Nervous System Divisions: CNS and PNS

Divisions of the Nervous System

• The nervous system is a complex network of cells and tissues.
• It coordinates the body's responses to internal and external stimuli.
• The central nervous system (CNS) and the peripheral nervous system (PNS) are the two main divisions.

Central Nervous System (CNS)

• The CNS includes the brain and spinal cord.
• It is the control center, integrating sensory information and dispatching commands.
• The brain is the body's most complex organ
• It handles higher functions like thinking, memory, emotion, and decision-making.
• The brain is divided into the cerebrum, cerebellum, and brainstem, each with specific functions.
• The spinal cord provides a communication pathway between the brain and the rest of the body.
• It is involved in reflexes, enabling rapid, automatic responses without brain involvement.

Peripheral Nervous System (PNS)

• The PNS consists of nerves outside the brain and spinal cord.
• The PNS connects the CNS to the limbs and organs.
• It serves as the communication system between the body and the CNS.
• The sensory division transmits sensory information from receptors to the CNS.
• The motor division transmits motor commands from the CNS to muscles and glands.
• The motor division is further divided into the somatic and autonomic nervous systems.

Autonomic Nervous System (ANS)

• The ANS controls involuntary bodily functions without conscious control.
• These functions include heart rate, digestion, and respiratory rate.
• The sympathetic nervous system activates the "fight or flight" response during stress.
• The sympathetic nervous system increases heart rate, dilates pupils, and redirects blood flow to muscles.
• The parasympathetic nervous system promotes relaxation and energy conservation.
• The parasympathetic nervous system slows heart rate, constricts pupils, and increases digestion.

Somatic Nervous System

• The somatic nervous system controls voluntary movements
• It transmits motor commands from the CNS to skeletal muscles.
• It allows conscious control over body movements like walking and talking.

Neuron Structure

• Neurons transmit electrical signals.
• Dendrites are short, branching extensions that receive signals.
• Dendrites convert incoming signals into electrical impulses directed towards the cell body.
• The cell body (soma) contains the nucleus and organelles.
• The cell body maintains the cell's metabolism and integrates signals from dendrites.
• The axon is a slender projection that transmits electrical impulses away from the cell body.
• Axons transmit impulses towards other neurons, muscles, or glands.
• The myelin sheath is an insulating layer around some axons.
• It increases the speed of electrical signal transmission and prevents signal loss.
• Oligodendrocytes in the CNS and Schwann cells in the PNS produce it.
• Nodes of Ranvier are gaps in the myelin sheath with concentrated ion channels.
• Nodes of Ranvier facilitate faster signal conduction via saltatory conduction.

Membrane Potential

• Membrane potential is the electrical charge difference across the cell membrane, essential for transmitting signals.
• Resting membrane potential is the electrical potential of a neuron at rest, around -70 mV.
• The sodium-potassium pump maintains it by moving 3 sodium ions out and 2 potassium ions into the cell.
• This creates a polarized state, with the inside more negatively charged.
• Ion channels are proteins allowing specific ions to pass.
• Ion gradients refer to the concentration difference of ions inside versus outside the cell.
• Sodium is more concentrated outside, and potassium is more concentrated inside.

Changes in Membrane Potential

• Membrane potential changes are crucial for transmitting electrical signals.
• Depolarization is when the membrane potential becomes less negative.
• This happens as sodium channels open, allowing sodium ions into the cell, which reduces the negative charge.
• Repolarization is the return to the resting membrane potential.
• Potassium channels open, allowing potassium ions to exit, restoring the negative charge.
• Hyperpolarization is when the membrane potential becomes more negative than resting potential.
• This happens when excess potassium ions leave or chloride ions enter, hindering the neuron from firing.

Graded and Action Potentials

• Graded potentials are small, localized changes in membrane potential in response to stimuli.
• The strength of the stimulus determines the size of the graded potential.
• Graded potentials are essential for initiating action potentials but cannot travel long distances.
• Action potentials are rapid, large changes in membrane potential traveling down the axon.
• It transmits electrical signals over long distances.
• Action potentials occur when a threshold is reached, opening sodium channels for rapid sodium influx.
• Repolarization then occurs as potassium ions exit.

Conduction of Action Potentials

• Saltatory conduction is when the action potential jumps from one node of Ranvier to the next along myelinated axons.
• Saltatory conduction significantly speeds up signal transmission.
• Continuous conduction occurs in unmyelinated axons, with continuous depolarization and repolarization along the axon's length.
• Continuous conduction is slower.

Nervous System Signaling

• The nervous system uses electrical and chemical signals to transmit information.
• Electrical signals are action potentials traveling along the axon to other neurons or target cells.
• Chemical signals involve neurotransmitters released into the synapse to transmit the signal to the next cell.

