-40 nm wide) between the presynaptic and postsynaptic neurons.
• serves as the space across which neurotransmitters travel to bind with receptors on the postsynaptic neuron. Although narrow, this gap is crucial for controlled signal transmission between neurons

3) Postsynaptic Neuron (Receiving Neuron):

• The neuron or target cell that receives the signal.
• Postsynaptic Membrane: Contains specialised receptor proteins that bind to neurotransmitters released from the presynaptic neuron. The binding of neurotransmitters to these receptors induces either excitatory or inhibitory responses in the postsynaptic neuron.
• Neurotransmitter Receptors: These are proteins embedded in the postsynaptic membrane that are highly specific to certain neurotransmitters. Once a neurotransmitter binds to its receptor, it triggers a change in the postsynaptic cell, either depolarizing or hyperpolarizing the membrane, depending on the type of receptor and neurotransmitter involved.

Steps in Synaptic Transmission

1. Action potential passes through the presynaptic neuron
2. This permits the vesicles (carrying neurotransmitters) to proceed to synaptic gap, where fusion occurs
3. Neurotransmitters released from presynaptic neurons across the synaptic cleft
4. Neurotransmitters received by receptors at postsynaptic neuron
5. Any neurotransmitter not taken by postsynaptic neuron, remains in synaptic gap; until these are ‘recycled’ by the presynaptic neuron or destroyed

Types

• Electrical synapse: Direct ion flow through gap junctions, fast and synchronous
• Chemical synapse: Uses neurotransmitters for flexible, slower transmission

Question 4

Differentiate between the central (CNS) and peripheral (PNS) nervous systems and subdivisions

Answer 4

Central Nervous System (CNS)

1) Brain:

• Forebrain: Contains the cerebral hemispheres, thalamus, and hypothalamus
• frontal lobe = primary motor cortex; executive functions; ability to think and consider.
• Parietal lobe = Primary somatosensory cortex (pain, touch and proprioception). General awareness and long-term memory storage
• Temporal lobe = Primary auditory cortex; primary olfactory cortex; heaviliy implicated in learning and memory. Medial to it, is the hippocampus
• Occipital lobe = Primary visual cortex.
• caudal to central sulcus = intake information
• rostral to central sulcus = act
• The cerebral cortex is the outer grey matter layer of the cerebral hemisphere, 80% of cortex functions in cognition
• Thalamus, hypothalamus, subthalamus and epithalamus make up the diencephalon
• Thalamus: Acts as a relay station for sensory and motor signals to the cerebral cortex
• Hypothalamus: ventral (below) to the thalamus Controls autonomic functions (e.g. hunger, thirst, temperature regulation) and links the nervous system to the endocrine system via the pituitary gland, out of the infundibulum
• corpus callosum: WM tract which connects the hemispheres
• Midbrain: Connects the forebrain to the hindbrain; involved in movement and auditory/visual processing
• Hindbrain: consists of the cerebellum, pons and medulla oblongata
• Cerebellum: Coordinates fine motor control, posture and balance. motor movements. Arbor vitae, growing evidence regarding the role of cerebellum has within cognitions
• Pons: Relays signals from the forebrain to the cerebellum and regulates respiration
• Medulla Oblongata: Controls vital involuntary functions such as heart rate, breathing and blood pressure,

The brain is made up of grey matter and white matter

• Grey Matter: Contains neuron cell bodies and is involved in processing information
• White Matter: Contains myelinated axons, allowing for faster signal transmission between brain regions and the spinal cord

2) Spinal Cord:

• Acts as a communication pathway between the brain and the body, relaying sensory input and motor commands.
• Protected by the vertebral column and consists of both grey and white matter
• Grey matter contains neuron cell bodies and is involved in reflex arcs
• White matter contains myelinated axons that transmit signals to and from the brain
• divided into segments: cervical, thoracic, lumbar, sacral and coccygeal, each giving rise to spinal nerves that innervate different parts

Peripheral Nervous System (PNS)

PNS is responsible for transmitting sensory and motor information between CNS and peripheral tissue. PNS has 2 primary divisions:

1) Somatic Nervous System (SNS):

• Controls voluntary movements by innervating skeletal muscles
• Composed of sensory (afferent) neurons that transmit signals from sensory receptors (skin, muscles) to the CNS, and motor (efferent) neurons that sends commands from the CNS to muscles for movement

2) Autonomic Nervous System (ANS):

• Controls involuntary functions such as heart rate, digestion and respiration
• Sympathetic nervous system: Prepare the body for fight or flight during stressful situations, increase heart rate, dilating pupils and inhibiting digestion
• Parasympathetic Nervous System: Promotes rest and digest activities, reducing heart rate, stimulating digestion and conserving energy
• Enteric Nervous System: Considered part of the ANS, it controls gastrointestinal functions independently of the CNS

Nerves in the PNS

Cranial Nerves

• 12 pairs of nerves that arise from the brain and brainstem
• Control functions related to the senses (vision, smelling, hearing, taste) and motor functions (eye movement, facial expressions)
• CN1 (Olfactory) and CN 2 (Optic) are often considered part of the CNS due to their direct connection to the brain, but functionally they are considered part of the PNS

Spinal Nerves:

• 31 pairs of nerves that emerge from the spinal cord, connecting the CNS to various parts of the body
• Sensory Division
• Motor vision

Differences

• CNS main components is the Brain and spinal cord vs PNS main components are cranial nerves, spinal nerves, ganglia
• CNS is encased in bone (skull, vertebral column) vs PNS not protected by bone; more vulnerable
• CNS function is to process information, integrates sensory input, and sends motor output vs PNS functions to relay information between CNS and the body
• CNS has limited capacity for regeneration (neurons are post-mitotic) vs PNS has greater capacity for axonal regeneration
• CNS is protected by BBB, meninges and cerebrospinal fluid (CSF) vs PNS is supported by Schwann cells for myelination and structural integrity

Question 5

Describe the CNS and PNS main regions/subdivisions

Question 6

Organisation of the PNS

1. Somatic (voluntary) component
2. Autonomic (involuntary) component

not anatomically separate