Human Anatomy and Physiology 2 - Nervous System Divisions (PPT/PDF)
Download notes (PPT/PDF) for Human Anatomy and Physiology 2, focusing on the divisions of the nervous system. Topics covered include: classification of the nervous system, the peripheral nervous system, the somatic nervous system, the autonomic nervous system, the sympathetic and parasympathetic nervous systems, autonomic motor pathways, autonomic receptors, and the differences between the somatic and autonomic nervous systems. Suitable for B.Pharm and related health science programs.
Keywords: Nervous System, Classification, Peripheral Nervous System, PNS, Somatic Nervous System, Autonomic Nervous System, ANS, Sympathetic, Parasympathetic, Motor Pathway, Autonomic Receptors, Somatic vs. Autonomic, Human Anatomy and Physiology, B.Pharm, PPT, PDF, Download.
Divisions of the Nervous System - Detailed Explanation
1. Classification of the Nervous System
The nervous system is broadly classified into two main divisions:
- Central Nervous System (CNS): Includes the brain and spinal cord. The CNS is the integrating and control center of the nervous system.
- Peripheral Nervous System (PNS): Includes all the nervous tissue outside the CNS (nerves, ganglia, sensory receptors). The PNS connects the CNS to the limbs and organs, serving as a communication relay.
2. Peripheral Nervous System (PNS)
The PNS is further divided into two main functional divisions:
- Sensory (Afferent) Division: Carries sensory information from receptors *to* the CNS. This includes somatic sensory information (from skin, muscles, joints) and visceral sensory information (from internal organs).
- Motor (Efferent) Division: Carries motor commands *from* the CNS to effector organs (muscles and glands). The motor division is further subdivided into:
- Somatic Nervous System (SNS)
- Autonomic Nervous System (ANS)
3. Somatic Nervous System (SNS)
The SNS controls *voluntary* movements of skeletal muscles. It has a single motor neuron pathway: a motor neuron in the CNS extends its axon directly to a skeletal muscle fiber. The neurotransmitter released at the neuromuscular junction is acetylcholine (ACh), which binds to nicotinic receptors on the muscle fiber, causing contraction.
4. Autonomic Nervous System (ANS)
The ANS controls *involuntary* functions, such as heart rate, digestion, respiration, glandular secretions, and smooth muscle contraction. It has a two-neuron motor pathway:
- Preganglionic Neuron: Cell body is in the CNS. Its axon extends to an autonomic ganglion.
- Postganglionic Neuron: Cell body is in the autonomic ganglion. Its axon extends to the effector organ (smooth muscle, cardiac muscle, or gland).
The ANS is further divided into two branches:
- Sympathetic Nervous System
- Parasympathetic Nervous System
5. Sympathetic and Parasympathetic Nervous Systems
These two divisions of the ANS often have opposing effects on target organs, providing a fine-tuned control mechanism.
- Sympathetic Nervous System: The "fight-or-flight" system. It prepares the body for stressful or emergency situations. Key effects include:
- Increased heart rate and contractility
- Increased blood pressure
- Bronchodilation (widening of airways)
- Increased blood glucose levels
- Dilation of pupils
- Redirection of blood flow to skeletal muscles
- Decreased digestive activity
Anatomy: Preganglionic neurons originate in the thoracolumbar region of the spinal cord (T1-L2). Ganglia are located near the spinal cord (sympathetic trunk ganglia and prevertebral ganglia). Preganglionic fibers are short; postganglionic fibers are long.
Neurotransmitters: Preganglionic neurons release acetylcholine (ACh). Most postganglionic neurons release norepinephrine (NE). Some postganglionic sympathetic neurons (e.g., those innervating sweat glands) release ACh.
- Parasympathetic Nervous System: The "rest-and-digest" system. It promotes energy conservation and maintenance functions. Key effects include:
- Decreased heart rate
- Increased digestive activity
- Constriction of pupils
- Bronchoconstriction (narrowing of airways)
Anatomy: Preganglionic neurons originate in the brainstem (cranial nerves III, VII, IX, X) and the sacral region of the spinal cord (S2-S4). Ganglia are located near or within the effector organs (terminal ganglia). Preganglionic fibers are long; postganglionic fibers are short.
Neurotransmitters: Both preganglionic and postganglionic neurons release acetylcholine (ACh).
6. Autonomic Motor Pathway
As mentioned above, the autonomic motor pathway involves two neurons:
- Preganglionic Neuron:
- Cell body in the CNS (brainstem or spinal cord).
- Axon extends to an autonomic ganglion.
- Releases acetylcholine (ACh).
- Postganglionic Neuron:
- Cell body in the autonomic ganglion.
- Axon extends to the effector organ (smooth muscle, cardiac muscle, or gland).
- Releases either norepinephrine (NE) (most sympathetic) or acetylcholine (ACh) (parasympathetic and some sympathetic).
7. Autonomic Receptors
Autonomic neurotransmitters bind to specific receptors on the effector cells:
- Cholinergic Receptors (bind ACh):
- Nicotinic Receptors: Found on all postganglionic neurons (both sympathetic and parasympathetic) and at the neuromuscular junction of skeletal muscle. Always excitatory.
- Muscarinic Receptors: Found on effector cells innervated by parasympathetic postganglionic neurons, and on effector cells innervated by sympathetic postganglionic neurons that release ACh. Can be excitatory or inhibitory.
- Adrenergic Receptors (bind NE and epinephrine):
- Alpha Receptors (α1, α2): Generally excitatory, but with some exceptions.
- Beta Receptors (β1, β2, β3): β1 receptors are generally excitatory (especially in the heart), β2 receptors are generally inhibitory (e.g., causing bronchodilation), and β3 receptors are involved in lipolysis.
8. Differences Between Somatic and Autonomic Nervous Systems
| Feature | Somatic Nervous System | Autonomic Nervous System |
|---|---|---|
| Effectors | Skeletal muscles | Smooth muscle, cardiac muscle, glands |
| Control | Voluntary | Involuntary |
| Number of Neurons in Pathway | One | Two (preganglionic and postganglionic) |
| Neurotransmitter at Effector | Acetylcholine (ACh) | Acetylcholine (ACh) or Norepinephrine (NE) |
| Effect on target tissue | Always excitatory | Excitatory or inhibitory |
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