Human Nervous System

Composition: The nervous system is a complex network consisting of the brain, the spinal cord, and a vast network of nerves.
Distribution: These nerves extend through all organs and muscles, reaching out to the skin.
Relationship with the Endocrine System:
- The nervous system and the endocrine system share many characteristics and act together to regulate the body.
- Response Speed: Endocrine responses develop more slowly compared to nervous system responses.
- Response Duration: Endocrine responses last significantly longer than those of the nervous system.

Sensory Input:
- Purpose: To monitor the body's internal state and its surrounding environments.
- Mechanisms: Detects changes through specialized sensory receptors that respond to stimuli.
- Stimuli Types: Includes light, sound, temperature, and pressure.
- Transmission: Sensory neurons, also known as afferent neurons, carry signals from these receptors to the Central Nervous System (CNS).
- Concrete Examples: Feeling the heat from a flame or noticing a loud, sudden noise.
Integration:
- Purpose: To process and interpret incoming sensory information to determine an appropriate response.
- Location: This process occurs within the brain and the spinal cord (CNS).
- Decision Making: Integration involves thinking, memory, and emotions, making decisions based on both current input and past experiences.
- Concrete Example: Deciding to withdraw your hand after the integration center processes the sensation of touching something hot.
Motor Output:
- Purpose: To initiate a physical response based on the processed information.
- Transmission: Signals are sent from the CNS to effectors, which are muscles or glands.
- Mechanisms: Uses motor neurons, also known as efferent neurons.
- Action Types: Triggers both voluntary and involuntary actions.
- Concrete Examples: Physically moving your hand away from a source of pain or an increase in heart rate during a stressful situation.

Definition: Neurons are the functional units or cells of the nervous system.
Scale: The nervous system is composed of billions of these neurons.
Function: Neurons carry the impulses responsible for the majority of the functions performed by the body.
Size and Variability:
- Neurons occur in a variety of sizes.
- They vary largely in length, ranging from a fraction of a millimetre to over $1\,m$ long.
Structural Components of a Neuron:
- Nucleus: Located within the cell body.
- Cell Body: The central part of the neuron.
- Dendrites: Branch-like structures that receive signals.
- Axon Hillock: The junction between the cell body and the axon.
- Axon: The long projection that conducts impulses away from the cell body.
- Axon Terminals: The ends of the axon where signals are transmitted to other cells.

Central Nervous System (CNS):
- Components: Formed by the brain and the spinal cord.
- The Brain: Located within the skull; it functions as the primary control center for most bodily functions.
- The Spinal Cord: A continuation of the brain that resides within the protection of the vertebral column.
Peripheral Nervous System (PNS):
- Components: Consists of personal nerves, specifically cranial and spinal nerves.
- Function: Acts as the communication line between the CNS and the rest of the body.

Definition: Three different membranes located between the soft brain tissue and the bones of the skull.
Coverage: These three layers form a continuous covering over the brain and the spinal cord within the vertebral column.
Layers of the Meninges:
- Pia mater (innermost layer).
- Arachnoid mater (middle layer).
- Dura mater (outermost layer).
Subarachnoid Space: The space around the spinal cord and brain that contains Cerebrospinal Fluid (CSF). This space is continuous between the brain and the spinal cord.

The Spinal Cord (Internal Structure):
- Grey Matter: Contains the cell bodies and dendrites of neurons.
- White Matter: Surrounds the grey matter and contains the myelinated axons of neurons.
Physical Extension: Extends from the brain down through the spinal canal, which is located inside the protective vertebral column.
Nerve Transmission: The spinal cord consists of millions of nerve fibers transmitting electrical information between the brain and the limbs, trunk, and organs.
Nerve Classifications in the Spinal Cord:
- Motor Neurons: Carry information from the brain to the muscles.
- Sensory Neurons: Carry information from the body back to the brain, including data on skin temperature, touch, pain, and joint position.

Afferent (Sensory) Division:
- Composed of sensory neurons.
- Conducts signals from receptors toward the CNS.
Efferent (Motor) Division:
- Composed of motor neurons.
- Conducts signals from the CNS away toward effectors.
Somatic Nervous System (SNS):
- Function: Controls skeletal muscle contractions.
- Voluntary Actions: Example: Moving an arm to pick up an object.
- Involuntary Actions: subconscious movements, such as moving a hand away from heat before pain is fully felt.
Autonomic Nervous System (ANS):
- Function: Controls involuntary responses and regulates the internal environment (homeostasis).
- Targets: Controls activity of smooth muscles, cardiac muscles, and glands.

Definition: An activity that occurs rapidly without conscious control.
Location: Many reflexes occur at the spinal cord level (e.g., the knee-jerk reflex).
The Reflex Arc Sequence:
1. Sensory Receptor detects stimulus.
2. Sensory Neuron transmits signal.
3. Spinal Cord processes information.
4. Motor Neuron transmits response signal.
5. Effector Organ performs the action.
Step-by-Step Example (Stepping on a pin):
1. A sharp pin touches pain receptors in the foot.
2. Nerve endings send information to special cells in the spinal cord, bypassing the brain for speed.
3. The spinal cord processes this and sends impulses down to the leg muscle; simultaneously, the brain is informed of the action.
4. The leg moves quickly to avoid injury.
Other Types of Reflexes: Withdrawal reflex, pupillary reflex, and blink reflex.

Vital Functions Regulated by the ANS:
- Heart rate and Blood pressure.
- Breathing rate and Body temperature.
- Digestion and Metabolism.
- Pupil dilation, Sweating, and Salivation.
Sympathetic Division (Fight or Flight):
- Activation: Occurs during periods of stress or perceived threat.
- Goal: Prepares the body to confront (fight) or flee (flight) the situation.
- Duration: Usually short-lived.
- Physical Effects: Increases heart rate, blood pressure, body temperature, and airflow through airways.
Parasympathetic Division (Feed and Breed / Rest and Digest):
- Activation: Most active during quiet, non-stressful conditions.
- Goal: Conserves energy and has a calming effect on the body.
- Functions: Regulates digestion and reproductive functions.
- Extreme Responses: Can be activated in perceived hopeless situations, leading to uncontrolled urination, defecation, or fainting.