Peripheral Nervous Syste/m - Comprehensive Notes

The nervous system consists of neural tissue, which is a group of specialized cells facilitating communication throughout the body.
Central Nervous System (CNS): Comprises the brain and spinal cord.
Peripheral Nervous System (PNS): Comprises nerve cells and ganglia.
A ganglion is a cluster of nerve cell bodies located outside the central nervous system.

Central Nervous System: It integrates and coordinates both voluntary and involuntary functions.
Peripheral Nervous System: It connects the body to the central nervous system.
The peripheral nervous system is further divided into the somatic nervous system and the autonomic nervous system.

Somatic Nervous System: Controls voluntary movement and sensation.
Autonomic Nervous System: Manages involuntary activities to maintain a stable internal environment. This system is further split into the sympathetic and parasympathetic nervous systems.
Sympathetic Nervous System: Primarily active during stressful situations, mediating fight-or-flight responses.
Parasympathetic Nervous System: Predominant during restful conditions, conserving energy and managing rest-and-digest mechanisms.

The peripheral nervous system performs two main functions dealing with voluntary actions:

Carries sensory signals from the body and external world to the CNS, resulting in conscious sensation.
Sometimes receives sensory input regarding pain from internal body organs

Carries motor signals dictating the brain’s desired response to the appropriate location.
Controls voluntary movement e.g., picking up a veggie burger, petting the cute dog.

The autonomic nervous system performs two main functions dealing with involuntary actions:

Carries motor signals that control autonomic activities to the appropriate organ.
Can help maintain homeostasis in our bodies and carry out certain reflexes.
- Speed/slow heart and breathing rates
- Increase/decrease digestive functions

Sympathetic: Fight or Flight
The sympathetic nervous system responds to a state of stress or fear.
For example, it may receive conscious sensation from the somatic nervous system about seeing a bear on a hike. Some responses will be:
- Increasing blood pressure and heart rate to deliver more blood to muscles
- Decreases digestive activity to conserve energy
- Dilates pupils to let in more light to see better
- Stimulates adrenal glands to secrete hormones that prolong other sympathetic responses

Parasympathetic: Rest and Digest
The parasympathetic nervous system works when there is no threat and the body can relax
For example, after avoiding the bear, the parasympathetic nervous system will:
- Decrease blood pressure and heart rate as the body no longer needs as much oxygen
- Increase digestive activity
- Constricts pupils

The PNS is composed of nerves (clusters of neurons) that extend out from the brain and the spinal cord.
- Spinal nerves carry signals to and from the spinal cord.
- Cranial nerves carry signals to and from the brain.
These nerves serve as communication lines (sensory and motor) and link every part of the body to the CNS.

There are 12 pairs of cranial nerves
They primarily serve the head and neck
- Exception: the vagus nerve which extends into the thoracic and abdominal cavities
Some are mixed nerves, some are sensory nerves, some are motor nerves.

There are 31 pairs of spinal nerves
Spinal nerves are named according to where they emerge from:
- Cervical
- Thoracic
- Lumbar
- Sacral
- Coccygeal
The spinal nerves are mixed nerves, in that they contain nerves that are part of the afferent and efferent pathways.
They are formed by the combination of the ventral and dorsal roots of the spinal nerves
The dorsal horns contains sensory neurons entering from the dorsal root. The cell bodies are located inside the dorsal root ganglion
The ventral horns contain cell bodies for motor neurons of the somatic nervous system. These cells send out their axons through the ventral root of the spinal cord
The dorsal and ventral roots fuse to form the spinal nerves

Recall:

3 neuron chain:
1. Sensory neuron from receptor cell to spinal cord
2. Interneuron goes up spinal cord to thalamus
3. Finally, the signal is relayed by a neuron from the thalamus to somatosensory cortex
After signal is transmitted to the brain, there is often a motor output that follows.

Somatic reflexes
- Reflexes that stimulate the skeletal muscles
- Involuntary, although skeletal muscle is normally under voluntary control
- Example: pulling your hand away from a hot object
- Incorporates spinal cord and PNS
Autonomic reflexes
- Regulate the activity of smooth muscles, the heart, and glands in response to external stimuli
- All involuntary reflexes
- Example: regulation of smooth muscles, heart and blood pressure, glands, digestive system
- Incorporates brainstem, spinal cord, and PNS

Reflexes occur over pathways called reflex arcs which are composed of five elements:

Sensory receptor: Receives a stimulus
Sensory neuron: Carries message from receptor to the integration center
Integration center (CNS): Grey matter in spinal cord processes information and directs motor output
Motor neuron: Carries message to an effector
Effector organ: Is the muscle or gland to be stimulated

Spinal reflexes are immediate, involuntary reactions brought on by the spinal cord that are advantageous for a person’s safety

Stimulus causes a painful sensation picked up by sensory receptor
Sensory neuron carries information to spinal cord
Interneurons in spinal cord integrate information and send information to motor neurons
Motor neuron stimulates appropriate muscle
Muscles contract, removing appendage or limb from stimulus