Excerpt from the course
Peripheral Nervous System
The peripheral nervous system encompasses all neural structures outside the brain and spinal cord: the sensory receptors, peripheral nerves and ganglia and efferent motor ending.
Sensory Receptors
Sensory receptors are part of a sensory neuron that responds to change in the environment and stimuli (environmental change and surroundings). These changes can present in the form of sound, pressure, chemical and other various forms. Sensory receptors and sensory neurons are designed to become aware of the electrical energy of the outside stimuli and respond to it. Once the response occurs, the sensory receptor will then relay it back to the nervous system via graded potentials that in turn trigger nerve impulses along the afferent fibres on the way to the CNS. When the stimulus ceases, the body ceases to notice this due to the receptor potential being deactivated.
Peripheral Nerves and Associated Ganglia
The PNS is divided into sensory (afferent) and motor (efferent) division. The nerves that carry the impulses both towards the CNS are sensory/afferent and nerves that carry impulses away from the CNS are motor/efferent. Peripheral nerves are classified as cranial or spinal nerves depending on whether they arise from the brain or spinal cord. Ganglia are neuron cell bodies that are associated with nerve fibres in the PNS. Ganglia associated with afferent nerve fibres contain just the cell bodies of sensory neurons and are known as dorsal root ganglia.
Motor Endings
These are the motor endings that activate effectors by releasing neurotransmitters.
Cranial Nerves
There are twelve pairs of cranial nerves associated with the brain that pass through various regions of the skull. Cranial nerves are mixed nerves and have many different functions.
Sensory, Motor and Integrated Systems
In this section we look at how certain facets work together to carry out the basic functions of receiving sensory input, integrating, associating and storing information, and transmitting motor impulses that result in movement or secretion. It is through this functioning of systems that the body is able to communicate and relate with each other. It is necessary to have these in place so if we feel pain; we can react to alleviate the discomfort (for example, moving our hand away from a fire).
Sensation is being aware of a change in the environment, whether that environment is external or internal. Each type of sensation such as touch, pain, hearing and vision is called a sensory modality. Each specific sensory neuron carries information for that particular sensory modality. Neurons carrying information for touch can only transmit impulses for touch, not pain or hearing
General Senses
The general senses are made up of both somatic senses and visceral senses. Somatic senses are include tactile sensations which are touch, pressure and vibration, thermal sensations, warm and cold, pain sensations and proprioceptive sensations which is perception of both moving and non moving body parts. Visceral senses are information about internal organs.
The general or somatic senses begin in the receptors located in the skin (cutaneous) or embedded in muscles, tendons, joints and the inner ear (proprioceptive). Anaesthesia means without sensation and when it is used it blocks sensation within the body and between systems. The sensations from the somatic receptors then cross over to the opposite side in the spinal cord or brainstem before going to the thalamus.
There are two general pathways from sensory receptors to the cortex: posterior column pathway (discriminative touch and proprioception) and the spinothalamic (anterolateral) pathway (pain and temperature). This input is then integrated in the Central Nervous System which is then conveyed by the motor pathway to create a response.
The integrative functions include such activities as memory, sleep and wakefulness, and emotional responses (limbic system). These functions are the subject of many studies and we are just beginning to understand the physiology and interdependency of these systems with the rest of the body. Of course we are aware when we have little sleep it can affect our emotional responses as well as our memory.
The Process of Sensation
The process begins in a sensory receptor (either a specialised cell or in the dendrites of a sensory neuron). The sensory receptor responds to stimulus or a different change in the environment. Once the receptor is stimulated, they will convert energy into a graded potential. Importantly, each sensory receptor can only transduce its own particular type of stimulus.
Once the message is received, a particular region of the CNS will receive and integrate the sensory nerve impulse and then integrate it into the cerebral cortex. Different sensory impulses from each part of the body arrive in a particular region of the cerebral cortex, which interprets the sensation as coming from the stimulated sensory receptor. Therefore, you seem to hear with your ears, or feel pain in a certain bodily part, when in fact it is your brain telling you about the action as it responds to the message.
Adaption in Sensory Receptors:
Over a certain amount of time, the generator or receptor potential decreases if a stimulus is constantly maintained. The nerve impulses in the first order neuron therefore decrease and adapt to the ongoing stimulus and because of this, the perception of a stimulus may decrease or fade even though the stimulus does not change.
There are two different types of adapting receptors, rapidly adapting and slowly adapting. Rapidly adapting receptors adapt quickly to a change, such as touch, smell and pressure. Slowly adapting receptors continue to trigger nerve changes for the duration of the stimulus such as pain and body positions
Special Senses
Special senses include the sensory modalities of smell, taste, hearing, vision and balance. These are all necessary if we are to notice specific alterations and changes in our environments. These sensations all play a role in deepening the impact of our surroundings such as experiencing people, food, music, or a rose. And sensation can also have a great effect on our organ systems. Seeing a full buffet, especially when we are hungry, can start the whole digestive process. Hearing a beautiful song can relax our cardiovascular system by affecting the autonomic nervous system.
Sense of Smell: Olfaction
The receptors for olfaction are known as bipolar neurons. These neurons are stored in the nasal epithelium with olfactory glands. Axons from olfactory receptors form the olfactory nerves which relay messages to olfactory bulbs. These then transmit to the limbic system and temporal and frontal lobes of the cerebral cortex
Sense of Taste: Gustation
The gustatory receptor cells are stored in the taste buds. Dissolved chemicals known as tastants stimulate gustatory receptors by either binding to receptors attached to G-proteins in the membrane, or passing through ion channels. Gustatory receptor cells then produce receptor potentials which cause the release of neurotransmitter. This will then stimulate nerve impulses in first order sensory neurons. Gustatory receptor cells trigger nerve impulses in cranial nerves VII, IX and I. Signals also pass to the medulla oblongata, thalamus and cerebral cortex (parietal lobe).
Vision
Refraction of light on the cornea and lens reflect an image on the retina, which focus an inverted image on the central fovea on the retina. Transduction of light energy into a receptor potential occurs in the outer segment of both rods and cones. Receptor potentials in rods and cones decrease the release of inhibitory transmitters, which induces graded potentials in bipolar and horizontal cells. Horizontal cells transmit inhibitory signals to bipolar cells, bipolar and amacrine cells transmit excitatory signals to ganglion cells, which depolarize and initiate nerve impulses. Ganglion nerve impulses are then conveyed into the optic nerve II fibre, to the thalamus and from there to the cerebral cortex
Hearing
Sound waves enter the external auditory canal, strike the eardrum and pass through the ossicle. They then pass through the vestibular membrane and scala tympani to increase pressure in the endolymph and vibrate the basilar membrane. Â Hair bundles on the spiral organ are finally stimulated. These hair cells convert vibrations into receptor potentials which release a certain neurotransmitter which initiates nerve impulses into first order sensory neurons. Sensory axons which are located in the cochlear branch terminate in the medulla oblongata. They then pass to the inferior colliculus, thalamus and temporal lobes of the cerebral cortex
Equilibrium
There are two different types of equilibrium, static and dynamic. Static is the maintenance of the position of the body (in general the head), usually the steady state of the body without movement and dynamic equilibrium is the maintenance of the body position (head in general) in response to sudden movements such as turning, speeding up and slowing down.
Iona Lister