Neurotransmitters and Synapses

• Neurotransmitters are chemical substances involved in signal transmission across synapses.
• Dopamine, serotonin, acetylcholine, and norepinephrine are examples.
• They bind to receptors on the postsynaptic cell, causing excitation (depolarization) or inhibition (hyperpolarization).
• Synapses are junctions between two neurons or a neuron and a target cell like a muscle or gland.
• A synapse contains a presynaptic terminal, synaptic cleft, and postsynaptic membrane.

Reflex Arcs

• Reflex arcs are pathways for producing a reflex action through a series of steps.
• Reflex actions allow the body to respond quickly to a stimulus without brain input.
• The receptor is a sensory organ or cell that detects the stimulus (e.g., pain, pressure, heat).
• The sensory neuron transmits the sensory information from the receptor to the spinal cord or brainstem.
• The integration center is in the spinal cord or brainstem.
• In the integration center the sensory neuron synapses with an interneuron or a motor neuron.
• The motor neuron transmits the response signal from the spinal cord or brainstem to the effector.
• The effector is the muscle or gland that carries out the reflex action.

Monosynaptic vs. Polysynaptic Reflexes

• Monosynaptic reflexes involve only one synapse between the sensory and motor neuron.
• An example of a monosynaptic reflex is the patellar (knee-jerk) reflex.
• Polysynaptic reflexes involve more than one synapse and include one or more interneurons.
• An example of a polysynaptic reflex is the withdrawal reflex.

Reaction Time

• Reaction time is the duration it takes for the body to respond to a stimulus.
• Voluntary reactions involve conscious processing of stimuli.
• The brain interprets the stimulus in voluntary reactions
• The motor cortex sends a signal to initiate a reaction.
• Voluntary reactions involve higher brain centers.
• Reflex reactions involve automatic, involuntary responses that bypass higher brain centers.
• Reflexes are processed directly in the spinal cord or brainstem.
• The patellar and withdrawal reflexes occur nearly instantly.

Differences in Processing Time

• Voluntary and reflex actions have significantly different processing times.
• Reflexes are fast as they involve a short pathway through the spinal cord or brainstem.
• Monosynaptic reflexes transmit signals directly from the sensory neuron to the motor neuron.
• The signal result in a very rapid response (less than 50 milliseconds).
• Voluntary actions need more time as the signal requires processing at higher brain centres
• The higher brain centres include the cortex.
• This makes voluntary actions slower compared to reflexes.

Brain Structures

• The brain is the control center, regulating body functions, processing sensory information, and enabling cognitive tasks.
• The brainstem connects the brain to the spinal cord and controls basic life-sustaining functions.
• These functions include heart rate, breathing, and digestion.
• The cerebellum is at the back of the brain.
• The cerebellum is responsible for coordination, balance, and fine motor control.
• The limbic system deals with emotion, memory, and arousal.
• The limbic system includes the hippocampus, amygdala, and hypothalamus.
• The cerebrum is the largest part of the brain, that is responsible for higher cognitive functions.
• Reasoning, memory, and voluntary movement are all higher cognitive functions.
• The cerebrum controls the opposite sides of the body.

Functions of the Major Brain Regions

• The brainstem is a basic part of the brain that is essential for survival.
• The brainstem connects to the spinal cord.
• It regulates vitals such as heart rate, blood pressure, breathing, and digestion.
• The medulla, regulates reflexes like coughing and swallowing.
• The pons regulates breathing and sleep cycles.
• The pons facilitates communication between the cerebellum and the rest of the brain.
• The midbrain influences vision, hearing, and motor control.
• The superior and inferior colliculi are located in the midbrain and deal with visual and auditory processing respectively.
• The cerebellum ensures smooth, coordinated movements.
• The cerebellum adjusts and refines motor commands from the brain.
• It coordinates voluntary movements, balance, posture, and fine motor control such as writing or playing instruments.
• The Hippocampus is crucial for creating new memories
• The Hippocampus is responsible for making short-term memories long-term.
• The Amygdala is involved in emotional memory and responses to stress.
• The Hypothalamus regulates body temperature, hunger, thirst, and the release of hormones.

Cerebrum and its Lobes

• The cerebrum is responsible for complex thought processes such as reasoning, problem-solving, memory, and learning.
• The frontal lobe is involved in decision-making, planning, problem-solving, and voluntary motor control.
• The prefrontal cortex handles executive functions such as judgment, impulse control, and social behavior.
• Broca's area is located in the Frontal lobe, which is involved in speech production and language expression.
• Broca's aphasia, caused by damage to Broca's area, results in difficulty speaking, but comprehension remains intact.
• Located on the sides of the brain, the Temporal lobe is responsible for auditory processing as well as memory.
• Wernicke's area helps with language comprehension.
• The Parietal Lobe processes sensory information of touch, temperature, and pain.
• The Parietal Lobe influences spatial orientation and coordination of movement.
• The Occipital lobe processes vision.
• The Occipital lobe contains the visual cortex, which interperates signals from the eyes.

Somatosensory Pathways

• The somatosensory system processes sensory information related to touch, pain, temperature, and body position.
• The sensory information from the body transfers to the brain through specialized pathways
• Primary sensory neurons detect sensory stimuli from sensory receptors.
• Primary sensory neurons then send signals to the spinal cord or brainstem.
• Secondary neurons transmit sensory signals from the spinal cord or brainstem to the thalamus.
• The thalamus relays information to specific brain areas for processing.
• Tertiary neurons bring the information from the thalamus to the cerebral cortex for conscious perception.

Transmission of Sensory Information

• Sensory information travels from receptors via the PNS to the CNS, where it is processed.
• Sensory receptors detect specific stimuli from the environment (e.g., mechanical pressure, temperature changes).
• If a stimulus is strong enough, it generates an action potential in the sensory neuron.
• The action potential is then transmitted to the CNS.
• The brain processes the signals received as action potentials.
• Signal processing allows the sensations of touch, pain, and temperature to be perceived.

Somatosensory Receptors

• Sensory receptors are specialized cells that detect different types of sensory stimuli.
• Mechanoreceptors detect mechanical stimuli.
• These stimuli include pressure, vibration, and stretch, locate in the skin, muscles, and internal organs.
• Merkel cells, Pacinian corpuscles, and Meissner's corpuscles are examples of Mechanoreceptors.
• Thermoreceptors detect temperature changes.
• Thermoreceptors are located in the body for the body to sense heat and cold.
• Nociceptors, located throughout the body, detect pain from physical damage or harmful stimuli.

Proprioception

• Proprioception is the ability to sense the position, movement, and orientation of the body and its parts.
• Proprioception assists with movement and balance.
• Proprioceptors, found in muscles, tendons, and joints, provide information about the position of body parts.
• Muscle spindles, Golgi tendon organs, and Joint receptors are examples of Proprioceptors.
• Proprioception maintains a sense of body position and makes adjustments for smooth movement and balance.

Pain Perception

• Pain alerts the body to potential or actual tissue damage.
• The perception of pain involves processes in the peripheral and central nervous systems.
• Nociceptive pain results from the activation of nociceptors due to damage/noxious stimuli.
• Nociceptive pain is localized and sharp, such as from a cut or burn.
• Neuropathic pain stems from nerve damage, leading to chronic pain that feels like burning, tingling, or shooting.
• Neuropathic pain can occur even without tissue injury, due to nerve dysfunction.

Two-Point Discrimination

• Two-point discrimination is the ability to distinguish two nearby stimuli on the skin.
• Two-point discrimination measures the density of sensory receptors throughout the body.
• Mechanoreceptors have a greater ability to perceive two-point discrimination.
• Mechanoreceptors provide higher acuity.
• Two point discrimination assists with fine motor control and spatial awareness e.g. reading braille.
• Sensory receptor density influences sensitivity of sensory perception. High receptor density in certain areas allows for finer discrimination of stimuli.
• The fingertips, lips, and tongue have a high density of sensory receptors, enabling discrimination.
• The back or legs have a lower receptor density, making them less sensitive to touch.

Vision and the Eye

• Vision is an important sense for perceiving the world through light.
• The eye detects light, focuses images, and sends signals to the brain for processing.
• The cornea is the dome-shaped outer layer of the eye which helps to focus light.
• The aqueous humor is a fluid between the cornea and the lens which maintains intraocular pressure.
• The pupil is a circular opening that regulates the amount of light entering the eye for processing.
• The iris contains muscles that control pupil size based on how much light is around.
• The lens changes shape to focus on objects, called accommodation.
• The ciliary body contains muscles that help change the shape of the lens.
• The retina receives the light and contains rods and cones.
• The optic nerve transmits visual information from the retina to the brain for processing.
• The macula is the central area of the retina, that is responsible for detailed central vision.
• The fovea is the small central region of the macula which provides the clearest vision.
• The vitreous humor is a gel-like substance that maintains the shape of the eye and allowing light to pass through to the retina.
• Light passes through the cornea and is bent to focus onto the retina.
• Light is absorbed when it hits the retina by photoreceptor cells, the signals are sent to the brain.
• Image processing occurs by the brain to create images from the signals.
• Rods are photoreceptors for vision in low-light conditions
• Cones are photoreceptors that function in bright light conditions.
• During Phototransduction Process light causes a chemical change that generates an electrical signal, which signals the brain.
• The fovea contains a high concentration of cones, which allows fine details to be viewed.